Influence of reflux ratio on the anaerobic digestion of pig manure in leach beds coupled with continuous stirred tank reactors

Influence of reflux ratio on the anaerobic digestion of pig manure in leach beds coupled with continuous stirred tank reactors

Waste Management 97 (2019) 115–122 Contents lists available at ScienceDirect Waste Management journal homepage: www.elsevier.com/locate/wasman Infl...

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Waste Management 97 (2019) 115–122

Contents lists available at ScienceDirect

Waste Management journal homepage: www.elsevier.com/locate/wasman

Influence of reflux ratio on the anaerobic digestion of pig manure in leach beds coupled with continuous stirred tank reactors Jie Yang, Dehan Wang ⇑, Zifeng Luo, Weishen Zeng College of Natural Resources and Environment, South China Agricultural University, Guangzhou, Guangdong 510642, PR China

a r t i c l e

i n f o

Article history: Received 27 April 2019 Revised 5 August 2019 Accepted 5 August 2019

Keywords: Pig manure Anaerobic digestion Methane yield Reflux ratio

a b s t r a c t The effect of reflux ratio on the anaerobic mono-digestion of pig manure (PM) in leach beds coupled with continuous stirred tank reactors (CSTRs) has been studied in this work, and contents of volatile fatty acids (VFAs) and biogas yields were determined for three groups of leach bed reactor (LBR) – CSTR systems. The obtained results indicated that the reflux of biogas slurry increased both the pH of the acid-producing phase and acetic acid yield and repeatedly degraded the refractory organic matter in the biogas slurry. The larger reflux ratio increased the inoculation volume and substantially enhanced the mass transfer process. The maximum values of the biogas and methane yields equal to 259.49 and 167.44 mL/g volatile solids, respectively, were achieved at a reflux ratio of 100%. Moreover, the weight of the PM leachate residue was reduced by 94.14%, and the total nutrient content (N + P2O5 + K2O) was relatively high (1.48%), which was suitable for vegetable seedling substrates. In conclusion, during the treatment of PM in LBR– CSTRs, the solid phase remains on the leach bed, and the leachate is supplied to a biogas tank, which effectively increases its stability of operation. Ó 2019 Elsevier Ltd. All rights reserved.

1. Introduction China is the largest pork-producing country on Earth. Due to the large-scale and intensive development of the pig industry, many pig farms generate excessive amounts of waste (Deng et al., 2017). Because the produced manure is typically cleaned by rinsing, its solid phase contains 60–90 wt% of water after solid-liquid separation (Adam et al., 2018). Hence, flocculants must be added to manure before composting (Roe and Cornforth, 2019) simultaneously with odour control agents, which significantly increases its treatment cost. Anaerobic digestion (AD) is an environment-friendly technology (Vasco-Correa et al., 2018), which not only reduces the cost of the treatment of livestock and poultry excrements and pollution degree of the environment caused by their disposal, but also produces valuable by-products such as clean energy (CH4 gas) and highquality fertilisers. Three different types of AD correspond to different concentrations of raw materials or total solids (TS): wet digestion (TS < 12%), semi-dry digestion (TS = 15–20%), and dry digestion (TS > 20%) (Di Maria et al., 2017; Nguyen et al., 2016). Compared with the traditional wet digestion process, the advantages of the

⇑ Corresponding author. E-mail address: [email protected] (D. Wang). https://doi.org/10.1016/j.wasman.2019.08.005 0956-053X/Ó 2019 Elsevier Ltd. All rights reserved.

semi-dry and dry digestions include high volume of the produced methane, low energy demands for heat preservation and heating, low yield of the biogas slurry, low moisture content in the biogas residue, and relatively easy treatment procedure, which made the AD technology popular among many researchers around the world (Jang et al., 2018; Sukasem et al., 2017). According to the literature (Scarlat et al., 2018), the majority of the recently built biogas plants in Europe utilise either the semi-dry or dry digestion technologies. Unfortunately, digestion and stirring are difficult to perform during the mass and heat transfer leading to acidification that negatively affects the biogas production efficiency. To overcome the extremely low hydrolysis rate of the dry litter, a highly efficient hydrolyser must be added to the system for the hydrolysis and acidogenesis of solid waste prior to methanogenesis in a digester. Continuous stirred tank reactors (CSTRs) are most commonly used for the biological production of biogas from waste (Soni-Bains et al., 2017). Due to the high solid contents in the livestock and poultry manures (20– 30 wt%), a series of pretreatment steps (such as dilution) must be conducted to meet the operational requirements of CSTRs. Leach bed reactors (LBRs), which can process high solid contents, are more suitable for the livestock and poultry waste treatment and generate leachates with relatively high contents of volatile fatty acids (VFAs) (Chen et al., 2014; Mengistu et al., 2015; Parawira, 2009). These reactors are commonly used in two-

