exploring forward osmosis systems for recovery …...exploring forward osmosis systems for recovery...

Exploring Forward Osmosis Systems for Recovery of Nutrients and

Water

Zhenyu Wu

Thesis submitted to the Faculty of the

Virginia Polytechnic Institute and State University

in partial fulfillment of the requirements for the degree of

Master of Science

in

Environmental Engineering

Zhen (Jason) He, Chair

Andrea M. Dietrich

Zhiwu (Drew) Wang

December 20, 2017

Blacksburg, Virginia

Keywords: Forward osmosis; Swine wastewater; Struvite; Nutrient recovery; Water recovery

Copyright 2017, Zhenyu Wu

Exploring Forward Osmosis Systems for Recovery of Nutrients and Water

Zhenyu Wu

ABSTRACT

Livestock wastewater contains a large amount of nutrients that are available for recovery.

In this study, a proof of concept process based on Forward Osmosis (FO) was proposed and

investigated for in-situ formation of struvite from digested swine wastewater. This FO sys-

tem took advantage of a drawback reverse solute flux (RSF) and used the reversed-fluxed

Mg2+ for struvite precipitation, thereby accomplishing recovery of both water and nutrient.

With 0.5 M MgCl2 as a draw solution, high purity struvite formed spontaneously in the

feed solution and the water flux through the FO membrane reached 3.12 LMH. The precip-

itated struvite was characterized and exhibited a similar composition to that of commercial

struvite. The FO system achieve > 50% water recovery, > 99% phosphate recovery (given

sufficient magnesium supply), and > 93% ammonium nitrogen removal from the digested

swine wastewater. The recovered products (both struvite and water) could potentially gen-

erate a value of 1.35 $ m−3. The results of this study have demonstrated the feasibility of

nutrient recovery from livestock wastewater facilitated by FO treatment.

Exploring Forward Osmosis Systems for Recovery of Nutrients and Water

Zhenyu Wu

GENERAL AUDIENCE ABSTRACT

Forward Osmosis (FO) effectively separates water from dissolved solutes with a semi-permeable

membrane. This separation feature can be used in real water body to recover nutrients, con-

centrate wastewater for further treatment and produce energy for power plant. And the

water body rich in nutrients induces the plants growth. These plants consume tons of oxy-

gen in the water which decrease biodiversity in the water body, cause new species invasion

and economical lose. The nutrients-rich water has caused trouble to our human being for

decades, and one of them is livestock wastewater. Specifically, in this study, the piggery

wastewater was used to be treated by FO system. FO has not been used to treat piggery

waste/wastewater without additive from previous literature review. In this study, a FO re-

actor was built up for in-situ nutrient recovery as struvite, which is a valuable slow-release

fertilizer. The experiments from this study proved the concept for in-situ struvite recovery

from digested livestock wastewater via FO treatment with simultaneous water recovery, and

will encourage further exploration of FO promoted resource recovery form wastes.

Acknowledgments

I would like to express my greatest gratitude and highest respect to my advisor, Dr. Zhen

(Jason) He, for giving me a great opportunity to do researches and training me with critical

thinking and problem solving skills, for giving substantial support to my life and career, for

giving me chances and guidance again and again when I encountered difficulties in research,

Ph.D. position hunting. Dr. He is admirable for his devotion to career, his professional

ethics, his kindness and his patience, inspiring me to develop myself and chase my career

goal in the future. I would also like to thank my committee, Dr. Andrea M. Dietrich and

Dr. Zhiwu (Drew) Wang for their time and support in the research efforts and their kindness

and understanding.

I would like to thank all the EBBL (Environmental Biotechnology & Bioenergy Lab) mem-

bers for giving me generous support so that I was able to sort out the repeated problems

happened in my research. Thank you for always being there to help and stand with me.

Special thanks to Mr. Shiqiang Zou, who always acted like a big brother, listened to my

troubles, gave me advices and encouraged me to move forward. Also thanks to Heyang Yuan

for the great help with my lab work.

I would like to thank my parents and other family members for unconditional support and

iv

encouragement during my one and half years in the United States. I am proud of being a

member in this family.

