Improving Post-Anaerobic Digestion of Full-Scale Anaerobic Digestate

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Improving post anaerobic digestion of full-scale anaerobic digestate using free ammonia treatment Kang Song, Senbati Yeerken, Lu LI, Jing Sun, and Qilin Wang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.9b00152 • Publication Date (Web): 06 Mar 2019 Downloaded from http://pubs.acs.org on March 7, 2019

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ACS Sustainable Chemistry & Engineering

Improving post anaerobic digestion of full-scale anaerobic digestate using free ammonia treatment†

Kang Song a, Senbati Yeerken a, b, Lu Li a, *, Jing Sun c, Qilin Wang d, *

a

State Key Laboratory of Freshwater Ecology and Biotechnology, Institute of

Hydrobiology, Chinese Academy of Sciences, Wuhan 430072, China b

c

University of Chinese Academy of Sciences, Beijing 100049, China State Key Laboratory of Pollution Control and Resources Reuse, College of

Environmental Science and Engineering, Tongji University, Shanghai 200092, PR China d

Centre for Technology in Water and Wastewater, School of Civil and Environmental

Engineering, University of Technology Sydney, Ultimo, NSW 2007, Australia

*Corresponding authors: Dr. Lu LI, Email: [email protected] Dr. Qilin Wang, Email: [email protected]

† Electronic supplementary information (ESI) available

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Abstract：

Post anaerobic digestion of full-scale anaerobic digestate (AD) is used in enhancing sludge reduction in some sewage treatment plants (STPs). However, the AD degradation was usually inhibited due to its slow hydrolysis rate and low degradability. This study presents an innovative pretreatment method by using free ammonia (FA, i.e. NH3) to improve post anaerobic digestion full-scale AD degradation. The FA treatment at over 360 mg NH3-N/L for 24 h has improved AD degradation with the highest solubilization (0.1 mg chemical oxygen demand (COD)/mg volatile solids (VS), at 850 mg NH3-N/L) being 5.3 times that without pretreatment (0.019 mg COD/mg VS). After 8 days post anaerobic digestion, unpretreated AD has degraded 8.5% while the FA pretreated AD at 360-850 mg NH3-N/L has degraded 9.9-10.9%, a relative increase of 14-22% was represented. The mathematical model captured well with the tested data with R2>0.994 in all cases, and the model revealed that AD degradation improvement was attributed to an increase in AD degradable percentage. Economic analysis shows that FA pretreatment method could be economically favorable in enhancing full-scale AD post anaerobic digestion.

Keywords: Anaerobic digestate, Free ammonia, Post anaerobic digestion, Sludge reduction, Methane production

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Introduction

Large amounts of sludge were produced during the biological nitrogen removal process in sewage treatment plants (STPs), its treatment and disposal is quite costly and commonly accounting for over 60% of the total operation cost in STPs [1-3]. Anaerobic digestion is a common method in sludge reduction and stabilization before its final disposal. However, the anaerobic digestate can still be further degraded through post anaerobic digestion to further reduce the amount of sludge that needs to be disposed of. Therefore, anaerobic digestate (AD) post anaerobic digestion has gain attentions in recent years and has been applied for further improving sludge reduction in practice. For example, an Australian local wastewater treatment plant achieved a ~5.5% degradation of AD through post anaerobic digestion [4]. However, the anaerobic digestate commonly has a poor biodegradability because the easily biodegradable part has already been degraded during anaerobic digestion [5-8]. Thus, suitable pretreatment process that can improve the biodegradability of AD was urgently needed.

There were plenty of pretreatment processes has been applied for improving the AD degradation, such as ultrasonic [9], persulfate and zero valent iron combination process [10], hydrothermal pretreatment [11, 12], free nitrous acid pretreatment [13], Fe(II) activated oxidation process [14], Fenton process [15] and so forth. Among which, Free ammonia (FA, NH3) as a renewable material can be directly collected from STPs’ anaerobic digestion liquor was widely studied in recent decades [16-18]. FA was reported