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phase digestion processes as hydrolysers (Demirer and Chen, 2008); furthermore, the filtration of biogas slurry by a leach bed can increase the utilisation rate of organic matter and promote the heat and mass transfer reactions (Korazbekova and Bakhov, 2014). Overall, coupling LBRs with CSTRs significantly enhances their performance. It was found previously that the anaerobic process contained three stages: hydrolysis, acidogenesis, and methanogenesis (Wang et al., 2018). In a two-phase digestion reaction, the first phase corresponds to hydrolysis (the rate-limiting step) combined with acidogenesis, and the second phase consists of methanogenesis (Zhen et al., 2017). On the one hand, the hydrolytic acidification products of complex organic compounds (such as acetic acid) serve as a substrate for the next stage of methylium production. On the other hand, the VFAs produced during acidification are difficult to remove and accumulate inside the matrix with a high solid content, which inhibits both the methanogenic and acidification reactions (Khan et al., 2016). Thus, the high hydrolysis rate during two-phase digestion would increase the efficiency of the subsequent methanogenic process. To accelerate the hydrolysis reaction, ensure a proper acid-base balance over the entire reactor volume, and increase the efficiency of the treatment of organic solid waste, methane effluent reflux is typically performed. Moreover, methane effluent contains a large number of microorganisms, and its reflux can increase the hydrolytic enzyme activity (Yoshida et al., 2017). Lee et al. (2010) studied the anaerobic digestion of high-temperature kitchen waste under three different loads and found that reflux produced both regulating and diluting effects. The influence of reflux on anaerobic digestion also depends on the digestive substrate. In the study of Jagadabhi et al. (2011), the combined utilisation of a leach bed and an upwelling anaerobic reactor for the anaerobic digestion of vegetables demonstrated that it was possible to produce a system buffer and prevent over-acidification by refluxing the effluent of the anaerobic reactor to the leach bed at various fresh water contents. Cavinato et al. (2011) found that when biological waste was used as the substrate, the backflow maintained the pH of the hydrogen-producing phase at a level of 5.5, while the biogas production rate increased. These results were particularly important for the long-term operation because the reflux of methane effluent could have a positive effect on the AD process. The reflux ratio utilised during the inoculation of methane effluent to the acidification reactor strongly affects the methane feed concentration, organic load, and hydraulic retention time as well as the interactions between various types of acid-producing bacteria (the greater reflux ratio corresponds to a larger degree of inoculation). However, the influence of reflux ratio on the efficiency of the anaerobic digestion of PM in the leach beds coupled with methanogenic reactors has not been investigated yet. In this work, AD was conducted at three different reflux ratios (50%, 75%, and 100%), and their impacts on the performances of the PM digestion in the leach beds coupled with CSTRs were examined at a temperature of 35 ± 1 °C. After the biogas production, biodrying of the PM leachate residue was performed, and the reductions of its weight and nutrient content were determined. As a result, the optimal reflux ratio of the combined process was obtained as a reference value for the efficient operation and process control of biogas-producing pig farms, and the methane yield of PM was significantly improved. This study describes a new process that integrates the digestion and composting of PM in a leach bed using an inorganic filtration medium. The leachate soaking at a biogas slurry reflux ratio of 100% reduces the odour emission and leads to a large methane yield of the PM digestion. Furthermore, no discharge of the biogas slurry is observed during the entire digestion process.

