Closing CO2 Loop in Biogas Production: Recycling Ammonia As

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Closing CO2 loop in biogas production: recycling ammonia as fertilizer Qingyao He, Ge Yu, Te Tu, Shuiping Yan, Yanlin Zhang, and Shuaifei Zhao Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b00751 • Publication Date (Web): 05 Jul 2017 Downloaded from http://pubs.acs.org on July 5, 2017

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Environmental Science & Technology

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Closing CO2 loop in biogas production: recycling ammonia as fertilizer

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Qingyao He a,b,c, Ge Yua,b, Te Tua,b, Shuiping Yan a,b,*, Yanlin Zhang a,b , Shuaifei Zhao c,**

3

a

4

District, Wuhan 430070, PR China

5

b

6

c

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* Email: [email protected];

8

** Email: [email protected]; phone: +61-2-9850 9672

College of Engineering, Huazhong Agricultural University, No.1, Shizishan Street, Hongshan

The Cooperative Innovation Center for Sustainable Pig Production, Wuhan 430070, PR China Department of Environmental Sciences, Macquarie University, Sydney, NSW 2109, Australia

9 10 11 12 13 14

* Corresponding author: Dr Shuaifei Zhao

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Email: [email protected]

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Tel.: +61 2 98509672

17 18 19 20 21 22 23 1

ACS Paragon Plus Environment


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Abstract

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We propose and demonstrate a novel system for simultaneous ammonia recovery, carbon capture,

26

biogas upgrading, and fertilizer production in biogas production. Biogas slurry pretreatment

27

(adjusting the solution pH, turbidity and chemical oxygen demand) plays an important role in the

28

system as it significantly affects the performance of ammonia recovery. Vacuum membrane

29

distillation is used to recover ammonia from biogas slurry at various conditions. The ammonia

30

removal efficiency in vacuum membrane distillation is around 75% regardless of the ammonia

31

concentration of the biogas slurry. The recovered ammonia is used for CO2 absorption to realize

32

simultaneous biogas upgrading and fertilizer generation. CO2 absorption performance of the

33

recovered ammonia (absorption capacity and rate) is compared with a conventional model absorbent.

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Theoretical results on biogas upgrading are also provided. After ammonia recovery, the treated

35

biogas slurry has significantly reduced phytotoxicity, improving the applicability for agricultural

36

irrigation. The novel concept demonstrated in this study shows great potential in closing the CO2

37

loop in biogas production by recycling ammonia as an absorbent for CO2 absorption associated with

38

producing fertilizers.

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TOC/Abstract art

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1. Introduction

50

Concerns on energy security and climate change associated with greenhouse gas emissions are of

51

growing interest. Energy is one of the most important and challenging needs of our time as it

52

powders our life 1. Global energy demands are increasing rapidly. According to the most recent world

53

energy outlook 2, the global energy demands are projected to increase by 30% to 2040. More than 85%

54

of our energy supply is now still based on fossil fuels

55

significant amounts of greenhouse gases, causing the critical concern on climate change.

56

As a renewable energy source, biogas is playing a vital role in supplying sustainable energy and

57

minimizing greenhouse gas emissions 4. Production of biogas through anaerobic digestion converts

58

food wastes, manure, and other organic wastes into CH4 (~ 60%), CO2 (~ 40%) and nutrient-rich

59

digestate 5. Biogas can be used for replacement of fossil fuels in power and heat generation. It has

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wider industrial applications (e.g. as gaseous vehicle fuel) after upgrading, compression and/or

61

liquefaction

62

technology combining bioenergy generation and carbon capture and storage (Bio-CCS) has been

63

identified as a promising way to achieve net negative CO2 emissions 10-12. In this system, biomass is

64

converted into energy and side-product CO2 is captured and stored.

65

After anaerobic digestion, the digestate (in addition to biogas) is generally mechanically separated

66

into solid and liquid fractions. The solid phase accounts for 0.5 - 15 wt%, and most of the digestate is

67

liquid, called biogas slurry (BS) 5, 13, 14. The quantity of BS in a biogas production plant is huge and it

68

is generated at rates of 0.05 - 0.1 m3 BS/m3 biogas

69

or fertilizer due to its richness in nutrients like nitrogen, phosphorus and potassium

6-8

1, 3

. Fossil fuel consumption produces

. Biogas production and CO2 emission reduction can be realized at the same 9. The

5, 13

. BS is traditionally used as a soil ameliorant

4


15

. However,

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direct use of BS to soil may cause environmental risks because of the high concentrations of

71

nutrients, especially high total ammonium nitrogen (TAN) concentration (0.5 - 5 g N L-1) 15-19.

72

Various methods have been employed to remove or recover TAN from BS to minimize risks of using

73

the nutrients

74

absorption 16, zeolite adsorption by ion exchange 24, co-precipitation with phosphate and magnesium

75

to form struvite

76

studies on the use of BS to capture CO2 to realize production of fertilizers and reduction in CO2

77

emissions. Actually, BS can be used as one type of “once-through” CO2 absorbents 26, and one of the

78

main reactions is: NH +4 → NH 3 → NH 4 HCO 3 27. After CO2 absorption, the liquid can be used as

79

fertilizers for agricultural irrigation.

80

Recently, algae cultivation for biogas upgrading combining utilization of nutrients in biogas slurry

81

has been studied to separate and utilize CO2 simultaneously

82

biogas by stabilizing CO2 into algae and produce valuable products, its efficiency needs to be

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significantly improved. Additionally, a large amount of fresh water is required to dilute the nutrients

84

in BS to a suitable level in algae cultivation when BS has a high concentration of TAN and turbidity

85

31

86

sodium bicarbonate and calcium carbonate tends to form over the desirable product (NH4HCO3) 32.

87

Therefore, superior processes to achieve efficient biogas CO2 separation and safe nutrient utilization

88

with BS is highly required.

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In this study, we demonstrate a novel system where multiple benefits, including biogas upgrading,

90

carbon capture, ammonia recovery, and fertilizer production can be achieved (Fig. 1). Briefly, after

15, 16, 20-22

. These technologies include reverse osmosis

20

21

, stripping

, and low pressure processes with gas permeable membranes

23

and acid

25

. However, few

28-30

. Although the process can upgrade

. Ion exchange may be feasible to recover ammonia from wastewaters for CO2 absorption; however,

5



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anaerobic digestion, vacuum membrane distillation (VMD) is used to recover ammonia from BS.

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Treated BS can be used for safe agriculture irrigation after TAN removal. Recovered ammonia is

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used for CO2 absorption to realize simultaneous biogas upgrading and fertilizer generation.

94

Ammonia recovery performances by VMD in terms of operational parameters, recovered ammonia

95

concentration and quantity are investigated. The phytotoxicity effect of biogas slurry before and after

96

ammonia removal is evaluated. CO2 absorption performance of the recovered ammonia (absorption

97

capacity and rate) is compared with a conventional model absorbent. Theoretical results on biogas

98

upgrading are also provided. The novel concept in this work has great promise in closing the CO2

99

loop in biogas production by recycling ammonia as fertilizers.