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effective in accelerating the disintegration of waste activated sludge (WAS), improving the WAS biodegradability and methane production from WAS anaerobic digestion [19, 20]. Wei et al. demonstrated that the free ammonia pretreatment of secondary sludge was effective in increasing the anaerobic methane production. The secondary sludge solubilisation with FA pretreated at 680 mg NH3-N/L was 10 times higher than without FA pretreatment. The hydrolysis rate and biochemical methane production was also largely improved with FA pretreatment at concentration 420-680 mg NH3-N/L. 85-680 mg NH3-N/L pretreatment for 24 h was able to enhance the primary sludge (PS) biochemical methane production by 8-17% [21, 22]. Xu et al. found that FA pretreatment with concentration over 189.4 mg NH3-N/L for 24 h improved the phosphorus releasement, which was attributed to the sludge cell extracellular polymeric substances (EPS) disintegration by FA effect [23]. Wang et al. applied CaO2 and FA combined pretreatment for sludge anaerobic fermentation. They found that the adding of FA enhanced the short chain fatty acid production, accelerate the sludge cell disintegration and sludge biodegradability [24].

These studies indicated that FA has the ability to break down extracellular and intracellular constituents, thereby improve the substrate amount and sludge solubilization, and furtherly improve the methane production. This led to a hypothesis that FA pretreatment on AD before post anaerobic digestion could be applied as a potential technology for enhancing sludge degradation and methane production. To validate this hypothesis, full-scale AD was pretreated by FA at a series concentration of 55, 360, 850

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mg NH3-N/L for 24 h with un-pretreated AD as control. Afterwards, the post anaerobic digestion of those AD was conducted for 8 days, the sludge solubilization, AD degradation and methane production was then tested and compared. Finally, the FA-based sludge pretreatment approach was evaluated from the economic point of view. This study for the first time proposed the FA pretreatment technology to improve AD degradation in post anaerobic digestion.

Materials and methods

Sludge sources

The full-scale AD was harvested from local STP with the total solids (TS) concentration at 28.5 ± 0.4 g/L and volatile solids (VS) at 21.1 ± 0.5 g/L, respectively. The TCOD (total chemical oxygen demand), SCOD (soluble chemical oxygen demand) and pH were 26.2 ± 0.3 g/L, 0.5 ± 0.1 g/L and 7.0 ± 0.2, respectively. Detail was shown in Table 1. The collected AD was immediately transported to the lab and then divided into two parts. One was kept in incubator (37 ºC) and applied as inoculum in the BMP tests described later. One part was stored in the refrigerator (4 ºC) before the FA treatment, and used as substrate in the following BMP tests.

AD pretreatment by FA

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The effectiveness of full-scale AD post anaerobic digestion with FA pretreatment was assessed by batch experiments. The parameters tested including SCOD, SKN (Soluble Kjeldahl Nitrogen), ammonia nitrogen (NH4+-N) and volatile fatty acid (VFA). Five reactors with 65 mL of AD in each reactors were used. The ammonium concentration was 1000 mg N/L in the AD. The pH of AD in three FA pretreatment reactors was adjusted and controlled with a programmable logic controller (PLC) at 8.0, 9.0 and 10.0 by 1 mol/L NaOH solution, and resulted in designated FA concentration of 55, 360 and 850 mg NH3N/L, respectively. The FA concentration was calculated by Equation (1) as follows:

S ×10pH/(Kb/Kw + 10pH)

(1)

STAN – Total ammonia nitrogen (TAN) concentration, Kb - ionization constant in ammonia equilibrium equation, Kw - water ionization constant. The Kb/Kw value was calculated by formula Kb/Kw = e6,344/(273+T) [25].

Besides that, a control reactor without any pretreatment was conducted. The pH of control reactor was maintained around 7.0 and 7.2 during the 24 h treatment time. The FA concentration in the control reactor was 6-10 mg NH3-N/L. Meanwhile, in order to investigate whether the increased AD degradation was due to the alkaline pretreatment, corresponding test was also conducted in parallel. The corresponding reactor was setup with alkaline (pH = 10.0) pretreatment only in absent of ammonium. Pretreatment by

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washing AD with pure water to remove the ammonium until non-detectable level was conducted, after which the pH were adjusted and controlled at 10.0 by NaOH solution with PLC. The reaction in all reactors were mixed at 500 rpm by magnetic stirrers, maintained at 25 ℃, lasted for 24 h. The main characteristics (i.e. VFA, VS, SCOD, SKN and NH4+-N) of AD in each reactor were measured in triplicate before and after pretreatment. The measured changes of VFA, VS, SCOD, SKN and NH4+-N during 24 h pretreatment were then calculated.

