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Treatment of domestic wastewater by an integrated anaerobic fluidized-bed membrane bioreactor under moderate to low temperature conditions
Bioresource Technology 159 (2014) 193–198
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Treatment of domestic wastewater by an integrated anaerobic fluidized-bed membrane bioreactor under moderate to low temperature conditions Da-Wen Gao ⇑, Qi Hu, Chen Yao, Nan-Qi Ren State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology, Harbin 150090, China
h i g h l i g h t s The accumulation of VFAs was affected by temperature significantly. Low temperature accelerated membrane fouling process. Proteins were the dominant EPSs causing membrane fouling at low temperature. Granular active carbon can mitigate membrane fouling via protein absorption.
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Article history: Received 8 January 2014 Received in revised form 19 February 2014 Accepted 22 February 2014 Available online 3 March 2014 Keywords: Domestic wastewater Resource recovery Anaerobic membrane bioreactor (AnMBR) Temperature Membrane fouling
a b s t r a c t The performance of a novel integrated anaerobic fluidized-bed membrane bioreactor (IAFMBR) for treating practical domestic wastewater was investigated at a step dropped temperature from 35, 25, to 15 °C. The COD removal was 74.0 ± 3.7%, 67.1 ± 2.9% and 51.1 ± 2.6% at 35, 25 and 15 °C, respectively. The COD removal depended both on influent strength and operational temperature. The accumulation of VFAs (Volatile Fatty Acids) was affected by temperature, and acetic acid was the most sensitive one to the decrease of temperature. The methanogenic activity of the sludge decreased eventually and the methane yield was dropped from 0.17 ± 0.03, 0.15 ± 0.02 to 0.10 ± 0.01 L/L d. And as compared with a mesophilic temperature, a low temperature can accelerate membrane biofouling. Proteins were the dominant matters causing membrane fouling at low temperature and membrane fouling can be mitigated by granular active carbon (GAC) through protein absorption. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Domestic wastewater has been regarded more as a resource rather than a waste (McCarty et al., 2011; Wang et al., 2012), especially for current world that faces severe risks such as climate changes, energy crisis and water scarcity. Anaerobic process, which has been widely used to treat high-strength industrial wastewater for energy reclaiming, is currently recognized as a promising technology for domestic wastewater treatment (Foresti et al., 2006; Seghezzo et al., 1998). Many full-scale anaerobic treatment plants have been set up in tropical countries, such as India, Colombia, and Brazil (Seghezzo et al., 1998; Florencio et al., 2001). Given the common perception that anaerobic bioreactors can be operate efficiently under the mesophilic (30–40 °C) or thermophilic (50–60 °C) temperature. However, the temperatures of domestic wastewaters ⇑ Corresponding author. Tel./fax: +86 451 86289185. E-mail address: [email protected] (D.-W. Gao). http://dx.doi.org/10.1016/j.biortech.2014.02.086 0960-8524/Ó 2014 Elsevier Ltd. All rights reserved.
in regions without hot climates are relatively low (average of 16 °C in the U.S.) (Smith et al., 2013), thus it’s quite practical to study anaerobic technologies in a lower temperature (<20 °C). Now the performance of anaerobic treatment for domestic wastewater at low temperatures is being closely focused (Bandara et al., 2012; Donoso-Bravo et al., 2013; Elmitwalli et al., 2002; Gao et al., 2011a). A technology for domestic wastewater treatment now actively being pursued is anaerobic membrane bioreactor (AnMBR), which allows high mixed liquor suspended solids (MLSS), enables high removal of organic matter and low production of excess sludge. The membrane leads to nearly absolute biomass retention, with the potential to generate a high quality effluent. Studies have been focused on assessing AnMBR performance for domestic wastewater treatment, which evaluated AnMBR performance at psychrophilic temperatures (Ho and Sung, 2010; Smith et al., 2013). Another alternative for anaerobic treatment of domestic wastewater at low temperatures could be the two-stage system, which consists of two high-rate reactors for allowing an increase in the