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2. Experimental

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2.1. Materials and raw biogas slurry pretreatment

102

Raw biogas slurry (RBS) was from a large-scale mesophilic anaerobic biogas digestion plant

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(digestion substrate: pig manure; digestion temperature: ~ 35 ºC), located at Caoda Village in

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Yingcheng City, Hubei Province, China. The collected RBS was stored anaerobically at ambient

105

temperature prior to experiments until no biogas was produced. Characteristics of the RBS measured

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at 15 ± 2 ºC are shown in Table 1. Detailed measurement methods can be seen in the supplementary

107

information.

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Table.1. Properties of the raw biogas slurry (RBS). Parameters

Values

Units

pH

7.87 ± 0.21

-

Electric conductivity (EC)

16.61 ± 0.32

mS·cm-1

Turbidity

976.96 ± 21.14

NTU

6


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Chemical oxygen demand (COD)

2911.98 ± 30.65

mg·L-1

Total ammonia nitrogen (TAN)

2.0 ± 0.06

g N·L-1

Total solids (TS)

4387 ± 54.37

mg·L-1

Total phosphorus (TP)

37.74 ± 0.014

mg·L-1

Volatile fatty acid (VFA)

0.011 ± 0.001

mg·L-1

109

Chemical additives: poly aluminium chloride (PAC), iron(III) sulfate (IS), sodium hydroxide

110

(NaOH), calcium oxide (CaO) and magnesium oxide (MgO) were employed for RBS pretreatment 33.

111

Undissolved solids and suspended solids were separated from the solution by centrifuging after

112

stirring for one hour. After these pretreatments, no pH adjustment was carried out. The supernatant

113

liquid (i.e. biogas slurry) was used in further measurement and vacuum membrane distillation

114

(VMD). Additional, TAN concentrations were also adjusted to different levels by diluting the BS or

115

adding ammonia into the BS to evaluate the effect of TAN concentration on ammonia recovery

116

performance in VMD. Properties of biogas slurries with different concentrations of TAN are shown

117

in the supplementary information (Table S1).

118

2.2. Membrane module and vacuum membrane distillation

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Table 2. Specifications of the hollow fiber membrane contactor. Parameters

Values

Units

Fiber inner diameter

200

µm

Fiber outer diameter

300

µm

Membrane pore size

80 - 90

nm

Membrane porosity

33

%

Module inner diameter

20

mm

Module outer diameter

22

mm

Number of fibers

500

-

Total length

0.7225

m

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Effective length

0.380

m

Contract area

0.12

m2

120

VMD was carried out using a hollow fiber membrane module with hydrophobic microporous

121

polypropylene membranes, supplied by Ningbo Moersen Membrane Technology Co., Ltd.

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Specifications of the membrane module are listed in Table 2. The VMD experimental setup for

123

ammonia recovery is schematically shown in Fig. 2. One litre biogas slurry solution was circulated

124

on the lumen side of the hollow fiber membrane by a peristaltic pump (Leifu YZ25, Baoding Leifu

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Fluid Science and Technology Co., Ltd., China) under stirring and heating. The liquid flow rate

126

varied from 15 to 60 mL/min by changing the rotation speed of the pump. A water bath with a heat

127

exchanger was used to maintain the feed solution temperature. The inlet and outlet temperatures of

128

the liquid were monitored by k-type thermocouples (30 - 70 °C). A vacuum pump (Yvhua Instrument

129

Co., Ltd, Gongyi, China) was used to generate the vacuum on the shell side of the membrane (2 - 15

130

kPa). The recovered ammonia and water vapor were condensed and collected.

131

In the first step, the solution flowed through the hollow fiber membrane contractor was not recycled

132

to the feed solution tank, and the effect of operating parameters on the total flux, ammonia flux and

133

ammonia loss was evaluated 34. In the second step, BS was used and the solution flowing through the

134

hollow fiber membrane contractor was recycled to the feed tank (Fig. 2). The operating parameters

135

were selected based on the results of the first-step experiment. Each experimental run lasted ~ 90

136

mins, and 5 ml concentrate was sampled at a time interval of 15 mins. The same volume of RBS was

137

added into the feed tank to minimize the influence of the feed loss on process performance. The

138

weights of the sampled concentrate and permeate were measured. The chemical composition of the

139

permeate was determined by the same methods used for RBS measurement. 8


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2.3. Recovered ammonia for CO2 absorption

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After 90-min VMD, pH values and CO2 loadings of the treated biogas slurry (TBS) and permeate

142

were tested. Both TBS and permeate were saturated with biogas (CH4 : CO2 = 6 : 4, vol./vol.) at

143

ambient

144

TBS with bases and VMD were decreased from 10.7 to 9.8. Then, they were further saturated with

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CO2 to reach pH values of ~ 7.0. CO2 saturation of the permeate is to evaluate the CO2 absorption

146

capacity. CO2 absorption capacities of the permeates were determined by comparing the total

147

inorganic carbon (TIC) variation before and after CO2 saturation. The TIC concentration was

148

determined with a TC/TN Analyzer (multi N/C 2100, Analytik Jena AG, German). Synthesized

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aqueous ammonia with a suitable TAN concentration based on the permeate from VMD was used to

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test the biogas CO2 absorption rate in a typical bubbling reactor

151

solution (200 g) via fritted glass in the middle of the solution in a 500 mL glass reactor vessel. Liquid

152

in the reactor was constantly stirred by a magnetic stirrer at 100 rpm to enhance the mass transfer of

153

CO2. A monoethanolamine (MEA) solution with the same molar concentration of aqueous ammonia

154

was used as the model absorbent for comparison. The absorption temperature was maintained at

155

25 °C and the biogas flow rate was fixed at 1 L·min−1 under the standard state.

156

2.4. Phytotoxicity test

157

For fertilizer application, phytotoxicity test of BS is necessary. After VMD, the TAN concentration

158

of the BS was reduced 0.4 g N·L-1 and the pH of the feed BS was adjusted with biogas (CH4 : CO2 =

159

6 : 4, vol./vol.) to be near neutral (pH 6.5 - 7.5). Phytotoxicity of the BS was evaluated by

160

germination test with mungbean seeds due to their fast growing rates. First, the BS was diluted into 7

temperature and atmospheric pressure to be near neutral (pH 6.5 - 7.5). The pH values of

9


35

. Biogas was bubbled into the


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different concentrations (C): 0, 50, 100, 200, 400, 800 and 1000 mL·L-1 (BS/total solution). Then, 10

162

mL solution was put in a 9-cm petri dish where 15 mungbean seeds were placed on a piece of filter

163

paper. Each treatment was replicated three times. The petri dishes covered with lids were placed in a

164

lightless incubator (MLR-350, Versatile Environmental Test Chamber) for seed germination at 25 ±

165

0.5 °C and a relative humidity of 80%. Water loss in each dish was monitored everyday by weighing,

166

and distilled water was added if necessary. Seeds were considered to germinate when the gemmule

167

length was over 2 mm, and germination experiments were terminated when the gemmule length of

168

the seed in the control solution (i.e. C = 0) was over 20 mm

169

hours. Finally, the percentage of seed germination was determined, and the lengths of roots and

170

shoots were also measured. The BS concentration causing a 50% inhibition (EC50) was considered

171

for phytotoxicity evaluation

172

and thus a low phytotoxicity.