2. Materials and methods 2.1. Substrate and inoculant PM: fresh pig manure with a TS content of 27.57 ± 0.98%, VS content of 78.46 ± 0.44%, and pH of 6.1 ± 0.05 was obtained from the Institute of Animal Husbandry, Guangdong Academy of Agricultural Sciences. Its total carbon content was 35.88 ± 0.21%, total nitrogen (TN) concentration was 2.09 ± 0.03%, total phosphorus (TP) content was 0.9 ± 0.01%, and total potassium concentration was 1.12 ± 0.13%, all of data were based on dry matter. Filtration medium: ceramsite round spheres with sizes of 2– 3 mm containing hard surface shells were provided by the Xuanyi Plant. They were fabricated from ceramic to ensure good water insulation and air retention properties, high strength, and high chemical and thermal stabilities. In previous studies on this topic (Yang et al., 2019), the parameters of three inorganic filtration media (ceramsite, perlite, and rubber particles) for LBR-CSTRs were compared. The obtained results revealed that the LBR-CSTR with the filtration medium composed of ceramsite exhibited the best performance. Its biogas yield and degradation degree of organic matter were 241.68 mL/g VS and 86.82%, which were 1.24 times and 19.49% higher than the values obtained in the control test, respectively. Biogas slurry was obtained from a laboratory CSTR reactor. Its pH was 7.85 ± 0.07, chemical oxygen demand (COD) was 579.71 ± 5.42 mg/L, NH3-N content was 310.33 ± 2.78 mg/L, and TN content was 492.31 ± 4.22 mg/L. Tap water: pH was 6.86 ± 0.67, EC was 139.80 ± 6.82 lS/cm, hardness was 34.08 ± 3.31. 2.2. Experimental setup Fig. 1A shows the schematic diagram of the test device, while Fig. 1B displays its actual photograph. The LBR including a filtration medium without anaerobic sludge and CSTR were manufactured from stainless steel and contained a hot water interlayer for controlling the reaction temperature. The LBR volume was 15 L, and a 210  210 mm plate was used to form a compartment with a volume of 1 L at the reactor bottom. The CSTR had a capacity of 40 L and effective volume of 32 L, and its outlet was connected to a wet anticorrosive gas flow meter (Changchun Alpha Meter Co., Ltd., model lmm-1). The other end of the flowmeter was connected to an aluminium foil gas sampling bag (Dalian Haide Technology Co., Ltd., 30 L). Tests were conducted simultaneously on three sets of the same equipment type. Fig. 1C shows the device utilised for the biodrying of the leachate residue of PM. Its air pump was controlled by a time control switch. 2.3. Experimental procedure Three LBR–CSTR systems (T1, T2, and T3) were simultaneously tested in batches at a temperature of 35 ± 1 °C. First, 1 L of the ceramsite filtration medium was placed on the sieve plate of the leach bed followed by the addition of 2 L (2200 g) of PM. Biogas slurries with various tap water contents and total volumes of 4 L were discharged from the CSTRs to the leach beds of all three groups every day, and their degrees of uniformity were maintained constant (in the first group, 2 L of biogas slurry was mixed with 2 L of tap water; in the second group, 3 L of biogas slurry was mixed with 1 L of tap water; and in the third group, 4 L of biogas slurry was supplied from the CSTRs without mixing with tap water). After 3 h of reaction, the resulting leachates were pumped into the CSTRs. Between different process stages, stirring was conducted

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for 10 min at a speed of 60 r/min using an agitator. Table 1 lists the parameters of all experimental setups used in this work. After the production of biogas in the batch test, the obtained PM leachate was stabilised via biodrying. The air was supplied by intermittent ventilation using a time-controlled switch. The ventilation frequency was 20 min/h, the ventilation rate was 12 L/min, and the biodrying time was 3 d. 2.4. Analytical methods PM was weighed by an electronic scale before and after the reaction. Samples of the leachate and biogas slurry with volumes of 15 mL were analysed daily for index determination. An APHS-3C precision pH meter was used for pH measurements. COD, NH3-N, TN, and TP contents were determined using the standard methods (APHA, 2011). The total volatile fatty acid (TVFA) concentrations in the studied samples were obtained by an Agilent 6890N gas chromatograph (Agilent Technologies, Santa Clara, CA, USA). The oven temperature for TVFA analysis was programmed to maintain a level of 150 °C for 2 min, after which its magnitude was increased to 190 °C at a rate of 4 °C/min and maintained at that level for 3 min. The injected sample volume was 0.006 mL. TVFAs, including acetic acid, propionic acid, isobutyric acid, butyric acid, isovaleric acid, nvaleric acid, isocaproic acid, n-caproic acid, and heptanoic acid were identified by comparing their retention times with those of a standard mixture. The methane gas produced in the digestion process was collected by a 30-L aluminium foil air bag; the daily methane concentration was determined by a Geotech Biogas 5000 biogas composition analyser; and the volume of the biogas produced each day was measured by a wet anti-corrosive gas flow meter (lmm-1 manufactured by Changchun Alpha Meter Co., Ltd.). The dried leachate residue was ground and screened, and its organic matter, N, P, and K contents were estimated according to the standard developed for organic fertilisers (NY 525-2012). The pH of the extracted solution was determined by the oscillating extraction with deionised water for 1 h at room temperature, W (g): V (mL) ratio of 1:10, and stirring speed of 200 r/min, while its electrical conductivity (EC) was measured by a DDS-11A conductivity meter. Sodium bicarbonate (0.05 mL) was utilised as the solvent, and the absorption values at 465 nm (E4) and 665 nm (E6) were obtained in the concentration range of 40– 200 mg/kg by a visible spectrophotometer (the E4/E6 ratio can be used as a quick indicator of the degree of maturity during the composting process). The removal rate of VS was calculated by the following three formulas:

A

Time Switch B

Air Pump

W reduction ¼

C Fig. 1. A diagram of the LBR-CSTR experimental setup (A) and its actual photograph (C). A diagram of the experimental biodrying reactor (B). H/C denotes the heat control system.

W initial  W final  100% W initial

TSremov al ¼

TSinitial  TSfinal  100% TSinitial

VSremov al ¼

VSinitial  VSfinal  100% VSinitial

where Wreduction is the weight reduction of the PM, Winitial is the initial weight of the PM, Wfinal is the final weight of the biologically dried product, TSremoval is the removal efficiency of the TS of the

Table 1 Operational parameters of different LBR-CSTR setups. Group

Reflux ratio (%)

PM mass (g)

Ceramsite volume (L)

Biogas slurry volume (L)

Tap water volume (L)

Soaking time (h)

Number of reflux cycles

T1 T2 T3

50 75 100

2200 2200 2200

1 1 1

2 3 4

2 1 0

3 3 3

1 1 1

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PM, TSinitial is the initial TS content of the PM, TSfinal is the final TS content of the biologically dried product, VSremoval is the removal efficiency of the VS of the PM, VSinitial is the initial VS content of the PM, and VSfinal is the final VS content of the biologically dried product. All chemical analyses were performed in triplicate, and average values with their standard deviations were reported.

As shown in Fig. 2C, the ammonia concentration in the leachate first increased and then decreased due to the dissolution of protein in the PM at the beginning of the batch test. As the reaction progresses, the concentration of ammonia nitrogen becomes more stable and significantly lower than the concentration of ammonia inhibitor (Poirier et al., 2016; Tian et al., 2018). The results reported by Hansen et al. (1998) showed that the biogas yield of the PM continuously stirred with the tank agitator was significantly lower than the potential biogas yield. In this study, the methane yield of the CSTR was only 77.58 mL/g VS. However, ammonia nitrogen does not inhibit the anaerobic digestion because PM is easy to acidify, leading to the accumulation of VFAs (up to 6322.32 mg/L), which have a negative effect on the anaerobic digestion process.

3. Results and discussion 3.1. Digestive performance of the LBR-CSTR system

8.2

8.2

8.0

8.0

7.8

7.8

7.6

7.6

7.4

7.4

7.2

7.2 7.0

7.0 1

3

5

A

7 9 11 Time d

13

15

1

17

1600

1400

1400

1200

1200

1000

1000 800 600 400

5

7 9 Time d

11

13

15

17

800 600 400 200

200

0

0

C

3

B

NH3-N (mg/L)

NH3-N (mg/L)

3.1.2. Influence of reflux ratio on VFA yield VFAs are products of the hydrolysis and acidification of macromolecular organics that occur during the anaerobic digestion process; they also serve as the substrate used by methanogens. The concentration of VFAs reflects the degree of acidification in an anaerobic digestion system (Chen et al., 2002). As shown in Fig. 3B, a relatively low concentration of acid is accumulated in the biogas slurry within the first 6 d of the experiment. The VFA concentrations in the leachates of the T1 and T3 systems reached maxima on the second day of the test, while that in the leachate of the T3 system first increased and then gradually decreased, which was consistent with the observed change in the concentration of acetic acid. Further, methanogens exhibited different utilisation rates for different organic acids. The degradation rates of mixed organic acids by the methanogens present in the reactor can be ranked as follows: acetic acid > ethanol > butyrate > propionic acid (Wang et al., 2009). As can be seen from Fig. 3B, the VFA utilisation rate of CSTR in the LBR leachate was nearly 100%, which was in good agreement with the content of VFA in Fig. 3C.

