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Effects of Percolate Recirculation on Dry Anaerobic Co-digestion of Organic Fraction of Municipal Solid Waste and Corn Straw Mingyu Qian, Yixin Zhang, Ruihua Li, Michael Nelles, Walter Stinner, and Yeqing Li Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b01869 • Publication Date (Web): 10 Oct 2017 Downloaded from http://pubs.acs.org on October 15, 2017

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Effects of Percolate Recirculation on Dry Anaerobic Co-digestion of Organic Fraction of Municipal Solid Waste and Corn Straw Mingyu Qian a,b,c, Yixin Zhang a, Ruihua Li a, Michael Nelles a, b, c, Walter Stinner c, Yeqing Li a,* a

State Key Laboratory of Heavy Oil Processing, Beijing Key Laboratory of Biogas Upgrading

Utilization, Institute of New Energy, China University of Petroleum Beijing (CUPB), No. 18 Fuxue Road, Changping District, Beijing, 102200, China b

Faculty of Agricultural and Environmental Sciences, University of Rostock, Justus-von-Liebig-

Weg 6, 18059, Rostock, Germany c

Biochemical Conversion Department, Deutsches Biomasseforschungszentrum gGmbH (DBFZ),

Torgauer Straße 116, D-04347 Leipzig, Germany * Corresponding author: [email protected]/[email protected]; phone: +861089739062, fax: +861089739062

ABSTRACT

Garage-type dry fermentation (GTDF) technology is an appropriate method to co-digest organic fraction of municipal solid waste (OFMSW) and corn straw. In this study, anaerobic digestion (AD) performance of lab-scale GTDF under different frequency of percolate recirculation (FPR, daily percolate recirculation volume to total percolate volume) of 0.3, 0.6, 1.2, 2.4 and 4.8 were 1

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investigated, and their effects on volatile fatty acids (VFA) and methane generation were analyzed. Results indicated that higher FPR positively contributed to the hydrolysis and acidogenesis by its inoculation effect and the enhancement of mass transfer, but its washing effect also led to a fast VFA accumulation and a higher peak of VFA concentration. Highest methane yield of 166.33 mL CH4/gVS was obtained at FPR of 0.3, which correspond to 66.6% of the biochemical methane potential (BMP) of the co-substrate. Additionally, in comparison to the AD performance of addition of solid digestate inoculum, a comparable methane yield can be reached through the mono-inoculation of percolate recirculation. Key words: Garage-type dry fermentation (GTDF); Co-digestion; Percolate recirculation; VFA generation; Methane production;

1. Introduction

With the rapid economic development and urbanization, China is facing the problem of waste disposal. In 2014, 653 cities and 1,596 counties annually generated 178.6 million tons and 245.2 million tons of municipal solid waste (MSW), respectively.1 The China National New Urbanization Plan (2014-2020) revealed that the percentage of the population living in cities increased from 19.4% in 1980 to 56.1% in 2015, and the urban population expanded from 191 to 771 million.2 The urban population will grow by another 100 million by 20203 and the annual generation of MSW in cities is expected to reach 200 million tons.4 During the urbanization process, a large number of new towns will be constructed. Because the new towns are still surrounded by arable land, the waste generated from the new towns presents a particular characteristics of mixture of urban waste and agricultural waste.5 The Chinese MSW contains high water content and high content of organic fraction of MSW (OFMSW) (50-60%), which has led to serious adverse effects in tranditional MSW 2


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treatment technologies (i.e., incineration and landfilling), for example dioxin and greenhouse gas release caused by insufficient burning, and abundant leachate generation from landfills. 6 Anaerobic Digestion (AD), as an engineered solution to degrade organic wastes and to produce biogas, is considered as a sustainable approach to treat waste, to produce clean energy and to recovery the organic matter which is the most important benefit in rural area.7 Thus, anaerobic co-digestion of OFMSW and agricultural waste (i.e. corn straw) is becoming a significant task for the successful urbanization in China. The Chinese OFMSW is characterized with high water content of >70% and low carbon to nitrogen ratio (C/N) of 13.7-20.1.8-10 Straw, as one of the most abundant substrates for biogas production in China , generates about 830 million tons annually,11 but its high C/N ratio and lignin content presents an inappropriate characteristic for AD.12 Pretreatment is widely applied to promote the biodegradability of straw, i.e. alkaline pretreatment or physical methods, which shows the feasibility but high cost.13 Thermophilic AD (55 ℃) has also been used to accelerate the hydrolysis of straw and increase the methane yield,14 and to decrease the viscosity of percolate for an efficient recirculation.15 However, under thermophilic condition, the ammonia inhibition is also higher than the mesophilic AD, especially for the digestion of substrate with a high content of organic nitrogen (i.e. chicken manure and OFMSW). Thus, co-digestion of OFMSW and corn straw, gives an efficient and economical approach of adjustment of C/N and TS content and increasing of methane production.16,17 AD can be classified by several parameters, e.g. based on total solid (TS) content of the input substrate and the number of the reaction phases.18 The dry fermentation is operating with a TS content >15% (w/w), of which garage-type dry fermentation (GTDF) is a batch system and consists of the solid digester for loading of the solid substrate and the percolate tank for storing the percolate. It characterized by some significant technical advantages, i.e., less use of water, higher tolerance to impurities and lower energy consumption.19 Because of these advantages, 3