173

2.5. Data analysis

174

The total flux ( J t ) in VMD can be expressed as:

175

Jt =

176

where ∆m (g) is total mass difference of BS during the operating time t (min), and A is the

177

membrane area (0.12 m2). The ammonia flux ( J TAN ) can be expressed as:

178

J TAN =

36

. The germination period was ~ 36

37, 38

. A high EC50 value indicates a high BS application concentration,

∆m Αt

(1)

V0 C 0 - Vt C t At

(2)

10


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179

where V0 and C0 are the initial volume (L) and TAN concentration (g N·L-1) of BS, respectively, Vt

180

and Ct are the volume (L) and TAN concentration (g N·L-1) at the time of t (min), respectively. The

181

loss of ammonia ( LTAN ) in ammonia recovery can be calculated by:

182

LTAN =

183

(3)

184

where Di and Vi are the TAN concentration (g N·L-1) and the volume (L) of the condensate, the value

185

of i means the sampling times (range from 1 to 6).

186

In seed germination, all the data are checked by Levene’s test for homogeneity of variances and

187

Kolmogorov–Smirnov test for data normal distribution

188

Dunnett’s test at the 5% level of significance. When a significant difference (p < 0.05) was detected

189

between treatments (germination with CO2-rich biogas slurry) and the control (germination with

190

distilled water), EC50 value of CO2-rich biogas slurry can be calculated based on the correlations

191

between the logarithmic concentration of biogas slurry (logC) and the inhibition ratio (IR). IR can be

192

calculated by:

193

= 1 − × 100%

194

where MGLB and MGLC are the mean gemmule length of the seeds in CO2-rich biogas slurry and

195

those in the control (mm), respectively.

196

3. Results and discussion

197

3.1. Pretreatment of raw biogas slurry (RBS)

V0 C 0 - Vt Ct -∑DiVi ×100% V0 C 0 - Vt Ct

37

. The endpoints are valuated using

(4)

11



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198

Pretreatment of RBS plays an important role in both improving separation efficiency of VMD for

199

ammonia recovery and producing solid digestate as fertilizers after phosphorus solidification.

200

Reducing total and suspended solid concentrations will help minimize the risk of membrane pore

201

blocking and fouling in VMD for ammonia recovery.

202

The pH value and temperature are the most dominant factors in determining the free ammonia

203

concentration in BS. The proportion of free ammonia in TAN as a function of pH and temperature

204

can be calculated by 16 (also plotted in Fig. S1 in the supplementary information):

205

[NH 3 ] =

[TAN] 1 + 10

(5)

4×10 −8 ×T 3 + 9×10 −5 ×T 2 − 0.0356×T +10.072 − pH

206

where [TAN] and [NH 3 ] are the concentrations of total ammonium nitrogen and free ammonia

207

respectively (g N·L-1). T is the temperature (°C).

208

In VMD, the feed temperature is better to be higher than 50 °C in order to maintain a sufficient vapor

209

pressure (i.e. driving force) on the feed side. In this study, we selected temperatures between 60 and

210

70 °C. Therefore, the pH of BS should be higher than 9 (ideally near 11) to maximize the free

211

ammonia content in TAN based on Fig. S1.

212

PAC and IS are two typical inorganic flocculants for reducing the contents of suspended solids and

213

phosphorus in BS 39. Alkalis (NaOH and CaO, MgO) were used to increase the pH of BS. Besides,

214

these alkalis can also help to reduce the suspended solids, total phosphorus (TP), chemical oxygen

215

demand (COD) and turbidity of BS 15, 33. The effects of chemical additives on water quality of the BS

216

are shown in Fig. 3. PAC and IS flocculants have minimal effects on the pH of the RBS, but can

12


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217

significantly decrease the phosphorus content, turbidity and COD of the RBS. Three alkalis (NaOH

218

and CaO, MgO) can improve the pH of RBS by providing hydroxyl ion. NaOH is most effective in

219

increasing the pH due to its superior solubility (Fig. 3a)

220

CaO in reducing the phosphorus content and COD.

221

The main form of nitrogen in the digestate is ammonium nitrogen as anaerobic digestion decomposes

222

organic nitrogen into ammonium nitrogen

223

HCO3− and CO32− . The acid-base equilibrium in BS is typically dominated by the solubility

224

equilibrium of NH3 and CO2

225

carbonate form when CaO and MgO are added into the BS. Then, the acid-base equilibrium in BS is

226

dominated by the hydrolysis of ammonia 22.

227

The additives reduce the phosphate content by conversing the soluble phosphate into insoluble solids.

228

The solid phase can be separated by sedimentation or filtration. Typically, phosphorus is recovered

229

from BS by co-precipitation with phosphate and magnesium to form struvite

230

additives except NaOH are effective in reducing the total phosphorus content (Fig. 3b). Apart from

231

struvite, other insoluble solids can form when PAC, IS and CaO are added into BS to introduce

232

dissolved cations Al3+, Ca2+, Fe3+ 42.

233

COD and turbidity of BS should also be reduced since BS will be safer for fertilizer applications and

234

algae cultivation 5. All the additives, particularly CaO, perform well in reducing COD and turbidity

235

of the BS. Considering the synergetic efficiency, we finally selected CaO as the reasonable additive

236

because of its excellent performance in improving pH (to maximize the free ammonia content in BS)

237

and reducing the phosphorus content, turbidity and COD (Fig. 3)

17

. However, NaOH is less effective than

40

. The anions that balance the ammonium ions are

16, 41

. Thus, precipitates like calcium carbonate and magnesium

13


33

20

. In this study, most

. CaO is a relatively cheap


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238

industrial product. To increase the pH values of BS higher than 10.7 so that more than 99% TAN can

239

transfer into free ammonia, the required CaO concentrations were 1.18, 1.18, 1.06, 1.09

240

(mol-CaO/mol-TAN) for BS with TAN concentrations of 1, 2, 3, 4 (g N·L-1), respectively. Thus, the

241

estimated molar ratio between CaO (required), NH3 (recovered) and CO2 (absorbed) is 1:1:1 in the

242

loop.

243

3.2. Effects of operating parameters on ammonia recovery performance in VMD

244

VMD has been employed for ammonia recovery from various wastewaters

245

studies were carried out to treat BS by VMD. In this study, the recovered aqueous ammonia contains

246

CO2 (< 0.1 mol-CO2/mol-N) and VFAs (including ethanol, acetic acid, propionic acid, and butyric

247

acid). The total concentration of the VFAs is < 0.03 mol/L, regardless of the concentration of TAN.

248

The effects of operating parameters on VMD performance in ammonia recovery are shown in Fig. 4.

249

The feed flow rate significantly increases mass transfer rates in VMD (Fig. 4a). With the increase of

250

the feed flow rate, the total flux and ammonia flux rise linearly. This means that the boundary layer

251

effect is very severe. Increasing the feed flow rate will accelerate the turbulence of the flow and thus

252

minimize the boundary layer effect, improving mass transfer rates

253

much higher than the ammonia flux. This could be caused by the reduced ammonia loading and

254

residence time at higher flow rate. The feed flow rate has minimal effects on the ammonia loss (~

255

10%).

