Digestate processing: a review

Paz Vilanova PlanaSupervisor: Prof. Dr.-Ing Bernd Noche

CONTENTS

• Researcher in charge

• Introduction

• Objectives

• Digestate processing

• Solid- liquid separation

• Stabilization and thermal drying technologies

• Nutrient recovery technologies

• Liquid purification and concetrate technologies

ECONOMIC

GEORAPHIC ANALYSIS

Research Fields

• Biogas

• Digestate management

• Distribution models/ Transport systems

• Biofertilizer

Research & Work Experience

• 2014- Department of Transport Systems and Logistics. UDE (Germany)

5.6 Digestate distribution for large biogas plants: Storage and Transport

• 2011-2013- Project officer Biogas company (Spain and The Netherlands)

• 2010-2011- Project officer Tipperary Energy Agency (Ireland)

• 2008-2009- Project officer Biogas company (Spain)

• 2008- Technical assistant Biogas pilot plant (Spain)

RESEARCHER IN CHARGE

Feedstocks & Digestion

Process Monitoring

Upgrading & Supply

Optimum Biogas Utilisation

INTRODUCTION

1. https://www.google.com/maps/d/viewer?mid=1Qf92NTQfp73mglljO7i9YdoOMKk. 2. http://biogazownierolnicze.pl/mapa-biogazowni. 3. http://www.czba.cz/en/map-of-biogas-plants/

• Biogas production by anaerobic digestion (AD) is a well-known source ofrenewable energy. In 2014 the total number of installed biogas plants was morethan 17.000.

↑ BIOGAS PRODUCTION ↑ DIGESTATE PRODUCTION

1 2 3

INTRODUCTION• Digestate is the liquid-solid by-product produced through

the AD of organic material (3.5 to 13% DM).

• The AD effluent is a nutrient- rich material, especially in

nitrogen (N), phosphorous (P) and potassium (K).

• Uses: organic fertilizer, soil conditioner, compost, land regeneration, solid fuel and

building material.

• Types depending on:

• physical properties: Solid fraction, liquid fraction or whole digestate.

• source of feedstock: Agriculture-based digestate,

digestate from food and municipal waste, and sludge from

waste water treatment plant.

• Digestate production: 20 m3/yr. per kWel installed.

LANDFILLWWTPBIOGAS PLANT

Image source: www.agrar.basf.de . Table source: Weiland, 2010

POWER(kWel)

DIGESTATE (m3/yr.)

DISTRIBUTION AREA(ha)

500 11.500 300

1.000 23.000 600

2.000 46.000 1.200

4.000 92.000 2.400

80.00 184.000 4.800

16.000 368.000 9.600

20.000 460.000 12.000

INTRODUCTION

DIGESTATE MANAGEMENT = STORAGE

+ PROCESSING + TRANSPORTATION + UTILIZATION + ECONOMICS + ENVIRONMENTAL QUALITY

• Digestate processing water reduction, proper storage, nutrient management, or/and improve quality.

OBJECTIVES• The main objective of this study is to identify and analyse the relevant processing

technologies for digestate which may affect its storage and its transport.

• The presentation shows part of the study carried out and based on an extensivereview of technical and scientific literature regarding manure, sludge and digestateprocessing technologies.

• The studied technologies pretend to:

• Reduce the volume of the digestate

• Increase the nutrient content on the digestate

• Reduce the logistic cost of the digestate

• Transformation of digestate into more valuable products

• Nutrients recovery

• Nitrogen and phosphorous reduction or removal

DIGESTATE PROCESSING

SOLID- LIQUID SEPARATION

Pictures: BIGATEC Ingenieurbüro für Bioenergie

SOLID- LIQUID SEPARATION

COAGULATION - FLOCCULATION

Addition of flocculants andpolymers to modify the state of theparticles, to increase separationefficiency.

SEDIMENTATION

Process by which particles settle in a basinor tank, by gravity to the bottom of theliquid and form a digestate of higher solidsconcentration.