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methanogenic activity (Elmitwalli et al., 2002; Alvarez et al., 2008). These studies examined the performances of different anaerobic reactor combination for domestic wastewater treatment at low temperatures. In view of the advantages of AnMBR, it makes good sense to try using membrane bioreactor as one of the combination reactors. In fact, several studies have used compact systems containing AnMBR to meet the more stringent environment regulation and recover resources (An et al., 2009; Kim et al., 2011). However, membrane bioreactor technology is still facing another serious problem, membrane fouling, which hindered the large-scale application of MBR (Gao et al., 2010; Meng et al., 2009). Membrane fouling can mainly be attributed to an accumulation of cells, extracellular polymeric substances (EPS) and soluble microbial product (SMP) on the membrane surface (Meng et al., 2009). Previous studies reported that granular activated carbon (GAC) or powder activated carbon (PAC) was adopted to reduce or alleviate the membrane fouling as they can effectively adsorb microbial metabolic products (Choo et al., 2000; Akram and Stuckey, 2008; Hu and Stuckey, 2007). Recently, fluidized GAC was used to alleviate the membrane fouling by scouring action on the membrane surface (Kim et al., 2011; McCarty et al., 2011). Based on the concept of AFBR and AnMBR, an integrated anaerobic fluidized bed membrane bioreactor (IAFMBR) was proposed for domestic wastewater treatment. In the previous study, the treatment efficiency of IAFMBR for practical domestic wastewater under different hydraulic retention times (HRTs) was discussed (Gao et al., 2014). As temperature was a vital factor in an anaerobic process, the IAFMBR application under moderate to low temperature was worth substantial attention. On this basis, a study was continued conducted to assess the feasibility of actual domestic wastewater treatment by an IAFMBR. The primary objectives of this study were to investigate the effect of temperature on the performance under relatively moderate to lower temperature. A mass balance on COD at different temperatures was conducted to assess the pathway of the organic. In addition, the membrane fouling at different temperatures was also studied in order to provide references for the practical project.
2. Methods 2.1. Reactor design A laboratory-scale integrated anaerobic fluidized bed membrane bioreactor (IAFMBR) which was made of 8 mm thick plexiglas plate and the total volume was 7.6 L with effective volume 5.8 L, consisted of three parts, i.e. outer tube, middle tube and inner tube (Gao et al., 2014). The outer tube was served as AFBR with granular activated carbon (200–300 g) as a carrier, and the inner tube performed as anaerobic membrane bioreactor (AnMBR) which installed hollow fiber membrane (Mitsubishi Rayon Co., Ltd. Tokyo, Japan). The designed membrane flux was 11.3 L/(m2 h) with a total area of 0.19 m2 and an average pore diameter of 0.4 lm. The operating flux was 7.1 L/(m2 h) at the a hydraulic retention time (HRT) of 6 h.
2.2. Feed stock The reactor was fed with synthetic wastewater containing acetate as a substrate at start-up period and then gradually fed with domestic wastewater (Gao et al., 2014). The actual domestic wastewater was daily collected from a septic tank located within a community near university campus (Harbin, China), with a pH of 7.18–7.99.
2.3. Seed sludge The reactor was inoculated with 5 L of waste sludge from a municipal wastewater treatment plant in Harbin, China, with an initial MLSS and MLVSS concentration of approximately 20,500 mg/L and 13,300 mg/L, respectively (MLVSS/MLSS = 0.65). 2.4. Operation of IAFMBR The reactor was equipped with a temperature sensor and a water-heating system for temperature control. The experiment was conducted at different temperatures, reducing from 35 °C, 25 °C to 15 °C. The reactor was operated at 35 °C for 31 days, for the following 33 days at 25 °C, and for 37 days at 15 °C. The water permeation was kept at about 23.2 L/d in IAFMBR, which corresponded to a hydraulic retention time (HRT) of 6 h. And 40 g GAC (10 30 mesh) was added into the inner tube (AnMBR) of IAFMBR to prevent membrane surface from the formation of biomass cake and clogging. The membrane module was cleaned before a new cycle each time (Gao et al., 2011b). 