assessment of sustainable technologies for pig manure

International Master of Science in Environmental Technology and Engineering

Master’s dissertation submitted in partial fulfilment of the requirements for the joint degree of

EU Erasmus+ Master course organized by

University of Chemistry and Technology, Prague, the Czech Republic

IHE Delft Institute for Water Education, Delft, the Netherlands

Ghent University, Ghent, Belgium

Academic year 2018-2020

Assessment of sustainable technologies for pig manure

valorization – a meta-analysis Ghent University, Ghent, Belgium

Vaibhav Shrivastava Thesis ID identifier: ES-IMETE.20-28

Promotors: Prof. Filip Tack, Prof. Erik Meers

Tutor: Caleb Elijah Egene

i

Copyright Statement

This is an unpublished M.Sc. dissertation and is not prepared for further distribution. The

author and the promoter give the permission to use this Master dissertation for consultation and

to copy parts of it for personal use. Every other use is subject to the copyright laws, more

specifically the source must be extensively specified when using results from this Master

dissertation.

The Promoter 1: The Author:

Prof. Filip Tack Vaibhav Shrivastava

The Promoter 2:

Prof. Erik Meers

ii

Preamble

This preamble concerns the impact of Coronavirus COVID-19 (outbreak in 2020) on the

master's dissertation work conducted on the initial topic of “Use of digestate and charred

digestate (biochar) to increase stable organic carbon in soils”.

The intended focus of the study was to employ the use of various valorization mechanisms for

the recovery of phosphorous from raw pig manure. Initially, for the month of February and

March 2020, the experiments with pig manure were conducted at the Faculty of Bioscience

Engineering (ECOCHEM lab), Ghent University. However, after the ban on 16th March 2020

on all lab activities at UGent, no further experiments could be carried out. Due to this

inaccessibility, the application and analysis of prepared samples in pot experiments could not

be continued, and not enough data was obtained from the previous experiments for proper

interpretation.

To deal with the following crisis, a reorientation towards a detailed critical literature-based

statistical study (meta-analysis) was proposed for the final thesis writing. This further led to

the modification of the master’s dissertation title to “Assessment of sustainable technologies

for pig manure valorization – a meta-analysis”. The current thesis focuses majorly on the trend

of nutrients recovery (particularly focusing on N and P) via use of various valorization

mechanisms over the span of last two decades.

This preamble was drawn up after consultation between the student and the supervisor and is

approved by both the parties.

The Promoters: The Author:

Prof. Filip Tack Vaibhav Shrivastava

Prof. Erik Meers

iii

Abstract (English)

During the last decades, the increasing concern about the environmental sustainability,

especially in terms of natural resources consumption and intensive agro-livestock practices,

became widely recognized. In this context, the production of fertilizers from waste resources

(manure) plays a crucial role. Moreover, the livestock sector produces an important source of

nitrogen and untapped nutrients whose direct use is restricted due to potentially negative

environmental consequences. Accordingly, there is an increasing interest in translating a

quantitative waste problem into an important recovery and reuse opportunity. The goal of this

study was to evaluate the valorization mechanisms related to different scenarios focused on pig

manure treatment: (a) ammonia stripping (b) pyrolysis and hydrothermal carbonization (c)

crystallization (d) solid-liquid separation and (e) biological processes. The following categories

were considered for assessment in accordance with literature: yearly distribution, country wise

distribution, recovered nutrients, technological feasibility, and economics. The reported factors

such as nutrient concentration in initial and final product were analyzed by the software Review

Manager 5.4 to identify the effectiveness of different valorization pathways in comparison to

initial products. Pyrolysis and crystallization have come forward as the most efficient options

for N and P recovery, respectively. Furthermore, an economic analysis was performed. The

price trend for fertilizers over the period of last 20 years has shown an overall growth trend,

attaining an all time high in the year 2009. Additionally, comparison between the cost-to-

benefit ratio of different valorization mechanism evaluated crystallization and pyrolysis with

the lowest payback periods and a good profit margin over initial products.

iv

Abstract (Dutch)

In de afgelopen decennia werd de toenemende bezorgdheid over de ecologische duurzaamheid,

vooral in termen van het gebruik van natuurlijke hulpbronnen en intensieve

landbouwhuisdieren, algemeen erkend. Hierbij speelt de productie van meststoffen uit

afvalbronnen (mest) een cruciale rol. Bovendien produceert de veehouderij een belangrijke

bron van stikstof en onaangeboorde nutriënten waarvan het directe gebruik beperkt wordt

vanwege mogelijk negatieve gevolgen voor het milieu. Dienovereenkomstig is er een

toenemende interesse om een kwantitatief afvalprobleem te vertalen naar een belangrijke kans

op terugwinning en hergebruik. Het doel van deze studie was om de valorisatiemechanismen

te evalueren die verband houden met verschillende scenario's gericht op de behandeling van

varkensmest: (a) strippen van ammoniak (b) pyrolyse en hydrothermische carbonisatie (c)

kristallisatie (d) vaste stof-vloeistofscheiding en (e) biologische processen . De volgende

categorieën kwamen in aanmerking voor beoordeling in overeenstemming met de literatuur:

jaarlijkse distributie, verdeling per land, teruggewonnen nutriënten, technologische

haalbaarheid en economie. De gerapporteerde factoren zoals nutriëntenconcentratie in begin-

en eindproduct werden geanalyseerd door de software Review Manager 5.4 om de effectiviteit

van verschillende valorisatiepaden in vergelijking met de oorspronkelijke producten te

identificeren. Pyrolyse en kristallisatie zijn naar voren gekomen als de meest efficiënte opties

voor respectievelijk N- en P-terugwinning. Verder is er een economische analyse uitgevoerd.

De prijsontwikkeling voor meststoffen over de periode van de afgelopen 20 jaar heeft een

algemene groeitrend laten zien, en bereikte een recordhoogte in het jaar 2009. Bovendien

evalueerde een vergelijking tussen de kosten-batenverhouding van verschillende

valorisatiemechanismen kristallisatie en pyrolyse met de laagste terugverdientijden en een

goede winstmarge ten opzichte van initiële producten.

v

Acknowledgement

I would like to express my appreciation and gratitude for the ones who have been supporting

and involving for the completion of this master thesis.

First and foremost, my praise is to ALLAH, the Almighty, the most Merciful and Beneficent,

for giving me opportunity, determination, and strength to do my research. His continuous grace

and mercy were with me throughout my life and ever more during the tenure of my research.

My sincere gratitude goes my promoter, Prof. Filip Tack and Prof. Erik Meers of the Laboratory

of Applied Analytical and Physical Chemistry, for allowing me to embark on this project. His

valuable guidance, advice and feedback during the course of this thesis helped make this work

a success.

I would like to thank my tutor, Caleb Elijah Egene, for his continuous assistance ranging from

setting up laboratory experiments, practical advice, editing of the thesis and full cooperation

during the entire period of this study. Even off-field, Caleb helped me by going off his way

many times, hence making the word “tutor” whole in every aspect. During my journey with

him, I not only earned a “Guru” but also a true friendship, which will hopefully last for our

lifetimes.

I must also thank European Union for providing me with opportunity to take part in Erasmus

Mundus and made it possible for me to study at Ghent University. Furthermore, I would also

like to thank all the staff members at UCT Prague, IHE Delft and Ghent University for

organizing an excellent IMETE programme.

Finally, this document will go incomplete without thanking my parents for their prayers,

support and encouragement that have helped me get to where I am today. And how can I forget

to thank the most priceless person, my friend, Izba Ali, who has stick to me through all thick

and thin and has been always been on my side. This is a debt that I can’t pay back in their entire

lifetime (but perhaps small installments in the form of cakes, chocolates and sweets can appease

a bit). I dedicate this work to all of them.

Vaibhav Shrivastava

vi

Acronyms and abbreviations

CI: Confidence Interval

df: Degrees of Freedom

DOC: Dissolved Organic Carbon

DOI: Digital Object Identifier

EEA: European Environmental Agency

EPA: Environment Protection Agency

EU: European Union

FAO: Food and Agriculture organization

of United Nations

HM: Heavy metals

HTC: Hydrothermal Carbonization

JSTOR: Journal storage

LPELC: Livestock and Poultry

Environmental Learning Community

MD: Mean Difference

MPCA: Minnesota Pollution Control

Agency

MT: Metric Tons

NPK: Nitrogen-phosphorus-potassium

NVZ: Nitrate Vulnerable Zones

OC: Organic Carbon

OM: Organic matter

p: Probability

RE: Recovery Efficiency

RevMan: Review Manager

SD: Standard Deviation

SP: Struvite precipitation

TKN: Total Kjeldahl nitrogen

UNDESA: United Nations Department of

Economic and Social Affairs

USGS: US Geological Survey

VMD: Vacuum Membrane Distillation

vii

Table of Contents:

Copyright Statement ................................................................................................................... i

Preamble .................................................................................................................................... ii

Abstract (English) .................................................................................................................... iii

Abstract (Dutch)........................................................................................................................ iv

Acknowledgement ..................................................................................................................... v

Acronyms and Abbreviations ................................................................................................... vi

List of figures ............................................................................................................................ ix

List of tables ............................................................................................................................... x

Table of Contents: ........................................................................................................................... vii

Introduction .................................................................................................................................... 1

Literature review ............................................................................................................................. 4

2.1 Manure as a fertilizer .............................................................................................................. 4

2.2 Relevance of N and its availability in the ecosystem .............................................................. 4

2.3 Relevance of P and its availability in the ecosystem............................................................... 6

2.4 Concerned legislation.............................................................................................................. 7

2.5 Use of manure for agriculture ................................................................................................ 9

2.6 Heavy metals ......................................................................................................................... 10

2.7 Factors affecting composition of pig manure ....................................................................... 11

2.8 Valorization technologies...................................................................................................... 14

2.8.1 Hydrothermal carbonization ......................................................................................... 14

2.8.2 Ammonia stripping ........................................................................................................ 14

2.8.3 Crystallization ................................................................................................................ 15

2.8.4 Pyrolysis ........................................................................................................................ 15

2.8.5 Mechanical Solid-liquid separation ............................................................................... 17

2.8.6 Biological Treatment ..................................................................................................... 17

Methodology ................................................................................................................................. 21

3.1 Data and literature sources .................................................................................................. 21

3.2 Study selection ...................................................................................................................... 21

3.3 Data extraction ..................................................................................................................... 22

3.4 Data analysis ......................................................................................................................... 22

3.5 Strategy for data assessment ................................................................................................ 22

3.5.1 Qualitative assessment ................................................................................................. 22

viii

3.5.2 Quantitative assessment ............................................................................................... 23

3.5.3 Heterogeneity assessment ............................................................................................ 23

3.6 Risk-of-Bias assessment ........................................................................................................ 24

Results and discussion .................................................................................................................. 25

4.1 Overview of the data collection ............................................................................................ 25

4.2 Data distribution ................................................................................................................... 27

4.2.1 Year wise distribution of studies ................................................................................... 27

4.2.2 Country wise distribution of studies ............................................................................. 28

4.2.3 Nutrient wise distribution of studies ............................................................................ 29

4.3 Risk-of-Bias ............................................................................................................................ 32

4.4 Quantitative analysis of studies ............................................................................................ 32

4.4.1 Ammonia Stripping ....................................................................................................... 32

4.4.2 Pyrolysis ........................................................................................................................ 33

4.4.3 Crystallization ................................................................................................................ 34

4.4.4 Hydrothermal Carbonization ........................................................................................ 35

4.4.5 Separation ..................................................................................................................... 36

4.4.6 Biological ....................................................................................................................... 37

4.5 Economics ............................................................................................................................. 39

4.5.1 Economic evaluation of fertilizers ................................................................................. 39

4.5.2 Economic assessment of available technologies .......................................................... 41

4.6 Overall techno-economic comparison .................................................................................. 43

Conclusions ................................................................................................................................... 44

References .................................................................................................................................... 45

Annex 1 ......................................................................................................................................... 60

Annex 2 ......................................................................................................................................... 64

ix

List of figures

Figure 1. Cycle for biogas generation and utilization (Shrivastava et al, 2019). ..................... 19

Figure 2. Data screening and selection steps for systematic review ........................................ 26

Figure 3. Distribution of concerned dataset across last two decades ....................................... 27

Figure 4. Country wise distribution of studies ......................................................................... 29

Figure 5. Comparison of RE various valorization technologies for nitrogen (x = mean, --- =

median and dot = datapoints) ................................................................................................... 30

Figure 6. Comparison of RE various valorization technologies for phosphorous (x = mean, ---

= median and dot = datapoints) ................................................................................................ 31

Figure 7. Risk-of-bias assessment for the dataset .................................................................... 32

Figure 8. Trend of valorization against initial product for nitrogen recovery across studies

(values in dg/L) ........................................................................................................................ 33

Figure 9. Trend of valorization against initial product for phosphorous recovery across

pyrolysis (values in mg/g) ........................................................................................................ 34

Figure 10. Trend of valorization against initial product for nitrogen recovery across

crystallization (values in mg/kg) .............................................................................................. 34


crystallization (values in mg/g). ............................................................................................... 35


hydrothermal carbonization (values in g/kg) ........................................................................... 36


hydrothermal carbonization (values in g/kg) ........................................................................... 36


separation processes (values in mg/dL) ................................................................................... 37


separation processes (values in mg/L) ..................................................................................... 37

Figure 16. Trend of valorization against initial product for nitrogen recovery across biological

processes (values in mg/L) ....................................................................................................... 38


biological processes (values in mg/L)...................................................................................... 38

Figure 18. Trend of market price (global) for fertilizers over the last 20 years (The World

Bank, 2020). ............................................................................................................................. 40

x

List of tables

Table 1. Phosphorus (total) application standards in Flanders region (Amery and Schoumans,

2014) .......................................................................................................................................... 8

Table 2. Nutrient application rate requirements for manure (MPCA, 2019) ............................. 9

Table 3. Manure production (liquid content) and nutrient content for major livestock species

(Shrivastava et al, 2019). ......................................................................................................... 10

Table 4. Quantity of nutrient elements excreted in manure of various animals during

reproduction cycles* ................................................................................................................ 13

Table 5. Treatment processes diversification in the selected dataset....................................... 27

Table 6. Economic comparison for various valorization technologies .................................... 42

1

Introduction

The European Union is the world's 2nd largest meat producer, behind China, with 63.85 million

tons contributing to about 14% of the world's production (Ritchie and Roser, 2019). The United

states comes 3rd in the list, with a meat production of 46.66 million tons. In January 2020, the

swine herd population in the world was estimated at 677.6 million animals (Shahbandeh, 2020).

