Bioresource Technology 153 (2014) 131–136
Contents lists available at ScienceDirect
Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Digestion and dewatering characteristics of waste activated sludge treated by an anaerobic biofilm system Tianfeng Wang a, Liming Shao b,c, Tianshui Li a, Fan Lü a,b, Pinjing He a,b,c,⇑ a
State Key Laboratory of Pollution Control & Resource Reuse, Tongji University, Shanghai 200092, PR China Institute of Waste Treatment and Reclamation, Tongji University, Shanghai 200092, PR China c Centre for the Technology Research and Training on Household Waste in Small Towns & Rural Area, Ministry of Housing and Urban-Rural Development of PR China (MOHURD), PR China b
h i g h l i g h t s Filler enhance conversion rates of sludge digestion. Pre-incubated filler improve dewaterability of digested sludge. Filler alone do not improve dewaterability of digested sludge.
a r t i c l e
i n f o
Article history: Received 7 October 2013 Received in revised form 20 November 2013 Accepted 24 November 2013 Available online 1 December 2013 Keywords: Waste activated sludge Anaerobic digestion Microorganism immobilization Filler Dewaterability
a b s t r a c t Immobilization of microorganisms for sludge anaerobic digestion was investigated in this study. The effects of filler properties on anaerobic digestion and dewaterability of waste activated sludge were assessed at mesophilic temperature in batch mode. The results showed that the duration of the methanogenic stage of reactors without filler, with only filler, and with pre-incubated filler was 39 days, 19 days and 13 days, respectively, during which time the protein was degraded by 45.0%, 29.4% and 30.0%, and the corresponding methane yield was 193.9, 107.2 and 108.2 mL/g volatile suspended solids added, respectively. On day 39, the final protein degradation efficiency of the three reactors was 45.0%, 40.9% and 42.0%, respectively. The results of normalized capillary suction time and specific resistance to filtration suggested that the reactor incorporating pre-incubated filler could improve the dewaterability of digested sludge, while the effect of the reactor incorporating only filler on sludge dewaterability was uncertain. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Owing to the high content of water and putrescible organic matter, waste activated sludge (WAS) generated from wastewater treatment plants must undergo some treatment to enhance its stability and reduce its corresponding volumes prior to final disposal. WAS treatment via anaerobic digestion (AD) results in a reduction of the sludge solids amount and a decrease in odor while produce methane (Duan et al., 2012; Young et al., 2013). Nevertheless, a long hydraulic retention time (HRT) is needed for sludge anaerobic digestion. Immobilization of microorganism can decrease the possibility of microbe losses, prolong solids retention times (SRT), benefit the formation and maintenance of preponderant organism ⇑ Corresponding author at: Institute of Waste Treatment and Reclamation, Tongji University, Shanghai 200092, PR China. Tel./fax: +86 21 65986104. E-mail addresses: [email protected], [email protected] (P. He). 0960-8524/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.biortech.2013.11.066
morphology, since large amounts of high-activity biomass are intercepted in biological treatment systems (Wang et al., 2010). Until recently, microorganism immobilization in anaerobic digester has mainly focused on liquid wastewater with low content of suspended solids (Ozgun et al., 2013). Anaerobic granulation and anaerobic filler biofilms are the main methods for microorganism immobilization (Wang et al., 2010). Comparatively, microorganism immobilization has seldom been investigated and applied to waste with high content of suspended solids owing to strict operating and separating condition. Wang et al. (2010) incorporated polyurethane foam matrices into an anaerobic sequencing batch reactor for the treatment of thermally hydrolyzed municipal biowaste (food waste, fruit–vegetable waste and dewatered sewage sludge), expecting to provide an adequate environment for microbe growth and retention, and to increase the possibility of high-efficiency anaerobic conversion. Gong et al. (2011) incorporated activated carbon fiber into an anaerobic reactor for treatment of cattle manure and obtained higher biogas and methane production than a control blank reactor
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over the long term. Romano and Zhang (2008) incorporated polyethylene cylinders into an anaerobic mixed biofilm reactor for treatment of mixture of onion juice and wastewater sludge and obtained efficient volatile solid reduction. These studies focused on the biowastes that were easily biodegradable or had been prehydrolyzed. In contrast, sludge organics were mainly distributed in the solid phase, which was generally unsuitable for application of the method of microorganism immobilization. Few relevant studies for sludge digestion by this method have been conducted. Nevertheless, since microorganism immobilization can enhance the degradation rate of dissolved organics and the degradation efficiency of total organics, it is possible to improve the dewaterability of digested sludge, which deserves detailed research. In this study, three identical reactors, one without filler, one with only filler, and one with pre-incubated filler, were used to anaerobically digest WAS. Dissolved organic matter (DOM), methane yield, particle size, extracellular polymeric substances (EPS), normalized capillary suction time (CST) and specific resistance to filtration (SRF) were used to investigate the effects of polyester nonwoven fabric as the filler on anaerobic digestion and dewatering characteristics of the digested WAS.