v

Contents

List of Figures viii

List of Tables x

1 Introduction 1

1.1 Demand for nutrients removal and recovery . . . . . . . . . . . . . . . . . . . 1

1.2 Introduction to agricultural waste/wastewater treatment methods . . . . . . 2

1.3 Introduction of Struvite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

1.3.1 Struvite Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . 5

1.3.2 Struvite Automatic Precipitation in Wastewater Environments . . . . 7

1.4 Introduction to Forward Osmosis (FO) . . . . . . . . . . . . . . . . . . . . . 7

1.5 Research Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

2 Feasibility study and performance study of the FO 11

2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

2.2 Methods and Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

vi

2.2.1 Digested swine centrate . . . . . . . . . . . . . . . . . . . . . . . . . 13

2.2.2 FO system setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

2.2.3 Experimental procedure . . . . . . . . . . . . . . . . . . . . . . . . . 15

2.2.4 Measurement and analysis . . . . . . . . . . . . . . . . . . . . . . . . 16

2.3 Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

2.3.1 Water recovery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

2.3.2 Characterization of struvite . . . . . . . . . . . . . . . . . . . . . . . 19

2.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

3 FO Optimization in Different Initial Draw Solution Conditions 22

3.1 Material and methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

3.2 Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

3.2.1 Effects of draw concentration . . . . . . . . . . . . . . . . . . . . . . 23

3.3 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

4 Perspectives 27

5 Bibliography 29

vii

List of Figures

1.1 Schematic of an upflow anaerobic sludge blanket reactor (UASB): Wastewater

enters the reactor from the bottom and flows upward. [1] . . . . . . . . . . . 2

1.2 Schematic diagram of the anaerobic migrating blanket reactor (1.Feed Tank;

2.Injection Pump Diaphragm; 3.AMBR Reactor; 4,6.Influents; 5,7.Effluents;

8.Biogas Output; 9.Gas Meter; 10.Mixers) [2] . . . . . . . . . . . . . . . . . . 3

1.3 Diagram of the stirred anaerobic sequencing batch reactor [3] . . . . . . . . . 4

1.4 Schematic diagram of a typical RO system [4] . . . . . . . . . . . . . . . . . 8

1.5 Schematic diagram of a typical FO system [4] . . . . . . . . . . . . . . . . . 9

2.1 Schematic of the forward osmosis treatment system . . . . . . . . . . . . . . 15

2.2 Water extraction performance of the FO with 0.5 M MgCl2 as a draw: (A)

volume change of the draw and feed solutions; (B) water flux; (C) pH and

conductivity change in both the draw and feed solutions before and after the

extraction; and (D) Ions concentrations in the feed solution before and after

the extraction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

viii

2.3 Characterization of struvite: (A) XPS results of precipitates collected in the

FO process system and commercial struvite crystal; (B) SEM of the pre-

cipitates under 5000x magnification with mapping; (C) SEM of commercial

struvite crystal under 5000x magnification; (D) SEM-EDS of the precipitates,

the same spot at the blue square in Fig. 2.3 B; (E) SEM-EDS of commercial

struvite crystal, the same spot at the blue square in Fig. 2.3 C. . . . . . . . 21

3.1 Effects of the draw concentrations (0.25 1 M): (A) water flux and accumu-

lated volume of recovered water (inset); (B) the pH of the draw and feed

solutions before and after FO process; (C) the conductivity of the draw and

feed solutions before and after FO process. . . . . . . . . . . . . . . . . . . . 25

3.2 Ion distribution affected by the draw concentrations: (A) the NH4+-N con-

centration; (B) the Mg2+ concentration; (C) the PO43– -P concentration. . . 26

ix

List of Tables

1.1 Properties of struvite [5] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2.1 Characteristics of the filtered AD effluent (average ± standard deviation from

3 measurements) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

x

Chapter 1

Introduction

1.1 Demand for nutrients removal and recovery

Coastal watersheds raise about 75% of the world’s human population and are expended by

unpredictable urban, agricultural and industrial pressure [6]. During the 1960s and 1970s,

eutrophication in water bodies has been increasing dramatically in many parts of the world

[7], especially in coastal watersheds areas. Eutrophication has impacts like harmful algal

blooms, hypoxia, finfish and shellfish kills, loss of higher plant and animal habitat [7], in-

crease turbidity of water bodies, decrease lifespan of lakes and exert adverse impacts to

ecosystems [8]. Through a life cycle analysis view [9], these impacts bring about $2.2 billion

loss per year into the United States economic [10]. This economic decreasing gives a hint of

the urgent of eutrophication management in water bodies.

People has recognized excessive nutrients like nitrogen (N) and phosphorus (P) discharged

into the water body is the main reason of eutrophication for a while [11]. But nutrients from

spots with different local environment (separated into point sources or non-point sources)

1

Zhenyu Wu Chapter 1. Introduction 2

have different bioavailabilities and lag times [8]. Domestic wastewater counts a large part to

nutrients discharge because the large volume and high nutrients loading. Other than that,

with the increasing in global crop (+82% for 2000-2050 [12]), run-off from agriculture start

to generate larger amount of nutrients (the global soil N surplus increased by a factore of

3.8 to 138 Tg · y−1 of N and P surplus increased by a factor of 5.5 to 11 Tg · y−1 of P [12])

due to fertilizer overuse.