Post anaerobic digestion assay by biochemical methane potential (BMP) test

After pretreatment, pH of all pretreated sludge was neutralized to 7.0 using hydrochloric acid (1 mol/L). The pretreated and untreated AD was digested in post anaerobic batch digesters, AD degradability was tested and compared. The methane production potential of post anaerobic digestion process was evaluated by BMP batch tests, details were referred from earlier studies [26, 27]. The working volume of BMP test serum vial is 100 ml, the test was lasted for 8 days, thus the sludge retention time as 8 days can be applied at full scale post anaerobic digestion process. The inoculum and AD was added at a VS ratio 2:1 (i.e. 65 ml inoculum and 35 ml AD), which is based on the previous study [23]. The blank set was prepared by 65 ml inoculum and 35 ml MilliQ water.

As shown in Table 2, Reactor 1 was served as a control reactor with untreated AD. Reactors 2, 3, 4 were served as experimental reactors, the FA added for AD treatment

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were 55, 360 and 850 mg NH3-N/L, respectively. Reactor 5 was served as the alkaline treatment reactor, which feed with 65 mL inoculum and 35 mL AD that pretreated with alkaline (pH = 10). Triplicate tests were conducted in each condition.

Chemical Analysis

The analysis of MLSS, MLVSS, SKN, NH4+-N, TCOD and SCOD were conducted according to standard methods [28]. For the measurement of SCOD, SKN, NH4+-N and VFA, samples were filtered by disposable syringe filter with pore size 0.45 um before test. VFA was measured by GC (Agilent, USA). The percentage of methane in the collected biogas were measured by GC-8A gas chromatograph (Shimadzu, Kyoto, Japan) equipped with thermal conductivity detector.

Mathematical analysis of BMP tests

The degradable percentage (B0) and hydrolysis rate (k) were used for assess the AD post anaerobic digestion degradation potential and kinetics. These two key parameters were evaluated by fitting AD degradation percentage experimental data to a first-order kinetic model [29] in Aquasim 2.1d, the equation was as shown in Equation (2):

Y(t) = B0 * (1 - e -kt)

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(2)

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Y(t) is the degradation percentage of AD (time t, %); B0 is the AD degradable percentage (%); k is the hydrolysis rate (d−1); t is the time (d). The residual sum of squares (RSS) between the real degradation percentage and predicted degradation percentage of the FA treated AD sludge was calculated, the minimal RSS condition was selected.

The change of AD solubilization (VFA, VS, SCOD, SKN and NH4+-N) during pretreatment period was calculated as a biomass specific value, which was divided by the corresponding VS of AD. The AD degradation percentage during post anaerobic digestion was calculated from the VS destruction in the digester by Equation (3):

Y(t) = B(t) / 380 × RAD

(3)

Y(t) is the AD degradation fraction at time t; B(t) is the biochemical methane production from the samples at time t, with subtracting methane produced from blank; 380 is the standard condition theoretical biochemical methane potential (25℃, 1 atm); RAD is VS to TCOD measured ratio of AD in this study.

Statistical analysis

The analysis of variance was used to evaluate the significance of the results in this study. P 0.05) in all pretreatment conditions tested. This indicated that the increased soluble substances was not able to be further biodegraded to generate NH4+-N or VFA. This could result from the inhibition effect of FA/alkaline on enzymes which were responsible to microbe acidogenesis and hydrolysis [31].

Effect of FA pretreatment on AD degradation

The degradation percentage of AD during 8 days post anaerobic digestion with and without FA pretreatment was displayed in Fig. 2. The FA pretreatment over 360 mg NH3N/L starts to have effect on AD degradation fraction increment (P0.994 in all cases. The model predicted degradable percentage of AD and the actual degradable AD value were shown in Fig. 4. The highest AD degradable percentage (B0) was 10.5±0.33% when the FA pretreatment concentration was 850 mg NH3-N/L as compared with other FA pretreatment test and the