256

Temperature is an important factor in both ammonia solution and VMD because it significantly

257

affects the ammonia solubility in the solution and vapor pressure of the solution. Both the total flux

14


34, 43, 44

. However, few

45, 46

. However, the total flux is

Page 15 of 36


258

and ammonia flux increase dramatically with the rise in feed temperature. However, flux

259

enhancement with temperature rise slightly becomes flat after 70 °C, particularly for the ammonia

260

flux. This result mainly is caused by the nearly saturated free ammonia concentration at pH 10.8 after

261

70 °C (Fig. S1). Additionally, increased temperature polarization and heat loss at high temperature

262

may also reduce the permeate flux

263

temperature, and then maintains relatively stable after 70 °C. The lowest ammonia degree is only 3.2%

264

when the feed temperature is 69 °C, indicating that a low ammonia loss can be achieved under

265

optimized conditions.

266

In VMD, a low absolute pressure on the permeate side has favorable eﬀects on mass transfer rates,

267

but it also causes high ammonia loss (Fig. 4c). Higher ammonia loss takes place due to the higher

268

ammonia vapor partial pressure caused by the higher free ammonia concentration in the solution and

269

lower absolute pressure generated by the vacuum pump based on the Henry’s law 49. As the absolute

270

pressure on the permeate side increases, the fluxes and ammonia loss reduce dramatically. In

271

practical operation, the vacuum pressure should be optimized to achieve reasonable ammonia flux

272

with minimal ammonia loss. As expected, ammonia flux increases significantly when the ammonia

273

concentration in the feed increases (Fig. 4d). However, increasing feed ammonia concentration also

274

leads to increased ammonia loss. The total flux does not change with the variation of the feed

275

ammonia concentration.

276

According to the experimental results obtained above, we select a feed temperature of 69 °C, an

277

absolute pressure of 10 kPa on the permeate side, and a feed flow rate of 60 ml·min-1 to maintain

278

reasonable ammonia fluxes with low ammonia loss. In VMD, the permeate can be condensed and

47, 48

. Ammonia loss decreases first with the rise in feed

15



34

279

collected before or after the vacuum pump

. Collecting the condensate at ambient or higher

280

pressure after the vacuum pump may be better than collection under vacuum condition at industrial

281

scale. However, collecting the condensate before vacuum pump is more suitable for bench-scale test,

282

because the vapor is likely to condense in the vacuum pump if collection after vacuum.

283

3.3. Performance of ammonia recovery from biogas slurry

284

When various TAN concentrations of BS are used in VMD, ammonia recovery performance is

285

shown in Fig. 5. As anticipated, the TAN concentration in the feed solution decreases with operating

286

time. The measured TAN concentrations are used to fit an exponential decay curve (i.e. 1st order

287

kinetics) with a high correlation coefficient (R2 > 0.98). The ammonia removal efficiency is

288

independent on the initial ammonia concentration under certain operational conditions. Thus, the

289

ammonia removal efficiency in this study is always about 75% (almost unchanged) when the feed

290

TAN concentration varies from 1 to 4 g N·L-1, agreeing with the characteristics of the 1st order

291

kinetics.

292

Based on the regression equations obtained in Fig. 5a, we calculate the theoretical recovered

293

ammonia (including concentration and volume) from a certain volume of BS with different initial

294

ammonia concentrations (Fig. 5b). For real BS, its TAN concentration varies from 1 to 4 g N·L-1, the

295

recovered ammonia concentration ranges from 3.9 to 18.3 g N·L-1, which can be used as a suitable

296

CO2 absorption solvent

297

solution and BS is around 1:5 (~ 20%), suggesting that the quantity of recovered ammonia is enough

298

for CO2 absorption.

50

. Fig. 5b also shows that the volume ratio between recovered ammonia

16


Page 16 of 36

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299

3.4. CO2 absorption performance of recovery ammonia solutions

300

The recovered ammonia is used to absorb CO2 from biogas. Fig. 6 shows the CO2 absorption

301

performance from biogas at 25 °C. There is a small amount of CO2 in the recovered ammonia before

302

absorption, which may slightly affects the CO2 absorption capability of the recovered ammonia.

303

However, its effect is anticipated to be limited as CO2 concentration in the condensate is much lower

304

(< 0.1 mol·mol-1) compared with the ammonia concentration

305

capability increases linearly with TAN concentration of the recovered ammonia. The maximum TAN

306

concentration of the recovered ammonia is about 18.3 gN L-1 (Fig. 5), and the maximum CO2

307

absorption capacity of the condensate is 0.846 gC·(gN)-1. This means the absorption capability of

308

recovered ammonia approaches to 0.987 mol-CO2·(mol-NH3)-1.

309

CO2 absorption rate is another important index when the condensate is used to absorb CO2 from

310

biogas. At a lower concentration (0.4 mol L-1) and lower CO2 loadings (< 0.4 mol mol-1), the

311

recovered ammonia has comparable absorption rates with MEA. The absorption rate of the MEA

312

solution reduces dramatically when the CO2 loading is higher than 0.3 mol mol-1. The absorption rate

313

of the recovered ammonia also decreases with the rise in CO2 loading. However, the reduction in the

314

absorption rate of the recovered ammonia is not as severe as that of the MEA solution. This suggests

315

that the recovered ammonia may has better absorption performance that the MEA solution at higher

316

CO2 loadings.

317

3.5. Biogas upgrading by recovered ammonia from BS

318

In biogas generation, many parameters, such as biogas production rates and TAN concentrations of

27

17


. As expected, CO2 absorption


Page 18 of 36

319

BS vary significantly due to the diversity of feedstocks and fermentation methods. For instance, the

320

biogas volumetric productivity is 2.3 m3· m−3· d−1 in the biogas plant at Alviksgården (Sweden),

321

while it is only 0.7 m3· m−3· d−1 in the biogas plant at Jintan (China)

322

digestion, TAN concentration can be higher than 5 g-N·L-1 in raw BS

323

anaerobic digestion, TAN concentration may be lower than 0.5g - N·L-1 in raw BS 19. Therefore, the

324

CH4 content after upgrading varies with a number of parameters, such as the total ammonia

325

concentration in BS, biogas productivity, and CO2 absorption performance when recovering

326

ammonia for biogas upgrading 32.

327

The CH4 content can be estimated by:

328

C CH 4 =

329

where Vbiogas is the production of biogas per day (m3·d-1), VCO2 is the absorbed CO2 per day (m3·d-1),

330

and ωCH

Vbiogasω CH 4 Vbiogas - VCO 2

4

51

. For food waste anaerobic 17, 18

, while for crop straw

(6)

× 100%

represents the volume fraction (%) of CH4.