1.2 Introduction to agricultural waste/wastewater treat-

ment methods

Figure 1.1: Schematic of an upflow anaerobic sludge blanket reactor (UASB): Wastewaterenters the reactor from the bottom and flows upward. [1]

One reason that causing eutrophication is agricultural waste/wastewater discharging. Live-


Figure 1.2: Schematic diagram of the anaerobic migrating blanket reactor (1.Feed Tank; 2.In-jection Pump Diaphragm; 3.AMBR Reactor; 4,6.Influents; 5,7.Effluents; 8.Biogas Output;9.Gas Meter; 10.Mixers) [2]

stock productions are increasing around the smaller urban areas and towns of the nations.

Since these agricultural wastes frequently are many times the magnitude of the wastes from

the community, they represent a challenge for satisfactory management [13]. Different meth-

ods were developed to treat agricultural wastes. Composting is a way of obtaining a stable

product from biological oxidative transformation, similar to that which naturally occurs in

the soil [14]. By-products from agricultural wastes are developing dramatically, especially

from solid waste. For meat industries, typical byproducts are: edible fats, animal feed,

fertilizers, sausage casings, surgical thread, pharmaceutical products and feather meal. For

plant industries, most of the wastes are basically cellulosic in nature, which are easier for

processing and utilization if in solid form [13]. For biological methane production from or-

ganic material in agricultural waste, several different reactors have been developed in the

past 30 years like: up-flow anaerobic sludge blanket (UASB) (Fig. 1.1), anaerobic migrating

blanket reactor (AMBR) (Fig. 1.2) and anaerobic sequencing batch reactor (ASBR) (Fig.


Figure 1.3: Diagram of the stirred anaerobic sequencing batch reactor [3]

1.3) [15]. For the agricultural wastewater reclamation, source control, advanced treatment

process flowschemes and other engineering controls all provide a feasibility for increased im-

plementation of water reuse applications [16].

Nutrition recovery is quite important for agricultural waste treatment. Many conventional

and emerging technologies have shown up in this field. In allusion to different compounds,

different methods are used [17]. For pectin recovery: freeze drying, acid-assisted extrac-

tion, centrifugation, sequential ethanol precipitation [18], laser ablation [19], microwave-

assisted extraction, Soxhlet extraction [20], microwave- & pressure-assisted extraction, fil-

tration, washing & centrifugation [21] are introduced. For phenols recovery: concentration,

acid-assisted extraction, ethanol precipitation, dilution, microfiltration, ultrafiltration [22],

drying, pressurized & superheated ethanol-assisted extraction [23], microwave-assisted ex-

traction [24], conventional solid-liquid extraction [25], resin adsorption, methanol elution,

evaporation and freeze-drying [26], water extraction & high voltage electrical discharge are

used [27]. For proteins recovery: ultrafiltration, diafiltration & drying [28], skimming, mi-


crofiltration & freeze drying [29], centrifugation & ion-exchange membrane chromatography

are researched. Also, for lactose recovery: stirring, crystallization or sonocrystallization &

ethanol precipitation [30] are investigated.

1.3 Introduction of Struvite

1.3.1 Struvite Characteristics

Struvite is an orthophosphate. It has magnesium (Mg2+), ammonium (NH4+), and phos-

phate (PO43– ) in equal molar concentrations. The general formula for the struvite group

minerals is AMPO4 · 6 H2O. A corresponds to potassium (K) or ammonia (NH3) and M

corresponds to magnesium (Mg), cobalt (Co), or Nickel (Ni) [31]. Struvite in the face of

a magnesium ammonium phosphate hexahydrate crystallizes as an orthorhombic structure

(i.e., straight prisms with a rectangular base). Table 1.1 complies the main chemical and

physical properties of struvite crystals. Struvite crystals occur automatically in various bio-

logical media. For example, it has been fixed in rotting organic material like guano deposits

and cow manure, which is a media that where Mg and P already present through the micro-

biological combination of ions from bacterial metabolisms [36]. In the medical field, struvite

has also often been studied as it can spontaneously develop calculi in human kidneys [37],

and as a method to entrap nitrogen in compost in soil science [38, 39].