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control test. The control test has a lowest degradable percentage, even lower than the alkaline treated test. This indicated that Alkaline treatment was able to improve the AD degradable percentage [33]. The alkaline treated AD shows lower B0 as compared with the FA pretreated AD, even with the situation of lowest FA concentration tested. This could draw the conclusion that FA pretreatment is able to increase the AD degradable percentage, this was in accordance with the results from Fig. 2. The AD hydrolysis rate value shows positive relatives with the degradation percentage as shown in Table 3. The highest FA pretreatment concentration 850 mg NH3-N/L shows a highest hydrolysis rate as 0.45 d-1. The FA pretreated AD at 55 to 850 mg NH3-N/L has a positive influence on k, increased by 19-45% as compared with control (from 0.31 d-1 to 0.37-0.45 d-1). This could conclude that FA pretreatment was able to increase the AD hydrolysis rate with the increment of FA concentration added, thus improved the AD degradation rate, which could finally result in enhance the methane production.

A potential FA pretreatment technology for improving AD digestion during post anaerobic digestion

This study revealed for the first time that FA pre-treatment at >360 mg NH3-N/L prior to post anaerobic digestion of AD could enhance the degradation of full-scale AD. Based on this result, the proposed FA-based pretreatment technology concept diagram of “close loop” in the STPs was displayed in Fig. 5. The renewable chemical FA could be produced from the anaerobic digestion liquor directly from the STP and storage in the FA

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pretreatment unit (a small and simple tank with mixing, easy to be operated in practical application). The AD produced from the anaerobic digestion process will enter the FA pretreatment unit (alkali will be add to achieve the desirable FA concentration at 360-850 mg NH3-N/L), then mix and react with FA. The FA pretreated AD will be send to the post anaerobic digestion system, where the FA treated AD could be degraded rapidly through the anaerobic digestion process, and the AD degradation could be enhanced.

Our study also shows that FA pre-treatment enhanced the AD degradation in post anaerobic digestion as compared with the case without pretreatment. Only alkali is needed for FA pretreatment since abundance of ammonium (~ 1.0 g N/L) were existed in the AD. FA pretreatment technology would be potentially more environmentally friendly and economically favorable as compared to the FNA pretreatment method reported by earlier studies, with no need to introduce extra nitrogen source.

To evaluate the potential economic benefits of the FA pre-treatment technology, a desktop scaling up economic analysis was performed in a STP (200,000 population equivalent) based on the experimental results obtained in this study. The detailed analysis data were shown in Table S1 in Supporting Information (SI). During the analysis, the post anaerobic digester was assumed as equipped FA pretreatment unit with FA concentration at 850 mg NH3-N/L. The AD degradations (VS basis) were assumed to be 10.1% and 8.6%, respectively, in the post anaerobic digesters with and without FA pretreatment based on the experimental results in this study. Economic analysis demonstrated that, after

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implementing FA pretreatment with FA produced on site, a cost saving of $8,000-20,000 per year relying on sludge transport and disposal cost was predicted. The FA pretreatment technology is economically favorable and available in reducing sludge production in the post anaerobic digester.

Conclusions

This study evaluated the feasibility of AD degradation enhancement in post anaerobic digestion by the proposed free ammonia (FA) pretreatment technology under Lab scale post anaerobic digestion assays. The main conclusions are as follows: 

The AD degradation by post anaerobic digestion with FA pretreatment at over 360 mg NH3-N/L was significantly improved; over 20% improvement was achieved for the full-scale AD tested in this study.



The cumulative methane production of AD by post anaerobic digestion was increased with FA pretreatment at above 360 mg NH3-N/L.



FA pretreatment is a potentially economically favourable technology for AD pretreatment prior to post anaerobic digestion.

ASSOCIATED CONTENT Supporting Information. Details of the economic analysis of post anaerobic digestion using free ammonia technology, one table is shown in Supporting Information.

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Acknowledgments

The research was supported by National Natural Science Foundation of China (Grant no. 41877344), and National Water Science and Technology Project (Grant no. 2018ZX07208001). Dr. Kang Song acknowledges the supports from One-Hundred Scholar Award (Y82Z08-1-401, Y75Z01-1-401) of Chinese Academy of Sciences.