2. Methods 2.1. Sewage sludge WAS was obtained from the sludge thickener of a local domestic wastewater treatment plant in Shanghai, China. The capacity of the plant was 75,000 m3/day and it used an anaerobic–anoxic–oxic process. The service population of the waste water treatment plant was about 200,000. The total suspended solids (TSS), volatile suspended solids (VSS) and protein in the WAS were 29,217 ± 20 mg/L, 20,219 ± 20 mg/L and 11,348.0 ± 15.7 mg/L respectively.
2.2. Experimental setup 2.2.1. Anaerobic reactors Three identical reactors were constructed of cylindrical plexiglass columns (Fig. 1), each having an internal dimension of 150 mm, a total height of 1500 mm, and a working volume of 15 L, one without filler (R1), one with only filler (R2), and one with filler that had been pre-incubated over 60 days (R3). During the pre-incubation period, R3 reactor was fed with glucose and sodium acetate with an organic loading rate (OLR) of 1 g COD/(Ld) and an HRT of 15 day and continuously produced methane over 15 days. Its methane production was near zero before sludge digestion. Three reactors were operated at temperatures of 35 ± 1 °C with 15 L WAS respectively, while keep the sludge suspension recycled internally in reactors by a peristaltic pump at a downward flow rate of 1.2 L/min.
2.2.2. Filler characteristics The filler in anaerobic reactors was wear-resistant and corrosion-resistant polyester nonwoven fabric that had been prewashed with distilled water until the concentration of dissolved organic carbon in (DOC) its eluate was below 1 mg/L by submerging 2.0 g filler in 100 mL distilled water and shaking at 120 rpm for 12 h. The dimensions of filler were about 5 mm 15 mm 15 mm, with a porosity of 96.8 ± 0.2% and BET surface area of 0.2063 m2/g. The total quantity of filler in R2 and R3 was 140.9 and 138.7 g respectively.
Fig. 1. Sketch of down-flow non-woven biofilm reactor.