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GTDF is becoming more common in Europe, and to be considered as a favorable solution to treat the stackable non-free flowing materials, such as OFMSW and straw. The straw can be used as the structure materials to keep the porosity of the stacked substrate for mitigating the problem of poor biogas release and the permeation of percolate caused by the high water content of Chinese OFMSW, which is a significant factor for the successful operation of GTDF.20 Conventional GTDF is operated by mixing a percentage of solid digestate as inoculum with substrate at startup phase of AD, to guarantee optimal conditions for methanogenic bacteria, shorten the start-up process, and mitigate the inhibition caused by VFA accumulation.21-23 Analysis in literature indicates that the ratio of the solid digestate (as inoculum) to substrate showed a large variation ranging from 1:2 to 5:1(TS basis) depend on different kinds of substrates.24,25 The use of solid inoculum caused a loss in the volume capacity of digester, and further caused a low efficiency of biogas production. In order to avoid this disadvantage, the use of percolate (or liquid digestate) recirculation has been studied, which is benefit to mass transfer, moisture increasing of substrate, and inoculating the substrate. Percolate irrigation causes a washing effect which contributes to the removal of inhibitor factors from the solid biomass to liquid phase.26 Previous study reported that the hydrolysis and biogas production was positively affected when the FPR was performed higher than 2 times per day.27 Rico et al. reported that when the FPR was 1-1.25 and the percolate recirculation rate in intermittent was increased, the stability and speed of the process can be improved.

15

In contrast, some studies also found that

high volume of percolate recirculation led to an acidification in digester and a lower methane yield.28,29 According to the literature, the percolate recirculation also supplied additional microorganisms as an inoculation effect, while the percolate to substrate ratio showed a large variation between 0.5 and 3.1 (w/w). 9,30,31 Di Maria et al. considered the economic point of view, and reported that the optimal range of percolate to substrate ratio was from 1.5 to 2.5.32 It is widely agreed that the recirculation of percolate affects the AD performance of GTDF. The 4


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objective of the present study was to investigate the washing effect and inoculation effect presented by percolate recirculation, and how it affects the VFA generation and methane production of co-digestion of OFMSW and corn straw.

2. Materials and Methods

2.1 Substrate and inoculum OFMSW used in this study was collected from the waste bins placed in the campus of China University of Petroleum Beijing and a community in Changping District, Beijing. The organic fraction was separated manually and consisted of mainly food wastes (i.e. waste from restaurant, canteen, and fruit- & vegetable-market). The corn straw was obtained from a field in Wei County, Hebei Province. The corn straw consisted of the stalks and leaves. The impurities (e.g., sand, plastic, bone and others) from OFMSW and corn straw were manually moved out. OFMSW and corn straw were manually cut into particle size of less 2 cm individually. The inoculum used in this study was the digestate collected from Asuwei MSW biogas plant in Changping District, Beijing (15 km away from our laboratory). The solid and liquid digestate was gotten by a manually pressing separation through a 200-mesh nylon filter. The TS and volatile solid (VS) content was 16.33 ± 0.16% and 9.50 ± 0.76% for solid digestate, 4.52 ± 0.84% and 1.43 ± 0.21% for liquid digestate, respectively. 2.2 Analytical methods The TS and VS content of substrate and digestate were determined according to standard methods.33 The content of hemicellulose, cellulose, and lignin were determined by semiautomatic fiber analyzer (ANKOM A200i USA) according to the neutral detergent fiber (NDF), acid detergent fiber and lignin (ADF/ADL) analysis.34 The pH value was measured by pH meter (Mettler Toledo, FE30K). Total ammonia nitrogen (TAN) was analyzed by Spectrophotometer (Hach DR-2800, USA) and Hach test kit. Volatile fatty acids (VFA) and total inorganic carbon 5


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(TIC) were determined by automatic potentiometric titrator (Mettler Toledo, T70) according to the two-step titration method.35 Single VFA (acetic, propionic butyric, isobutyric, pentanoic and isopentanoic acid) was analyzed by gas chromatography (GC) (FULI, 9790Ⅱ) equipped with a flammable ionized detector (FID) and a DB-FFAP capillary column using argon gas (Ar) as carrier gas. Biogas was collected by aluminum foil gas bag and its volume was measured daily by wet tip gas meter (LMF-1) and was reported at standard temperature and pressure (i.e. STP=0℃ and 1.013 × 105 Pa). Biogas was sampled by a syringe for composition (CO2, CH4, N2, and O2) analysis by another GC (FULI, 9790Ⅱ) equipped with a thermal conductivity detector (TCD) and a 2m stainless column packed with Porapak Q (50/80 mesh). Argon gas was used as carrier gas at a flow rate of 30 mL/min. The operational temperature at the injection port, the column oven and the detector were 150℃, 130℃ and 160℃, respectively. The BMP tests were conducted in the Automatic Methane Potential Test System (Bioprocess, AMPTS II) under mesophilic conditions of 37 ℃ for 50 days. Cumulative VFA production expressed in mg was calculated as Cumulative VFA production (msum)= CD1*VD1 + CD1*VD1 +…+ CD45*VD45

(1)

where CD1 to CD45 represents the daily VFA concentration in mg/L; VD1 to VD45 represents the volume of the percolate, as the percolate recirculation is a closed system, the V is set as 500 mL (the volume of sample extraction is neglected). Ammonia-N concentration expressed in g/L was calculated as Ammonia-N concentration = 10pH-pKa*TAN/ (1+10pH-pKa) (2) where pKa = 9.2. 2.3 Lab-scale GTDF reactor