2.5. Analytical methods COD and MLSS were tested according to the standard methods (APHA, 2001). DO and pH were monitored by Handheld MultiParameter Instruments (pH/Oxi 340i, WTW, Germany). The volume of biogas production was measured daily at room temperature using a wet gas meter. The concentration of acetic acid, propionic acid, butyric acid and valeric acid in effluent samples was determined by gas chromatography (HP7890 Agilent Technologies, Palo Alto, CA) equipped with a flame ionization detector (GC-FID) (Gao et al., 2011a). Biogas content (methane, carbon dioxide and hydrogen) was determined by gas chromatography according to the literature (Gao et al., 2011a). For the methane dissolved in effluent, a calculation based on the Henry’s Law was considered to compensate under estimation total methane production, assuming that methane in effluent was saturated (Gao et al., 2011a). The solubility constant in each temperature period was dependent on methane content in biogas and experience data obtained from the literature (Perry and Chilton, 1973). The extracellular polymeric substance (EPS) and soluble microbial product (SMP) samples were collected according to the previous study (Gao et al., 2011b), and then extracted based on the reference (Malamis and Andreadakis, 2009). The content of polysaccharides was tested by the phenol–sulphuric acid method (Dubois et al., 1956). Proteins’ concentration was measured by the Modified BCA kit (Sangon, China). 3. Results and discussion 3.1. The performance of IAFMBR 3.1.1. COD removal The influent concentration fluctuated greatly, in the range of 247–449 mg/L for COD (Fig. 1). In general, there was no significant decrease in the COD removal for IAFMBR when temperature dropped from 35 °C to 25 °C. When temperature dropped further to 15 °C, the COD removal decreased obviously (Fig. 1). The performance of IAFMBR at different temperatures was summarized in Table 1. The respective COD removal was 74.0 ± 3.7%, 67.1 ± 2.9% and 51.1 ± 2.6% when temperature decreasing from 35, 25 to 15 °C. The volumetric COD removal rate was 0.95, 0.81 and 0.73 gCOD/L d at 35, 25 and 15 °C, respectively. The COD removal efficiency was the highest at 35 °C, and then dropped with the decreasing temperature. In contrast, one point should be noted,
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set, a portion of refractory organics could be further removed biologically by microorganism on the membrane surface (Gao et al., 2014).
40
400
30
300 20 200 10
100
0
0 0
20
40 60 Time (day)
80
100
Fig. 1. COD concentrations of influent and effluent of AFBR and the effluent of AnMBR at different temperatures.
there was an obvious increase (from 1.21 to 1.44 gCOD/L d) for volumetric COD loading from 25 to 15 °C simultaneously. There was a lower microbial activity at 15 °C compared with 25 °C. At this point, the increase in volumetric loading was negative on COD removal, and therefore, COD removal efficiency decreased. Individually, the role of outer tube (AFBR) and inner tube (AnMBR) of IAFMBR on COD removal was also studied when temperature dropped (Fig. 1). The COD removal efficiency of AFBR was 61.2 ± 2.9%, 58.0 ± 4.3% and 44.2 ± 2.4% while the average CODeffluent was 124.9 ± 13.4, 126.9 ± 22.2, and 200.7 ± 22.1 mg/L at 35, 25, and 15 °C, respectively (Fig. 1). For AnMBR, COD removal declined from 32.9 ± 9.5% to 21.1 ± 7.8% when temperature dropped from 35 to 25 °C. At 15 °C, the COD removal was 12.3 ± 2.7%. The result showed a better performance at a moderate temperature than low temperature for both AFBR and AnMBR. At the same temperature, AFBR played a significant role in COD removal but AnMBR was limited. The results can be explained from the difference between physical and biological mechanisms of membrane in COD removal (Baek and Pagilla, 2006). When the mixed liquor is passed through the membrane filter, some suspended organic fractions can be rejected by smaller pore, and soluble organic substances can be adsorbed on the membrane surface. These are the physical mechanisms in membrane system. The biological mechanism is the biological COD removal occurring in the bioreactor. Generally, there was a physical barrier against the soluble and insoluble organic-carbon for membrane filter, and the soluble organics was adsorbed on the membrane surface. Then the biological COD removal occurs in the membrane surface. In this study, as COD removal was mostly carried out in AFBR, the physical-COD removal in AnMBR was reduced and the organic on membrane surface was limited. Therefore, it probably happened that there was no abound biomass on the membrane surface. When temperature dropped below 15 °C, the biological-COD removal on membrane surface was weaker. However, once a longer HRT was