2.3. Analytical methods The sludge samples were collected from the sampling port (on the top of reactors) using a peristaltic pump. The sludge samples were first centrifuged at 2000g for 10 min after which the supernatant was filtered through a 0.45 lm microfiber filter. The filtrate corresponded to the liquid samples in this study. 2.3.1. EPS extraction The sludge samples were subjected to ultrasound at 20 kHz and 480 W for 10 min in an ice bath. Following ultrasound, the suspensions were centrifuged at 12,000g for 10 min. The bulk solution was then collected as the EPS. The EPS was filtered through a 0.45 lm microfiber filter before analysis. 2.3.2. Physico-chemical analysis DOC, inorganic carbon (IC) and total nitrogen (TN) of liquid samples were analyzed using a TC/TN analyzer (TOC-V CPN, TNM-1, Shimadzu, Japan). Kjeldahl nitrogen (KN) and ammonia nitrogen (AN) were analyzed using an auto Kjeldahl determination system (8400, FOSS, Sweden) for sludge samples and liquid samples. Protein of sludge, EPS and liquid sample were calculated by 6.25 multiplying the concentration of organic nitrogen (ON), which determined by subtracting AN from KN. Polysaccharide of EPS and liquid sample were measured by the anthrone method using
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glucose as a standard (Gaudy, 1962). Volatile fatty acids (VFA) of liquid sample and gas composition of biogas were measured using an Agilent 6980N (Agilent, USA) and a Kechuang GC9800 (Kechuang, China) gas chromatograph, respectively. Total suspended solids (TSS) and volatile suspended solids (VSS) were measured according to the Standard Methods (APHA, 1998). The particle size of the sludge flocs was determined with an EyeTech instrument (EyeTech, Ankersmid, USA). Sludge dewaterability was obtained using a capillary suction time (CST) instrument (Model 319, Trition, UK) and a specific resistance to filtration (SRF) instrument (PS-WN-066, Daming, China). All tests were conducted in duplicate. The surface area of filler was measured using a surface analyzer (ASAP2020, Micromeritics, USA) at 77 K. The morphology of the filler was observed with a fluorescence microscope (DMI4000B, Leica, Germany).
3. Results and discussion 3.1. Degradation efficiency of sludge protein Since the protein accounted for the 56.1% of the VSS of WAS in this study, protein degradation was essential to the removal of sludge organics (Table A1 was the TSS concentration and table A2 was the VSS concentration). Owing to the final metabolic product of anaerobic digestion of organic nitrogen being ammonia nitrogen, incremental ammonia nitrogen divided by the initial organic nitrogen was defined as the degradation efficiency of protein in this study (Fig. 2). On day 20, the degradation efficiency of protein in R1, R2 and R3 was 27.4%, 30.0% and 33.8%, respectively. Owing to the presence of filler and inoculum in R3, the degradation rate of protein in R3 was the highest in the initial stage. Accordingly, the degradation rate of protein in R1 was lowest in the initial stage. On day 39, the degradation efficiency of protein in R1, R2 and R3 was 45.0%, 40.9% and 42.0%, respectively. Because parts of sludge of R2 and R3 were adhered onto the surface of filler, the protein degradation efficiency of R1 became the highest in the final stage. Compared with R2, there was some anaerobic active sludge in the surface filler of R3 at the beginning of digestion. Owing to the lower incremental attachment of filler of R3 (Table 1), the protein degradation efficiency of R3 was higher than that of R2.
Fig. 2. Evolution of the degradation efficiency of protein.
Table 1 Attachment of filler in anaerobic reactors (g dry solid/g filler). Attachment of filler
R2
R3
At beginning of digestion At end of digestion
0 1.80 ± 0.12
1.57 ± 0.02 2.62 ± 0.15
3.2. Evolution of DOC Dissolved organic carbon (Fig. 3) included VFA carbon, protein carbon, polysaccharide carbon and other organic carbons (the general chemical formula of protein and polysaccharide was used to calculate the organic carbon content separately). The maximum DOC of R1, R2 and R3 was 1642.8, 1333.0 and 672.2 mg/L on day 3, respectively. Owing to lack of filler and inoculum, there was a 4 day lag phase of R1 for the proliferation of methanogens (Fig. 4). VFA accumulated in the first 3 days and was gradually metabolized over the remainder of the period. There was also a 4 days lag phase of R2 for the proliferation of methanogens. VFA accumulated in the first 3 days and was gradually metabolized with methanation over the remainder of the period. However, owing to the presence of filler, higher hydraulic shear force resulted in the formation of biofilm (Brito and Melo, 1999). The VFA concentration of R2 was less than that of R1 in the later stage of digestion. Owing to the better metabolic conditions, VFA in R3 was gradually converted into biogas. VFA only slightly accumulated in the first 3 days and gradually metabolized in the next few days with methanation.
Fig. 3. Evolution of DOC.