331

= 22.4

332

C NH 3 and β are the total ammonia content (kmol) in BS and effective factor, respectively. In this

333

study, we select β = 0.9 according to the base experiment. Vbiogas and C NH can be calculated from:

(7)

3

334

Vbiogas = γ VP

335

TNH 3 =

336

where VP is the volume of the fermentation tank (m3), γ is the biogas volumetric productivity

(8)

VP × [TAN] HRT

(9)

18


Page 19 of 36


337

(m3· m−3· d−1), and HRT is the hydraulic retention time in fermentation (d).

338

The variation in CH4 content with biogas volumetric productivity (γ), hydraulic retention time (HRT)

339

and total ammonia content (TAN) is displayed in Fig. 7. Higher CH4 content can be achieved at

340

higher TAN concentration and lower HRT, particularly when the biogas volumetric productivity is

341

low. The CH4 content after biogas upgrading can be up to over 95% when the biogas volumetric

342

productivity is relatively low (e.g. 0.5 or 1.0 m3·m−3·d−1). This phenomenon is mainly caused by the

343

low biogas production but high total ammonia content during fermentation

344

recovering ammonia from BS for biogas upgrading can be more effective for small biogas plants

345

with low productivities and high TAN concentrations in BS. To maximize the CH4 content in biogas,

346

it is also favorable to reduce the HRT. These results offer important insights into biogas upgrading

347

with recovered ammonia.

348

The theoretical evaluation in Fig. 7 is based on several assumptions: (i) the fermentation tank is 1000

349

m3, a typical up-flow anaerobic sludge blanket process is used, and the solid concentration in

350

fermentation is 8%

15

, (ii) the values of ω CH

4

19

. It suggests that

and ωCO in typical biogas are 60% and 40%, 2

351

respectively 8, (iii) the biogas volumetric productivity (γ) ranges from 0.5 to 2.0 m3·m−3·d−1 51, (iv)

352

HRT varies from 10 to 30 days 13, (v) TAN concentration in BS ranges from 1.0 to 5.0 gN L-1 17, 18.

353

3.6. Phytotoxicity of BS after ammonia removal

354

Without ammonia removal, BS can be used absorb CO2 and then used as liquid fertilizers

355

However, the CO2 absorption rate and capacity are very low, and the untreated BS can cause severe

356

phytotoxicity 27. Seed germination and root elongation tests have been used as simple, sensitive and

19


52

.


Page 20 of 36

357

cheap environmental bioassay methods for phytotoxicity evaluation 37, 53, 54.

358

Fig. 8 and Table 3 compare the phytotoxicity of the BS before and after ammonia removal. When the

359

concentration of the BS is below 50 mL·L-1, both raw BS and treated BS have little phytotoxicity to

360

gemmule growth. As the BS concentration increases to 100 mL·L-1, the raw BS starts to inhibit the

361

growth of the gemmule. When the concentration increases to 200 mL·L-1, raw BS seriously inhibit

362

the germination of the seeds, while the treated BS does not display any phytotoxicity. At higher BS

363

concentrations, the BS after ammonia removal also shows much lower phytotoxicity than the raw BS.

364

These results suggest that ammonia removal by VMD can effectively reduce the phytotoxicity of BS

365

in fertilizer application. However, dilution may still be required before irrigation since the TAN in

366

BS cannot be completely removed. Improving the ammonia recovery in VMD is necessary to further

367

minimize the phytotoxicity of BS.

368

Table 3. Phytotoxicity comparison of the biogas slurry before and after ammonia removal in terms of

369

EC50. Biogas slurry

Regression equations between IR and logC

R2

EC50 ± standard deviation (mL·L-1)

Before ammonia removal

y = -7.168 +

104.508 1 + 103.097×( 2.102 x )

0.976

126.436 ± 15.107

After ammonia removal

y = -6.088 +

84.111 1 + 108.343×( 2.603− x )

0.916

400.644 ± 32.002

370

y is the inhibition ratio (IR, %); x is the logarithmic concentration of the BS (logC) in mL·L-1.

371

Evaluation of EC50 also confirms that the treated BS has much lower phytotoxicity in agricultural

372

application (Table 3). EC50 value of the raw BS is about 126 mL·L-1, while that of the treated BS is

373

about 400 mL·L-1. This means the raw BS is very toxic and it can cause 50% inhibition even after 20


Page 21 of 36


374

diluting 8 times, which significantly limits its on-site agricultural application or even direct discharge.

375

After ammonia removal by VMD, the treated BS becomes much less toxic, and can be used for

376

irrigation after minor dilution.

377

Reduction in phytotoxicity of the BS is mainly attributed to the removal of ammonia in the solutions

378

as high ammonia concentration does harm to the organism 17. The main difference between the raw

379

BS and treated BS is the TAN concentration. The TAN concentration of the BS reduces from 2 to 0.4

380

g·L-1 after VMD. After ammonia removal, the BS shows much better applicability in agricultural

381

application due to the reduced phytotoxicity (Fig. 8). Although P and N in BS are reduced after

382

pretreatment and VMD, they cannot be completely removed. Besides, treated BS still cannot be

383

discharged into water body because of its high COD content. Treated BS used for irrigation can

384

further minimise its environmental risks. Furthermore, other trace components, such as gibberellin,

385

indoleacetic acid and humic acid in BS can promote the growth of crops. Overall, employing VMD

386

to recover/removal ammonia from BS has great significance because it not only offers a new

387

absorbent for biogas upgrading but also improve the applicability of the BS for agricultural

388

irrigation.

389

The novel concept demonstrated in this study shows great potential in closing the CO2 loop in biogas

390

production by recycling ammonia as an absorbent for CO2 absorption associated with producing

391

fertilizers. In the future, continuous large scale demonstrations combining biogas production,

392

upgrading, fertilizer generation and application are required to further validate the system.

393

Acknowledgements

21



394

The authors thank the financial support from the National Natural Science Foundation of China

395

(51376078) and Open Research Fund Program of Collaborative Innovation Center of Membrane

396

Separation and Water Treatment (2016YB01). Mr. Qingyao He acknowledges the support from

397

China Scholarship Council (CSC) for studying at Macquarie University in Sydney (201606760032).

398

References

399

1.

400

technologies. Prog. Energy Combust. Sci. 2011, 37, (1), 15-51.

401

2.

402

3. Luque, R.; Herrero-Davila, L.; Campelo, J. M.; Clark, J. H.; Hidalgo, J. M.; Luna, D.; Marinas, J.

403

M.; Romero, A. A., Biofuels: a technological perspective. Energy Environ. Sci. 2008, 1, (5), 542-564.

404

4.

405

2010, 85, (4), 849-860.

406

5.

407

for agricultural digestate valorization: current situation and perspectives. Energy Environ. Sci. 2015,

408

8, (9), 2600-2621.

409

6.

410

Biorefin. 2009, 3, (1), 42-71.

411

7.

412

biomethane. Biomass Bioenergy 2011, 35, (5), 1633-1645.

413

8.

414

absorption of carbon dioxide for biogas upgrading. Chin. J. Chem. Eng. 2016, 24, (6), 693-702.

415

9.

416

selective natural amino acid salt in gas-liquid membrane contactor. Chem. Eng. Process. 2014, 85,

417

(0), 125-135.

