Closing CO2 Loop in Biogas Production: Recycling Ammonia As

Jul 5, 2017 - Various methods have been employed to remove or recover TAN from BS to minimize risks of using the nutrients.(15, 16, 20-22) These techn...
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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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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,**

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a

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District, Wuhan 430070, PR China

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b

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c

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* Email: [email protected];

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** 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

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* Corresponding author: Dr Shuaifei Zhao

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Email: [email protected]

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Tel.: +61 2 98509672

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Abstract

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

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

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

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

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

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

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

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

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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

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

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

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

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producing fertilizers.

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TOC/Abstract art

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1. Introduction

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

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

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

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

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of our energy supply is now still based on fossil fuels

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

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

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

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

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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

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liquefaction

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

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

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

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

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

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

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

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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

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. However,

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

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

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

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the nutrients

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

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to form struvite

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

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

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

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fertilizers for agricultural irrigation.

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

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has been studied to separate and utilize CO2 simultaneously

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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

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

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

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

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with BS is highly required.

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

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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,

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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.

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

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

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

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

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

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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

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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

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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

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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

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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

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

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

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

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

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

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

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

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

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in the supplementary information (Table S1).

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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

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

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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

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

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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

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

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

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

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

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

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

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

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

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

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

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

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

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

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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

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

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ambient

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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

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

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

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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

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

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

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

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

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

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2.4. Phytotoxicity test

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

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

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

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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

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. 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

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

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

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

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

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

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

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

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

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

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for phytotoxicity evaluation

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and thus a low phytotoxicity.

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2.5. Data analysis

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

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Jt =

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

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

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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)

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

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

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

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LTAN =

183

(3)

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

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

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

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Kolmogorov–Smirnov test for data normal distribution

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

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

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

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

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calculated by:

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 = 1 −  × 100%

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

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those in the control (mm), respectively.

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3. Results and discussion

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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)



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

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

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

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blocking and fouling in VMD for ammonia recovery.

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

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

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

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[NH 3 ] =

[TAN] 1 + 10

(5)

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

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

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

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

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

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

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ammonia content in TAN based on Fig. S1.

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

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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

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

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

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

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

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CaO in reducing the phosphorus content and COD.

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

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organic nitrogen into ammonium nitrogen

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

224

equilibrium of NH3 and CO2

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

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dominated by the hydrolysis of ammonia 22.

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

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

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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

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

232

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

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

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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

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20

. In this study, most

. CaO is a relatively cheap

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

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

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(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

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loop.

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

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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.

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

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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%).

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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

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. However, the total flux is

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

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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

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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.

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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

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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

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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

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

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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

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3.4. CO2 absorption performance of recovery ammonia solutions

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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

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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)

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(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

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.

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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

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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

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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

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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).

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

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

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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

-

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

-

(c)

1.0

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

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

1.0 1.5 2.0 Flocculant dosage (g/L)

2.5

1000

3.0

0.0

0.5

1.0 1.5 2.0 Flocculant dosage (g/L)

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.

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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

Total flux Ammonia flux Ammonia loss

12 10 8 6 4

0.8

55

60 65 70 o Feed temperature ( C)

75

Total flux Ammonia flux Ammonia loss

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 )

Ammonia loss degree (%)

2

12

-2

0.2

Ammonia flux (g m min )

10

14

Ammonia flux (g m min )

4

20 0.4

Total flux (g m min )

6

0.6

-1

8

30

-2

10

0.8

-2

12

1.0

18 16

Total flux (g m min )

-2

-1

Total flux (g m min )

-1

Ammonia flux (g m min )

14

(b)

40

Total flux Ammonia flux Ammonia loss

Ammonia loss degree (%)

1.0

16

Ammonia loss degree (%)

(a)

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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.

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Environmental Science & Technology

-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

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-1

(a)

TIC concentration in condensate (gC L )

Environmental Science & Technology

-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).

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Environmental Science & Technology

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).

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(a) 30

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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

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