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The lab-scale GTDF reactor consisted of a digester and a percolate tank. The digester was made up of a vertical cylinder with a hermetic cap. The effective volume of the digester was 800 mL, with a height of 150 mm and an internal diameter of 100 mm (Fig. 1). On the cap, the holes were made for extracting the biogas by gas bag and for recirculating the percolate by a peristaltic pump (Longer, BT300-IF). The percolate was distributed evenly by a distribution system installed on the cap. To separate the solid particle from the percolate, a filter with a porosity of 3 mm was fitted at the bottom of digester. The biogas produced from the percolate tank was also collected to another gas bag. The percolate was collected at the bottom of digester and flowed into a percolate tank connected by silicone tube. The effective volume of percolate tank was 500 mL. 2.4 Experimental setup The experiments of co-digestion of OFMSW and corn straw were performed in the lab-scale GTDF reactor in two different runs. Run 1 was for investigating how the FPR (frequency of percolate recirculation) affected the VFA generation and biogas production. Each digester was loaded with 150 g of mixed substrate (120 g of OFMSW and 30 g of corn straw, based on wet weight). Run 2 was for investigating the effect of addition of solid digestate (as solid inoculum) on VFA generation and biogas production. The digester was loaded with 75 g of mixed substrate (60 g of OFMSW and 15 g of corn straw) and 75 g of solid digestate (based on wet weight). FPR was defined as the daily recirculation volume to the total percolate volume. In run 1, when the ratio of daily percolate recirculation volume to loading quantity (v/w) was set as 1:1, 2:1, 4:1, 8:1 and 16:1, the daily percolate recirculation volume was 150, 300, 600, 1200 and 2400 mL (compared with 150g of loading quantity in all digesters), which corresponded to five FPRs of 0.3 (R1), 0.6 (R2), 1.2 (R3), 2.4 (R4) and 4.8 (R5) (compared with the total percolate volume of 500mL), respectively. In run 2, a FPR of 1.2 was set for maintaining the humidity of substrate and promoting the mass transfer. The reactor was labelled as R3s. 7


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In all runs, the percolate recirculated as 4 minutes per hour and for 8 hours per day (in total 32 minutes per day). The experiments were conducted for 45 days. The digesters and percolate tanks were placed in the biochemical incubator under mesophilic condition (37℃). The percolate was sampled from the percolate tank to analyze the pH, VFA, single VFA, TIC and TAN. Biogas was collected by gas bags to measure its production and composition, and the biogas/methane yield was gotten by dividing its volume with the amount of VS of substrates. 2.5 Statistical analysis All the physical and chemical analyses were performed in triplicate and the mean values and standard errors were presented. The statistical differences were analyzed by MS Excel 2010.

3. Results and Discussion

3.1 Characteristics of substrate and digestate Table 1 presents the characterization of OFMSW and corn straw in term of TS, VS, elemental compositions and BMP. The collected OFMSW was at an acid pH of 6.34 and a VFA concentration of 1243 ± 128 mg/L, which indicated that the OFMSW degraded in waste bins before it was collected. The OFMSW was characterized by a low TS and C/N ratio. Corn straw used in this study was dry straw, with TS of 84.4% and a higher C/N ratio. Co-digestion of OFMSW and corn straw gave an approach to adjust the C/N ratio and TS content. The corn straw shows a high lignin content. Bioconversion of lignin to biogas is a problem, but this is beneficial to keep the porosity structure of the stacked substrate in digester which is required for biogas release and percolate permeation in GTDF system. Considering all these factors, the ratio of OFMSW to corn straw was set at a ratio of 4:1. Theoretical calculation showed that this ratio provided a C/N ratio of approximately 24.1, TS of 30.6% and density of co-substrate of 0.24 g/cm3.

8


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3.2 Effect of different FPR on VFA generation 3.2.1 VFA generation The VFA and TIC concentration (Fig.2a), pH (Fig.2b) and TAN (Fig.2c) of the percolate was analyzed in order to study the effects of different FPR on VFA generation. At the startup phase, the measurement was more frequent (every 2 days), and then the frequency was adjusted to every 4-6 days after 21 days of experiment. The initial VFA and TIC concentration of liquid-digestate was 593±31 mg/L and 6712±42 mg CaCO3/L. After the recirculation process started, the VFA concentration increased sharply in first 7 days. The peak of VFA concentration of 17,691 mg/L was reached when the FPR was 4.8 times (R5), followed by 9,080 mg/L in R4, 8,497 mg/L in R3, 7,686 mg/L in R2 and 7,427 mg/L in R1. These results showed that the higher was the FPR, the higher was the peak of VFA concentration. In R5, The pH of percolate decreased to an acid pH of 5.74. The gas production was only observed in the first three days, and it consisted of a large proportion of CO2 (>50%) and H2 (>20%), but no methane was detected. An acidification phenomenon in percolate was observed in R5, and then the experiment was terminated at day 13th. For the FPR of 0.3, 0.6 and 1.2 (R1-R3), the peak of VFA concentration was reached at day 7, but FPR of 2.4 times was at day 3. Afterwards, VFA concentration started to decrease in all reactors. The cumulative VFA production (calculated based on Eq.1) in R1-R5 was 18665 mg, 18172 mg, 18431 mg, 21822 mg and 40806 mg, respectively. Analysis of variance, the cumulative VFA production in R1-R4 was similar (p-value > 0.05), but the difference between R4 and R5 was significant with a p-value of 0.027 (< 0.05). It indicated that the VFA generation was not significantly increased when the FPR was less than 2.4. 3.2.2 Influence of TAN on the buffer The VFA/TIC was used as an indicator of process stability. The peak of VFA/TIC was observed at the startup phase, afterwards, the VFA/TIC gradually recovered to a normal range