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List of Tables and Figures Table 1 Key characteristics of the AD and digesting sludge used in the experiments (mean  95% confidence interval from triplicate measurements) Table 2 Experimental design of the post anaerobic digestion assays Table 3 Determined hydrolysis rate (k) and degradable percentage (Y) of AD with and without pretreatment using the first-order kinetic model (with 95% confidence intervals) Fig. 1 Changes in SCOD, SKN, NH4+-N and VFA concentrations in AD with and without pretreatment for 24 h. Error bars represent 95% confidence intervals of triplicate tests Fig. 2 Degradation of AD (on the basis of VS) with and without pretreatment during the 8 days post anaerobic digestion period. Error bars represent 95% confidence intervals of triplicate tests Fig. 3 Cumulative methane production from AD with and without pretreatment during the 8 days post anaerobic digestion period. Error bars represent 95% confidence intervals of triplicate tests Fig. 4 The actual degradation percentage of AD (Y(t)) and kinetic model predicted Y(t) Fig. 5 Conceptual graph of the proposed FA pretreatment technology for improving AD degradation in post anaerobic digestion

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Table 1 Key characteristics of the AD and digesting sludge used in the experiments (mean  95% confidence interval from triplicate measurements) Parameter Total Solids (TS) (g/L) Volatile Solids (VS) (g/L) Total Chemical Oxygen Demand (TCOD) (g/L) Soluble chemical oxygen demand (SCOD) (g /L) pH

AD 28.5  0.4 21.1  0.5 26.2  0.3 0.5  0.1 7.0  0.2

Table 2 Experimental design of the post anaerobic digestion assays Reactor Function Experimental conditions R1 Control 65 mL digesting sludge + 35 mL untreated AD R2 FA1 65 mL digesting sludge + 35 mL FA-treated AD at 55 mg NH3-N/L R3 FA2 65 mL digesting sludge + 35 mL FA-treated AD at 360 mg NH3-N/L R4 FA3 65 mL digesting sludge + 35 mL FA-treated AD at 850 mg NH3-N/L R5 Alkaline 65 mL digesting sludge + 35 mL alkaline-treated AD at pH 10.0 R6 Blank 65 mL digesting sludge + 35 mL supernatant from original AD Table 3 Determined hydrolysis rate (k) and degradable percentage (B0) of AD with and without pretreatment using the first-order kinetic model (with 95% confidence intervals) Degradable percentage Hydrolysis rate AD degradation R2 (B0, %) (k, d-1) 7.95±0.16 0.31 0.998 Control 8.4±0.23 0.37 0.996 FA1 9.5±0.23 0.40 0.997 FA2 10.5±0.33 0.45 0.995 FA3 8.05±0.29 0.36 0.994 Alkaline

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Fig. 1 Changes in SCOD, SKN, NH4+-N and VFA concentrations in AD with and without pretreatment for 24 h. Error bars represent 95% confidence intervals of triplicate tests

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12 Degradation fraction of ADS (%)

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Control FA1 FA2 FA3 Alkaline

10 8 6 4 2 0 0

2

4

6

8

Digestion time (day)

Fig. 2 Degradation of AD (on the basis of VS) with and without pretreatment during the 8 days post anaerobic digestion period. Error bars represent 95% confidence intervals of triplicate tests

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80 Control FA1 60 Cumulative methane production (L CH4/kg VS added)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


FA2 FA3 Alkaline

40

20

0 0

2

4

6

8

Digestion time (day)

Fig. 3 Cumulative methane production from AD with and without pretreatment during the 8 days post anaerobic digestion period. Error bars represent 95% confidence intervals of triplicate tests

27



Fig. 4 The actual degradation percentage of AD (Y(t)) and kinetic model predicted Y(t)

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Page 28 of 30

Page 29 of 30

Wastewater

Aeration tank

Sedimentation tank

inlet

Digested liquor

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Excess sludge

Effluent

Thickened

CH4

Alkali

CH4

production

adding

production

sludge

treated

Anaerobic

ADS

digestion FA

Sludge cake

Sludge

Post

Pretreatment

Dewatering

Anaerobic

Unit

digestion

Fig. 5 Conceptual graph of the proposed FA pretreatment technology for improving AD degradation in post anaerobic digestion

29



For Table of Contents Use Only

FA pretreatment technology provides an innovative pathway in sludge reduction and methane production improving, which demonstrates the principles of green engineering.

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Page 30 of 30

Improving Post-Anaerobic Digestion of Full-Scale Anaerobic Digestate

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