This activity resulted in generation of 178 million m3 per year of swine manure in Europe alone,

a volume that would fill up to 71,000 Olympic-sized swimming pools (European commission,

2018). This has shifted focus of environmental enthusiasts on manure related problems such as

emissions of greenhouse gases from livestock production, runoff, over fertilization of soils and

leaching. It is estimated that pig supply chains generate 0.7 gigatons of CO2 equivalent per

year, representing 9% of total emissions from the livestock sector of world (MacLeod et al,

2013). Furthermore, emissions from cattle production contributes to 41% of total emissions

from livestock sector.

Pig manure is rich in organic matter and nutrients including nitrogen, phosphorus, and

potassium. However, use of pig manure for agriculture is only possible when it is a balance

with the number of livestock. The EU Nitrate directive recommends maximum dosage quantity

as 170 kg of nitrogen per ha per annum (Council Directive 91/676/EEC). When animal

husbandry is intensive, production of pigs with enhanced diets will release large quantities of

nitrogen and phosphorus and lead to accumulation of copper and zinc, fed to pigs, in the soil.

Such imbalances can lead to severe environmental problems affecting soil quality (drainage,

plants toxicity, pathogenic activity), water (nitrate pollution of the groundwater or

eutrophication of the surface water) and air (bad odours, greenhouse gas emissions) (Camargo-

Valero et al, 2015). On the other hand, production of conventional synthetic fertilizers has

tremendous environmental impacts. Alone, fertilizer industry accounts for 1.2% of the overall

energy consumption of the planet. Ammonium is the basic component for nitrogen fertilizer

production, but the process of its production is very energy intensive (36.6 GJ/t NH3) and

produces high CO2 emissions (1,966.8 kg CO2 eq/kg) and nitrogen oxides (European

commission, 2018).

For dealing with the problems of manure management and fertilizer production sustainability,

different valorization approaches of manure have been proposed. More rudimentary

approaches involve designing storage such that the release of greenhouse gases is significantly

reduced. However, more focus has recently been given to the application of different

2

technologies to valorize pig manure as organic soil amendments or mineral fertilizer

substitutes.

Among the alternatives for pig manure treatment, aerobic composting is a relatively simple

operation that yields a valorized product with high nutrient content (Chen et al, 2010). Another

useful method to treat pig manure is biogas production through anaerobic digestion. The

process of anaerobic digestion can also yield renewable energy (clean and cheap methane), soil

conditioner and liquid fertilizer. Moreover, the biogas production could be enhanced by the

anaerobic co-digestion technique compared to the single-substrate digestion (Wang et al,

2012). However, the anaerobic digestion is a complex process and microbial reactions can be

inhibited by many factors such as sunlight-dark conditions. Recycling of pig-manure can also

be achieved by applying it as a fertilizer for pond carp production (Zoccarato et al, 1995).

Furthermore, it can be incorporated into algal biomass production, via aerobic fermentation,

which can be used for animal feeding (Martin, De la Noüe and Picard, 1985). Swine manure

derived biochar through slow pyrolysis has the potential to be used as an excellent soil

amendment (rich in NPK) as well as a cheap adsorbent to immobilize contaminants (Tsai et al,

2012; Zhang et al, 2013).

The goal of this study is to quantitatively assess and compare the techno-economic

sustainability of existing valorization technologies for pig manure by pooling the results of

several previously conducted studies.

The specific objectives of this study are to:

• Summarize the state of the art of options for the down processing and valorization of

pig manure.

• Evaluate and compare the various transformed products based on the recovery

efficiency and mean difference obtained for various nutrients.

• Quantitatively compare the replacement value of products derived from pig manure for

mineral N and P fertilizers based on nutrient availability and nutrient use efficiency.

This work is organized under 5 chapters. Introduction (chapter 1) provides a brief summary

and statistical data about the background information on manure, problem statement and

objectives of the thesis. Literature Review (chapter 2) consists with an overview of the

environmental relevance and significance of nutrients (nitrogen and phosphorus) and

importance of manure in agricultural application and nutrient valorization. It also describes

various legislations and other concerns regarding agricultural application of manure

3

Methodology (chapter 3) describes the procedure of data extraction from various online

databases as well as the inclusion/exclusion criteria. Furthermore, procedure of data

interpretation through RevMan and other methods are also explained. Results and Discussion

(chapter 4) provides a summary of the research trends on the basis of year wise and country

wise studies and explains the significance of the results obtained from the statistical analysis

(through RevMan and recovery efficiency) of the collected dataset. A brief discussion on

economics of various technologies is also performed. Conclusion (chapter 5) conveys the

conclusion of this study discussing best valorization technology for different nutrients.

4

Literature review

2.1 Manure as a fertilizer

Manure is in many cases the main fertilizer source for farmers, which has long been closing

the nutrient cycling. However, the overall sum of nutrients in regions with intensive livestock

farming systems often exceed the nutrient requirements of the crops. In these areas, the excess

nutrients such as nitrogen (N) and phosphorus (P) results in their high concentrations in soil,

surface waters, and atmosphere (Velthof et al, 2014). The demands for minerals, on the other

hand, are high, and many minerals, such as phosphorus (P) and potassium (K), now mined, are

becoming increasingly scarce. Therefore, nutrient removal and recovery from manure has

recently received a lot of attention in regions with intensive livestock farming such as The

Netherlands and Belgium (esp. Flanders) (Schoumans et al, 2015).

In order to boost water quality and to compel the member states to set up an action plan, the

European Nitrates Directive was been implemented in 1991 (EEA, 2020). Manure production

was developed in these vulnerable areas since the 1990's to extract excess nitrogen or export

nutrients to less nutrient-dense areas (Velthof et al, 2014). Now, with the high costs of fossil-

based fertilizer and lower stocks of phosphorus and potassium, the focus of the farmers and

fertilizers producers is towards shifting from the manure processing to more advanced

techniques for nutrient recovery.

In the recent times, the extraction of nutrients from pig slurry has gained a strong interest

among researchers. However, handling and application of the valorized product always remain

a question of interest. Nowadays, the most effective method of handling pig slurry is to

biologically treat the thin fractions and dry them or composting the dried solid fraction and

export them to nutrient deficient areas (Schoumans et al, 2015). The composting and pelleting

processes are well known and relatively easy to execute. However, in contrast to raw manure,

the costs of transport are not much lower and much compost in crop farms is denied because it

has a low N/P ratio that does not meet the needs of the majority of arable crops. Therefore, it

is more interesting to valorize the components of manure into valuable products (Schoumans

et al, 2015).

2.2 Relevance of N and its availability in the ecosystem

Nitrogen is an abundant element on Earth; it makes up 78.1% of Earth's atmosphere and is an

essential nutrient for all forms of life. Most of this Nitrogen is available in the form of

unreactive nitrogen (N2) that most living organisms cannot use. However, part of it, which is

5

fixed by natural or anthropogenic processes [N, which includes nitrogen oxides (NOX), reduced

nitrogen (NHX), nitrous oxide (N2O), nitric acid (HNO3), and other organic and inorganic

forms] is available for use by living organisms (Stevens, 2019). The quantity of N released into

the aquatic environments from human activities has increased over the past century to such a

degree that it is now beyond natural fixation, resulting in a more than doubled global cycling

of nitrogen (anthropogenic N production, 210 Tg N per year; natural N production, 203 Tg N

per year) (Stevens, 2019). In many parts of the world, due to this increase in nitrogen fixation,

air, water and soil pollution has become an important aspect of concern.

Nitrogen use is, however, substantially disproportionate worldwide. In countries outside the

Organization for Economic Co-operation and Development (OECD) and major emerging

economies, the amount of nitrogen taken up by the crops remains low. Not only are there

insufficient fertilizers, the available nutrients are also inefficient in usage. Sub-Saharan Africa

provides an appropriate example. In 2012, nutrient-poor soils yielded an average of 1 metric

ton (MT) ha−1 for grain crops, with fertilizer use averaging 9 kg ha−1 of the cultivated soil

(Stevens, 2019). On the other hand, Asia, where there are major emerging economies, has

achieved crop yields exceeding 4.5 MT ha−1 with fertilizer applications of 96 kg ha–1. Lack of

nitrogen obviously contributes to significant problems in fulfilling people's nutritional

requirements. These problems are just as difficult to address as the problems that nitrogen

pollution causes in other parts of the world.

For the preparation of chemical fertilizers, the Haber Process, also called the Haber-Bosch

Process is employed. It is a complex chemical procedure which uses nitrogen from the air and

combines it with hydrogen to produce ammonia under high pressures and temperatures. Today,

this ammonia is the basis of the widely used synthetic nitrogen fertilizer worldwide (Gellings

and Parmenter, 2016). Haber processes produce approximately 500 million tons of fertilizer

per year (453 billion kilograms). This fertilizer helps feed about 40% of the population of the

planet. Even after so many advantages, on the curse side, we have several socio-environmental

issues with the production of nitrogen fertilizers using this technology (Gellings and Parmenter,

2016). First, this is a high energy process and takes up about 1 – 2% of the world’s energy

supply every year (Harford, 2017). According to 2017 statistics, this number corresponds to

around 500 TWh. Furthermore, fertilizers seep into rivers which causes algae blooms. In turn,

bacteria feed on the dead algae using up oxygen in the water, as a result killing aquatic animals.

This further contributes in causing ocean dead zones.

6

2.3 Relevance of P and its availability in the ecosystem

Phosphorus is the 11th most common element found on Earth with a concentration in the crust

of about 1 g/kg and it is safe to say that is fundamental to all biotic species (Tiessen 2008). It

is important for creating DNA, cell membranes, and also for the creation of human bones and

teeth. This is important for food production because it is one of three nutrients used in

commercial fertilizer (nitrogen, potassium, and phosphorus). It is not present free in nature but

is widely dispersed, generally as phosphates, in many minerals. Inorganic phosphate rock

partially consisting of apatite is the source of most of the world’s phosphorus up to date

(Tiessen 2008).

In agriculture and food production, around 90% of the world's mined phosphate rock is used,

more as fertilizer, less as animal feed and food additives (FAO 2020). Moreover, the market

polarity makes this resource more inaccessible due to political or geographical reason.

According to the US Geological Survey (USGS), about 50% of the global reserves of

phosphorus are with the Arab nations. Major apatite deposits are also found in China, Russia,

Morocco and USA (in Florida, Idaho, Tennessee and Utah). Moreover, the phosphorus that is

available in the nature cannot be extracted up to the demands of the modern world due to

physical, economic, energy or legal constraints (USGS 2020). In addition to this, majority of

this extracted phosphorus is lost in the processing. About 30 – 40% is lost during mining and

processing and another 50% is wasted in the complexity of the food chain (Renee, 2013). Just

20% of phosphorus in phosphate rock enters global food consumption. Much of the excess

phosphorus from farm or fertilizer runoff or from phosphates in detergent and soda washed

into drains reaches our rivers, lakes and oceans and results in eutrophication (Renee, 2013). It

is a severe form of water contamination in which algae blooms produce, then die, consume

oxygen, and create a "dead zone" in which nothing can survive. There are over 400 dead coastal

areas at river mouths that are rising at a rate of 10% per decade (Renee, 2013).

The current extractable phosphorus reserves, according to the reliable estimates show the

capability of phosphate rock resources to last an additional 300 – 400 years. With a world

population predicted to hit 9 billion by 2050 and needing 70% more food than we currently

generate, and a rising global middle class that consumes more meat and milk, phosphorus is

crucial to global food security (Renee, 2013). Yet there are no international bodies or laws that

control capital from global phosphorus. In these crucial times, the experts are looking at the

other sources of phosphorus which can be used to meet the global P demand in the future.

7

Animals and people excrete nearly all of the phosphorus they consume in food. For ages,

animal manure has been applied to soil for improving the nutrient composition of soil, aiding

crop production. Even today, in some parts of the world (e.g. Vietnam), manure is directly

applied to the soil as a fertilizer (Shrivastava et al, 2019). However, this practice is mostly done

in developing countries or in others with fragile or undrafted legislation.