Source image: Takamoto biogas

SCREW PRESS

Digestate flows through ascrew which is located insideof a tube, and solids arecollected on the screw whilethe liquids pass through.Solids retained on thescreen are pressed to theend and discharged.

SOLID- LIQUID SEPARATION

Source image: Euroby

BELT PRESS

Digestate passes throughtwo horizontal andtensionate belts which pressdigestate and water issqueeze out. The belts areturning on rollers to movethe digestate.

DECANTERCENTRIFUGE

High speed mechanicalseparator which usescentrifugal force to separatesmall particles and colloids,and liquid material.

Source image: FAN Source image: Alfa laval

SOLID- LIQUID SEPARATIONTechnology

TS (%)

N total(%)

P(%)

Energyconsumption

kWh/m3

Volumen reduction

(%)

Investment cost€

Operational cost€/m3

Data source

Flocculation 70 43 79 20-22 22 50.000 0,8

Zhang et al., 1998, Hjort et al., 2010, Rico et al., 2007, Foged et al., 2010, Vanoti et al., 2005, Estevez Rodriguez et al., 2005, Heviánková et al., 2015, Garcia et al., 2007, Luckert et al., 2004. European Commission

Sedimentation 5 - - 0,0 – 0,1 - 17.000 -Levasseur et al., 2004. European Commission

Screw press 20-48,1 5-28 7-40,8 0,1-1,1 5-2520.000-35.000

0,25- 0,8

Møller et al., 2000, Buton et al., 2003, Frost et al., 2007, Westerman et al., 2005, Verhoeven et al., 2013, Hjorth et al., 2009, Luostarinen et al., 2011, Levasseuret al., 2004, Horst et al., 2013. European Commission, Fuchs and Drosg et al., 2010

Belt press 25- 65 30- 32 29-70 0,5- 2 2975.000-125.000

1,5

Vanotti et al., 2009, Buton et al., 2003; Frost et al., 2007, Horst et al., 2013, Levasseur et al., 2004, Foged et al., 2010, Hjorth et al., 2009, European Commission

Decanter centrifuge 50-71 12,5-44 27,1-93 2- 7 13-29 100.000 0,6-2,3

Hjorth M et al., 2010, Horst et al., 2013, Buton et al., 2003, Frost et al., 2007, Luostarinen et al., 2011. European Commission, Møller et al., 2001

SCREW PRESS < BELT PRESS < DECANTER CENTRIFUGE

STABILIZATION AND DRYING TECHNOLOGIES

STABILIZATION TECHNOLOGIESCOMPOSTING

Micro-organisms degrade and transform organic matter into CO2 and water. The aerobicdecomposition produces heat which cause the evaporation of the water which, together withthe breakdown of organic matter, result in a mass- and volume reduction.

TechnologyTS (%)

Energyconsumption

kWh/t

Volumen reduction

(%)

Investment cost€

Operational cost€/t

Data source

Composting 40-65 - 40-50 40.000-300.00 20

Ministry of Agriculture, Food and Fisheries. 1996Barrington et al., 2002, de Guardia et al., 2010, European commission, Zhang, et al., 2011, Levasseur et al., 2004, CBMI at al., 2010, Rehl et al., 2011, Sherman et al., 1999

Image source: popedouglasrecycle

DRYING TECHNOLOGIESBELT DRYER

Thermal energy derived from the CHPwhich is used to evaporate watercontented in the digestate.

SOLAR DRYING

Thermal energy derived from the sunwhich is used to evaporate watercontented in the digestate.

Picture source: Ecopreneur

Picture source: Stella

TechnologyInitial TS

(%)TS (%)

Energyconsumption

kWh/t

Volumen reduction

(%)

Investment cost€

Operational cost€/t

Data source

Solar drying 20 - 2000,6-3,5 t

water/m2 - -Vetter et al., 2006, Rehl et al., 2011,

Sypuła et al., 2013

Belt dryer 20 85-90 15-18 30- 40150.00-250.000

6,8kTBL ( 2008) European commission,

Rehl et al., 2011

NUTRIENT RECOVERY TECHNOLOGIES

NUTRIENT RECOVERY TECHNOLOGIESAMMONIA STRIPPING

Process which removes and recovers theammoniacal N. The total ammonium N istransferred to a gaseous form ammonia (NH3) viastripping, and after that it is absorbed in a solutionof sulfuric acid.