pH

pH

3.1.1. Influence of reflux ratio on pH and NH3-N content pH is an important parameter of the anaerobic digestion process, and acid-producing bacteria exhibit strong adaptability to pH generally varying from 4.5 to 8.0 (Seda et al., 2013). As shown in Fig. 2A and B, the pH fluctuations of the LBR-CSTR systems observed at three different reflux ratios with little volatility during the entire experiment. On the second day of the reaction, the pH of the leachate in setup T3 reached its lowest magnitude, while the pH values of the leachates in setups T1 and T2 reached minima on the third day. After eighth days, the overall pH trend observed for the studied three systems was T3 > T2 > T1, and the biogas slurry was slightly alkaline. Refluxing not only increased the degree of alkalinity, but also promoted the penetration of anaerobic microorganisms into the acid-producing phase, thus increasing the buffering capacity of the system. Finally, the pH values of T1, T2, and T3 were stable and equal to about 7.4, 7.5 and 7.7, respectively, indicating that the reflux of the biogas slurry produced a buffering effect on the pH of the PM percolation bed and that the buffering capacity increased with an increase in the return flow rate because the treatment of manure with the water-diluted biogas slurry at a small reflux ratio reduced the pH of the system.

1

3

5

7 9 Time d

11

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15

17

1

D

3

5

7

9

11

13

15

17

Time d

Fig. 2. pH values of the leachates (A) and biogas slurries (B) in setups T1, T2, and T3. NH3-N concentrations of the leachates (C) and biogas slurries (D) in setups T1, T2, and T3.

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6000

6000

5000

5000 VFA (mg/L)

VFA (mg/L)

J. Yang et al. / Waste Management 97 (2019) 115–122

4000 3000 2000

4000 3000 2000 1000

1000

0

0 1

A

3

5

7 9 Time d

11

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17

1

B

3

5

7 9 Time d

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13

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17

2500 Valeric Acid

2000

Isovaleric Acid

Butyric Acid

Propanoic Acid

Acetic Acid

ρ mg/L

1500 1000 500 0 T1T2T3 T1T2T3 T1T2T3 T1T2T3 T1T2T3 T1T2T3 T1T2T3 T1T2T3 T1T2T3 T1T2T3 T1T2T3 T1T2T3 T1T2T3 T1T2T3 T1T2T3 T1T2T3 T1T2T3

C

1

2

3

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5

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7

8

9 Time d

10

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17

18000 16000 14000 12000 10000 8000 6000 4000 2000 0

18000 16000 14000 12000 10000 8000 6000 4000 2000 0

COD (mg/L)

COD (mg/L)

Fig. 3. VFA concentrations in the leachates (A) and biogas slurries (B) of the T1, T2, and T3 systems. VFA compositions in the T1, T2, and T3 systems (C).

3

5

7

9 11 Time d

COD removal rate (%)

1

13

15

17

1

3

5

1

3

5

7

9 11 Time d

13

15

17

100 90 80 70 60 50 40 30 20 10 0 7

9

11

13

15

17

Time d Fig. 4. COD concentrations of the leachates (A) and biogas slurries (B) in the T1, T2, and T3 systems. COD removal rates measured for the T1, T2, and T3 systems (C).

The PM in the leachate bed demonstrated acetic acid-type fermentation, and the produced VFA mainly contained acetic acid on the 7th day of the experiment suggesting that the organic acids in the leachate could be easily consumed by methanogens. Therefore,

the biogas slurry reflux during methanogenesis can apparently promote the acidification of the PM percolation bed and increase the VFA yield. The higher is the reflux ratio, the stronger is the observed effect.