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after day 13. In the first 11 days of AD, the VFA/TIC was >1, but a neutral pH was kept (Fig. 2b). An increasing of TAN concentration in the percolate during the AD process was observed. TAN tended to decrease at the beginning, with the exception of the treatment FPR of 1.2. TAN increased because of N release during degradation of organic material. In this study, the TAN concentration increased from initial concentration of 1.26g/L to 1.39-1.54 g/L (R1-R4, respectively) at the end of experiment. Ammonia released from organic material acts as a base, dependent on the pH. The ammonia can be transferred to ammonium, and as a consequence this provides the buffer. According to Eq. (2), the initial ammonia concentration in percolate was 52 mg/L. In order to investigate the buffer contribution of ammonia, ammonia concentration in R1-R4 at day 13 was calculated, because since day 13, the VFA/TIC was recovered to a normal range in all reactors. At day 13, the ammonia concentration in percolate was 58 mg/L in R1, 71 mg/L in R2, 67 mg/L in R3, and 77 mg/L in R4, which contributed to the buffer of 170 mg CaCO3/L in R1, 209 mg CaCO3/L in R2, 198 mg CaCO3/L in R3, and 225 mg CaCO3/L in R4. These results indicated that the production of ammonia could lead to an increasing of buffer system and contribute to the pH at a neutral level.27 3.2.3 Single VFA analysis The single VFA in R2 and R4 was analyzed and showed as percentage in Fig. 3. The overall trend in R2 and R4 was similar, that acetic acid, butyric acid and propionic acid were the main acidogenesis products. A major variation of acetic acid was observed at the startup phase (first 3 days), and then the concentration of acetic acid and butyric acid decreased gradually and tended to stabilize until day 25. Afterwards, the propionic acid and butyric acid increased and acetic acid cannot be detected in percolate, correspondingly the biogas production slowed down (Fig. 4). The production and accumulation of propionic acid could cause a failure of AD.36 The acetic acid and butyric acid was about 52.7-85.7% in R2 and was about 68.8-83.1% in R4 during the first 25 10


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days, which showed typical butyric acid fermentation.37 Comparing between R2 and R4, the percentage of butyric acid in R4 was higher than that in R2. In R2, the percentage of butyric acid was from 3.5% to 6.4% (exception of the data on day 12 of 12.25%) from day 3 to day 18, and the percentage pf acetic acid during this period was 49.1%-65.2%. In R4, the percentage of butyric acid was from 15.2% to 27.8% (exception of the data on day 15 of 4.6%) from day 3 to day 18, and the percentage of acetic acid during this period was 45.8%-54.8%. These findings indicated that the conversion from butyric acid to acetic acid was lower in R4 than in R2. The reason is probably the low biological activity of acetogens caused by the lower pH in R4, as showed in Fig. 2b. It is well known that the acetogens are sensitive to pH. These findings indicated the percolate recirculation can positively influence the hydrolysis and acidogenesis caused by its inoculation effect. Analysis in literature showed that the promotion of hydrolysis and acidogenesis by percolate recirculation is attributed to different mechanisms, i.e. the increasing of moisture of substrate, improvement of mass transfer, 23 and enhancement of incubation.9 Higher ratio of FPR provided more microorganisms into the digester in a short time, and the increasing of moisture content also improved the conditions for the growth of microorganisms. Additionally, the high FPR also contributed to the mass transfer and to enhance the distribution of microorganisms, and the substrate at the bottom of digester had more chance to contact microorganisms. When the FPR was conducted at a high rate, the washing effect also presented.26 The VFA generated from digester was moved to percolate in a shorter time, which led to a fast VFA accumulation and a high peak of VFA concentration. 3.3 Effect of different FPR on methane production The curves of methane production in R1-R4 (Fig. 4) showed similar trends that a lag phase of about 7 days was found. During the lag phase methane concentration in biogas was low. It started to gradually increase after 7 days and reached 50% after 22 days, and then the methane concentration remained stable between 55% and 59% in all the reactors till the end of AD. The 11