STRUVITE PRECIPITATION

Struvite precipitation recoversnitrogen and phosphorous in form ofamorphous magnesium nitrogen-phosphate salt called struvite

Image source: tecnium

TechnologyEfficiency

(%)Energy

consumptionkWh/t

End productInvestment cost

€Operational cost

€/tData source

Ammonia stripping 60-90 14-50Water ammonia (25 – 35 %

ammonia)Ammonium sulphate fertiliser

0.4-0.5M 4- 5,45Liao et al., 1995, Zeng, et al., 2006 , Bonmati et al.,2003, CBMI 2010, kTBL, 2008, Saracco et al., 1994,Fuchs et al., 2013, Eupean commission

Struvite precipitation80 P

23-30 Nhigh Struvite high high

CBMI 2010, Fuchs et al., 2013, European commission. Martin et al., 2008, Kern et al., 2008

CONCENTRATE AND LIQUID PURIFICATION TECHNOLOGIES

CONCENTRATE AND LIQUID PURIFICATION TECHNOLOGIES

EVAPORATION

Evaporation of the water content in digestateusing thermal energy, (CHP) increasing bothnutrient and solids concentration. During theprocess high temperatures will causeammonia losses and the vapor phase iscondensed.

MEMBRANE FILTRATION

A membrane is a physical barrier functioningas a molecular sieve which retainscontaminants yet allowing water to permeatethrough. Types: UF (10 to 100 nm): retainingsoluble macromolecules and larger particles.RO (pore size below 1 nm) : retaining smallmolecules and ions.

Image source: HRS

TechnologyEfficiency

(%)Energy consumption

kWh/t

Volumen reduction

(%)

Investment cost€

Operational cost€/t

Data source

Evaporation 20 100-350 90 250.000 10-40Fuchs et al., 2013, European commission, kTBL, 2008, Rehl et al., 2011

Membrane filtration 50 UF: 0,2-1.0, RO:1,5-10 99% OM 250.000 - 370.000 1,0-7FAKHRU et al., 1994, Fuchs et al., 2013, Europeancommission, CBMI (2010), kTBL, 2008, Rehl et al., 2011

DISCUSSIONTECHNOLOGY BENEFITS LIMITATIONS

FLOCCULATION - Increase solids removal (45% - 85% by use of flocculants)- Total cost

- High consumption of agents- Solid phase high concentrations of iron or aluminium, and the liquid phase has high concentrations of sulphates or chlorides (Country restrictions for further use)

SEDIMENTATION - Low energy input- Reduces transportation cost- Low operation cost

- Low hydraulic loading rates- Remove solids from tank every 1 to 2 months.- Poor removal of small suspended solids- Large space requirements (3 or more tanks)

SCREW PRESS - Low cost- Low energy requirements- Simple solution- Often present in farms

- Smaller particles (diameter of 0.5–1 mm) cannot be separated- Necessary high fibre content in digestate- Daily cleaning

BELT PRESS - High part of phosphorus remains in solid phase- Simple storage facilities

- Regular maintenance and management- High investment cost- High use of flocculants

DECANTER CENTRIFUGE - Separate most of phosphorus in the digestate into the solid fraction- Efficient in separate small particles and colloids (0.02 mm)- Simple storage facilities

- High investment cost- Necessary high flow of digestate

COMPOSTING - Volume reduction (up to 50%)- Stabilization and sanitation of the solid fraction.- Increases concentration of nutrients in the solid fraction- Quantifiable incomes: Sales of compost

- Pre-treatment: solid liquid separation- Specific machinery (tractor, solid- liquid separator)- Space requirement- Heavy metals concentrate in compost- Low C/N relation- Nitrogen loss (30-50% N) . Emissions to air as NH3 - Efficient moisture content range of 40- 60 %. < 40% slower, microbial activity decreases (no activity <15 %)

