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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.
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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-
Zhenyu Wu Chapter 1. Introduction 3
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.
Zhenyu Wu Chapter 1. Introduction 4
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-
Zhenyu Wu Chapter 1. Introduction 5
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].
Zhenyu Wu Chapter 1. Introduction 6
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]
Zhenyu Wu Chapter 1. Introduction 7
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
Zhenyu Wu Chapter 1. Introduction 8
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],
Zhenyu Wu Chapter 1. Introduction 9
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
Zhenyu Wu Chapter 1. Introduction 10
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
Zhenyu Wu Chapter 2. Feasibility study and performance study of the FO 13
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).
Zhenyu Wu Chapter 2. Feasibility study and performance study of the FO 14
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.
Zhenyu Wu Chapter 2. Feasibility study and performance study of the FO 15
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.
Zhenyu Wu Chapter 2. Feasibility study and performance study of the FO 16
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
Zhenyu Wu Chapter 2. Feasibility study and performance study of the FO 17
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
Zhenyu Wu Chapter 2. Feasibility study and performance study of the FO 18
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
Zhenyu Wu Chapter 2. Feasibility study and performance study of the FO 19
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
Zhenyu Wu Chapter 2. Feasibility study and performance study of the FO 20
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.
Zhenyu Wu Chapter 2. Feasibility study and performance study of the FO 21
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
Zhenyu Wu Chapter 3. FO Optimization in Different Initial Draw Solution Conditions 24
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.
Zhenyu Wu Chapter 3. FO Optimization in Different Initial Draw Solution Conditions 25
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.
Zhenyu Wu Chapter 3. FO Optimization in Different Initial Draw Solution Conditions 26
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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