In the context of the EU countries, phosphate rock is classified as a critical raw material by the

European Commission (European Commission, 2019). The phosphorus is completely imported

to serve the necessary demands therefore making it vulnerable to market fluctuations in

fertilizer and mineral P prices (The World Bank, 2020). Many studies have been done in recent

years to understand the P-flow analysis (PFA) that provide insight into how people use and

reuse P and how P on various spatial scales is lost to the environment (Chowdhury et al, 2014).

PFA findings indicate that significant amounts of P are lost outside agriculture, by wastewater

and biodegradable solid waste, in manufacturing, consumption and waste handling sectors.

National PFA studies show that there are significant variations in phosphorus flow within

countries and within regions. However, the research on the extraction of phosphorus from

different sources is developing rapidly. Municipal wastewater, sewage sludge and animal

manure are some of the promising sources of P recycling to replace P derived from phosphate

rocks (Ehmann et al, 2017).

2.4 Concerned legislation

Due to the environmental threats (such as organic micropollutants and pathogens) associated

with sewage sludge, its agricultural application is prohibited by legislation or is not practiced

in many European countries. Owing to these possible environmental and health threats,

acceptance of direct applications and thus direct P recovery in many European countries is poor

or declining (Ott and Rechberger, 2012). For the latest alternative methods of handling waste,

such as cement co-incineration, caloric power plants and waste incinerators, P is irreparably

lost. Moreover, one of the biggest flaws in the current EU legislation regarding the soil

application of manure is its “status”. The manure, even after any kind of treatment or digestion

is particularly considered as waste, which further limits its application as a fertilizer alternative.

However, this legislation is under revision currently at the EU, increasing the chances of more

usage of manure in the future applications.

One way to reduce the direct risk of P losses to surface waters is to restrict the P application by

setting maximum application levels. In addition, general legislation regarding the use of

manure or fertilizers can indirectly restrict the use of P in agriculture. Manure typically has a

8

N/P ratio of 2 to 8, as opposed to the N/P ratio of 7 – 11 required by crops for efficient growth

(Amery, 2014). This results in an excess of P if the crop's N requirement is met using only

manure. The application of manure in (NVZ) is limited to 170 kg N/ha/yr or a higher value in

case of derogation. It helps to restrict the application of P to soil, depending on the N/P ratio

of the manure applied (Amery, 2014). The limits may have a standard value (which are

established according to local legislations) but may also be distinguished with respect to crop

type, soil phosphorus status or even crop yield. Most laws apply to all the types of (acceptable)

phosphorus application but often only the use of manure or inorganic fertilizer is regulated.

If the case of Belgium, a new Manure Decree comes into effect in Flanders every four years as

a means of enforcing the Nitrates Directive. This new Manure Decree (Anon, 2011) tackles the

distribution of phosphorus and nitrogen. The current application standards generally fluctuate

between 65 and 95 kg P2O5/ha/yr depending upon the type of crop. The average application

levels for phosphorus are around 5 kg P2O5/ha/yr less than the crop's general uptake of

phosphorus, resulting in a low, negative injection of phosphorus into the soil (Amery, 2014).

Phosphorus can be supplied to the soil in the form of compost, organic materials or natural

fertilizers (only chemical P fertilizers cannot be used by derogation farms). Table 1 shows the

directive principles for various crops.

Table 1. Phosphorus (total) application standards in Flanders region (Amery and Schoumans, 2014)

Crop

Application limits (kg P2O5/ha/y)

2011-

2012

2013-

2014 2015-2016* 2017-2018*

Grassland (only mowing) 95 95 95 90

Grassland (not only mowing) 90 90 90 90

1 cut grass/rye + maize 95 95 95 90

Maize 80 80 75 70

Winter wheat - triticale 75 75 70 70

Other cereals 75 70 70 70

Other crops 75 65 55 55

* limits for 2015-2018 are indicative

Just 40 kg of P2O5/ha/yr can be added to soils found in saturated phosphate areas. No

phosphorus can be used on soils located in security zone category 1 i.e. the drinking water

extraction areas. No phosphorus can be added to these agricultural soils in vulnerable natural

areas (Anon, 2011).

9

In case of United States, the legislations define the limits in the form of Animal Units. The

overall application levels of manure on all soils are limited by crop-available nitrogen.

Nevertheless, criteria for the phosphorus-based rate must also be met in certain specific

circumstances (MPCA, 2019). Table 2 shows the summary of nutrient application rate

requirements for manure.

Table 2. Nutrient application rate requirements for manure (MPCA, 2019)

Nutrient type Application rate requirement

Nitrogen (N) a) Cannot exceed crop N needs for non-legumes.

b) Cannot exceed crop N removal for legumes.

Phosphorus (P) a) No long-term soil P build-up near waters.

b) Manure management plan with P management strategy required

if applying to extremely high P soils and facility is over 300 animal

units.

Potassium (K) No restrictions in rule.

There is no legislation in India governing the use of P fertilizer, as a result of which the soil

check for P showed a moderate status in cultivated soils whereas the P deficiency in soils was

maximum two decades earlier. The excess N-P-K complex fertilizer is more of a compulsion

than an exception in most irrigated soils due to extensive ads from chemical fertilizer

manufacturers and traders to improve their industry with complete disregard for soil and water

quality and the related environmental pollution problems. Sadly, soil testing for P or any

nutrient for that matter is not needed to decide on crop need or otherwise fertilization. Thanks

to the increase in chemical fertilizer prices that ultimately swayed farmers from opting for

organic fertilization such as composting, vermicomposting, green manure, green leaf manure,

oil cakes etc. (Reddy, 2015). Legislation to encourage the use of chemical fertilizers and

pesticides, herbicides are imperative for every nation, if not the planet, to ameliorate the quality

of natural resources and the life of all Earth's species.

2.5 Use of manure for agriculture

As per 2017 census, over 115 million tons of nitrogen (Mt N) has been supplied as an

input into agricultural soils by animal manure globally (FAO, 2019). 75 % (88 Mt N) of

this animal manure is left permanently in paddocks and pastures. One quarter (27 Mt N

yr-1) was treated in manure management systems and applied as fertilizer; and another one

10

third of N deposited or applied to soils (34 Mt N) was lost to leaching (FAO, 2019). More

than 50% of all animal manure was produced by cattle. The overall livestock manure N

production increased by 0.6%, compared with the mean growth of 0.9% for previous ten-

year cycle (2008-2017). The average cumulative growth rate was 1.3% per year, between

1961 to 2017. In 2017, the production of cattle manure was 75% higher than in 1961. In

2017, Brazil (7.9 Mt N from beef cattle), the United States of America (3.0 Mt N from

beef cattle) and China (2.0 Mt N from goats) reported the highest N inputs of pasture by

country and animal type (FAO, 2019). Furthermore, the largest inputs of manure treated

and applied to soils by country and animal type were in China (1.4 Mt N from pigs), India

(1.0 Mt N from buffalo) and again China (0.9 Mt N from chicken) (FAO, 2019).

Table 3. Manure production (liquid content) and nutrient content for major livestock species (Shrivastava et al,

2019).

Animal Manure Produced

(kg yr-1)

Nutrient Concentration (kg 1000 l-1)

Total N Total P NH3-N

Swine (Farrow-finish) 17007 3.4 1.3 1.9

Dairy (Cow) 24490 3.7 0.8 0.7

Dairy (heifer) 11338 3.8 0.7 0.7

Beef (Cow) 13605 2.4 0.8 0.8

Poultry (layer) 59 6.8 2.7 4.4

Poultry (broiler) 38 7.6 2.1 1.6

Poultry (turkey) 128 6.4 2.1 1.9

2.6 Heavy metals

Pig manure is a sustainable nutrient source that has been commonly used as organic fertilizer

in agriculture. Nevertheless, the presence of antibiotic residues and heavy metals (HM) in pig

manure restricts its direct land use (Wang et al, 2016). Trace elements may be taken up from

the soil by plants or biota and cycle in biological tissues before ultimately returning to the soil

through decaying biological remains (Tack, 2010). Under nominal concentrations, HM such as

Cu, Zn, Se, etc. are important for the plant life cycle, but the metals become toxic to the edaphic

organisms and crops above different threshold ranges. HM are absorbed by the root and are

translocated to the shoot by different transport mechanisms that mediate the movement of non-

essential HMs such as Pb and Cd in certain circumstances (Holzel et al, 2012). Trace metals

like Cu and Zn (Cd appears as a contamination in Zn feed) are used globally to prevent

11

infectious diseases in intense animal breeding and as promoters of growth (Cang et al, 2004;

Mccarthy et al, 2013). Mineral feed contributes to emissions of Cd Pb, As and Hg. Other

potential sources of HMs, such as Cu, Cd, Cr, Zn and Ni, are added for the prevention of

corrosion in metal structures, ash alteration of manures for odour control and lime alteration

for disinfection purposes (Anonymous, 2004). Such heavy metals can accumulate in soil, up

taken by plants and affect animal and human health through the food chain (Buelna et al, 2008;

Qureshi et al, 2008). The prolonged application of HM into the soil can also reduce the soil

buffering capacity, and thus contribute to permanent soil and groundwater pollution. While

certain mechanisms of heavy metal tolerance may be linked to mechanisms of antimicrobial

resistance (Akinbowale et al, 2007; Bass et al, 1999), heavy metals may select bacteria that are

resistant to antibiotics. This was observed in the case study done by Berget et al (2005), where

copper-resistant soil-isolated bacteria that were substantially more resistant to antibiotics (e.g.

ampicillin, tetracycline, sulfone-amides, chloramphenicol) than copper-sensitive strains. For

the remediation of heavy metals from pig slurry, composting and anaerobic digestion

approaches have typically been employed (Marcato et al, 2008; Lu et al, 2015), however these

demonstrate other limits, such as the heavy metals can only be fixed and the total heavy metals

cannot be reduced. Organic modifications such as effective application of cattle manure have

also been reported to regulate heavy metals (HM) content in soil (Wei et al, 2015). Nonetheless,

applying organic matter (OM) as soil enhancers is an effective technique for heavy metal

immobilization by complexion components, thus potentially decreasing plant uptake.

Pig manure has a higher Cu and Zn content than cattle manure in general. After they have been

applied to the soil, HMs are subject to reactions that alter the plants' bioavailability. Moreover,

adsorption on to clay and/or organic, precipitation as insoluble salts (carbonate and phosphate)

and changes in the oxidation state of HM affects its solubility (Provolo et al, 2018).

2.7 Factors affecting composition of pig manure

The composition of manure from livestock and within different lots of a specific domestic

animal varies greatly. Azevedo & Stout (1974) has confirmed that certain variations in manure

nutrient concentrations have been caused due to factors such as digestion processes and feed

preferences of various animal species (Table 4). For instance, due to the ability of the ruminants

to extract the organically bound P from plant feeds, its manure generally has lower P value in

comparison to poultry and swine manure. Based on overall composition, liquid fraction of

swine manure contains on an average of 9% of total N by mass and 4% dry matter in

comparison to 7% and 10% is case of liquid fraction in cattle manure (Evans et al, 1977).

12

Furthermore, as per the data collected by Azevedo and Stout stated in Table 3, 10 hogs produce

as much as 80% of manure N as two beef cows. The concentration of different dietary salts and

other mineral additives (Cu, As and other minerals) in swine manure are directly affected by

the concentration of the aforementioned minerals in swine rations (Sutton et al, 1976; Brumm

& Sutton 1979; Sutton et al, 1984a). Thus, high use of manure from such sources can therefore

be harmful to plants and can reduce productivity of the soil.

It is stated by Sutton et al (1984a) that swine manure from 0.5% salt averaged diet is more than

twice the sodium content in comparison with diet containing 0.2% salt. The use of manure,

however, did not contribute to soil pollution or phytotoxicity. Wallace et al (1960) reported

addition of dietary copper to swine rations at levels of 125 mg/L to 250 mg/L to stimulate

growth. However, a 16 to 32-fold increase in Cu concentration in manure has been observed in

the study done by Kornegay et al (1976); where swine fed with a 250–300 mg/L Cu diet.

The housing system, the manure collection, storage, and handling process may also influence

the manure composition (Szoegi, Vanotti and Hunt, 2015). Usually 50% of N in pig manure

slurry is present in the form of ammonium nitrogen and the remaining 50% in the form of

organic nitrogen. (Choudhary et al, 2015). The organic nitrogen constitutes microbial N, labile

organic N and stable organic. Beauchamp (1983) determined that approximately 20% of

organic N will be mineralized and usable throughout the growing season, and that 25% of

ammonia will be volatilized, resulting in a net supply of approximately 50% of N used for crop

growth. According to the work done by Nasiru et al (2014), the method of storage of manure

and the treatment process contributes to about 10 – 30% ammonia nitrogen losses from manure.

Moreover, manure storage leads to gases such as ammonia, nitrous oxide, and methane in the

atmosphere (Kulling et al, 2001). The amount of gas emissions depends on the digestibility of

feed and the general output of animals. The lowest loss was from closed shelters such as aerated

storage and the maximum loss was the open storages like feed-lot surfaces and anaerobic

lagoons. Al-Kanani et al (1992) reported that use of sphagnum peat moss reduced NH3 losses

by 75%. Nasiru et al (2014) suggested the use of vermicast for the reduction of ammonia

nitrogen losses. Losses relating to P and K are mostly minimal only contributing to 5 – 10%.