Table 1.1: Properties of struvite [5]

Nature Mineral salt

Chemical Name Magnesium ammonium phosphate hexahydrate

Formula MgNH4PO4 · 6 H2O

Aspect White glowing crystal [31]

Structure Orthorhombic (space group Pmn21 ): regular PO43 octahedra,

distorted Mg(H2O)62+ octahedral,

and NH4 groups all held together by hydrogen bonding [32]

Molecular weight 245.43 g ·mol–1

Specific gravity 1.711 (ρ = 1.711 g · cm–3 [33])

Solubility Low in water: 0.018 g · 100 ml–1 at 25°C in water

High in acids: 0.033 g · 100 ml–1 at 25°C in 0.001 N HCl;

0.178 g · 100 ml–1 at 25°C in 0.01 N HCl [34]

Solubility constant 10−13.26 [35]


1.3.2 Struvite Automatic Precipitation in Wastewater Environ-

ments

In the water / wastewater treatment area, struvite as a scale problem has been known for

a while. Rawn et al. mentioned the appearance of a crust of crystalline material in areas

of a pipe that was carrying supernatant liquors and identified it as magnesium ammonium

phosphate in a purity ratio of 96% in their research about digestion system [40]. Borgerding

confirmed struvite as a source of scale deposits in wastewater treatment plants (WWTP) in

1972 [33]. In 1963, He identified struvite on the walls of an anaerobic digestion system at the

Hyperion treatment plant in Los Angeles. Because the struvite deposit was dissolved after

an acidic treatment, this scale problem was considered to be solved successfully. Unluckily,

it reappeared and reduced the diameter of pipes significantly in the same plant a few years

later. Since then, struvite as a scale agent had been the theme of several researches [41, 42],

but most authors have considered struvite more a problem to eliminate than a product of

economic interest [5].

1.4 Introduction to Forward Osmosis (FO)

Osmosis itself has been used for desiccating foods since the early days of mankind. Con-

ventionally, “osmosis is defined as the net movement of water across a selectively permeable

membrane driven by a difference in osmotic pressure across the membrane” [43]. Nowadays,

osmosis phenomenon has been largely invested in numerous disciplines including: water

treatment, food processing, power generation and novel methods for controlled drug release.

In the field of water treatment, reverse osmosis (RO) 1.4 is generally a more familiar process

than osmosis. RO uses hydraulic pressure to oppose, and exceed, the osmotic pressure of an


Figure 1.4: Schematic diagram of a typical RO system [4]

aqueous feed solution to produce purified water [44]. Compare RO and osmosis: in RO, the

applied pressure drives the mass transfer through the membrane; in osmosis 1.5, the osmotic

pressure itself drives mass transfer through the membrane.

In water/wastewater treatment field, osmosis (now referred as forward osmosis (FO)) has

been used in several different conditions, which includes treating industrial wastewater (at

bench-scale) [45, 46, 47], concentrating landfill leachate (at pilot- and full-scale) [48, 49, 50],


Figure 1.5: Schematic diagram of a typical FO system [4]

and to treat liquid foods in the food industry (at bench-scale) [51, 52, 53, 54, 55, 56, 57, 58,

59].

Forward osmosis (FO) membrane can be used to concentrate digested swine waste sludge

[60], which is favor of struvite recovery. A natural osmotic pressure gradient across a piece

of semi-permeable membrane is utilized by FO to drive water migration from a feed solution

(high-water potential) to a draw solution (low-water potential) [43]. When MgCl2 is used in


draw solution in FO process, the salt flux from draw to feed can increase the Mg2+ concen-

tration on the feed side. Also the protons will diffuse from the feed to draw side will increase

the pH on the feed side, which is beneficial for struvite formation.

1.5 Research Objectives

This work aims to present a proof of concept for in-situ struvite formation by FO treatment

with the specific objectives:

1. Recover ammonium and phosphorus from digested swine wastewater with reverse

fluxed magnesium;

2. Examine the effects of the MgCl2 draw concentration on recovery of struvite and water.

Chapter 2

Feasibility study and performance

study of the FO

2.1 Introduction

The booming agriculture development has successfully met the mounting food demand in

the 21st century while generating a large amount of nutrient-rich wastes/wastewater [61].

Livestock waste/wastewater, such as swine and cow manure, is a major type of agriculture

waste/wastewater and rich in ammonium, phosphorus, and metal ions like magnesium [62].

Direct discharge of livestock waste/wastewater can lead to potential environmental pollution

in natural water bodies, and hence proper treatment should be conducted. Current treat-

ment approaches for livestock waste/wastewater focus on removal of organics and nutrient

via biological processes [63]. A popular treatment method, anaerobic digestion (AD), can ef-

fectively reduce organic concentration and recover useful bioenergy as biogas [64]. However,

AD cannot realize nutrient recovery, and its effluent often contains a high concentration of

nutrients. Therefore, there is a need to develop a cost-effective and energy-efficient approach

11

Zhenyu Wu Chapter 2. Feasibility study and performance study of the FO 12

to recover nutrients from AD treated livestock wastewater.