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3.4. Morphology of filler and biofilm After digestion, the filler of R2 and R3 had similar morphology, being porous on inside and compact outside (Figs. A1a and A1b). Since the substrate used in the reactors had a high solid content of sludge, the outside of the filler in R2 and R3 was a compact layer of organic materials that should be the combined action of filtration and adhesion. Compared with R2, the organic on surface of filler of R3 was more tightly adhered (Figs. A1c and A1d), and the attachment of filler of R3 was greater than that of R2 (Table 1). These findings indicate that the filler of R3 was pre-incubated under anaerobic conditions for several days, which likely resulted in a thicker gel layer on the surface of the filler (Ersahin et al., 2012). Accordingly, when compared with the filler of R2, the filler of R3 had greater attachment potential in this experiment. At the end of the digestion period, the majority of solids were accumulated on the filler (Table 1). Fig. 4. Evolution of methane yield.
The dissolved polysaccharide concentration of the three reactors was maintained at low levels throughout the digestion process, which was consistent with the results of a prior study (Shao et al., 2013). Due to high first-order rate coefficient, polysaccharide was easily hydrolyzed and metabolized (Christ et al., 2000). In the first stage, dissolved protein in R1 was greater than that in R2 and R3. The hydrolysis rate of protein is significantly influenced by the available electron donors (Henze and Mladenovski, 1991). Owing to the lack of filler and inoculum, a higher protein level of R1 was caused by the imbalance of hydrolysis and methanation. Due to the filler improving the degree of turbulence of flow around biofilm, the low concentration of dissolved protein of R2 and R3 showed that hydrolysis is the limiting factor. Other organic materials, such as lipids and humics were also present in the reactor. As the digestion process proceeded, lipid was metabolized with VFA, polysaccharide and protein. Meanwhile, humics was gradually generated (Shao et al., 2013). Therefore, other materials gradually increased. 3.3. Methane yield Evolution of the methane yield of the three reactors during anaerobic digestion is shown in Fig. 4. There was a 4 day lag phase for R1 and R2, but no lag phase for R3. Moreover, production rates were near zero (
Contents lists available at ScienceDirect
Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Digestion and dewatering characteristics of waste activated sludge treated by an anaerobic biofilm system Tianfeng Wang a, Liming Shao b,c, Tianshui Li a, Fan Lü a,b, Pinjing He a,b,c,⇑ a
State Key Laboratory of Pollution Control & Resource Reuse, Tongji University, Shanghai 200092, PR China Institute of Waste Treatment and Reclamation, Tongji University, Shanghai 200092, PR China c Centre for the Technology Research and Training on Household Waste in Small Towns & Rural Area, Ministry of Housing and Urban-Rural Development of PR China (MOHURD), PR China b
h i g h l i g h t s Filler enhance conversion rates of sludge digestion. Pre-incubated filler improve dewaterability of digested sludge. Filler alone do not improve dewaterability of digested sludge.