However, these losses increase significantly in case of open-lot or lagoon handling systems

where 40 – 50% of P can be lost to runoff and leaching (Sutton et al, 1984a).

The rate, time and method of applying manure varies, including weathering, soil

characteristics, crop type and mineralization rate of nutrients. When integrated immediately

after land application to mitigate loss of nutrients, the maximum nutrient gain from swine

13

manure is obtained. For minimizing the localized salt concentration, the uniform application of

manure must be practiced, especially in the case of Na which can significantly reduce

germination and crop yields. In case of severe odour problems resulting because of manure,

knifing the manure into the soil is recommended. However, this procedure also limits the rate

of manure application. In the study performed by Dickey and Vanderholm (1975), 30 – 90%

losses of NH3 − N from manure were observed with ploughing down respectively.

Table 4. Quantity of nutrient elements excreted in manure of various animals during reproduction cycles*

Number and type of animal, length

of production period, and animal

weight during production period

Element excreted (kg)

N P K

1000 broilers: 10 weeks: 0-1.8 kg 70 14 23

100 hens: 365 days: 2.3 kg 57 20 19

10 hogs: 175 days: 14-91 kg 52 13 15

10 beef: 365 days: 181-500 kg 64 13 66

2 dairy cows: 365 days: 544 kg 64 13 66

*Adapted from Webber et al (1968) as in Azevedo & Stout (1974)

In case of the areas with high rainfall and highly permeable soils, the nutrient availability of

the crop could be maximized by application of manure close to the planting date. However,

germination and growth in the planting process may be reduced immediately after application

of heavy manure due to the accumulation of salt. In the areas with low temperature climates,

application of manure may not be possible in early spring, because of the possibility to be

frozen in the pit, particularly when it is stored in open lagoons. The fall–winter application may

cause a decrease of 25 – 30% N from the manure, a longer field duration will allow soil micro-

organisms to decompose the manure and make the nutrient more available to spring seeded

crops (Madison et al, 1995). The loss of N from fall–winter application of manure can be

minimized either by injecting it into the soil or by addition of a nitrification inhibitor. To obtain

the best possible results from manure application, the rate should be such that the amount of

available nutrients is equal to the amount required by the crop. However, a problem is

encountered as only 45% of N is mineralized in the first year of application.

A study done by Larson (1991) also shows that even after the period of 5 years, only 80% of

the total N is mineralized from the manure. Alternatively, almost all the P and K present in

manure are available at the time of application. Low permeability for heavy-structured soils

and low decomposition levels further corresponds to lower rates of manure application in

comparison to highly permeable coarser textured soils, promoting faster decomposition of

14

manure. However, high levels of application of manure in coarse soil textured can pollute

groundwater by nutrient release, while high application rates of manure on heavy textured soil

may be beneficial because of the high nutrient holding capacity of these soils. Manure should

not be applied to snow or frozen land, particularly if the soil is subjected to accelerated spring

runoff. Furthermore, the leaching of N contaminating the groundwater can be prevented by

restricting on use of the heavily manured fields during summers.

2.8 Valorization technologies

2.8.1 Hydrothermal carbonization

Hydrothermal carbonization (HTC) was first introduced by Friedrich Bergius in 1913. It is

often used to convert waste feedstock/biomass (f) into a solid fuel of relatively high calorific

value as compare to brown coal (Oliveira et al, 2013). The process of HTC involves the

temperature treatment of organic matter in saturated conditions, generally rich in moisture

content, at around 180-250℃ (Román et al, 2012) and 2-10 MPa of pressure for a certain time.

Thus, HTC is a combination of processes such as dehydration, hydrolysis, polymerization and

decarboxylation (Román et al, 2012). Although, the rudimentary resemblance of HTC lies with

torrefication (dry pyrolysis), but, in comparison to torrefication, it requires less time and can

be employed for wide range of biomass types. In general, this process leads to an increase in

carbon content of the matter while lowering its oxygen and hydrogen share and the final solid

product is termed as hydrochar. The hydrochar can be used as soil amendment whereas the

liquid products of HTC are usually acidic and contain organic and inorganic compounds (e.g.,

nitrogen, phosphorus and some mineral matter originated from raw material) (Oliveira et al,

2013). The key advantage of the HTC is its ability to transform wet biomass into highly useful

and carbonaceous solid fuel without depending upon the initial drying (Román et al, 2012).

However, the installation cost and sophisticated maintenance requirements are among the main

concerns of this technology.

2.8.2 Ammonia stripping

Ammonia stripping (AS) works on the principle of mass transfer, where wastewater is brought

in contact with air to strip the ammonia gas present in it. Ammonia in wastewater is found in

the form of ammonium ions and ammonia gas and their relative concentration depends upon

the pH and temperature of the wastewater (Wang, 2018) The ammonia stripping is carried out

in two steps. First, the influent, containing high nitrogen content, is ammonified and

anaerobically predigested (3 – 5 days). In this step the ammonification bacteria break down

15

proteins and ammonium ions (NH4+) are formed. The second step is stripping, ammonium ions

are transformed into ammonia (NH3) by adjusting the pH up to 10 (Guštin and Marinšek-Logar,

2011). The increase in pH gives rise to increase in ammonia gas formation, therefore lime is

typically used to raise the pH value of the wastewater prior to stripping step (Wang, 2018).

Along with raising the pH of the influent, lime (calcium oxide) also generates calcium

carbonate which act as a coagulant for particulate matter. Ammonia is then recovered in the

form ammonium sulphate [(NH4)2SO4] by scrubbing with acidic solution.

2.8.3 Crystallization

Struvite precipitation is a well-recognized process for the recovery of phosphorus. This method

not only solves the problem of environmental pollution caused by the discharge of phosphorus

into natural streams but can also be utilized as a useful resource in the form of fertilizer (Suzuki

et al, 2007). Numerous studies have been carried out on the topic of struvite precipitation of

swine wastewater and it has been identified that phosphate recovery depends on the

concentration of magnesium, ammonium and phosphate ions as well as pH, N/P ratio and

influent flow rate (Kozik et al, 2013; Desmidt et al, 2013; Tansel, Lunn and Monje, 2018). The

addition of magnesium salts along with pH adjustment for struvite precipitation is among the

most commonly employed approach (Suzuki et al, 2007; Kruk, Elektorowicz and

Oleszkiewicz, 2014).

2.8.4 Pyrolysis

Pyrolysis (PL) is a process of thermal decomposition where lignocellulosic biomass is

degraded under oxygen-deficient conditions and inert environment. At around 350 – 550℃ the

organic matter in the biomass starts to degrade and can persist until 700 – 800℃ in the absence

of oxygen (Bridgwater, Meier and Radlein, 1999; Bridgwater and Peacocke, 2000). The end

products of this process include biochar, bio-oil and gases (e.g., CH4, H2, CO2 and CO and their

relative proportion depends upon various parameters such as the temperature, heating rate,

residence time, pressure, types of precursors and reactor configuration. Depending upon the

form of final products, pyrolysis can be categorized into two types; (i) slow pyrolysis (yields

heat and biochar) and (ii) fast pyrolysis (yields bio-oils along with biochar) (Zaman et al, 2017).

Unlike pyrolysis, the gasification process allows the biomass to react with a regulated oxygen

amount with steam or air (Brewer 2012; Mohan et al, 2014). In gasification, the biomass is

heated to a temperature higher than 800°C with an average residence time ranging between

several seconds to multiple hours (Inyang and Dickenson 2015).

16

Biochar may be defined as “the product of thermal degradation of organic materials in the

absence of air (pyrolysis) and is distinguished from charcoal by its use as a soil amendment”

(Lehmann and Joseph, 2009). It could be used for soil improvement, due to its water holding

properties and the improvement of nutrient content, thus increasing the overall soil fertility and

crop productivity.

Biochar has gained significant recognition in recent years for its multi-functionality including

carbon sequestration and soil fertility enhancement (Laird et al, 2010), bio-energy production

(Field et al, 2013), and environmental remediation via adsorption process (Mohan et al, 2014).It

also provides a stable pool of organic carbon for extended periods, contributing to the beneficial

effects of organic matter in soils (Tack and Egene, 2019). This may therefore decrease the

solubility of pollutants, mobilize and leach from soil solids, and minimize the movement of

pollutants to other compartments of the environment and dispersion into adjacent areas (Tack

and Egene 2019). The production of biochar from biomass (agriculture residues, forest waste,

manure, food waste etc.) can substantially reduce the quantity of waste produced in the

environment (Ahmad et al, 2013). Furthermore, greenhouse gases emissions are reduced by

biochar application due to its composition as a high carbon content material (Lehmann et al,

2007, Liu et al, 2014). Biochar could also be used as a sorbent for contaminants, including

organic and inorganic contaminants, due to its high surface area and large pore volume (Zhang

and Ok, 2014).

Lehmaan and Joseph (2009) performed experiments using manure-based biochar has obtained

high degree of usable nutrients that can increase crop productivity. The increased crop

productivity is also attributable to the reduction of nutrient leaching and the increase in

microbial activities following the use of biochar for soil application (Verheijen et al, 2010,

Lehmann et al, 2011). However, when applying a biochar form such as solid waste-derived

biochar, careful consideration should be taken as the heavy metal contents or toxic compounds

can severely inhibit the mechanisms of soil organisms (Ahmad et al, 2014a).

Biochar is typically formed by slow pyrolysis, rapid pyrolysis and gasification, differentiated

with heating, temperature and residence time (Ronsse et al, 2013, Inyang and Dickenson,

2015). In slow pyrolysis, biomass is heated at a slow pace from 5 to 30 min in the absence of

oxygen to 400 to 500oC (Mohan et al, 2014). This process leads to secondary reactions

(between the vapor and solid phase) that can allow the production of carbonated solids to

increase or optimize the production of biochar (Brewer, 2012). In comparison, during fast

pyrolysis, the biomass is heat-degraded for about 2 seconds during a short residence time

17

(Inyang and Dickenson, 2015). It aims to rapidly extract vapors and aerosol components in

order to maximize the output of liquid (bio-oil) (Brewer, 2012).

2.8.5 Mechanical Solid-liquid separation

When the liquid and solid fractions of raw manure or digestate are separated through

mechanical separation, it results into the enrichment of nitrogen (as well as potassium) in the

liquid phase and the enrichment of phosphorus and organic matter in the solid phase. This way,

a high amount of phosphorus can be concentrated in a small volume. This technique is generally

applied as a pre-treatment step in nutrient recovery processes. However, this technique can also

help in substantial manure management. Separation can be performed by various technologies

such as screw press, centrifuge of belt press (Hjorth et al, 2010). The N-containing liquid

fraction can be applied on cultivable land which helps to reduce the usage of mineral fertilizer.

Whereas the P-rich solid fraction can be employed in regions where P-content in soil is low or

carbon demand is high.

2.8.6 Biological Treatment

2.8.6.1 Algal uptake

Nutrient uptake by microalgae from organic waste is another potential technique of nutrient

recycling from manure. This a sustainable and environmentally friendly technique where

nutrient assimilation into algal biomass give rise to high quality fertilizers. Manure digestate is

typically a favorable medium to cultivate microalgae for the production of biofertilizers

because it has less contamination compared to raw manure and has comparatively high N and

P concentrations. Similarly, this technique can also be used for nutrient extraction from the

liquid fraction of digestate. In the recent past, several studies have demonstrated the potential

of dry algal biomass, generated from the treatment of anaerobically digested manure, as a

probable fill-in for commercial fertilizers (Mulbry et al, 2005; Veronesi et al, 2015; Uggetti et

al, 2014).

2.8.6.2 Composting

Composting is defined as the biological degradation process of heterogeneous solid organic

materials under controlled moist, self-heating, and aerobic conditions to obtain a stable material

that can be used as organic fertilizer (Lobo and Dorta, 2019). The cycle involves decomposing

organic material into a humus-like substance which could act as a nutrient source supporting

plant growth (Lal, 2013). Composting includes the following three components: human

18

control, aerobic conditions, and internal biological heat production. For an effective working,

it requires the counterbalance of four essential counterparts:

• Carbon — The microbial oxidation of carbon produces the heat. The compounds with

high carbon content tend to be brown and dry.

• Nitrogen — for growing and replicating more carbon oxidizing species. The high

nitrogen products, including fruit and vegetables, appear to be green (or colorful) and

wet.

• Oxygen — for oxidizing the carbon, the decomposition process.

• Water — to maintain the activity of microorganisms without inhibiting aerobic

conditions.

The composting is mostly preferred with an optimal C:N ratio of around 25:1. Rapid

composting is encouraged by having a C/N ratio of approximately 30 or less (Lal, 2003). Above

30, the substrate is nitrogen starved, below 15 it is likely to outgas a portion of nitrogen as

ammonia.

Composting is used as an important recycling tool for local-generated livestock waste and

offers an environmentally friendly alternative method for organic waste disposal as it

contributes to organic waste stabilization and usage. Many studies indicate that the application

of mature compost to agronomic soils improves production because of its characteristics such

as high plant nutrient content and retention of moisture. In a study performed by Huang et al

(2004) reported that, at the initial C/N of 30, the co-composting of pig manure with sawdust

provided a maturity for the compost after 49 days. With manual turning, the stable compost

produced could be used for organic farming or as a soil amendment. However, treatment at a

low initial C/N of 15 significantly influenced behaviours of many important parameters such

as high dissolved organic carbon (DOC) and soluble NH4+ during the co-composting.