Compared to cow manure, swine wastes have a higher water content; nutrients can be re-

covered from swine wastewater as struvite, a valuable slow-release fertilizer [15]. Usually,

external addition of magnesium would be necessary to precipitate struvite. A previous study

reported the use of MgO-contained wastewater as a source of magnesium for struvite precip-

itation and removal of total ammonia nitrogen from swine wastewater [65]. The produced

struvite from a synthetic swine wastewater was applied to treat antibiotics by absorbing

tetracyclines [66]. Struvite was also recovered from swine wastewater by using a microbial

fuel cell that could generate electricity and provide a high pH solution for precipitation

[67]. Those and other prior studies have provided a strong proof of struvite recovery from

swine wastewater and also identified the key challenges to its implementation, for example

magnesium dosage [68]. Reduced magnesium dosage could be achieved with a higher initial

phosphorus concentration (e.g. > 100 mg L−1 P) if swine wastewater is properly concen-

trated. Meanwhile, onset of slow struvite crystallization is revealed at a pH above 7 [41],

while fast struvite precipitation tends to happen under a more basic environment (a solution

pH over 8.0). Therefore, direct struvite precipitation from AD treated swine wastewater

(that usually has a solution pH of 6.9-7.8 [69]) would be rather difficult. High-efficient stru-

vite precipitation can be achieved by adding additional chemicals, e.g. sodium hydroxide in

current practice to increase pH, leading to elevated cost and footprint [70].

Forward osmosis (FO) has been studied to concentrate digested wastewater for enhanced

struvite recovery [60], but it has not been employed to treat swine wastewater for nutri-

ent recovery. FO takes advantage of an osmotic pressure gradient across a semi-permeable

membrane to drive water migration from a feed solution (high-water potential) to a draw

solution (low-water potential) [43]. This water migration can concentrate the feed solution


and possibly benefit further treatment with reduced footprint and chemical use (e.g., pH

adjustment). In this study, a FO system with MgCl2 as a draw solute was investigated to

achieve simultaneous nutrient and water recovery from digested swine wastewater. When

MgCl2 is used as a draw solute to concentrate swine centrate (feed), both the reverse salt

flux of Mg2+ from the draw to the feed and pH increase via concurrent protons diffusion

from the feed to the draw (to balance charge [60]) can help realize in-situ struvite formation.

In this way, no additional Mg or pH adjustment will be needed. This work aims to present a

proof of concept for in-situ struvite formation by FO treatment with the specific objectives:

(1) recover ammonium and phosphorus from digested swine wastewater with reverse fluxed

magnesium; and (2) examine the effects of the MgCl2 draw concentration on recovery of

struvite and water.

2.2 Methods and Analysis

2.2.1 Digested swine centrate

The supernatant of digested swine wastewater (referred as centrate unless otherwise stated)

was collected from an anaerobic digester (AD) at North Carolina Agricultural and Techni-

cal State University and was conserved under 4 °C. Before applying as the feed solution,

the centrate was filtered by a 0.45µ m membrane (FisherbrandTM) to remove suspended

particles, thereby mitigating potential membrane fouling. It should be noted that the use

of filtration would increase the cost of struvite produced and may be replaced with gravity-

driven settlement in practical applications. The filtered centrate contained PO43– (166.50

mg L−1), NH4+ (413.33 mg L−1), Mg2+ (15.07 mg L−1), and Cl– (104.77 mg L−1), and had

a pH of 7.4 (Table 2.1).


Table 2.1: Characteristics of the filtered AD effluent (average ± standard deviation from 3measurements)

Parameters Values Units

COD 4166.67 ± 144.29 mg L−1

Conductivity 4.66 ± 0.02 mS cm−1

pH 7.39 ± 0.00 -

PO43– -P 166.50 ± 2.15 mg L−1

NH4+-N 413.33 ± 0.33 mg L−1

Mg2+ 15.07 ± 5.44 mg L−1

Cl– 104.77 ± 10.58 mg L−1

2.2.2 FO system setup

A bench-scale FO system was built (Fig. 2.1) and contained one piece of cellulose triacetate

(CTA) membrane with a total surface area (S) of 30 cm2 (Hydration Technologies Inc.,

Albany, OR, USA). The FO membrane was installed with its active layer facing the feed

(FO mode), creating an identical volume of 48 mL for each of the feed and draw chambers.

The CTA membrane had a suggested operating pH range of 2-12 by its manufacturer [71].

Both draw and feed solutions were recirculated by using pumps (50 mL min−1, 4.24 cm s−1)

between the chamber and an external reservoir bottle (300 mL). The FO system was operated

under a batch mode (24 h interval), and samples (5 mL) were taken directly from the reservoir

bottles at 0 h and 24 h for water quality analysis. The experiment was operated without

any biological processes and under temperature control for 22 ± 2 °C.