a r t i c l e
i n f o
Article history: Received 7 October 2013 Received in revised form 20 November 2013 Accepted 24 November 2013 Available online 1 December 2013 Keywords: Waste activated sludge Anaerobic digestion Microorganism immobilization Filler Dewaterability
a b s t r a c t Immobilization of microorganisms for sludge anaerobic digestion was investigated in this study. The effects of filler properties on anaerobic digestion and dewaterability of waste activated sludge were assessed at mesophilic temperature in batch mode. The results showed that the duration of the methanogenic stage of reactors without filler, with only filler, and with pre-incubated filler was 39 days, 19 days and 13 days, respectively, during which time the protein was degraded by 45.0%, 29.4% and 30.0%, and the corresponding methane yield was 193.9, 107.2 and 108.2 mL/g volatile suspended solids added, respectively. On day 39, the final protein degradation efficiency of the three reactors was 45.0%, 40.9% and 42.0%, respectively. The results of normalized capillary suction time and specific resistance to filtration suggested that the reactor incorporating pre-incubated filler could improve the dewaterability of digested sludge, while the effect of the reactor incorporating only filler on sludge dewaterability was uncertain. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Owing to the high content of water and putrescible organic matter, waste activated sludge (WAS) generated from wastewater treatment plants must undergo some treatment to enhance its stability and reduce its corresponding volumes prior to final disposal. WAS treatment via anaerobic digestion (AD) results in a reduction of the sludge solids amount and a decrease in odor while produce methane (Duan et al., 2012; Young et al., 2013). Nevertheless, a long hydraulic retention time (HRT) is needed for sludge anaerobic digestion. Immobilization of microorganism can decrease the possibility of microbe losses, prolong solids retention times (SRT), benefit the formation and maintenance of preponderant organism ⇑ Corresponding author at: Institute of Waste Treatment and Reclamation, Tongji University, Shanghai 200092, PR China. Tel./fax: +86 21 65986104. E-mail addresses: [email protected], [email protected] (P. He). 0960-8524/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.biortech.2013.11.066
morphology, since large amounts of high-activity biomass are intercepted in biological treatment systems (Wang et al., 2010). Until recently, microorganism immobilization in anaerobic digester has mainly focused on liquid wastewater with low content of suspended solids (Ozgun et al., 2013). Anaerobic granulation and anaerobic filler biofilms are the main methods for microorganism immobilization (Wang et al., 2010). Comparatively, microorganism immobilization has seldom been investigated and applied to waste with high content of suspended solids owing to strict operating and separating condition. Wang et al. (2010) incorporated polyurethane foam matrices into an anaerobic sequencing batch reactor for the treatment of thermally hydrolyzed municipal biowaste (food waste, fruit–vegetable waste and dewatered sewage sludge), expecting to provide an adequate environment for microbe growth and retention, and to increase the possibility of high-efficiency anaerobic conversion. Gong et al. (2011) incorporated activated carbon fiber into an anaerobic reactor for treatment of cattle manure and obtained higher biogas and methane production than a control blank reactor
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over the long term. Romano and Zhang (2008) incorporated polyethylene cylinders into an anaerobic mixed biofilm reactor for treatment of mixture of onion juice and wastewater sludge and obtained efficient volatile solid reduction. These studies focused on the biowastes that were easily biodegradable or had been prehydrolyzed. In contrast, sludge organics were mainly distributed in the solid phase, which was generally unsuitable for application of the method of microorganism immobilization. Few relevant studies for sludge digestion by this method have been conducted. Nevertheless, since microorganism immobilization can enhance the degradation rate of dissolved organics and the degradation efficiency of total organics, it is possible to improve the dewaterability of digested sludge, which deserves detailed research. In this study, three identical reactors, one without filler, one with only filler, and one with pre-incubated filler, were used to anaerobically digest WAS. Dissolved organic matter (DOM), methane yield, particle size, extracellular polymeric substances (EPS), normalized capillary suction time (CST) and specific resistance to filtration (SRF) were used to investigate the effects of polyester nonwoven fabric as the filler on anaerobic digestion and dewatering characteristics of the digested WAS.
2. Methods 2.1. Sewage sludge WAS was obtained from the sludge thickener of a local domestic wastewater treatment plant in Shanghai, China. The capacity of the plant was 75,000 m3/day and it used an anaerobic–anoxic–oxic process. The service population of the waste water treatment plant was about 200,000. The total suspended solids (TSS), volatile suspended solids (VSS) and protein in the WAS were 29,217 ± 20 mg/L, 20,219 ± 20 mg/L and 11,348.0 ± 15.7 mg/L respectively.
2.2. Experimental setup 2.2.1. Anaerobic reactors Three identical reactors were constructed of cylindrical plexiglass columns (Fig. 1), each having an internal dimension of 150 mm, a total height of 1500 mm, and a working volume of 15 L, one without filler (R1), one with only filler (R2), and one with filler that had been pre-incubated over 60 days (R3). During the pre-incubation period, R3 reactor was fed with glucose and sodium acetate with an organic loading rate (OLR) of 1 g COD/(Ld) and an HRT of 15 day and continuously produced methane over 15 days. Its methane production was near zero before sludge digestion. Three reactors were operated at temperatures of 35 ± 1 °C with 15 L WAS respectively, while keep the sludge suspension recycled internally in reactors by a peristaltic pump at a downward flow rate of 1.2 L/min.