Moreover, the resulting compost's high electrical conductivity values must be reduced to levels

that would not impose an inhibition on plant development.

2.8.6.3 Anaerobic Digestion

Anaerobic digestion of animal manure is an alternative way to treat large volumes of organic

waste and its related issues in feeding lots and limited feeding operations (Ileleji, Martin and

Jones, 2015). There are majorly four steps involved in the process of anaerobic digestion

(figure 1):

19

• Hydrolysis - Biopolymers are decomposed to monomeric building blocks or other

dissolvable basic products. Fats decompose to fatty acids, carbohydrates as for example

polysaccharides are transformed in monosaccharides or oligosaccharides and the

proteins in pectin respectively (Shrivastava et al, 2019).

• Acidogenesis - simple monomers are converted into fatty and carbonic acids, as for

example butyric, propionic and acetic acid, then in lower alcohols - ethanol.

• Acetogenesis – A biological reaction where the lower fatty and carbonic acids as well

as the lower alcohols are converted in acetic acid.

• Methanogenesis – A biological reaction where the acetic acids are converted into

methane (Shrivastava et al, 2019).

Figure 1. Cycle for biogas generation and utilization (Shrivastava et al, 2019).

1. Digestate

Increasing numbers of anaerobic digestion plants in Europe have resulted in increased nutrient-

rich digestate production with great potential as fertilizer for arable land. Anaerobic digestion

of animal manure prior to being used as a fertilizer is usually considered favourable, since the

digestate generated has greater proportions of mineralized, plant-available nutrients than

untreated manure (Insam et al, 2015). Other environmental benefits such as increase in soil

carbon and reduction in atmospheric carbon levels, soil erosion (runoff) and nitrate leaching

20

further makes it an attractive option (LPELC, 2020). The digestate nutrient composition varies

with the substrate processed by the biogas plant and may contain compounds stimulating or

inhibiting soil microbial activity (Risberg et al, 2017). The composition of the digestate

depends on the origin of the ingoing substrate and on the management digestion cycle. Factors

like animal form (omnivore, ruminant, etc.), sex, species, age, and diet, as well as geographic

and environmental conditions strictly changes the composition of digestate. According to the

study done by Arthurson (2009), the proportion of ammonium (NH4+) is generally higher in

digestate than in the organic substrate going into the anaerobic digestion (AD) process. Another

study performed by Risberg et al (2017) displayed higher ammonium levels and lower organic

carbon levels in digestates as compared with pig slurry and cattle manure. In addition, digestate

usually induced increased potential for ammonium oxidation, while pig slurry and cattle

manure caused substantially more respiration. This trend indicates that for heavy soils with a

high level of clay and carbon, the digestate may be better alternative, whereas for lighter sandy

soils with less organic carbon the slurry and cattle manure are more efficient.

2. Biogas

Biogas is a mixture of methane, carbon-dioxide, sometimes nitrogen, hydrogen, hydrogen-

sulphide, ammonia, and other remainder-gases, which are produced by micro-organisms in

anaerobic environment from organic materials. The biogas mainly constitutes of methane

corresponding to around 45 – 70%, followed by CO2 between 35 – 50%. In a study done by

Nagy and Wopera (2012) at Hungary, the anaerobic digestion was carried out in a batch reactor

with continuous mixer at a thermophilic temperature of 54°C. For a hydraulic loading of

100,000 m3 pig slurry/yr, the estimated biogas production was about 1,024 m3/day. The

methane content of the produced biogas was normally 60%, which is proper for the work of

gas engines. With the gas engines cogeneration can be achieved. Through the cogeneration on

the one hand heat can be produced and on the other hand electricity can be generated. If this

biogas is used to release heat by burning, a corresponding energy of 22,022 MJ/day will be

obtained. The electrical equivalence of this heat will be 6,117 kWh/day (Nagy and Wopera,

2012). Moreover, for a higher biogas yield, the co-digestion (involving two or more type of

organic matter) is mostly preferred to maintain the C/N ratio in the feedstock. In another study

performed by Vasmara et al (2015), wheat straw was pre-treated with lignocellulosic fungi and

then is co-digested with pig slurry. The co-digestion in this case helped to reduce the maximum

methane yield time from 35 to 21 days with an average increase of 37% in biogas production.

21

Methodology

The meta-analysis of research studies was carried out according to the guidelines suggested by

the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA)

statement (www.prismastatement.org). Moreover, the principles of random effects model were

employed for this meta-analysis.

3.1 Data and literature sources

The studies were collected from various online databases such as Web of Science, Journal

Storage (JSTOR), ScienceDirect and Google Scholar on the bases of relevance to the concerned

topic. The approach for ferreting out the relevant studies from these databases was based on

the subject headings such as resource recovery, manure management, etc. and the

corresponding key words. To obtain more topic-oriented results, additional filters were

involved in the methodology. These filters contained search terms such as (1) manure, (2)

valorization and (3) nutrients. Furthermore, the manual selection of studies has also been done,

however this did not lead to inclusion of any further studies.

3.2 Study selection

The selection of the articles/studies from the initial database for further assessment first

proceeded via elimination of duplicates. For this purpose, studies were entered in a web

application named Kopernio, which facilitates in the blinded screening. After removing the

duplicates from the data, the remaining articles were first assessed based on the eligibility of

titles and abstracts of these studies. This further helped in labelling the articles as “included”

or “excluded”. Later on, the following conditions were selected and applied as a criterion to

find out the appropriate studies for a meta-analysis.

a. The study had to refer to pig manure.

b. The study must report the initial and final concentrations of nutrients or a

combination of either one of the above two values and the recovery efficiency.

c. The language of study could be one of the following: English, Dutch, French,

German, or Spanish.

The research studies that did not meet the aforementioned inclusion criteria, were excluded

from this study. The exclusion criteria adopted, for labelling a study as ineligible, comprised

of the following points.

http://www.prismastatement.org/

22

a. Manure Type: Any other source of manure (such as cattle, poultry) were not

considered

b. Studies reporting anaerobic digestion for biogas generation were excluded (as the

primary focus of the study was nutrient recovery).

c. The studies dealing with piggery wastewater were not included.

d. Research articles available in only Japanese or Chinese languages (i.e. no

translation available) were omitted.

e. Studies reporting only recovery/valorization efficiency were eliminated.

f. The studies for which full text could not be obtained from any source or the studies

with irrelevant abstract were discarded.

3.3 Data extraction

The data extraction was started from the shortlisted papers for which a Microsoft Excel data

sheet file was created for recording the results. The extracted data contained following

information about the studies: (1) recovered nutrients - nitrogen, phosphorous, organic carbon

or potassium, (2) technology implemented, (3) scale of implementation: lab scale, pilot scale

and full scale, (4) final concentration of nutrients, (5) flow/volume of test, (6) nutrient recovery

efficiency, (7) location of study, (8) duration of study and (9) authors names and DOI of study.

3.4 Data analysis

The dataset was sorted by nutrient extracted and filtered by valorization technique used. Using

this, a database of six main treatment technologies were identified for inclusion: hydrothermal

carbonization (HTC), ammonia stripping, pyrolysis, Crystallization, Biological processes and

Separation techniques. For each valorization mechanism, the reported recovery efficiency (%

recovery) was recorded. When percentage recovery was not directly given in the study, they

were calculated from the influent and effluent concentrations where provided.

3.5 Strategy for data assessment

3.5.1 Qualitative assessment

The data mined from the eligible studies will be described in a narrative style. This will

summarize the characteristics of the studies in terms of year wise trend in technological shift,

country wise technological preference and nutrient wise distribution of studies. A 95%

confidence interval is a range of values that you can be 95% certain contains the true mean of

the population.

23

3.5.2 Quantitative assessment

The evaluation of data will further be conducted in a statistical meta-analysis using Review

Manager (RevMan) computer program version 5.4 (The Cochrane Collaboration, 2020). The

meta-analysis will only be performed when three or more studies that had data points for

recovery of phosphorus and/or nitrogen for a particular technology of interest. For a particular

technology, the initial and valorized product among the studies were compared with the help

of forest plots. In this method, the differences in means between experimental (represents

valorized product concentration) and control groups (initial concentration) will be divided by

pooled standard deviation (SD) of the two groups to convert all outcome measures on to a

standardized scale with a unit of SD incorporating a random effects modelling approach. The

random effects model accounts for the intra-study and inter-study variability. The 95%

confidence intervals were considered to ensure the certainty that range of values contains the

95% values of the true mean of the entire dataset. Additionally, various units (such as mg/L,

dg/L, etc) were used in order to plot the graphs in the limits set up by RevMan (-1000 to 1000).

Furthermore, the data for REs were evaluated on box and whisker plots to summarizes the set

of data measured on an interval scale, which facilitates an explanatory data analysis. The graph

represents an overall shape of the distribution, the central value, and its variability. The value

is considered to be “satisfactory” when the condensed box is obtained for a relatively high

number of studies. For the economics, data on capital, operational and maintenance cost was

collected for each technology and evaluated. For some of the technologies, the data on value

of final product is also compiled.

3.5.3 Heterogeneity assessment

Inevitably, the studies delved out for the meta-analysis will differ from each other due to factors

including the diversity in slurry composition, and geographical distribution. The variability

among the studies is termed as heterogeneity and manifests the extent to which the results of

these studies are consistent (Cochrane, 2020). The evaluation of heterogeneity between studies’

valorization effects was determined with the help of probability (p) and percentage of variance

(I2) values. A low p value indicates that the observed variation in the estimates of effect is not

due to chance alone. However, a significant p value does not always indicate the absence of

heterogeneity because of few numbers of comparisons or small sample size contribute to false

results. Therefore, an additional measure I2 was used to assess heterogeneity. I2 describes the

percentage of total variation across the studies and is calculated by taking the difference

24

between Cochran's homogeneity test statistic (Q) and degrees of freedom (df). A value of 75

or more will be considered as an indication of heterogeneity (Deeks, Higgins and Altman 2008;

Borenstein et al, 2010).

3.6 Risk-of-Bias assessment

The quality of every study in the dataset was estimated using the Revised Cochrane risk-of-

bias tool for randomized trials, which majorly uses the 5 domains: 1) randomization process;

2) deviations from the intended interventions; 3) missing outcome data; 4) measurement of the

outcome; and 5) selection of the reported result. Each study has been categorized for the risk-

of-bias, divided into the sections of “low risk” (green), “some concern” (yellow) and “high

risk” (red). However, the overall risk for a particular study depends upon the overall effect of

all domains contributing equally for the final risk determination (Cochrane, 2020). For the

concerned meta-analysis, the 4 factors were used for the risk determination: 1) randomization

process; 2) missing outcome data 3) measurement of the outcome; and 4) selection of the

reported result. The overall judgement for the risk is defined as follows 1) If all the domains

are categorized as low risk then the complete study will be defined “low risk-of-bias”; 2) the

overall risk will be assigned as “some concerns” if the result in any one of the domains was

assigned “some concerns” 3) the overall risk will be high if the result in any of the domains

will be assigned as “high risk-of-bias” or if the results of multiple domains were assigned

“some concerns” (Cochrane, 2020).

25

Results and discussion

4.1 Overview of the data collection

A total of 52 studies published between 2002 and 2020 were selected. Prior to the initial

screening, the raw database consisted of 3337 articles. Out of these, 1457 studies were

identified as duplicate sources among the various databases. Further screening (explained in

the Figure 2) led to a final database consisting of records of 52 studies that reported the possible

valorization percentages of nitrogen and phosphorous by the application of various

technologies. A complete list of all the studies containing the details of the extracted data from

these studies is given in Appendix 1.

The shortlisted research studies were sorted in 3 phases.

• Phase 1: Dataset (n = 52) was sorted according to the scale of a study. It was observed

that the about 88% of the studies were performed on the lab/batch scale and only 6

studies (combined) were done for pilot- and full- scale respectively.

• Phase 2: In this phase, the categorization of the data was based on the type of nutrients

extracted or removed in the study. The studies reporting phosphorous recovery were 43

while 33 were carried out to determine nitrogen recovery.

• Phase 3: The scrutinization was done based on the type of technology used for the

treatment purpose. A comprehensive search in this phase divided all the nutrient

recovery technologies in six major treatment processes – hydrothermal carbonization,

pyrolysis, struvite precipitation, biological processes, separation techniques and

ammonia stripping (Table 5).

During the scrutinization of data, it was observed that some of the studies mentioned the use

of more than one treatment technique for the nutrient recovery process (number of studies - 7).

Adding to this, some studies also considered the use of different manure fractions to access the

nutrient recovery process (number of studies – 4).

26

Figure 2. Data screening and selection steps for systematic review

Iden

tifi

cati

on

Sc

reen

ing

Elig

ibili

ty

Incl

ud

ed

Records identified through database searching (n = 3,325)

Web of Science (n = 1,072)

JSTOR (n = 1,096)

Science Direct (n = 1,157)

Additional records identified through

other sources (n = 12)

Records identified for screening

(n = 3,337)

Records after duplicates removed

(n = 1880)

Records excluded (n = 1,457)

Full-text articles excluded

(n = 225) Reasons:

• Irrelevant abstract

(n = 157)

• Review papers (n = 23)

• Language (n = 5)

Full-text articles assessed for eligibility

(n = 317)

Studies included in risk of bias assessment

(n = 52)

Studies included in meta-analysis

(n = 52)

27

4.2 Data distribution

An overview distribution of the final dataset (Table 5) based on the number of studies for

various recovery technologies is shown. Other details such as RE, location, year, etc. has been

provided in Annex 1.