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3.1.3. Influence of reflux ratio on COD As shown in Fig. 4A and B, the COD of the leachate in setup T3 reached the peak value on the second day of the experiment. Although the biogas slurry was diluted with water during the treatment in setups T1 and T2, the COD of the T1 slurry was similar to that of the T2 slurry at the later process stage. Generally, the COD of biogas slurry reflects the amount of compounds that can be oxidised and indirectly – the degradation of organic matter after the leachate enters the CSTR. When biogas slurry has a high COD value, the amounts of the organic matter degraded during anaerobic digestion and produced biogas are low, indicating that the digestion process is severely inhibited, whereas in the opposite case, the degradation process proceeds smoothly with intense biogas production. It was found previously that the use of cow manure as a fermentation raw material combined with the reflux of biogas slurry increased the COD removal rate (Awasthi et al., 2018) because cow dung contained high contents of cellulose, hemicellulose, and lignin, which were difficult to degrade via anaerobic fermentation. During the continuous reflux of biogas slurry, the undegraded cellulose and other refractory materials are continuously supplied to the digester for further digestion. At a higher reflux ratio, the refractory materials are gradually degraded, and the COD removal rate is further increased. Therefore, the reflux ratio of biogas slurry strongly influences the removal rate of COD after the anaerobic digestion of PM. Fig. 4C shows the COD removal rates measured for the three systems. With the increase of the reflux ratio of the biogas slurry, the COD removal rate apparently increased. The average of COD removal rate of T3 is highest of 55.94%.

A

20 18 16 14 12 10 8 6 4 2 0

300 250 200 150 100

1

3

5

7 9 Time d

11

13

B

15

Content of CH4 %

Cumulative CH4 yeild mL/g VS

100 90 80 70 60 50 40 30 20 10 0

C

3.1.5. COD, NH3-N, TN, and TP values of CSTR effluent after biogas production As shown in Table 2, after the completion of the batch biogas production, the COD, NH3-N, and TN values of the CSTR effusive water can be ranked as T3 > T2 > T1 for the three studied systems. First, because the reflux ratio of T1 is the smallest one, fresh water

Cumulative biogas yeild mL/g VS

Daily yield of biogas L

3.1.4. Influence of reflux ratio on methanogenesis As shown in Fig. 5, the CSTR produced gas continuously for 16 d in setup T1 and for 15 d in setups T2 and T3. The greater is the reflux ratio of the biogas slurry, the higher is the biogas yield, suggesting that some organic compounds in the biogas slurry depicted in Fig. 4A and B degraded more to generate biogas. By taking the two-phase fermentation process as an example, the amount of

the biogas formed in the process of acid production apparently increased after the reflux of the biogas slurry because the methanogens contained in the slurry returned to the anaerobic environment and became active again. Before the completion of the anaerobic process (when the organic matter is not fully degraded), the biogas slurry reflux can increase the biogas production. The largest methane yield (167.44 mL/g VS) was obtained in setup T3, which was higher than the value of 131.4 mL/g VS estimated for the PM in Asia (Shen et al., 2018). The cumulative methane productions in setups T1 and T2 increased by 15.43% and 15.10%, respectively. On the fifth day of the treatment in the T3 system, the methane content during biogas production reached a peak value of 80.40%. Duan et al. (2019) conducted continuous fermentation of the PM with a TS of 3–8% during the complete mixing in a CSTR reactor; the resulting methane content ranged from 58.75% to 73.44%. According to the results of previous studies, reflux can positively affect the anaerobic digestion process without increasing the treatment cost as a cheap leachate treatment method (see Supplemental files). The effluent backflow of the methane reactor was inoculated into the acidification reactor, and the mass transfer process was enhanced by increasing the backflow ratio. Moreover, the larger reflux ratio indicates a larger inoculation quantity. The change in the reflux ratio affects the feed concentration, organic load, and hydraulic retention time of the methane reactor during operation and also makes various bacterial groups interact alternately with each other during the reaction. In this study, the treatment in the T3 system not only increased the biogas and methane yield of PM, but also prevented possible secondary pollution by the biogas slurry in the batch test.

1

3

5

7 Time d

9

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15

D

50 0 1

3

5

7 9 Time d

11

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15

180 160 140 120 100 80 60 40 20 0 1

3

5

7

9

11

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15

Time d

Fig. 5. Daily yields (A), cumulative yields (B), CH4 concentrations (C), and cumulative CH4 yields (D) of the biogases produced in setups T1, T2, and T3.