418

10. Platform, E. B. T., Biomass with CO2 capture and storage (Bio-CCS)-The way forward for

Ghoniem, A. F., Needs, resources and climate change: Clean and efficient conversion

IEA, World Energy Outlook 2016; http://www.worldenergyoutlook.org/publications/weo-2016/.

Weiland, P., Biogas production: current state and perspectives. Appl. Microbiol. Biotechnol.

Monlau, F.; Sambusiti, C.; Ficara, E.; Aboulkas, A.; Barakat, A.; Carrere, H., New opportunities

Abatzoglou, N.; Boivin, S., A review of biogas purification processes. Biofuels, Bioprod.

Ryckebosch, E.; Drouillon, M.; Vervaeren, H., Techniques for transformation of biogas to

Abdeen, F. R. H.; Mel, M.; Jami, M. S.; Ihsan, S. I.; Ismail, A. F., A review of chemical

Yan, S.; He, Q.; Zhao, S.; Wang, Y.; Ai, P., Biogas upgrading by CO2 removal with a highly

22


Page 22 of 36

Page 23 of 36


419

Europe. Report, 2012.

420

11. Hughes, A. D.; Black, K. D.; Campbell, I.; Davidson, K.; Kelly, M. S.; Stanley, M. S., Does

421

seaweed offer a solution for bioenergy with biological carbon capture and storage? Greenhouse

422

Gases: Sci. Technol. 2012, 2, (6), 402-407.

423

12. Kemper, J., Biomass and carbon dioxide capture and storage: A review. Int. J. Greenhouse Gas

424

Control 2015, 40, 401-430.

425

13. Aramrueang, N.; Rapport, J.; Zhang, R., Effects of hydraulic retention time and organic loading

426

rate on performance and stability of anaerobic digestion of Spirulina platensis. Biosyst. Eng. 2016,

427

147, 174-182.

428

14. Li, Y.; Park, S. Y.; Zhu, J., Solid-state anaerobic digestion for methane production from organic

429

waste. Renewable Sustainable Energy Rev. 2011, 15, (1), 821-826.

430

15. Sheets, J. P.; Yang, L.; Ge, X.; Wang, Z.; Li, Y., Beyond land application: Emerging technologies

431

for the treatment and reuse of anaerobically digested agricultural and food waste. Waste Manage.

432

2015, 44, 94-115.

433

16. Bonmatı,́ A.; Flotats, X., Air stripping of ammonia from pig slurry: characterisation and

434

feasibility as a pre- or post-treatment to mesophilic anaerobic digestion. Waste Manage. 2003, 23, (3),

435

261-272.

436

17. Walker, M.; Iyer, K.; Heaven, S.; Banks, C. J., Ammonia removal in anaerobic digestion by

437

biogas stripping: An evaluation of process alternatives using a first order rate model based on

438

experimental findings. Chem. Eng. J. 2011, 178, 138-145.

439

18. Serna-Maza, A.; Heaven, S.; Banks, C. J., Biogas stripping of ammonia from fresh digestate

440

from a food waste digester. Bioresour. Technol. 2015, 190, 66-75.

441

19. Li, H.; Tan, F.; Ke, L.; Xia, D.; Wang, Y.; He, N.; Zheng, Y.; Li, Q., Mass balances and

442

distributions of C, N, and P in the anaerobic digestion of different substrates and relationships

443

between products and substrates. Chem. Eng. J. 2016, 287, 329-336.

444

20. Uludag-Demirer, S.; Demirer, G. N.; Chen, S., Ammonia removal from anaerobically digested

445

dairy manure by struvite precipitation. Process Biochem. 2005, 40, (12), 3667-3674. 23



446

21. Masse, L.; Massé, D. I.; Pellerin, Y.; Dubreuil, J., Osmotic pressure and substrate resistance

447

during the concentration of manure nutrients by reverse osmosis membranes. J. Membr. Sci. 2010,

448

348, (1–2), 28-33.

449

22. Tao, W.; Ukwuani, A. T., Coupling thermal stripping and acid absorption for ammonia recovery

450

from dairy manure: Ammonia volatilization kinetics and effects of temperature, pH and dissolved

451

solids content. Chem. Eng. J. 2015, 280, 188-196.

452

23. Ukwuani, A. T.; Tao, W., Developing a vacuum thermal stripping - acid absorption process for

453

ammonia recovery from anaerobic digester effluent. Water Res. 2016, 106, 108-115.

454

24. Milan, Z.; Sánchez, E.; Weiland, P.; de Las Pozas, C.; Borja, R.; Mayari, R.; Rovirosa, N.,

455

Ammonia removal from anaerobically treated piggery manure by ion exchange in columns packed

456

with homoionic zeolite. Chem. Eng. J. 1997, 66, (1), 65-71.

457

25. Vanotti, M. B., Szogi, A.A., Use of gas-permeable membranes for the removal and recovery of

458

ammonia from high strength livestock wastewater. Proceedings of the Water Environment Federation,

459

Nutrient Recovery and Management. WEF, Alexandria, VA, 2011b, 9.

460

26. He, Q.; Yu, G.; Wang, W.; Yan, S.; Zhang, Y.; Zhao, S., Once-through CO2 absorption for

461

simultaneous biogas upgrading and fertilizer production. Fuel Process. Technol. 2017, 166, 50-58.

462

27. Yan, S.; Zhang, L.; Ai, P.; Wang, Y.; Zhang, Y.; Li, S., CO2 Absorption by Using a low-cost

463

Solvent: Biogas Slurry Produced by Anaerobic Digestion of Biomass. Energy Procedia 2013, 37,

464

2172-2179.

465

28. Xu, J.; Zhao, Y.; Zhao, G.; Zhang, H., Nutrient removal and biogas upgrading by integrating

466

freshwater algae cultivation with piggery anaerobic digestate liquid treatment. Appl. Microbiol.

467

Biotechnol. 2015, 99, (15), 6493-6501.

468

29. Zhao, Y.; Ge, Z.; Zhang, H.; Bao, J.; Sun, S., Nutrient removal from biogas slurry and biogas

469

upgrading of crude biogas at high CO2 concentrations using marine microalgae. J. Chem. Technol.

470

Biotechnol. 2015, 91, 1113-1118.

471

30. Zhao, Y.; Sun, S.; Hu, C.; Zhang, H.; Xu, J.; Ping, L., Performance of three microalgal strains in

472

biogas slurry purification and biogas upgrade in response to various mixed light-emitting diode light 24


Page 24 of 36

Page 25 of 36


473

wavelengths. Bioresour. technol. 2015, 187, 338-345.

474

31. Uggetti, E.; Sialve, B.; Latrille, E.; Steyer, J.-P., Anaerobic digestate as substrate for microalgae

475

culture: the role of ammonium concentration on the microalgae productivity. Bioresour. technol.

476

2014, 152, 437-443.

477

32. McLeod, A.; Jefferson, B.; McAdam, E. J., Biogas upgrading by chemical absorption using

478

ammonia rich absorbents derived from wastewater. Water Res. 2014, 67, 175-186.