DISCUSSIONTECHNOLOGY BENEFITS LIMITATIONS

DRYING TECHNOLOGIES - Volume reduction (40%)- Moderate-high concentration of nutrient (N and P)- Homogeneous and sterilized end product (Organic matter stabilization, pathogens and seeds removal)- Possibility of pelletization after drying- Solar drying technology : space requirement- Reduces transportation cost

- High investment cost- High energy consumption for belt dryer (15-18 KW/m3 )- Pre-treatment: solid liquid separation- Solar drying: space requirement (thinner layer, lower amount of moisture, shorter time required to reduce humidity)- Potential risk of air pollutant emissions (dust, ammonia, and other volatile substances). Installations of dust filter andwasher/scrubber units necessary- Treatment of the condensate- Heavy metals in the dried product

AMMONIA STRIPPING - Easy operation- Nutrient recovery (N, P and K in concentrate fraction)- Nutrient concentration (ammonia water 25- 35 % ammonia)- Volume reduction- Pure nitrogen fertilizer produced and standardized Nconcentration- The concentrate is also partially hygienized- Easier management of end products- The volatility of ammonia improved by increasing temperature and pH- Recovering of condensates ( reduction ammonia emissions)- Quantifiable incomes: 0.35 €/kg

- High investment cost- High energy consumption- Only applicable to liquid fractions- Solid- liquid pretreatment required- High maintenance and cleaning- Clog of the column- Concentration of heavy metals in condensate- Condensates treatment depending on the quality

STRUVITE PRECIPITATION

- Nutrient recovery (P and possibility of N)- High quality fertilizer: Struvite (MgNH4PO4 6H2O) - Production of a stable fertilizer- Reduces transportation cost- Nutrient concentration - Low retention times

- No industrial level- High investment cost- High energy consumption- High cost of chemical reactive and interactions with Ca and K- Risk of ammonia volatilization - Maintenance costs

DISCUSSION

TECHNOLOGY BENEFITS LIMITATIONS

EVAPORATION - Nutrient recovery (N, P and K in concentrate fraction)- Nutrient concentration- Volume reduction- Nitrogen remains in the concentrate - The concentrate (where N is contained) is also hygienized(according to the operation time/temperature).

- High investment cost- Economically feasible for large biogas plants (>700 kW)- High energy consumption (670 kW/ t evaporated water)- High temperature required (boiling point)- Solid- liquid pretreatment ( required high dry matter content, > 30% )- Heavy metals concentrated in the concentrate ( limiting use)-Condensates treatment

MEMBRANEFILTRATION

- Viruses removal (UF)- Pure water- 50 % treatment of whole digestate

- Economically feasible for large biogas plants (>700 kW)- Membrane fouling - Membranes can filter only dissolved material - Treatment of liquid part- 50 % of the digestate is accumulated as by-products - High pressure

CONCLUSIONS• Digestate processing is one of the most important aspects regarding digestate management, due to the

capacity to reduce cost of transport and storage

• As well digestate processing is an interesting alternative to improve the quality of digestate, so they should beincluded in the viability study for new biogas plants

• Simple processing is feasible for small biogas plants and it is a solution for biogas plants located in areas withsurplus of P

• Complete conditioning are high technology which entail high investment cost, huge energy requirement, highmaintenance cost, and large amount of chemical reagents which rarely is viable for a single biogas plant.

• Centralized digestate processing installations are a possible solution where there is a surplus of nutrients, andhigh density of biogas plants and farms. These installations could treat digestate from a specific area formdifferent biogas plants.

• Further research to improve energy consumptions should be carry out, to improve the viability of processingtechnologies.

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Thanks for listening

This project has received funding from the European Union’s Seventh Framework Programme for research, technological development and demonstration under grant agreement n. 316838

Project coordinated by the QUESTOR Centre at Queen’s University Belfast www.qub.ac.uk/questor