Table 5. Treatment processes diversification in the selected dataset

Treatment technology Number of studies*

Hydrothermal Carbonization 9

Ammonia Stripping 7

Pyrolysis 10

Crystallization 12

Separation 5

Biological 9

Total studies 52

*See annex 1 for further details of studies

4.2.1 Year wise distribution of studies

The studies were first categorized according to publication year in classes of 3-year periods

(Figure 3).

Figure 3. Distribution of concerned dataset across last two decades

0

2

4

6

8

10

12

14

16

18

2018 - 20202015 - 20172012 - 20142009 - 20112006 - 20082000 - 2005

Nu

mb

er o

f st

ud

ies

Years

Hydrothermal Carbonization Ammonia Stripping Pyrolysis Crystallization Separation Biological

28

Moreover, the data has been further subdivided into the employment of technology for each

range of years. This will help in gaining an insight of the range of technology used for various

years.

The number of studies on nutrient recovery over the span of years increased exponentially from

2000-2020 (Figure 3). Furthermore, a higher focus has been shifted towards the varied range

of technologies over the time period. Precisely, crystallization and ammonia stripping has

shown a growth of 53% and 50% over the last 5 years. Additionally, the development of novel

separation technologies (membrane distillation, electrodialysis reversal) also gained attention,

contributing to around 16% of the studies observed over the last 5 years. However, biological

methods for nutrient recovery showed a constant trend, contributing to ~23% of total studies

done in the last 3 years.

This exponential increase in studies over the last two decades could be explained by the

following points:

• The recent increase in environmental, economic, geopolitical and social concerns on

the use of nutrients for the short- and long-term in the field of agriculture. These factors

further put pressure on the need to reassess the way how plants and crops access their

required nutrients.

• The industrial, sanitation and green revolutions have altered human use of available

nutrients, shifting from close-looped nutrient management systems to unsustainable

linear paths (Ashley, Cordell and Mavinic, 2012). This has led to a global

environmental challenge of nutrient pollution and scarcity.

• The use of nutrients as a ‘strategic’ commodity is now in trend at the marketplace,

creating polarization of powers and the autocracy in the global trade.

• The focus of the recent governmental policies on the subject of “resource recovery” and

“waste valorization” has further intensified the research.

4.2.2 Country wise distribution of studies

The dataset is distributed based on the number of studies performed in the various geographical

location for the valorization of pig manure (Figure 4). Europe and China dominate the current

research for the nutrient’s recovery, contributing to around 53% and 29% of the total studies

respectively. In Europe, Denmark, Spain, and Belgium are the leading researchers adding to

21% of the total studies. Additionally, the other minor part of the studies is shared in between

29

USA, Japan, Taiwan and Vietnam, whose combined studies results in around 18% of the total

studies in the dataset.

Figure 4. Country wise distribution of studies

Furthermore, the use of crystallization and pyrolysis techniques were majorly focused for

nutrient recovery, especially in Europe. The use of struvite recovery alone results in around

20% and 22% of the studies performed in Europe and China. Interestingly, even though China

shares a large percentage in total studies, only a single study has been considered establishing

pyrolysis for the nutrient recovery at the database.

The data interpretation suggests that the number of studies is directly proportional to the

amount of pigs per head in different countries (China - 310 million pigs, market share of nearly

50%; EU - 148 million pigs) (Shahbandeh, 2020). Furthermore, USA, Japan and Vietnam also

follow the same trend.

On the contrary, studies for pyrolysis were not the part of the dataset for China. The most

plausible explanation is the implementation of the China’s national biochar programme, which

was signed and ratified in 2015, but the initial works began at late 2017 (Biochar International,

2020). This could further explain the smaller number of studies coming from China in the

sector of biochar.

4.2.3 Nutrient wise distribution of studies

The studied data were processed based on the recovery efficiencies for the nutrients. For this,

the studies were subdivided for the individual RE of each nutrient.

0

5

10

15

20

25

30

China Europe Others USA

Nu

mb

er o

f st

ud

ies

Hydrothermal Carbonization Ammonia Stripping Pyrolysis Crystallization Separation Biological

30

4.2.3.1 Nitrogen (N)

The technological distribution for different studies concerned with the RE for nitrogen was

plotted. The hydrothermal carbonization shows an interquartile range between 23% and 56%,

with a mean removal efficiency of 42% for all the studies. Furthermore, the whiskers

correspond with the value of 12% and 67%. In the case of ammonia stripping (mean – 83%)

and crystallization (mean – 84%), the whiskers show a value of 98-55% and 97-56%, with an

interquartile range of 96-68% and 96-67%. Moreover, the upper and the lower quartile values

for separation were recorded to be 78% and 42%, with a mean value of 64% over the set of 8

data points.

Figure 5. Comparison of RE various valorization technologies for nitrogen (x = mean, --- = median and dot =

datapoints)

From the above data, the use of ammonia stripping and crystallization appears to show

successful recovery efficiency for N. This could be the reason behind the exponential increase

in ammonia stripping and crystallization across the dataset. Ammonia stripping is a mechanical

procedure and creates no backwash or regeneration (EPA, 2020). However, a drawback of this

technology is that it is highly temperature dependent and energy requirements could be a

burden. This issue could be resolved by the use of solar heating of the wastewater before

entering the ammonia stripper (Melgaço, Meers and Mota, 2020). Overall, modified ammonia

stripping is interesting alternative for pig slurry processing, especially for the recovery of N.

31

4.2.3.2 Phosphorous (P)

The data for the RE of phosphorous across the 55 data points was plotted (Figure 6). The

crystallization technology had the data points (21), which corresponds to 39% of the total points

selected for this study. Additionally, the whiskers for the crystallization (mean – 80%)

corresponds to 98-40% with an interquartile range of 93% to 69%. On the other hand, the

hydrothermal carbonization shows a same whisker and interquartile ranges (between 95 to 60

percent) due to the inadequate number of data points for phosphorous recovery (5 points).

However, the most varied range of values has been shown by the separation technology (mean

– 48%), with an interquartile range of 84% to 13% and whiskers in 90-10% range.

Figure 6. Comparison of RE various valorization technologies for phosphorous (x = mean, --- = median and dot

= datapoints)

The crystallization and pyrolysis have come forward as the best technology for the valorization

of P. In the studies for crystallization, the recovery of P via struvite precipitation (crystallization

in Figure 6) showed the most successful results. However, Kozik et al (2013) reported that the

presence of organic impurities may affect the final size and shape of struvite crystals).

Furthermore, to reduce the cost of struvite production, the use of magnesium compounds like

MgCO3 and Mg(OH)2 is required, however those reagents decrease the purity of the struvite

(Huang et al, 2012). However, when used as a fertilizer, it prevents toxification by regulating

leaching of minerals into the soil, making it fit as a valorized product.

32

4.2.3.3 Potassium (K) and Organic Carbon (OC)

The data for potassium and organic carbon was very limited as only 13 data points were

available. Thus, a detailed RE graph was not prepared. For both nutrients, the hydrothermal

carbonization has shown appreciable results. In case of potassium, an average RE of 37% was

obtained for HTC and 16% for OC.

4.3 Risk-of-Bias

The risk of bias has been evaluated for the study (Figure 7) based on four parameters. Nearly

84% of the studies show “low risk”, 14% with “some concerns” and merely 2% shows “high

risk”.

Figure 7. Risk-of-bias assessment for the dataset

Most of the medium and high-risk studies concerns with the irregularity in reporting of the

results or error in measuring the final outcome. This estimate has been performed by comparing

among the values of particular technology in the dataset. If the concerned value is “too high”

or “too low” in comparison to the overall mean, the study will be placed under “risk” (See

annex 2 for detailed Risk-of-Bias assessment).

4.4 Quantitative analysis of studies

4.4.1 Ammonia Stripping

Due to the limited data availability on other nutrients, only recovery of nitrogen is considered

in the data analysis. However, 3 data points were found across the studies for phosphorous

0 10 20 30 40 50 60 70 80 90 100

Randomization process

Mising outcome data

Measurement of the outcome

Selection of the reported result

Overall Bias

Low risk Some concerns High risk

33

recovery with an average RE of 95%. Seven studies with 9 data points were available about

nutrient recovery via ammonia stripping (Figure 8).

Figure 8. Trend of valorization against initial product for nitrogen recovery across studies (values in dg/L)

The effect was found from 7 studies with a mean difference of 25 dg/L (95% CI: 10, 40) and

an overall effect of Z = 3.3% (p = 0.0008). This value of MD shows a significant inclination

towards the valorized product in comparison with initial (raw) product. This is due to ability to

form chemical building blocks (generally ammonium sulfate) which is suitable for production

of fertilizers and other chemical products (Brienza et al, 2020). The value of I2 corresponds to

maximum heterogeneity. The significantly higher values of MD reported by the studies of

Huang et al (2019) and Ippersiel et al (2012) intensifies heterogeneity. This is due to influence

of process variables (temperature, pH, scale, air-to-water ratio) resulting in varied SD.

Excluding these values, an I2 of 85% will be obtained.

4.4.2 Pyrolysis

Data about pyrolysis were collected from a total of 10 studies across 19 data points. Moreover,

a variety of biochar synthesized from a temperature ranging between 350oC to 850oC was taken

into account. The recovery for phosphorous is only considered for data analysis as insufficient

data points available in case of nitrogen recovery (only 1 data point for N). For phosphorous,

6 data studies across 12 data points were analyzed on RevMan (Figure 9). Furthermore, an

average RE of 45-47% was obtained for potassium across 5 data points.

A random effect with MD of 19 mg/g (95% CI: 13, 25) and an overall effect of Z = 6.3 (p <

0.00001) were determined. In comparison with the initial product, valorized product has shown

more favourability. This is because organic P in biomass proceed through compositional

transformations during process, in which breakdown of phytate (phytic acid bound to mineral)

occurs followed by the reoccurrence of inorganic metal-P consortiums (Sun et al, 2018).

34

Figure 9. Trend of valorization against initial product for phosphorous recovery across pyrolysis (values in

mg/g)

However, unlike ammonia stripping, the resulting heterogeneity in this case is not because of

variability in published data. If the specific case of Uchimiya and Hiradate (2014) (4 data

points) is considered for heterogeneity assessment, an overall I2 = 97% is obtained. Even

though the actual reason of this is unknown, differences in valorization temperatures, gas flows

and particle heating time could result in a significant effect on the I2 value.

4.4.3 Crystallization

The valorization of pig manure via crystallization contributes to maximum number of studies

in the dataset (n = 12). Furthermore, the studies correspond to 29 data points (23 for

phosphorous (Figure 11) and 6 for nitrogen (Figure 10) which are around 25% of all data points

considered in the entire meta-analysis. Additionally, 2 datapoints have been removed

considering the “high risk” in risk-of-bias studies.

Figure 10. Trend of valorization against initial product for nitrogen recovery across crystallization (values in

mg/kg)

For nitrogen recovery, a MD of 295 mg/kg (95% CI: 155, 435) is obtained with an overall

effect of Z = 4.1 (p < 0.0001). Even though the results favor the valorized product to be more

feasible (Figure 9), the overall reliability on the data is less as the dataset only considers 3

publications. This could also be one of the reasons of high heterogeneity across the studies. On

the other hand, recovery of N in process is significant as reaction dynamics for struvite

precipitation also requires NH3 to be present.

35

Figure 11. Trend of valorization against initial product for phosphorous recovery across crystallization (values

in mg/g).

For the phosphorous recovery via crystallization, a total of 21 data points across 10 papers were

observed. An average MD of 400 mg/kg (95% CI: 380, 420) is obtained with an overall effect

of Z = 39 (p < 0.00001) (Figure 11). As an interpretation, the use of precipitation using oxides

and sulphates of Al, Ca and Mg is found to be particularly relevant. However, the software is

only designed for considering the statistical values (input by user) and no co-relation has been

designed for considering intangibles factors (such as environmental effects, feasibility, capital

costs, etc.) while comparing the overall difference. Furthermore, a heterogeneity resulting due

to variation of factors such as molar ratio of Mg:NH4:P, pH and aeration rate value across

database.

4.4.4 Hydrothermal Carbonization

The use of hydrothermal carbonization for valorization was evaluated across 9 studies with 19

data points (8 for phosphorous, 6 for nitrogen, 3 for potassium and 2 for organic carbon). For

P, average MD of 0.22 g/kg (95% CI: 0.15, 0.28) is obtained with an overall effect of Z = 6 (p

< 0.00001) (Figure 12). This value shows slight favourability of valorized product over initial

product. Furthermore, the heterogeneity of the database could be reduced to 90% if the two

highest values corresponding to Song et al (2019) and Wang et al (2020) are not considered.

The variation in database is a result of initial synthesis temperature for the production of

hydrochars (ranging between 150oC – 350oC).