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fast methane production started when VFA concentration decreased and TIC increased. As Figs. 2-3 showed, the higher was the peak of VFA concentration (higher VFA/TIC ratio), the slower and less was the methane production. The methane production, in term of the quantity and rate, it was R1＞R2＞R3> R4, which was attributed to a fast recovery of pH and VFA/TIC ratio. The peak of daily methane yield in R1 (7.24 mL/d/gVS) and R2 (7.06 mL/d/gVS) was higher than in R3 (5.17 mL/d/gVS) and R4 (4.78 mL/d/gVS), and the peak of methane yield also resulted in the highest cumulative methane production in R1 (7.050 L), followed by R2 (6.572 L), R3 (4.971 L) and R4 (4.020 L) (Table 3). An opposite results on cumulative biogas production in R1 and R2 was showed in Fig. 4b, that the cumulative biogas production in R1 (14.82 L) was lower than in R2 (15.39 L). The cumulative biogas production in R3 and R4 was 11.24 L and 9.69 L, respectively, which were lower than R1 and R2. This phenomenon displayed that reactor with higher ratio of FPR had lower methane production. The inhibition of methanogens caused by accumulation of VFA38 and the low conversion of butyric acid to acetic acid might be the reasons for the low methane production. BMP test of OFMSW and corn straw are showed in Table 1. Based on the BMP results, theoretic BMP of the co-substrate was calculated as 249.67 mL CH4/gVS. Compared to this value, all the reactors showed lower methane yields. The maximum methane yield was 166.33 mL CH4/gVS in R1, which correspond to 66.6% of the theoretic BMP. It is well known that the insufficient mass transfer of dry fermentation, compared to the wet fermentation, is the main reason for the low biogas yield, which could explain the low biogas yield in all the reactors. In some previous studies, the methane yield of Chinese OFMSW was reported from 245 to 518 mL CH4/gVS, and the methane yield of corn straw was from 125 to 466 mL CH4/gVS in China.6, 12, 39 However, these studies were all conducted in a wet AD system. So far, very few studies were conducted on dry anaerobic co-digestion of OFMSW and corn straw. Some studies on dry codigestion of similar substrate were reported. Xu and Li reported that the methane yield of co12


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digestion of expired dog food and corn stover was 304.4 L/kgVS.40 Massaccesi et al. found that co-digestion of OFMSW and bark can get a methane yield of 55.62 L/kgVS.41 Zhu et al. studied the dry AD of corn stover and a methane yield of 222 L/kgVS was achieved.42 The results in our study showed a comparable methane yield to previous studies. The characteristics of the co-substrate of OFMSW and corn straw were measured before and after the AD process (Table 2). The initial TS and VS of input (co-substrate) were analyzed, and ranged from 31.56 ± 0.78 % to 33.42 ± 0.66 % of TS, and from 29.50 ± 0.76% to 30.77 ± 0.42% of VS, in R1-R4, respectively. 150 g of the co-substrate was filling into the digester with a given height of 8 cm, and the stack density of the co-substrate was calculated about 0.24 g/cm3. After the AD process, the same parameters of output (the solid digestate from digester) were analyzed again. Results showed that, (1) the height of the stack decreased from 8 cm to 5-5.3 cm; (2) the fresh mass of the substrate increased from 150g to 183.42-216.6 g; and (3) the density of the substrate also increased from 0.24 g/cm3 to 0.45-0.52 g/cm3. These changes were due to a water content increasing through percolate recirculation. Cumulative methane production in digester and percolate tank was measured individually (Table 3). The percentage of methane produced from percolate tank to the total methane production was about 38.1% in R4, and in R1-R3 the percentage was only 26.1%, 25.7% and 25.5%, respectively. This phenomenon indicated that increasing of FPR caused a higher proportion of the methane produced from the percolate tank. The reason might be the washing effect discussed above. Compared with R1-R3, more VFA produced from digester was flushed into the percolate tank by a stronger washing effect in R4, which led to more methane produced from percolate tank. 3.4 Effect of addition of solid inoculum on VFA generation and methane production In order to increase the methanogens and stabilize the AD process of GTDF, traditional

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inoculation method of addition of solid digestate at startup phase was common reported. In this paper, the results of VFA generation and biogas production during the AD process with inoculation by addition of solid digestate and by mono-inoculation of percolate recirculation were compared, to investigate the inoculation effect of percolate recirculation (Fig. 5). In R3s, the percolate recirculation was also conducted with a FPR of 1.2, to keep the water content and mass transfer. As can be seen in Fig. 5a, at day 8, VFA concentration decreased to 3540 mg/L and the TIC concentration increased to 9800 mg/L. VFA/TIC ratio of 0.36 was in an appropriate range (i.e. less than 0.4), which showed a stable AD process.6 Compared with the result of R3, that the VFA concentration decreased to 2246 mg/L and TIC increased to 7963 mg/L, and the VFA/TIC of 0.28 was reached at day 16 (results in R1, R2 and R4 were similar). The peak of VFA concentration was 8497 mg/L in R3 and 8923 mg/L in R3s, and the cumulative VFA production was 18431 mg in R3 and 18576 mg in R3s, which did not show a significant difference. These results indicated that the addition of solid digestate could contribute to the stability of GTDF process, in terms of pH and VFA/TIC ratio, one possible reason might be that the solid digestate provided a higher initial alkalinity and microorganism quantity, compared with the provision from liquid digestate.23 A conclusion can also be raised that the percolate recirculation showed a similar inoculation effect to the addition of solid digestate, in term of the hydrolytic and acidogenic bacteria, because of the similar results about the peak concentration and cumulative production of VFA between R3 and R3s. In R3s the biogas production started from the 1st day of AD process, which showed a shorter lag phase than in R3. However, the biogas quality, in term of methane concentration, was very poor during the first 22 days in R3s. The methane concentration started to more than 50% after 22 days, which showed the same result as in R3. After 45 days of AD, total methane production in R3s was 3.11L, and after a calculation, the methane yield was 145.90 mL CH4/gVS. Compared with the methane yield of 117.15 mL CH4/gVS in R3, addition of solid digestate caused the 14


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increasing of methane yield by 19.7%. However, it was also found that the methane yield in R3s was 12.3% less than in R1 (166.33 mL CH4/gVS), which indicated that an appropriate inoculation by only recirculation of percolate can reach a comparable result compared with the inoculation by addition of solid digestate.43 In R3s, The cumulative methane production in digester and percolate tank was analyzed individually. Approximately 0.32 L of methane was produced from percolate tank, and the ratio of methane production from percolate tank to the total methane production was only 10.3%, which showed a big less than the reactors without the inoculation of solid digestate (data showed in chapter 3.3). This result indicated that addition of solid digestate promoted the methane production in digester. The reason might be that the solid digestate provided more methanogens which promote a faster conversion from VFA to methane in digester, and less VFA was flushed into the percolate tank.