2.2.2. Filler characteristics The filler in anaerobic reactors was wear-resistant and corrosion-resistant polyester nonwoven fabric that had been prewashed with distilled water until the concentration of dissolved organic carbon in (DOC) its eluate was below 1 mg/L by submerging 2.0 g filler in 100 mL distilled water and shaking at 120 rpm for 12 h. The dimensions of filler were about 5 mm 15 mm 15 mm, with a porosity of 96.8 ± 0.2% and BET surface area of 0.2063 m2/g. The total quantity of filler in R2 and R3 was 140.9 and 138.7 g respectively.
Fig. 1. Sketch of down-flow non-woven biofilm reactor.
2.3. Analytical methods The sludge samples were collected from the sampling port (on the top of reactors) using a peristaltic pump. The sludge samples were first centrifuged at 2000g for 10 min after which the supernatant was filtered through a 0.45 lm microfiber filter. The filtrate corresponded to the liquid samples in this study. 2.3.1. EPS extraction The sludge samples were subjected to ultrasound at 20 kHz and 480 W for 10 min in an ice bath. Following ultrasound, the suspensions were centrifuged at 12,000g for 10 min. The bulk solution was then collected as the EPS. The EPS was filtered through a 0.45 lm microfiber filter before analysis. 2.3.2. Physico-chemical analysis DOC, inorganic carbon (IC) and total nitrogen (TN) of liquid samples were analyzed using a TC/TN analyzer (TOC-V CPN, TNM-1, Shimadzu, Japan). Kjeldahl nitrogen (KN) and ammonia nitrogen (AN) were analyzed using an auto Kjeldahl determination system (8400, FOSS, Sweden) for sludge samples and liquid samples. Protein of sludge, EPS and liquid sample were calculated by 6.25 multiplying the concentration of organic nitrogen (ON), which determined by subtracting AN from KN. Polysaccharide of EPS and liquid sample were measured by the anthrone method using
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glucose as a standard (Gaudy, 1962). Volatile fatty acids (VFA) of liquid sample and gas composition of biogas were measured using an Agilent 6980N (Agilent, USA) and a Kechuang GC9800 (Kechuang, China) gas chromatograph, respectively. Total suspended solids (TSS) and volatile suspended solids (VSS) were measured according to the Standard Methods (APHA, 1998). The particle size of the sludge flocs was determined with an EyeTech instrument (EyeTech, Ankersmid, USA). Sludge dewaterability was obtained using a capillary suction time (CST) instrument (Model 319, Trition, UK) and a specific resistance to filtration (SRF) instrument (PS-WN-066, Daming, China). All tests were conducted in duplicate. The surface area of filler was measured using a surface analyzer (ASAP2020, Micromeritics, USA) at 77 K. The morphology of the filler was observed with a fluorescence microscope (DMI4000B, Leica, Germany).
3. Results and discussion 3.1. Degradation efficiency of sludge protein Since the protein accounted for the 56.1% of the VSS of WAS in this study, protein degradation was essential to the removal of sludge organics (Table A1 was the TSS concentration and table A2 was the VSS concentration). Owing to the final metabolic product of anaerobic digestion of organic nitrogen being ammonia nitrogen, incremental ammonia nitrogen divided by the initial organic nitrogen was defined as the degradation efficiency of protein in this study (Fig. 2). On day 20, the degradation efficiency of protein in R1, R2 and R3 was 27.4%, 30.0% and 33.8%, respectively. Owing to the presence of filler and inoculum in R3, the degradation rate of protein in R3 was the highest in the initial stage. Accordingly, the degradation rate of protein in R1 was lowest in the initial stage. On day 39, the degradation efficiency of protein in R1, R2 and R3 was 45.0%, 40.9% and 42.0%, respectively. Because parts of sludge of R2 and R3 were adhered onto the surface of filler, the protein degradation efficiency of R1 became the highest in the final stage. Compared with R2, there was some anaerobic active sludge in the surface filler of R3 at the beginning of digestion. Owing to the lower incremental attachment of filler of R3 (Table 1), the protein degradation efficiency of R3 was higher than that of R2.