Figure 2.1: Schematic of the forward osmosis treatment system

2.2.3 Experimental procedure

The feasibility test of in-situ nutrient recovery was examined with 100-mL 0.50 M MgCl2

(conductivity 53.40 ± 0.02 mS cm−1) as the draw solution and 200-mL filtered centrate

as the feed solution in the FO system. For comparison, a control test was also performed

by directly adding 56.08 mg MgCl2 (the equivalent of 0.5 M MgCl2 solution) to 200 mL

filtered centrated to secure a Mg : P molar ratio of 1.2 : 1; its solution pH was adjusted

to 8.5 ± 0.1, an ideal pH for struvite precipitation [72], by adding 1 M sodium hydroxide

(NaOH) dropwise.


2.2.4 Measurement and analysis

Typical water quality parameters were analyzed by using standard methods according to

US Environmental Protection Agency (EPA) [73]. The concentrations of PO43– -P, NH4

+-N,

and COD (chemical oxygen demand) were measured by using a spectrophotometer following

the manufacturer instruction (DR 890, HACH Company, USA). The concentrations of Cl– ,

F– , NO2– , NO3

– , SO42– , Ca2+, Mg2+, K+ and Na+ were quantified by using an ion chro-

matography (Dionex LC20 ion chromatography, Sunnnyvale, USA). The solution pH and

conductivity were measured by a benchtop pH meter (Oakton Instruments, Vemon Hills, IL,

USA) and a conductivity meter (Mettler-Toledo, Columbus, OH, USA), respectively. The

morphology and element composition of the precipitates obtained from the feed reservoir

were analyzed by using scanning electron microscopy (SEM) coupled with energy dispersive

spectroscopy (EDS, Quanta 600 F FEI, USA) coated with platinum/palladium (80.0 % :

20.0 %, wt%). The crystal structure was analyzed by powder X-ray diffraction (XRD) mea-

surements performed using a Rigaku MiniFlex 600. The commercial struvite crystal with a

purity of 99.9 % (Alfa Aesar, Lancashire, U.K.) was used as the XRD spectrum reference.

Water flux was determined by weight change of the draw solution via an electronic bal-

ance (Scort Pro, Ohous, Columbia, MD, USA) connected with a data logger. The permeate

water flux (Jw, L m−2 h−1, LMH) of the FO system was calculated using Eq. 2.1:

Jw =mt+∆t,D −mt,D

ρ · S · ∆t(2.1)

where mt,D (g) represents the mass of draw solution at a specific time t (h), S (m2) is the

total surface area of the FO membrane, ρ (g mL−1) is water density and ∆t (h) stands for

operating time (24 h). Forward solute flux (FSF, mmol m−2 h−1 [74]), indicating penetration


of ions from the feed to the draw side through the FO membrane, was calculated using Eq.

2.2:

FSF =VD × Cf,D

S × ∆t(2.2)

where VD (L) is the final volume of draw solution, Cf,D (mmol L−1) is final molar concen-

tration in the draw. The reverse solute flux (RSF, mmol m−2 h−1) of draw solute ions (i.e.

Mg2+, Cl– ) from the draw to the feed side was calculated according to the increment of the

corresponding concentration in the final feed solution, by the following equation:

RSF =Ct × Vt + n− Co × Vo

S × ∆t(2.3)

where Vo and Vt (L) is the original and final feed volume, respectively; Co and Ct (mmol L−1)

is the original and final feed concentration, respectively; and n (mmol) represents the con-

cerned ions amount (including Mg2+ and Cl– in this study) in the struvite (n equals 0 for

Cl– ).

2.3 Results and Discussion

2.3.1 Water recovery

Water recovery from swine wastewater was first examined in the FO system with 0.50-M

MgCl2 as the draw solution. A total of 119.9 mL water was extracted within 24 h (Fig. 2.2

A), rendering a 60.0 % water recovery efficiency. The obtained maximum water flux (3.1

LMH) was comparable with that reported in a previous FO study concentrating digested

municipal sludge using MgCl2 as draw ( 3.0 LMH) [60]. Water flux was then gradually re-

duced to 1.6 LMH at the end of a batch (Fig. 2.2 B), due to the diminished osmotic pressure


Figure 2.2: Water extraction performance of the FO with 0.5 M MgCl2 as a draw: (A)volume change of the draw and feed solutions; (B) water flux; (C) pH and conductivitychange in both the draw and feed solutions before and after the extraction; and (D) Ionsconcentrations in the feed solution before and after the extraction.

difference across the FO membrane and thereby reduced driving force. Continuous dilution

via permeated water also led to gradual decrease of conductivity in the draw solution (Fig.

2.2 C). As a result of both the concentrating effect and RSF, conductivity on the feed side

was increased. Increased pH was observed for both the feed and draw solutions. The pH

increase on the feed side was mainly due to proton diffusion towards the draw side, for bal-

ancing the charge difference caused by Mg2+ ions diffusion from the draw to feed side [60].