36

Figure 12. Trend of valorization against initial product for nitrogen recovery across hydrothermal carbonization

(values in g/kg)

The phosphorous recovery indicates an overall MD of 3.6 g/kg (95% CI: 2.8, 4.5) is obtained

with an overall effect of Z = 8.8 (p < 0.00001) (Figure 13). This is due to the ability of

phosphorus being contained within the solid hydrochars which could be later leached out using

acid extraction (Heilmann et al, 2014). Even though only a slight inclination towards valorized

product is shown by the study (which is limited to nutrients recovery), the hydrochars could be

used for a variety of applications ranging from production of biofuel to use as an adsorbent

(Libra et al, 2011). However, in the case of Qiao et al (2011), the inverse in the favorability

could be observed which an average mean corresponding to -7.9 g/kg. This is because of

disruption of cells due to treatment, releasing phosphorous from the biomass waste into the

supernatant liquid.

Figure 13. Trend of valorization against initial product for phosphorous recovery across hydrothermal

carbonization (values in g/kg)

4.4.5 Separation

The study mentioned under the separation process combines the data in quick wash process,

distillation, and dialysis for efficient recovery of phosphorous and nitrogen. A total of 5 studies

consisting of 14 data points (7 for nitrogen and 7 for phosphorous).

In case of nitrogen (Figure 14), an average MD of 28 mg/dL (95% CI: -84, 28) is obtained with

an overall effect of Z = 0.9 (p = 0.3). The overall comparison listed initial product as a better

37

option over valorized product due to the quick wash process, which leaves most of the nitrogen

in the washed manure solids (LPELC 2019). However, the data studies also focus on two more

technologies, distillation and dialysis, which gives a positive MD strengthening valorized

product. Furthermore, if the consideration of intangible elements (socio-economic aspects) is

taken, the use of vacuum membrane distillation (VMD) was used without pH adjustment and

proved successful for nitrogen recovery, overall favouring the valorized product (Shi et al,

2020).

Figure 14. Trend of valorization against initial product for nitrogen recovery across separation processes (values

in mg/dL)

For phosphorous (Figure 15), an average MD of 6.2 mg/L (95% CI: -30.7, 18.2) is obtained

with an overall effect of Z = 0.50 (p = 0.62). The heterogeneity could further be reduced

significantly by excluding the study performed by Shi et al (2018). The technologies such as

quick wash process have shown a significant recovery of 90% of phosphorous, however the

co-recovery of nitrogen was not effectively analyzed (Szoegi, Vanotti and Hunt, 2015). The

phosphorous recovery could be significantly increased, if membrane processes concentrates

phosphate which could be coupled with other processes like crystallization (Xie et al, 2014).

Figure 15. Trend of valorization against initial product for phosphorous recovery across separation processes

(values in mg/L)

4.4.6 Biological

For the biological processes, the combination of studies done under composting, anaerobic

digestion (using digestate as a nutrient source), nutrients uptake by algae and others are

38

evaluated. In total, a database consisting of 9 studies with 21 data points (8 for nitrogen and 9

for phosphorous).

Figure 16. Trend of valorization against initial product for nitrogen recovery across biological processes (values

in mg/L)

For the nitrogen recovery, the analysis has shown a significant focus towards the use of

valorized product. The average MD of 5.2 mg/L (95% CI: 1.9, 8.5) is obtained with a

heterogeneity of I² = 100% (p < 0.00001) (Figure 16). Furthermore, the overall effect Z has a

value of 3.1 (p < 0.002). It is important to note that the number of studies done on nutrient

recovery from algal uptake were significantly low. A possible explanation of this could be the

inefficiency of algal species in the selective recovery of nutrients from pig slurry. This is due

to dependency of alage on factors like contamination, inconsistent slurry components, and

unstable biomass production.

Figure 17. Trend of valorization against initial product for phosphorous recovery across biological processes

(values in mg/L)

An average MD of 1.7 mg/L (95% CI: -2.5, -0.9) is obtained with a heterogeneity of I² = 100%

(p < 0.00001) (Figure 17). Furthermore, the overall effect Z has a value of 4.2 (p < 0.0001).

The analysis of data suggests that the use of biological methods for phosphorous recovery has

not shown significant results across the studies. However, for process of composting, a slight

inclination towards the valorized product is seen (Raza et al, 2019; Zhang and He, 2005) due

to formation of phosphorus pentoxide (P2O5). This results in an increase of P content in

compost (Hanrinth and Polprasert, 2016).

39

4.5 Economics

The feasibility of a proposed solution for a nutrient recovery goes hand in hand with the

economical assessment and cost benefit achieved by it. In this section, a brief overview on the

market demand of the fertilizers as well as the economic assessment of the proposed technology

will be discussed.

4.5.1 Economic evaluation of fertilizers

The value of global market for the consumption of phosphate rock was marked at 24.4 billion

US dollars and is expected to continue growing at a cumulative average growth rate of 7.1%

from 2019 to 2025 (GVR, 2019). The use of phosphate-based chemicals in production of

detergents, pesticides, metal coatings, toothpaste, matches, and water softeners is one of the

reasons of this growth (especially in underdeveloped and developing countries). However, the

agricultural industry drives the rock’s market, surging over 4% from 1.2% in last decade (GMI,

2019). The expected increase is a result of two major factors:

• The rise in usage of fertilizers due to increasing food production. It is expected that

nearly 68% of the world’s population will live in urban areas by 2050. The total world

fertilizer market is estimated at 200 billion US$ (20 – 25 billion €/year for European

Union) (UNDESA, 2020). That of phosphate fertilizers is around 50-70 billion

US$/year. This particular factor also puts external pressure on demand for processed

food, hence propelling the growth of phosphate rock market (UNDESA, 2020).

• Phosphate rock is also used as a preservative in food industry (Indexmundi, 2020) for

maintaining taste and increasing shelf life of processed food. Its growth is further

expected to rise exponentially in upcoming decade.

Furthermore, the addition of phosphate in animal feed also contributes significantly to

increasing market demand. The revenue of animal feed segment is expected to grow by 8.5%

in between 2020 to 2025 (The World Bank, 2020). In feed, it is mainly used in forms of

monocalcium phosphate and calcium phosphate. However, use of phytate as an alternative for

feeds could further reduce the pressure on market demand for phosphorous rock.

The growth of phosphate market is noted to be highest in regions of Asia-Pacific. Increasing

awareness about product among the farmers, growing demand for harvesting and rising

agriculture in countries such as India and China, whose major part of gross domestic product

depends upon agriculture (13% for India) is also a significant factor to this increase.

40

Figure 18. Trend of market price (global) for fertilizers over the last 20 years (The World Bank, 2020).

The fertilizer prices overall showed a rather constant trend throughout the span of 20 years

(Figure 18), except for a sudden spike in 2007-2009. In October 2007, fertilizers prices rose

rapidly making an all-time high within a period of 10 months. Prices were driven essentially

by a supply imbalance and rapidly increasing demand, especially in Asia. At a point of time,

available supply could not fulfil the required demand of fertilizers, especially in China and

India. Furthermore, use of fertilizers to produce biofuels in Brazil, Europe and US also

contributed to this factor (IDFC, 2008). Increased livestock production created more demand

for grain and thus for fertilizers. Grain reserves became historically low and prices rose sharply

(IDFC, 2008). Another factor was China's imposition of high tariffs on fertilizer exports and

devaluation of the U.S. dollar in 2007 and 2008 (The World Bank, 2008). Energy prices also

increased, affecting overall cost of production of ammonia (IDFC, 2008).

Following the rapid increase, a sharp decline was also noticed in late 2008. This was due to the

collapse of the global credit market, trade recession, and slowdown in world economic growth

which led to a fall in demand of fertilizers and accumulation of stocks (IDFC, 2008). However,

the price of potash continued to be relatively high due to its shortage and difficulties in

transporting (IDFC, 2008).

0

100

200

300

400

500

600

700

800

Jul-

00

Jul-

01

Jul-

02

Jul-

03

Jul-

04

Jul-

05

Jul-

06

Jul-

07

Jul-

08

Jul-

09

Jul-

10

Jul-

11

Jul-

12

Jul-

13

Jul-

14

Jul-

15

Jul-

16

Jul-

17

Jul-

18

Jul-

19

Jul-

20

Pri

ces

(EU

R/M

T)

Years

Urea triple superphosphate (TSP)

Rock Phosphate Potassium Chloride

diammonium phosphate (DAP)

41

4.5.2 Economic assessment of available technologies

• Ammonia Stripping: From study done by Errico et al (2018), the total capital

investment is about 4.87 million EUR (total bare module cost of about 1.95 million

EUR) treating 30,000 kg/h of digestate. Furthermore, the cost of chemicals and energy

were found out to be less than 1 million EUR. Having the average recovery efficiency

of 97% and current ammonium sulphate price of 462 EUR/MT, the overall benefit of

1.4 EUR per m3 of digestate is obtained (Errico et al, 2018). In another study, Hildago

et al (2002) suggested 1927 EUR/kg-raw manure treatment cost by using ammonia

stripping.

• Pyrolysis: The total capital cost for setting up a pyrolysis plant for biochar production

from manure was estimated to be 90,000 EUR for a daily inflow of 230 kg/h (Li et al,

2016). Furthermore, the yearly operation and maintenance costs corresponds to around

60,000 EUR/year. Considering the market price of biochar at 50 EUR/MT, the yearly

profit of 24,000 EUR could be achieved (Li et al, 2016).

• Struvite Precipitation: According to the study performed by Shu et al (2006), a total

profit of ranging from -8000 EUR to $90000 EUR/yr (the negative value indicated a

loss) for an average inflow of 50000 m3/d. Assuming overall capital and operation and

maintenance costs are calculated to be 2 million EUR, the payback period will be less

than 5 years. However, in another study done by Yetilmezsoy et al (2017), the average

cost for recovering struvite was 1753 EUR/kg.

• Hydrothermal Carbonization: The cost of setting up an hydrothermal carbonization

plant for production of biofuel for 110 MW coal power plant was estimated to be around

10 million EUR with an average operation and maintenance costs (excluding raw

materials) 6 million EUR/year MT (Saba, McGaughy and Reza, 2019). The breakdown

selling price of the fuel obtained from this source ranges between 100 to 113 EUR/MT

(Saba, McGaughy and Reza, 2019).

42

Table 6. Economic comparison for various valorization technologies

Ammonia

Stripping

Pyrolysis Struvite

Precipitation

Hydrothermal

Carbonization

Capital Costs 4-5 million EUR 1–15 million

EUR

1-1.5 million EUR 10 million EUR

Operational

Costs (per year)

1 million EUR 1-8 million

EUR

1 million EUR 5-8 million EUR

Profit (per year) 1.5-1.7 million

EUR

1-5 million

EUR

50000 – 1 million

EUR

1-3 million EUR

Payback Period 5-10 years 5-12 years 2-5 years 10-12 years

Recovered

Products

Ammonia N, P, K,

biofuels

N, P, K Biofuel, Carbon

The economics of a particular technology depends on various factors such as scale,

transportation costs, availability of raw materials, price of raw material, choice of reactor,

energy input requirement and the requirement of separation processes to retrieve products.

Hence, it is not possible to accurately predict cost effectiveness of any process without a

detailed techno-economic analysis comparing all process on a homogenized scale.

However, a brief comparison in between the technologies (Table 6) predicts pyrolysis and

struvite precipitation as most promising valorization mechanisms. This is due to their

comparative low capital and maintenance costs, high profit percentage and smaller payback

periods. Furthermore, the availability of multiple nutrients from both the processes adds to the

benefit. However, in some of the cases of pyrolysis, capital cost could rise as high as 12 million

EUR and overall cost of production (per kg biochar) is significantly high. However, these

processes are coupled with the production of biofuel making overall product economical.

For standalone recovery of nitrogen, ammonia stripping proves to be one of the most efficient

methods. Furthermore, initial capital cost is relatively inexpensive considering the treatment

scale. However, operational and maintenance cost could be significantly high as it is an energy

intensive process. However, these costs could be further reduced if the influent is preheated by

using the waste heat (of system) or renewable energy source. Additionally, ammonia stripping

43

could also be coupled up with technologies like crystallization for the recovery of other

nutrients, hence increasing the cost benefits.

4.6 Overall techno-economic comparison

Presently, only crystallization, ammonia stripping, pyrolysis, and separation techniques (such

as membrane distillation) have been applied at full-scale for nutrient recovery from pig manure.

While the investment costs of ammonia stripping are relatively low, its operational

expenditures are much higher than crystallization. The main operational costs for

crystallization are related to dosing of chemicals – such as calcium and magnesium hydroxides,

while main costs for stripping are air requirements and energy. Optimization of these

parameters is therefore of high interest.

The valorization by using pyrolysis seems as the promising technology, because of potential

benefits of soil carbon sequestration, heavy metal immobilization, improvement in soil quality,

increased crop yields, mitigation of nutrient leaching, and organic contaminant remediation.

However, it can be questioned under ecological assessment whether pyrolysis of a sustainable

treatment option is (due to involvement of combustion process) and if this should be

encouraged or not. Furthermore, techniques such as composting is relatively simple, but the

overall cost of this treatment is still significant as large surface areas are required, making its

potential implementation very region-specific. Further research to improve the economic and

technical feasibility of this technology is recommended.

Lastly from a technical point of view, further calibration in all technologies is required to

minimize operational and maintenance costs, specifically for chemicals and energy, in order to

produce high-quality bio-based fertilizers. Furthermore, struvite precipitation, ammonia

stripping and pyrolysis came forward as best available and most established technologies for

nutrient recovery in terms of fertilizer marketing potential, technical performance, and overall

economics.