479

33. Lei, X.; Sugiura, N.; Feng, C.; Maekawa, T., Pretreatment of anaerobic digestion effluent with

480

ammonia stripping and biogas purification. J. Hazard. Mater. 2007, 145, (3), 391-397.

481

34. Fang, M.; Ma, Q.; Wang, Z.; Xiang, Q.; Jiang, W.; Xia, Z., A novel method to recover ammonia

482

loss in ammonia-based CO2 capture system: ammonia regeneration by vacuum membrane

483

distillation. Greenhouse Gases: Sci. Technol. 2015, 5, (4), 487-498.

484

35. Yan, S.; He, Q.; Zhao, S.; Zhai, H.; Cao, M.; Ai, P., CO2 removal from biogas by using green

485

amino acid salts: Performance evaluation. Fuel Process. Technol. 2015, 129, 203-212.

486

36. An, J.; Zhou, Q.; Sun, Y.; Xu, Z., Ecotoxicological effects of typical personal care products on

487

seed germination and seedling development of wheat (Triticum aestivum L.). Chemosphere 2009, 76,

488

(10), 1428-1434.

489

37. Pan, M.; Chu, L. M., Phytotoxicity of veterinary antibiotics to seed germination and root

490

elongation of crops. Ecotoxicol. Environ. Saf. 2016, 126, 228-37.

491

38. Richter, E.; Roller, E.; Kunkel, U.; Ternes, T. A.; Coors, A., Phytotoxicity of wastewater-born

492

micropollutants-Characterisation of three antimycotics and a cationic surfactant. Environ. Pollut.

493

2016, 208, (Pt B), 512-22.

494

39. Drosg, B.; Fuchs, W.; Al Seadi, T.; Madsen, M.; Linke, B., Nutrient Recovery by Biogas

495

Digestate

496

http://www.iea-biogas.net/files/daten-redaktion/download/Technical%20Brochures/NUTRIENT_RE

497

COVERY_RZ_web1.pdf.

498

40. Batstone, D. J.; Hülsen, T.; Mehta, C. M.; Keller, J., Platforms for energy and nutrient recovery

499

from domestic wastewater: A review. Chemosphere 2015, 140, 2-11.

Processing.

Technical

report,

IEA

25


Bioenergy

2015,


500

41. Guštin, S.; Marinšek-Logar, R., Effect of pH, temperature and air flow rate on the continuous

501

ammonia stripping of the anaerobic digestion effluent. Process Saf. Environ. Prot. 2011, 89, (1),

502

61-66.

503

42. Yildiz, E., Phosphate removal from water by fly ash using crossflow microfiltration. Sep. Purif.

504

Technol. 2004, 35, (3), 241-252.

505

43. El-Bourawi, M. S.; Khayet, M.; Ma, R.; Ding, Z.; Li, Z.; Zhang, X., Application of vacuum

506

membrane distillation for ammonia removal. J. Membr. Sci. 2007, 301, (1–2), 200-209.

507

44. Chunrui Wu, H. Y., Zhengang Li & Xiaolong Lu, Ammonia recovery from high concentration

508

wastewater of soda ash industry with membrane distillation process. Desalin. Water Treat. 2016, 57,

509

(15), 6792-6800.

510

45. Zhao, S.; Feron, P. H. M.; Xie, Z.; Zhang, J.; Hoang, M., Condensation studies in membrane

511

evaporation and sweeping gas membrane distillation. J. Membr. Sci. 2014, 462, (0), 9-16.

512

46. Zhao, S.; Wardhaugh, L.; Zhang, J.; Feron, P. H. M., Condensation, re-evaporation and

513

associated heat transfer in membrane evaporation and sweeping gas membrane distillation. J. Membr.

514

Sci. 2015, 475, (0), 445-454.

515

47. Zhao, S.; Cao, C.; Wardhaugh, L.; Feron, P. H. M., Membrane evaporation of amine solution for

516

energy saving in post-combustion carbon capture: Performance evaluation. J. Membr. Sci. 2015, 473,

517

(0), 274-282.

518

48. Zhao, S.; Feron, P. H. M.; Cao, C.; Wardhaugh, L.; Yan, S.; Gray, S., Membrane evaporation of

519

amine solution for energy saving in post-combustion carbon capture: Wetting and condensation. Sep.

520

Purif. Technol. 2015, 146, (0), 60-67.

521

49. Renard, J. J.; Calidonna, S. E.; Henley, M. V., Fate of ammonia in the atmosphere-a review for

522

applicability to hazardous releases. J. Hazard. Mater. 2004, 108, (1), 29-60.

523

50. Bonet-Ruiz, A.-E.; Plesu, V.; Bonet, J.; Iancu, P.; Llorens, J., Preliminary technical feasibility

524

analysis of carbon dioxide absorption by ecological residual solvents rich in ammonia to be used in

525

fertigation. Clean Technol. Environ. Policy 2015, 17, (5), 1313-1321.

526

51. HUA Jing, T. Z., LU Xiaohua,YANG Zhuhong,WANG Changsong,, Effect of waste heat 26


Page 26 of 36

Page 27 of 36


527

recovery on net biogas yield in thermophilic biogas plants. CIESC Journal 2014, 65, (5), 5.

528

52. Chi, Z.; O'Fallon, J. V.; Chen, S., Bicarbonate produced from carbon capture for algae culture.

529

Trends Biotechnol. 2011, 29, (11), 537-41.

530

53. Tam, N. F. Y., Tiquia, S.,, Assessing toxicity of spent pig litter using a seed germination

531

technique. Resour., Conserv. Recycl. 1994, 11, 261-274.

532

54. Di Salvatore, M.; Carafa, A. M.; Carratù, G., Assessment of heavy metals phytotoxicity using

533

seed germination and root elongation tests: A comparison of two growth substrates. Chemosphere

534

2008, 73, (9), 1461-1464.

535

27



Page 28 of 36

Figure captions Fig. 1. A novel system to achieve biogas upgrading, carbon capture, ammonia recovery and fertilizer production in biogas production. Fig. 2. Schematic diagram of the vacuum membrane distillation setup for ammonia recovery from biogas slurry. Fig. 3. Effects of flocculants (PAC: Poly aluminium chloride and IS: Iron(III) sulfate) and alkalis (NaOH, CaO and MgO) on (a) pH, (b) total phosphorus concentration, (c) turbidity, and (d) chemical oxygen demand (COD) of the biogas slurry. Fig. 4. Effects of (a) feed flow rate, (b) temperature, (c) pressure on the permeate side, and (d) feed ammonia concentration on the total flux, ammonia flux and ammonia loss. Fig. 5. Ammonia recovery performance from biogas slurry (BS): (a) experimental TAN concentrations in the feed tank with time variation, (b) theoretical recovered ammonia (including concentration and volume) from a certain volume of BS with different initial ammonia concentrations based on the experimental performance in Fig. 4a (assuming a 90% recovery). Experimental conditions: temperature 69 °C, pressure on the permeate side 10 kPa, and feed flow rate 60 ml·min-1. Fig. 6. CO2 absorption performance from biogas at 25 ºC: (a) absorption capacity of recovered ammonia, and (b) comparison in absorption rates of recovered ammonia and a model absorbent (MEA: monoethanolamine). Fig. 7. Biogas upgrading performance combining CO2 absorption with recovered ammonia from biogas slurry at different biogas volumetric productivities (γ): (a) γ = 0.5 m3 m-3d-1, (b) γ = 1.0 m3 m-3d-1, (c) γ = 1.5 m3 m-3d-1, and (d) γ = 2.0 m3 m-3d-1. HRT: hydraulic retention time (d). Fig. 8. Phytotoxicity of biogas slurry before and after ammonia removal: (a) effect of biogas slurry concentration on gemmule length of the mungbean, and (b) inhibition ratio as a function of the logarithmic concentration of biogas slurry (logC).