Fig. 2. Evolution of the degradation efficiency of protein.
Table 1 Attachment of filler in anaerobic reactors (g dry solid/g filler). Attachment of filler
R2
R3
At beginning of digestion At end of digestion
0 1.80 ± 0.12
1.57 ± 0.02 2.62 ± 0.15
3.2. Evolution of DOC Dissolved organic carbon (Fig. 3) included VFA carbon, protein carbon, polysaccharide carbon and other organic carbons (the general chemical formula of protein and polysaccharide was used to calculate the organic carbon content separately). The maximum DOC of R1, R2 and R3 was 1642.8, 1333.0 and 672.2 mg/L on day 3, respectively. Owing to lack of filler and inoculum, there was a 4 day lag phase of R1 for the proliferation of methanogens (Fig. 4). VFA accumulated in the first 3 days and was gradually metabolized over the remainder of the period. There was also a 4 days lag phase of R2 for the proliferation of methanogens. VFA accumulated in the first 3 days and was gradually metabolized with methanation over the remainder of the period. However, owing to the presence of filler, higher hydraulic shear force resulted in the formation of biofilm (Brito and Melo, 1999). The VFA concentration of R2 was less than that of R1 in the later stage of digestion. Owing to the better metabolic conditions, VFA in R3 was gradually converted into biogas. VFA only slightly accumulated in the first 3 days and gradually metabolized in the next few days with methanation.
Fig. 3. Evolution of DOC.
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3.4. Morphology of filler and biofilm After digestion, the filler of R2 and R3 had similar morphology, being porous on inside and compact outside (Figs. A1a and A1b). Since the substrate used in the reactors had a high solid content of sludge, the outside of the filler in R2 and R3 was a compact layer of organic materials that should be the combined action of filtration and adhesion. Compared with R2, the organic on surface of filler of R3 was more tightly adhered (Figs. A1c and A1d), and the attachment of filler of R3 was greater than that of R2 (Table 1). These findings indicate that the filler of R3 was pre-incubated under anaerobic conditions for several days, which likely resulted in a thicker gel layer on the surface of the filler (Ersahin et al., 2012). Accordingly, when compared with the filler of R2, the filler of R3 had greater attachment potential in this experiment. At the end of the digestion period, the majority of solids were accumulated on the filler (Table 1). Fig. 4. Evolution of methane yield.
The dissolved polysaccharide concentration of the three reactors was maintained at low levels throughout the digestion process, which was consistent with the results of a prior study (Shao et al., 2013). Due to high first-order rate coefficient, polysaccharide was easily hydrolyzed and metabolized (Christ et al., 2000). In the first stage, dissolved protein in R1 was greater than that in R2 and R3. The hydrolysis rate of protein is significantly influenced by the available electron donors (Henze and Mladenovski, 1991). Owing to the lack of filler and inoculum, a higher protein level of R1 was caused by the imbalance of hydrolysis and methanation. Due to the filler improving the degree of turbulence of flow around biofilm, the low concentration of dissolved protein of R2 and R3 showed that hydrolysis is the limiting factor. Other organic materials, such as lipids and humics were also present in the reactor. As the digestion process proceeded, lipid was metabolized with VFA, polysaccharide and protein. Meanwhile, humics was gradually generated (Shao et al., 2013). Therefore, other materials gradually increased. 3.3. Methane yield Evolution of the methane yield of the three reactors during anaerobic digestion is shown in Fig. 4. There was a 4 day lag phase for R1 and R2, but no lag phase for R3. Moreover, production rates were near zero (