4. Conclusions

In this study, GTDF was used to investigated the effect of frequency of percolate recirculation (FPR) on methane production from co-digestion of OFMSW and corn straw. Results showed that FPR could play positive effects on the hydrolysis and acidogenesis, but it also caused the accumulation of VFA and incresaed the peak of VFA concentration. These results led to negative effects on acetic acid conversion and methane production. Highest methane yield of 166.33 mL CH4/gVS was obtained at FPR of 0.3. Tranditional GTDF with an inoculation by addition of solid digestate was conducted as well, which showed its advantages on stability and a short lag phase, but the methane yield of 145.90 mL CH4/gVS showed a similar result to the mono-inoculation of percolate recirculation. These results indicated that only using the percolate recirculation can be a feasible inoculation way. 15


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5. Acknowledgements

This work was supported by National Natural Science Foundation of China (No. 51508572), Science Foundation of China University of Petroleum, Beijing (No. 2462014YJRC034 & No. C201604) and Co-construction Project of Beijing Municipal Education Commission.

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6. References

[1] Ministry of Housing and Urban-Rural Development of the People’s Republic of China, China

Urban-Rural

Construction

Statistical

Yearbook;

http://www.mohurd.gov.cn/xytj/tjzljsxytjgb/index.html. 2014 [2] National Bureau of Statistics of the People’s Republic of China, China Statistical Yearbook; http://www.stats.gov.cn/tjsj/ndsj/2016/indexch.htm. 2016 [3] Zhu,

N.Z.,

Xinhuanet

China

unveils

land-centered

urbanization

plan;

http://news.xinhuanet.com/english/china/2014-03/16/c_133190495.htm. 2014. [4] Zhou, H.; Meng, A.H.; Long, Q.H.; Zhang, Y.G. An overview of characteristics of municipal solid waste fuel in China: Physical, chemical composition and heating value. Renew Sustain Energy Rev. 2014, 36, 107-122. [5] He, P.J.; Zhang, H.; Lv, F.; Shao, L.M. Treatment of household waste in small towns of China: status, basic conditions and appropriate modes. J Agri Resource Environ. 2015, 32(2), 116-120 (in Chinese with English abstract). [6] Liu, X.; Gao, X.B.; Wang, W.; Zheng, L.; Zhou, Y.J.; Sun, Y.F. Polit-scale anaerobic codigestion of municipal biomass waste: focusing on biogas production and GHG reduction. Renew Energy. 2012, 44, 463-468. [7] Pazera, A.; Slezak, R.; Krzystek, L.; Ledakowicz, S.; Bochmann, G.; Gabauer, W.; Helm, S.; Reitmeier, S.; Marley, L.; Gorga, F.; Farrant, V.S.; Kara, J. Biogas in Europe: food and beverage (FAB) waste potential for biogas production. Energy Fuels. 2015, 29, 4011-4021. [8] Browne, J.D.; Allen, E.; Murphy, J.D. Improving hydrolysis of food waste in a leach bed reactor. Waste Manage. 2013, 33(11), 2470-2477. [9] Xu, S.Y.; Karthikeyan, O.P.; Selvam, A.; Wong, J.W.C. Microbial community distribution and extracellular enzyme activities in leach bed reactor treating food waste: effect of

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different leachate recirculation practices. Bioresour Technol. 2014, 168, 41-48. [10] Cabbai, V.; Ballico, M.; Aneggi, E.; Goi, D. BMP tests of source selecteed OFMSW to evaluate anaerobic codigestion with sewage sludge. Waste Manage. 2013, 33, 1626-1632. [11] Wang, X.M.; Li, Z.F.; Zhou, X.Q.; Wang, Q.Q.; Wu, Y.; Saino, M.; Bai, X. Study on the biomethane yield and microbial community structure in enzyme enhanced anaerobic codigestion of cow manure and corn straw. Bioresour Technol. 2016, 219, 150-157. [12] Croce, S.; Wei, Q.; D’Imporzano, G.; Dong, R.J.; Adani, F. Anaerobic digestion of straw and corn stover: the effect of biological process optimization and pre-treatment on total biomethane yield and energy performance. Biotechnol Advances. 2016, 34, 1289-1304. [13] Li, Y.Q.; Zhang, R.H.; He, Y.F.; Liu, X.Y.; Chen, C.; Liu, G.Q. Thermophilic solid-state anaerobic digestion of alkaline-pretreated corn stover. Energy Fuels. 2014, 28, 3759-3765. [14] Fu, S.F.; Wang, F.; Yuan, X.Z.; Yang, Z.M.; Luo, S.J.; Wang, C.S.; Guo, R.B. The thermophilic (55 ℃ ) microaerobic pretreatment of corn straw for anaerobic digestion., Bioresour Technol. 2015, 175, 203-208. [15] Rico, C.; Montes, J.A.; Muñoz, N.; Rico, J.L. Thermophilic anaerobic digestion of the screened solid fraction of dairy manure in a solid-phase percolating reactor system. J.Cleaner Production, 2015, 102, 512-520. [16] Li, Y.Q.; Zhang, R.H.; Liu, X.Y.; Chen, C.; Xiao, X.; Feng, L.; He, Y.F.; Liu, G.Q. Evaluating methane production from anaerobic mono- and co-digestion of kitchen waste, corn stover, and chicken manure. Energy Fuels. 2013, 27, 2085-2091. [17] Yong, Z.H.; Dong, Y.L.; Zhang, X.; Tan, T.W. Anaerobic co-digestion of food waste and straw for biogas production. Renew Energy. 2015, 78, 527-530. [18] Madsen, M.; Holm-Nielsen, J.B.; Esbensen, K.H. Monitoring of anaerobic digestion process: a review prospective. Renew. Sust. Energy Rev. 2011, 15, 3141-3155. [19] Li, Y.B.; Park, S.Y.; Zhu, J.Y. Solid-state anaerobic digestion for methane production from 18