The increase in the draw pH was driven by NH4+ flux from the feed and loss of Cl– ions


from the draw to the feed. Membrane fouling was observed on the feed side of membrane (a

thin brown cake layer) and could be effectively controlled by simply physical flushing [75].

In addition to water flux, ions also migrated across the FO membrane during the pro-

cess. The FSF of NH4+ was 1.86 mmol m−2 h−1 after 24 h, rendering a loss rate of 2.5 %.

Ion penetration via FSF could cause potential pollution on the feed side, requiring further

purification treatment and increasing operation cost towards water reuse [76]. Multivalent

ions with larger hydrated radii would be more difficult to diffuse through the FO membrane,

presenting a reduced FSF and lower loss rate than that of the monovalent ions (NH4+) [77].

As a result, no PO43– -P was detected in the draw solution via FSF (i.e. negligible loss rate).

The RSF of Mg2+ from the draw to feed solution was 63.87 mmol m−2 h−1, among which

25.0 % was precipitated as struvite. The RSF of Cl– was 82.49 mmol m−2 h−1, higher than

that of Mg2+. This RSF difference could be caused by a higher concentration gradient of

Cl– across the membrane than that of Mg2+.

2.3.2 Characterization of struvite

Successful struvite precipitation was observed at the bottom of the feed solution container.

The composition of the precipitated crystals was characterized by using XRD and SEM-EDS

coupled with mapping. XRD showed several peaks between 20° and 40° (2 theta degree) with

well-detected intensity (Fig. 2.3 A), indicating a morphous diffraction being consistent with

that of standard struvite samples. The EDS revealed major components in the precipitates

to be P (28.6 %, wt%) and Mg (20.9 %, wt%) (Fig. 2.3 B red circle and Fig. 2.3 D red

peaks), which were close to that of the standard commercial struvite (32.9% P and 22.2%

Mg, Fig. 2.3 C red circle and Fig. 2.3 E red peaks). Impurities were determined to be

negligible with a tiny amount of Cl, P and Mg (Fig. 2.3 B blue square and Fig. 2.3 D


blue peaks). The SEM image of the precipitates exhibited some well-regulated crystals (Fig.

2.3 B), consisted of a mixture of regular prismoid crystals with an average size of ∼ 50µm

coated with a flat structure, similar to the standard struvite morphology owned orthorhmbic

structure, as showed in Fig. 2.3 C [78]. All these results have demonstrated the feasibility

of in-situ formation of high-purity struvite from swine centrate in the FO system. However,

the high RSF resulted in a good amount of Mg that was not used for struvite precipitation.

Thus, Mg dosage should be optimized.

2.4 Conclusion

In this study, in-situ struvite precipitation from digested swine wastewater has been accom-

plished by FO treatment using MgCl2 as a draw solute. Rather than adding MgCl2 directly

to the centrate, the FO system made it possible for nutrients like PO43– -P and NH4

+-N

be combined with reverse-fluxed Mg2+ and collected as struvite. The collected struvite was

confirmed for its composition by SEM-EDS and XPS.


Figure 2.3: Characterization of struvite: (A) XPS results of precipitates collected in theFO process system and commercial struvite crystal; (B) SEM of the precipitates under5000x magnification with mapping; (C) SEM of commercial struvite crystal under 5000xmagnification; (D) SEM-EDS of the precipitates, the same spot at the blue square in Fig.2.3 B; (E) SEM-EDS of commercial struvite crystal, the same spot at the blue square in Fig.2.3 C.

Chapter 3

FO Optimization in Different Initial

Draw Solution Conditions

3.1 Material and methods

The digested swine centrate, FO system setup, measurement and analysis was the same

as mentioned in section 2.1, 2.2 and 2.4. The test for draw optimization was performed

with varied draw solution concentration at 0.25, 0.50, 0.75 and 1.00 M (triplicates) and a

fixed draw recirculation flow rate of 50 mL min−1 (4.24 cm s−1). The precipitants at the

bottom of the feed reservoir was collected via filtration, washed twice with deionized water

(DI water), and then dried in the fume hood for further analysis. For physical cleaning and

fouling control, the FO membrane was washed by DI water for 30 min with a recirculation

rate of 200 mL min−1 (16.98 cm s−1) after each batch test.

22

Zhenyu Wu Chapter 3. FO Optimization in Different Initial Draw Solution Conditions 23

3.2 Results and Discussion

3.2.1 Effects of draw concentration

Different draw concentrations were successively evaluated in terms of water recovery per-

formance, solution pH, and conductivity. A MgCl2 concentration higher than 0.50 M was

sufficient to reach the maximum water recovery capacity for this FO system, i.e. ∼ 120

mL after 24-h operation (Fig. 3.1 A inset). The maximum water flux was in a positive

correlation with initial draw concentration (Fig. 3.1 A), due to a higher water driven force

created under an enhanced osmotic pressure [43]. These results implied that after the MgCl2

concentration reaches 0.5 M, the dilution rate can reach 120 % for the draw side.