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J. Yang et al. / Waste Management 97 (2019) 115–122 Table 2 COD, NH3-N, TN, and TP values of the final effluents in the studied LBR-CSTR systems. Group

COD (mg/L)

NH3-N (mg/L)

TN (mg/L)

TP (mg/L)

T1 T2 T3

104.35 ± 2.11b 463.67 ± 7.53a 463.77 ± 8.94a

351.07 ± 3.44b 350.53 ± 4.65b 612.62 ± 8.77a

379.78 ± 4.62c 452.75 ± 5.22b 721.10 ± 7.96a

79.73 ± 0.92a 32.38 ± 0.66b 11.78 ± 0.23c

Note: Data in the table are the Means ± SD, the different normal letters in the same column indicate significant difference among treatments at 0.05 level (n = 3), the same as below.

Table 3 Organic removal efficiencies obtained for the treated PM and characteristics of the biodrying products. W initial W final Reduction TS initial TS final Removal VS initial VS final Removal pH (g) (g) (%) (g) (g) efficiency (%) (g) (g) efficiency (%) T1 2200 T2 2200 T3 2200

150 139 129

93.18 93.68 94.14

476.96 476.96 476.96

250.3 170.28 78

47.52 64.3 83.65

361.01 361.01 361.01

83.23 75.11 32.94

76.94 79.2 90.88

EC (ms/cm)

E4/E6 Organic materials Nutrient (g/kg) (N + P2O5 + K2O, %)

7.38 ± 0.05 1.22 ± 0.04 128.22 ± 6.10 7.36 ± 0.02 1.31 ± 0.06 115.61 ± 2.19 7.49 ± 0.03 1.47 ± 0.02 99.23 ± 0.99

1.08 ± 0.04 1.26 ± 0.08 1.48 ± 0.07

2.08 2.18 2.31

nutrient content (N + P2O5 + K2O) was relatively high (1.48%), which was suitable for vegetable seedling substrates.

dilutes the biogas slurry, resulting in the low contents of pollutants in the effluent (its TP concentrations varied as T1 > T2 > T3). It was also found (Yilmazel and Demirer, 2011, 2013) that anaerobic digestion strongly influenced the removal of phosphorus from the biogas slurry. Therefore, during the treatment at a high reflux ratio (T3), phosphorus adsorption and removal effects were observed due to the anaerobic bacterial activity.

The authors declare that there are no conflicts of interest regarding the publication of this article.

3.2. Characteristics of PM leachate residue

Acknowledgements

As shown in Table 3, the weight of the PM in the LBR-CSTR systems with three different reflux ratios decreased after their anaerobic digestions from the initial 2200.00 g to 150.00, 139.00, and 129.00 g, corresponding to the reductions of 93.18%, 93.68%, and 94.14%, respectively (here, the treatment in the T3 setup produced the most significant effect). After the biological drying of the PM leachate residue during the three treatments, the pH values of the obtained products were neutral. Because the leachate residue of the treated PM has been partially stabilised by the anaerobic digestion, the rapid decrease in the water content of the leachate residue during the biological drying process can lead to its more effective land use. Owing to the largest biogas production rate of PM in the T3 system, its organic matter content is relatively low, but its nutrient amounts are the highest ones. The E4/E6 ratio represents the relative content of humic acid in the product (Chen et al., 1977). When its magnitude drops to 1.5–1.9, the product completely decomposes. In this work, the E4/E6 ratio of the PM leachate residue in setup T3 was slightly higher than the values obtained for the T1 and T2 systems.

This study was supported by the Special Fund Project for Agricultural Development and Rural Work in the Guangdong Province, China (grant numbers 2017LM4169 and 2017LM2149).

4. Conclusion During the treatment of PM in LBR-CSTR systems, the solid substances remain on the leach bed, and the leachate is supplied to the biogas tank, which effectively solves its operation problems. In this study, the effluent backflow of the methane reactor was inoculated into the acidification reactor. Because the larger reflux ratio required a greater inoculation volume, the mass transfer process was enhanced with an increase in the reflux ratio. The LBR-CSTR system with a reflux ratio of 100% produced 10.63 g/L of VFA due to acetate fermentation. The largest methane yield of PM (167.44 mL/g VS) was greater than the estimated methane production rate of the PM in Asia (131.4 mL/g VS). Moreover, the weight of the PM leachate residue was reduced by 94.14%, and the total

Declaration of Competing Interest

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