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organic waste. Renewable Sustainable Energy Rev. 2011, 15, 821-826. [20] Yazdani, R.; Barlaz, M.A.; Augenstein, D.; Kayhanian, M.; Tchobanoglous, G. Performance evaluation of an anaerobic/aerobic landfill-based digester using yard waste for energy and compost production. Waste Manage. 2012, 32, 912-919. [21] Pognani, M.; D’Imporzano, G.; Minetti, C.; Scotti, S.; Adani, F. Optimization of solid state anaerobic digestion of the OFMSW by digestate recirculation: a new approach. Waste Manage. 2015, 35, 111-118. [22] Schievano, A.; D’Imporzano, G.; Malagutti, L.; Fragali, E.; Ruboni, G.; Adani, F. Evaluation inhibition condition in high-solid anaerobic digestion of organic fraction of municipal solid waste. Bioresource Technol. 2010, 101, 5728-5732. [23] Yang, L.C.; Xu, F.Q.; Ge, X.M.; Li, Y.B. Challenges and strategies for solid-state anaerobic digestion of lignocellulosic biomass Renew Sustain Energy Rev. 2015, 44, 824-834. [24] Wu, D.; Lv, F.; Shao, L.M.; He, P.J. Effect of cycle digestion time and solid-liquid separation on digestate recirculated one-stage dry anaerobic digestion: Use of intact polar lipid analysis for microbes monitoring to enhance process evaluation. Renew Energy. 2017, 103, 38-48. [25] Xu, F.Q.; Wang, F.; Lin, L.; Li, Y.B. Comparison of digestate from solid anaerobic digesters and dewatered effluent from liquid anaerobic digesters as inocula for solid state anaerobic digestion of yard trimmings. Bioresour Technol. 2016, 200, 753-760. [26] Michele, P.; Giuliana, D.; Carlo, M.; Sergio, S.; Fabrizio, A. Optimization of solid state anaerobic digestion of the OFMSW by digestate recirculation: a new approach. Waste Manage. 2015, 35, 111-118. [27] Pezzolla, D.; Di Maria, F.; Zadra, C.; Massaccesi, L.; Sordi, A.; Gigliotti, G. Optimization of solid-state anaerobic digestion through the percolate recirculation. Biomass and Bioenergy. 2017, 96, 112-118. [28] Cysneiros, D.; Banks, C.J.; Heaven, S.; Karatzas, K.-A.G. The effect of pH control and 19


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‘hydraulic flush’ on hydrolysis and volatile fatty acids (VFA) production and profile in anaerobic leach bed reactors digesting a high solids content substrate. Bioresour. Technol. 2012, 123, 263-271. [29] Degueurce, A.; Trémier, A.; Peu, P. Dynamic effect of leachate recirculation on batch mode solid state anaerobic digestion: Influence of recirculated volume, leachate to substrate ratio and recirculation periodicity. Bioresour Technol. 2016, 216, 553-561. [30] Korazbekova. K.U.; Bakhov, Z.K. Performance of leach-bed reactor with immobilization of microorganisms in terms of methane production kinetics. J. Biol. Sci. 2014, 14(4), 258-266. [31] Kusch, S.; Oechsner, H.; Jungbluth, T. Effect of various leachate recirculation strategies on batch anaerobic digestion of solid substrates. Int. J. Environ. Waste Manage. 2012, 9(1-2), 69-88. [32] Di Maria, F.; Sordi, A.; Micale C. Optimization of solid state anaerobic digestion by inoculums recirculation: the case of an existing mechanical biological treatment plant. Appl Energy. 2012, 97, 462-469. [33] APHA. Standard Methods for the Examination of Water and Wastewater, 21th ed. American Public Health Association. Washington DC, 2005. [34] Van Soest, J.; Robertson, P.V.; Lewis, B. Methods for dietary fiber, neutral detergent fiber, and nonstarch polysaccharides in relation to animal nutrition, J. Dairy Sci. 1991, 74 (10) 583-3597. [35] McGhee, T.J. A method for approximation of the volatile acid concentrations in anaerobic digesters. Water Sewage Works. 1968, 115, 162-166. [36] Pullammanappallil, P.C.; Chynoweth, D.P.; Lyberatos, G.; Svoronos, S.A. Stable performance of anaerobic digestion in the presence of a high concentration of propionic acid. Bioresour. Technol. 2001, 98, 1774-1780. [37] Ren, N.; Liu, M.; Wang, A.; Ding, J.; Li, H. Organic acids conversion in methanogenic-phase 20