Overdose of Mg on the feed side (via RSF) could happen with a higher draw concentration

and hence a larger reverse MgCl2 leakage, leading to reduced pH of the feed solution resulting

from potential formation of Mg(OH)2 (Fig. 3.1 B). The conductivities of the feed and

the draw before and after the reaction showed a clear trend of incensement (Fig. 3.1 C).

This trend is the result of extra MgCl2 addition in the draw solution, which boosted the

conductivity on the draw side. Ions involved in struvite formation could provide information

of nutrient recovery efficiency of this FO system, including NH4+-N, PO4

3– -P, and Mg2+.

Comparable NH4+-N removal rates of ∼ 93% were observed in the feed for all the draw

concentrations (Fig. 3.2 A). However, NH4+-N loss from the feed to the draw increased

from 1.8% to 3.6% when the MgCl2 concentration was increased from 0.25 M to 0.75 M,

leading to an elevated FSF (up to 2.72 mol m−2 h−1) at a higher draw concentration. The

utilization rate of Mg2+ diffused from the draw to the feed was decreased at a higher MgCl2

concentration. The Mg2+ concentration change (Fig. 3.2 B) represented 24.27, 63.87, 155.79


and 181.71 mmol m−2 h−1 RSF, indicating 65.8 %, 25.0 %, 10.3 % and 8.8 % of the diffused

Mg2+ utilization rate. For PO43– -P, a removal rate higher than 99.0 % could be achieved

once the MgCl2 concentration was higher than 0.5 M (Fig. 3.2 C). No loss of PO43– -P from

the feed to the draw was observed. The MgCl2 concentration of 0.50 M could achieve a low

NH4+-N loss rate and the highest PO4

3– -P removal rate, but its Mg2+ utilization rate was

much lower than that of 0.25 M. It should be noted that excess Mg2+ in the final concentrated

feed solution could be recycled via mixing with fresh feed solution in the following batch test

(i.e. a closed-loop system) [78].

3.3 Conclusions

The draw concentration had a major impact on nutrients and water recovery efficiencies.

The results of this study have provided a proof of concept for in-situ struvite recovery from

digested livestock wastewater via FO treatment with simultaneous water recovery, and will

encourage further exploration of FO promoted resource recovery from wastes.


Figure 3.1: Effects of the draw concentrations (0.25 1 M): (A) water flux and accumulatedvolume of recovered water (inset); (B) the pH of the draw and feed solutions before andafter FO process; (C) the conductivity of the draw and feed solutions before and after FOprocess.


Figure 3.2: Ion distribution affected by the draw concentrations: (A) the NH4+-N concen-

tration; (B) the Mg2+ concentration; (C) the PO43– -P concentration.

Chapter 4

Perspectives

Extensive economic analysis would not be possible for a proof of concept study, and thus

a preliminary analysis of potential economic benefit on the recovered products (water and

struvite) from the swine centrate was conducted. When 0.5 M MgCl2 was used as the draw

solution, approximately 285 g struvite and 835.9 kg water could be recovered from one cubic

meter of centrate, based on a PO43– recovery rate of 97.8 % and an average water extraction

rate of 2.32 LMH. Assuming the unit price of struvite and water to be 0.35 $ kg−1 and

0.0015 $ kg−1 (2017 market price), respectively, the total value would be 1.35 $ m−3. This

economic benefit will become higher with the increased price of struvite crystal and also

benefit from waste treatment. In this preliminary assessment, the manpower costs, as well

as the energy consumption of pump and magnetic stirrer were not taken into account.

Despite the promise of the proposed method, several challenges must be properly addressed

towards future development. First, although no obviously fouling was observed in this study,

the foreseeable fouling problem in a close-chamber FO system is not negligible. Appropriate

fouling control and membrane cleaning strategies including physical and chemical methods

27

Zhenyu Wu Bibliography 28

need to be developed. Second, separation of struvite crystals from centrate requires further

investigation. This process is important to remove impurities from the final product and

will require additional energy input/operation. Third, as the SEM-EDS showed, the stru-

vite collected from this FO system was coated by MgCl2, which decreased the purity of the

recovered struvite. The accurate purity grade of the struvite was not analyzed and should

be assessed in the future study. The purity of struvite has a great impact on its price and

fertilizer efficiency. Last but not least, the remaining magnesium from RSF should be further

utilized for struvite precipitation from continuous treatment of digested swine wastewater.

Chapter 5

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