28


Page 29 of 36


Fig. 1. A novel system to achieve biogas upgrading, carbon capture, ammonia recovery and fertilizer production in biogas production.

29



Page 30 of 36

Fig. 2. Schematic diagram of the vacuum membrane distillation setup for ammonia recovery from biogas slurry.

30


Page 31 of 36

Environmental Science & Technology -

Equivalent OH dosage (mol/L) 0.2 0.4 0.6 0.8

-

0.0

NaOH CaO MgO

13 12

pH

1.0

(b)40 Total phosphorus content (mg/L)

(a)

Equivalent OH Dosage (mol/L) 0.2 0.4 0.6 0.8

PAC IS

11 10 9 8

0.0

0.5

1.0 1.5 2.0 2.5 Flocculant dosage (g/L)

0.0

35 NaOH CaO MgO

30 25 20

PAC IS

15 10 5 0

3.0

0.0

0.5

0.0

1.0 1.5 2.0 Flocculant dosage (g/L)

2.5

3.0

-


-

(c)

1.0


1.0

(d)

1000

0.0

1.0

3000

2500 COD (mg/L)

Turbidity (NTU)

800 600 400

NaOH CaO MgO

200

PAC IS

0

0.0

0.5

2000

NaOH CaO MgO PAC IS

1500


2.5

1000

3.0

0.0

0.5


2.5

3.0

Fig. 3. Effects of flocculants (PAC: Poly aluminium chloride and IS: Iron(III) sulfate) and alkalis (NaOH, CaO and MgO) on (a) pH, (b) total phosphorus concentration, (c) turbidity, and (d) chemical oxygen demand (COD) of the biogas slurry.

31



0.0 10

16

1.0

-1

Ammonia flux (g m min )

-2

60

0.8

10 8 6 4

30

20 0.4

0.0

10

2.5

5.0 7.5 10.0 12.5 Pressure on permeate side (kPa)

15.0

20 0.4 10

0.2

Ammonia loss degree (%)

-1 -2

-1 -2

0.6

0.0 50

(d)

1.0

18 16

2 0

4

0

40

0.6

0.2

6

30

2

0

-2

-1

Total flux (g m min )

30 40 50 -1 Feed flow rate (mL min )

Total flux Ammonia flux Ammonia loss

14 12

20

8

14

-1

(c)

0

10

0.8

40


12 10 8 6 4

0.8

55

60 65 70 o Feed temperature ( C)

75


0 80

40

30

0.6 20 0.4 10

0.2

2

0

0

0.0

1

2 3 4 -1 Feed ammonia concentration (g L )


2

12

-2

0.2


10

14


4

20 0.4


6

0.6

-1

8

30

-2

10

0.8

-2

12

1.0

18 16


-2

-1


-1


14

(b)

40



1.0

16


(a)

Page 32 of 36

0

Fig. 4. Effects of (a) feed flow rate, (b) feed temperature, (c) pressure on the permeate side, and (d) feed ammonia concentration on the total flux, ammonia flux and ammonia loss.

32


Page 33 of 36


-1

TAN concentration in feed (gN L )

(a)

-1

4.0

(-0.019x)

TAN=1 gN L -1 TAN=2 gN L -1 TAN=3 gN L -1 TAN=4 gN L

3.5 3.0

y=0.88e (-0.015x) y=1.93e (-0.016x) y=2.95e (-0.016x) y=3.86e

2.5 2.0 1.5 1.0 0.5 0.0

-1

Initial TAN concentration in BS (gN L ) 4 1 3 2

(b)

0

0

20

40 60 Time (min)

Condensate Treated BS

80

100

TAN =18.3 gN L

-1

-1

TAN =13.1 gNL

TAN =9.3 gN L

-1

TAN = 3.9 gN L

-1

100 200 300 400 500 600 700 800 900 1000 Liquid volume (mL)

Fig. 5. Ammonia recovery performance from biogas slurry (BS): (a) experimental TAN concentrations in the feed tank with time variation, (b) theoretical recovered ammonia (including concentration and volume) from a certain volume of BS with different initial ammonia concentrations based on the experimental performance in Fig. 4a (assuming a 90% recovery). Experimental conditions: temperature 69 °C, vacuum pressure on the permeate side 10 kPa, and feed flow rate 60 ml·min-1. 33


-1

(a)

TIC concentration in condensate (gC L )


-1 -1

CO2 absorption rate (mmol L s )

(b)

30

Page 34 of 36

Before CO2 absorption After CO2 absorption

25

2

y=0.053x, R =0.85 2 y=0.846x, R =0.98

20 15 10 5 0

0

5 10 15 20 25 30 35 -1 TAN concentration in condensate (gN L )

1.6

-1

0.4 mol L MEA -1 1.0 mol L MEA -1 0.4 mol L NH3

1.4 1.2

-1

1.0 mol L NH3

1.0 0.8 0.6 0.4 0.2 0.0 0.0

0.2

0.4 0.6 0.8 -1 CO2 loading (mol mol )

1.0

Fig. 6. CO2 absorption performance from biogas at 25 ºC: (a) absorption capacity of recovered ammonia, and (b) comparison in absorption rates of recovered ammonia and a model absorbent (MEA: monoethanolamine).

34


Page 35 of 36


Fig. 7. Biogas upgrading performance combining CO2 absorption with recovered ammonia from biogas slurry at different biogas volumetric productivities (γ): (a) γ = 0.5 m3 m-3d-1, (b) γ = 1.0 m3 m-3d-1, (c) γ = 1.5 m3 m-3d-1, and (d) γ = 2.0 m3 m-3d-1. HRT: hydraulic retention time (d).

35



(a) 30

Page 36 of 36

RBS: raw biogas slurry TBS: treated biogas slurry

Gemmule length (mm)

25 20 15 10 5 0

0

50

100

400

200

800

1000

-1

Biogas slurry concentration (mL L ) Gemmule inhibitation (% of control)

(b)

RBS TBS Regression for RBS Regression for TBS

100 80 60 40 20 0 -20 -40

1.6

1.8

2.0

2.2 2.4 log C

2.6

2.8

3.0

3.2

Fig. 8. Phytotoxicity of biogas slurry before and after ammonia removal: (a) effect of biogas slurry concentration on gemmule length of the mungbean, and (b) inhibition ratio as a function of the logarithmic concentration of biogas slurry (logC). 36


Closing CO2 Loop in Biogas Production: Recycling Ammonia As

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