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reactor of the two-phase anaerobic process. Chin. Environ. Sci. 2003, 24, 89. [38] Brown, D.; Li, Y.B. Solid state anaerobic co-digestion of yard waste and food waste for biogas production. Bioresource Technol. 2013, 127, 275-280. [39] Liu, G.Q.; Zhang, R.H.; EI-Mashad, H.M.; Dong, R.J. Effect of feed in inoculum ratios on biogas yields of food and green wastes. Bioresour Technol. 2009, 100, 5103-5108. [40] Xu, F.; Li, Y.B. Solid-state co-digestion of expired dog food and corn stover for methane production. Bioresour Technol. 2012, 118, 219-226. [41] Massaccesi, L.; Sordi, A.; Micale, C.; Cucina, M.; Zadra, C.; Di Maria, F.; Gigliotti, G. Chemical characterisation of percolate and digestate during the hybrid solid anaerobic digestion batch process. Process Biochem. 2013, 48, 1361-1367. [42] Zhu, J.Y.; Yang, L.C.; Li, Y.B. Comparison of premixing methods for solid-state anaerobic digestion of corn stover. Bioresour Technol. 2015, 175, 430-435. [43] Kusch, S.; Oechsner, H.; Jungbluth, T. Biogas production with horse dung in solid-phase digestion systems. Bioresour Technol. 2008, 99, 1280-1292.

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Table Table 1 chemical characterization of OFMSW and corn straw a

a

Parameter

OFMSW

Corn straw

pH

6.34 ± 0.02

n.d.

TS (%)

17.18 ± 0.31

84.40 ± 0.51

VS (%)

15.04 ± 0.28

80.12 ± 0.42

Carbon (% of TS)

44.37 ± 1.55

60.80 ± 0.30

Nitrogen (% of TS)

3.49 ± 0.51

1.19 ± 0.21

Cellulose (% of TS)

n.d.

36.44

Hemicellulose (% of TS)

n.d.

27.82

Lignin (% of TS)

n.d.

4.28

VFA (mg/L)

1243 ± 128

n.d.

TIC (mg CaCO3/L)

430 ± 27

n.d.

BMP (mL/gVS)

293.44 ± 63.72

216.79 ± 27.21

n.d., not determined. except for cellulose, hemicellulose and lignin, each value represents the average of three replications ± standard error.

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Table 2 characteristics of co-substrate between OFMSW and corn straw before and after AD process Group R1

R2

R3

R4

TS (%)-substrate

33.22 ± 0.51

32.42 ± 0.66

31.56 ± 0.78

32.81 ± 0.89

VS (%)-substrate

30.29 ± 0.91

30.77 ± 0.42

29.59 ± 0.95

29.50 ± 0.76

Height (cm)

8

Weight (g)

150

Density (g/cm3)a

0.24

Input

Output TS (%)-digestate

18.49 ± 1.28

19.92 ± 1.06

18.68 ± 1.30

20.35 ± 1.01

VS (%)-digestate

9.71 ± 0.40

9.73 ± 0.71

12.26 ± 0.27

10.53 ± 0.72

Height (cm)

5.3 ± 0.2

5.0 ± 0.1

5.3 ± 0.1

5.2 ± 0.2

Weight (g)

216.60 ± 8.2

183.76 ± 10.5

193.51 ± 7.7

213.42 ± 11.9

Density (g/cm3) a

0.52

0.47

0.46

0.45

a the density of co-substrate was based on calculation

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Table 3 cumulative methane production in digester and percolate tank Group Methane production (L)

R1

R2

R3

R4

Digester

5.210 ±0.27

4.882 ±0.24

3.701 ±0.15

2.490 ±0.22

Percolate tank

1.840 ±0.06

1.690 ±0.08

1.270 ±0.06

1.530 ±0.05

In total

7.050 ±0.33

6.572 ±0.32

4.971 ±0.21

4.020 ±0.27

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Figures

Fig.1 Schematic diagram of lab-scale GTDF reactor (1.percolate tank, 2.digester, 3.distribution system, 4. peristaltic pump, 5.gas bag), biogas flow:

, liquid flow:

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Fig. 2. The operational parameters in R1-R5 under different FPR, (a) VFA and TIC trends (■ VFA FPR=0.3, ☆VFA FPR=0.6, ◇VFA FPR=1.2, ▲VFA FPR=2.4, ▼VFA FPR=4.8; □TIC FPR=0.3, ○TIC FPR=0.6, ×TIC FPR=1.2, △TIC FPR=2.4, ▽TIC FPR=4.8); (b) pH trends (■ FPR=0.3, ○FPR=0.6, ×FPR=1.2, ▲FPR=2.4; ★FPR=4.8); and (c) total ammonia nitrogen (TAN) trends (■FPR=0.3, ○FPR=0.6, ×FPR=1.2, ▲FPR=2.4).

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Fig. 3. Volatile Fatty Acids (VFA) speciation in the percolate in R2 and R4.

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Fig. 4. In R1-R4 under different FPR, cumulative biogas production (■FPR=0.3, ●FPR=0.6, ◆ FPR=1.2, ▲FPR=2.4) and methane production (□FPR=0.3, ○FPR=0.6, ◇FPR=1.2, △ FPR=2.4).

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Fig. 5. The operational parameters in R3s (a, ▲pH, ▼TAN, ■VFA, ○TIC) and methane/biogas production (b, ■cumulative biogas production, ○cumulative methane production)

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