44

Conclusions

A meta-analysis and short economic evaluation were conducted for different valorization

mechanisms which have stated raw pig manure as a limiting product for fulfilling the nutrients

requirements as a fertilizer. All valorization mechanisms showed appreciable results for N and

P recovery with average REs of 79% and 85%. Recent studies (year 2015-2020) are mainly

focused towards crystallization (15%), ammonia stripping (15%) and pyrolysis (15%) along

with increasing trend of hydrothermal carbonization (25%). Furthermore, the recent studies

were led by the parts of Europe (Denmark, Spain and Belgium; 55% of total studies) and from

the parts of Chinese subcontinent (28% of total studies). Only 7 studies have demonstrated the

performance at pilot or full scale. Moreover, most technologies have reported to obtain

remarkable recovery efficiencies for standalone N or P, however, only crystallization has

shown >90% RE for both at the same time. Furthermore, technologies such as ammonia

stripping and pyrolysis show best results for standalone recovery (>96%). The data analysis

done by RevMan showed an overall effect in the range of 0.5 – 32 for all the cases, confirming

the statistical significance of the database. Additionally, values of RevMan shown significant

inclination towards the valorized product in comparison with initial (raw) product.

Furthermore, the relative probability was noted to be <0.05 for all the technologies (except

separation), hence confirming the significance that the dataset contains the experimental values

and the values are not merely obtained by chance. Additionally, the economic aspect was also

evaluated, in which pyrolysis and crystallization has come forward as the most promising

techniques with the payback period of nearly 5-7 years with an initial capital and operation and

maintenance costs ranging between 3 – 5 million EUR. Overall assessment categorizes struvite

precipitation, ammonia stripping and pyrolysis came forward as best available and most

established technologies for nutrient recovery in techno-economic aspects. However, further

calibration in all technologies is recommended to minimize operation and maintenance costs

along with promoting the use of multiple valorization techniques (in combination) to enhance

the overall RE.

45

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Annex 1

Ammonia stripping

Technology implemented Scale Type of manure fraction Sample

size

Recovery

efficiency

Duration

of study Year Location Reference

Ammonia stripping and vacuum

evaporation Lab Liquid fraction digestate

1L and 100

ml

95% 10 h 2016 Shunyi, Beijing. Li et al (2016)

Ammonia fermentation and air

stripping Lab Digestate 500 ml 87% 24 h 2019 Tsuchiura(Ibaraki, Japan) Huang et al (2019)

Ammonia stripping Lab

Fresh slurry 55% 225 mins

2012 Catalonia (Spain) Laureni et al (2013) Digested slurry 79%

Partially digested slurry 57%

Air-recirculated stripping Lab Digested pig slurry 3 L 98% 8 h 2015 Rongchang, PR China Liu et al (2015)

Ammonia stripping Lab Raw pig manure 8 L 90% 2012 Quebec, Canada. Ippersiel et al (2012)

Ammonia stripping and struvite Lab Digestate 1 L 91%

3 h 2015 Beijing, China Huang et al (2015) 97%

Ammonia stripping Lab 90% 2015 Spain Hidalgo et al (2016)

Pyrolysis


size

Recovery

efficiency

Duration


Pyrolysis (700℃) Lab Pig manure 35% 2014 Denmark Christel et al (2014)

Pyrolysis (600℃) Lab Pig manure 93% 2016 Denmark Christel et al (2016) 95%

Pyrolysis (500℃)

Lab Pig manure

78%

2014 USA Uchimiya and Hiradate

(2014)

Pyrolysis (650℃) 85%



Pyrolysis (400℃) Lab Pig manure

95% 4h 2015 China Jin et al (2016)

Pyrolysis (350℃) Lab Pig manure 93% 2016 Denmark Christel et al (2016)

Pyrolysis (600℃) Lab Pig digestate 160 mL 56% 2013 Swiss Troy et al (2014)

Fast pyrolysis (500℃)/acid

extraction Lab Biochar 0.2g 90% 120 min 2013 Southern Netherlands Azuara et al (2013)

61

Pyrolysis Lab Pig manure (420℃) 5g 92% 20 min 2012 Italy Marchetti et al (2012)

Pyrolysis (450℃) Lab Pig manure 95% 1 h 2017 Belgium Maggen et al (2017)

Hydrothermal


size

Recovery

efficiency

Duration


Hydrothermal carbonization Lab Pig manure 600 mL 95%

1h 2016 UK Ekpo et al (2016) 45%

Hydrothermal carbonization Lab Pig manure 20% 2018 Spain Gasco et al (2018)

Hydrothermal carbonization Lab Pig manure

130 mL

2011 China Qiao et al (2011)

Hydrothermal carbonization Lab Pig manure 94% 2018 Germany Li et al (2019)

Hydrothermal carbonization Lab Pig manure digestate 200 mL 95% 2020 China Zhang et al (2020)

Hydrothermal carbonization Lab Pig manure

67%

2017 Japan Huang et al (2017) 15%

Microwave-assisted hydrothermal

carbonization Lab Hydrochar (250℃) 40 ml

33% 30min 2020

Nanchang City,Jiangxi

Province,China Wang et al (2020)

78%

Hydrothermal liquefaction Lab Biocrudes (350℃) 7g 42% 60 min 2020 Aalborg, Denmark Conti et al (2020)

Hydrothermal carbonization Lab Hydrochar (220℃) 75g 12%

1 h 2019 Lin’an City, Zhejiang Song et al (2019) 73%

Struvite recovery


size

Recovery

efficiency

Duration


Precipitation via Ca(OH)2 and

Al2(SO4)3 Lab

Slurry

500 ml

40-75%

2 min 20

sec 2019 Bergamo, Italy Cattaneo et al (2019)

Digestate 60-75%

Digestate stripped 50-90%

Cattle and pig digestate 65-70%

Cattle and pig digestate

stripped 70-85%

Struvite recovery and anaerobic

digestion Lab Digested pig slurry 600 ml

89% 48 h 2015 Barcelona, Spain Cerrillo et al (2015)

98%

Struvite recovery Lab Raw pig manure 200 mL 69%

4 h 2019 Valencia (Spain) Luján-Facundo et al

(2019) 56%

62

Struvite crystallization Lab

TP = 85% 2018 China Ma et al (2018) TN = 71%

Ammonia stripping and struvite

ppt Lab Digestate 1 L

91% 3 h 2015 Beijing, China Huang et al (2015)

97%

Struvite precipitation Lab Digestate 3L 90% 15 h 2009 Hangzhou, Zhejiang

province. Perera et al (2009)

Struvite precipitation Lab Pig slurry 100 ml 95%

2017 France Piveteau et al (2017) 89%

Struvite precipitation Lab Pig slurry

95%

2013 France Daumer et al (2013)

95%

95%

95%

Struvite precipitation Pilot Pig manure 93% 2002 USA Burns and moody (2002)

Struvite precipitation Pilot Pig manure 90% 2012 USA Ackerman, Cicek and

Oleszkiewicz (2012)

Struvite precipitation Lab Pig manure 400ml 97% 2003 USA Burns et al (2003)

Struvite precipitation Lab Pig manure 65% 2014 France Achat et al (2014)

Separation processes


size

Recovery

efficiency

Duration


Quick wash process Lab Fresh pig manure 64 g 90% 1 h 2015 Florence Co., SC, USA Szoegi, Vanotti and Hunt

(2015)

Elco-ecopurin® Full

scale Pig slurry

42%

2002

Modena, Italy

Martinez-Almela and

Barrera (2005)

11%

53%

Barcelona, Spain 14%

71%

Murcia, Spain 28%

65%

Goshen, USA 30%

Vacuum membrane distillation Lab Raw biogas slurry 60-360

ml/min 60% 120 min 2020

Ezhou City, Hubei

Province, China Shi et al (2020)

Electrodialysis reversal Lab Digestate 2L 100%

9 h 2019 Galway, Ireland Shi et al (2019) 84%

63

Bipolar membrane electrodialysis

(BMED) system Lab Pig Manure hydrolysate 5L

78% 48 h 2018 Galway, Ireland Shi et al (2018)

75%

Biological methods


size

Recovery

efficiency

Duration


Vermicompost with pig manure

(VPM) Pilot Crop residue + PM 2g

25% 2 months 2019 Purple Soil, Yanting Raza et al (2019)

90%

Anaerobic digestion Lab Digestate 130 mL 90%

21 days 2016

Wang et al (2016) 89%

Duckweed ponds Full

Scale Digestate 3 m3

84% 134 days 2012 Southern Brazil Mohedaro et al (2012)

86%

Composting (mechanical turning) Full

scale

1 month stored slurry, fresh

slurry 4.8 m3

170 days 2017 Almería,

Spain Saez et al (2017)

Catalytic ozonation ± SBR Lab Digested slurry 1L 90%

4 days 2020 Maogang farm, Guangzhou Zeng et al (2020)

Struvite recovery anaerobic

digestion Lab Digested pig slurry 600 ml

89% 48 h 2015 Barcelona, Spain Cerrillo et al (2015)

98%

Algal accumulation Lab Pig manure

85%

2006 USA

Kebede-Westhead,

Pizarro and Mulbry

(2006) 92%

Composting Lab Pig manure

39%

1996 New Zealand Bhamidimarri and Pandey

(1996) 51%

Composting Lab Pig manure

42%

2005 China Zhang and He (2006) 94%

64

Annex 2

Study ID Randomization

process

Missing outcome

data

Measurement

of the outcome

Selection of the

reported result Overall Bias

Huang et al 2019 Low Low Low Low Low

Li et al 2016 Low Low Low Low Low

Laureni et al (2013) Low Low Low Low Low

Liu et al (2015) Low Low Low Low Low

Ippersiel et al (2012) Low Low Low Low Low

Huang et al (2015) Low Low Low Low Low

Hidalgo et al (2016) Low High High Low High

Christel et al (2014) Low Low Low Low Low

Christel et al (2016) Low Low Low Low Low

Uchimiya and Hiradate (2014) Low Low Low Low Low

Jin et al (2016) Low Some concerns Low Low Some concerns

Troy et al (2014) Low Some concerns Low Low Some concerns

Azuara et al (2013) Low Some concerns Low Some concerns Some concerns

Maggen et al (2017) Low Low Low Low Low

Marchetti et al (2012) Low Some concerns Some concerns Low Some concerns

Cattaneo et al (2019) Low Low Low Low Low

Cerrillo et al (2015) Low Low Low Low Low

Luján-Facundo et al (2019) Low Low Low Low Low

Ma et al (2018) Low High Some concerns Some concerns High

Perera et al (2009) Low Low Low Low Low

65

Piveteau et al (2017) Low Low Low Low Low

Daumer et al (2013) Low Low Low Low Low

Burns and moody (2002) Low Low Low Low Low

Ackerman, Cicek and Oleszkiewicz (2012) Low Low Low Low Low

Burns et al (2003) Low Low Low Low Low

Achat et al (2014) Low Low Some concerns Some concerns Some concerns

Szoegi, Vanotti and Hunt (2015) Low Low Low Low Low

Martinez-Almela and Barrera (2005) Low Low Low Low Low

Shi et al (2020) Low Low Low Low Low

Shi et al (2019) Low Some concerns Some concerns Low Some concerns

Shi et al (2018) Low Low Low Low Low

Raza et al (2019) Low Low Low Low Low

Wang et al (2016) Low Low Low Low Low

Mohedaro et al (2012) Low Some concerns Some concerns Low Some concerns

Saez et al (2017) Low Low Low Low Low

Zeng et al (2020) Low Low Some concerns Low Some concerns

Kebede-Westhead, Pizarro and Mulbry (2006) Low Low Low Low Low

Bhamidimarri and Pandey (1996) Low Low Low Low Low

Zhang and He (2006) Low Low Low Low Low

Ekpo et al (2016) Low Low Low Low Low

Gasco et al (2018) Low Low Low Low Low

Qiao et al (2011) Low Low Low Low Low

Li et al (2019) Low Low Low Low Low

66

Zhang et al (2020) Low Low Low Low Low

Huang et al (2017) Low Low Low Low Low

Wang et al (2020) Low Some concerns Some concerns Low Some concerns

Conti et al (2020) Low Low Low Low Low

Song et al (2019) Low Some concerns Some concerns Low Some concerns

Assessment Details

1. Bias arising from the randomization process:

a. the allocation sequence was random.

b. The allocation sequence was adequately concealed.

c. baseline differences between intervention groups suggest a problem with the randomization process.

2. Bias due to missing outcome data

a. data for this outcome were available for all, or nearly all, participants randomized.

b. (if applicable) there was evidence that the result was not biased by missing outcome data.

c. (if applicable) missingness in the outcome was likely to depend on its true value (e.g. the proportions of missing outcome data, or

reasons for missing outcome data, differ between intervention groups).

3. Bias in measurement of the outcome

a. the method of measuring the outcome was inappropriate.

b. measurement or ascertainment of the outcome could have differed between intervention groups.

c. outcome assessors were aware of the intervention received by study participants.

d. (if applicable) assessment of the outcome was likely to have been influenced by knowledge of intervention received.

67

4. Bias in selection of the reported result

a. the trial was analysed in accordance with a pre-specified plan that was finalized before unblinded outcome data were available for

analysis.

b. the numerical result being assessed is likely to have been selected, on the basis of the results, from multiple outcome

measurements within the outcome domain.

c. the numerical result being assessed is likely to have been selected, on the basis of the results, from multiple analyses of the data.

assessment of sustainable technologies for pig manure

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