3.1.2. Biogas production The composition of biogas was methane, carbon dioxide, hydrogen, hydrogen sulfide, etc. During the whole trial, the biogas was collected after absorption by sodium hydroxide solution, so the gas measured by wet gas meter can be seen gaseous methane yield. Table 2 summarized the methane yield when temperature dropped from moderate to low temperature. The methane yield was 0.19 ± 0.04, 0.19 ± 0.03 to 0.14 ± 0.03 L/gCODremoval d at 35, 25 and 15 °C, respectively. Generally, the biogas yield was increased with increasing organic loading at the same temperature (An et al., 2009; Elmitwalli et al., 2002). In this study, as the influent was practical domestic wastewater, the organic load changed randomly. The volumetric organic loading was 1.29 ± 0.15, 1.21 ± 0.14 to 1.44 ± 0.15 gCOD/L/d at 35, 25 and 15 °C, respectively. 3.1.3. VFAs accumulation The VFAs accumulated more with the stepwise drop of temperature from 35, 25 to 15 °C, and the total VFAs production in the effluent for each temperature was about 32.83, 41.51, and 71.99 mg/L. The concentrations of acetic acid, propanoic acid, butyric acid and valeric acid at a different temperature were shown in Fig. 2. Acetic acid was the most sensitive one to the temperature decrease, whose concentration at 15 °C was nearly three times as that at 35 °C in the effluent of both AFMR and AnMBR. Acetic acid took up 57.1%, 70.7%, and 73.0% of the total VFAs in AFBR effluent at 35, 25 and 15 °C, separately. For AnMBR, the ratio was 56.2%, 69.7% and 72.7%, correspondingly. A conclusion can be drawn that the low-temperature had a strong negative effect on the metabolic
VFAs concentraon (mg/L)
COD (mg/L)
Effluent of AFBR Temperature
Temperature ( º C)
Influent of AFBR Effluent of AnMBR
500
90 80 70 60 50 40 30 20 10 0
Valeric acid
Butyric acid
Propionic acid
Acec acid
AFBR AnMBR AFBR AnMBR AFBR AnMBR 35
25
15
Fig. 2. The variation of VFAs at different temperatures (average value) in AFBR and IAFMBR.
Table 1 COD removal efficiency of IAFMBR at different temperatures. Temperatures (°C)
HRT (h)
Water inflow (L/d)
Influent (COD g/d)
Effluent (COD g/d)
Volumetric COD loading (gCOD/L/d)
Volumetric COD removal (gCOD/L/d)
Removal efficiency per unit volume (%)
35 25 15
6 6 6
23.2 23.2 23.2
7.49 ± 0.88 7.00 ± 0.79 8.35 ± 0.86
1.96 ± 0.41 2.31 ± 0.39 4.09 ± 0.49
1.29 ± 0.15 1.21 ± 0.14 1.44 ± 0.15
0.95 ± 0.08 0.81 ± 0.07 0.73 ± 0.06
74.0 ± 3.7 67.1 ± 2.9 51.1 ± 2.6
Table 2 The methane production at different temperatures. Temperatures (°C)
COD removal (COD g/d)
Methane yield (L/d)
Volumetric methane yield (L/L/d)
Methane conversion amount (L/gCODremoval/d)
35 25 15
5.51 ± 0.63 4.62 ± 0.36 4.22 ± 0.49
1.01 ± 0.16 0.88 ± 0.13 0.58 ± 0.07
0.17 ± 0.03 0.15 ± 0.02 0.10 ± 0.01
0.19 ± 0.04 0.19 ± 0.03 0.14 ± 0.03
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activity of mesophilic methanogens. The concentrations of propionic acid in effluent also gradually increased, with slight variation compared with acetic acid. Both butyric acid and valeric acid in the effluent were comparatively low, and the concentrations were nearly unchanged. The same acetic acid-accumulating phenomenon was also pointed in other anaerobic reactors (Gao et al., 2011a; McHugh et al., 2004). 3.2. Pathway of organic matter in IAFMBR The pathways of CODinfluent in IAFMBR can be summarized as CODVFAs, CODmethane and CODbiomass&others. CODVFAs included the COD of acetic acid, propionic acid, butyric acid and valeric acid in the effluent. CODmethane was the part of organic matter that both measured in biogas and dissolved in the effluent. CODbiomass&others was the COD of other patterns, which included the organic that utilized for biomass formation, converted to trace amounts of CO2, H2 or other types of VFAs, and unbiodegradable organic matter but can be measured as a part of COD. The pathway of the organic in IAFMBR was obviously changed with temperature (Fig. 3). About 13.6%, 16.9% and 24.7% of CODinfluent were transferred to VFAs at 35, 25 and 15 °C, separately, and about 53.1%, 55.4% and 39.5% were removed in the form of methane, respectively. The part of CODVFAs in the effluent gradually increased as the temperature was reduced. While lowering operational temperature from 35 to 15 °C, the accumulation of acetic acid in the effluent increased sharply (the contribution of acetic acid for CODVFAs was 56.2% and 72.7%). A logical explanation to acetic acid accumulation should be the decrease of methanogens activity. However, it was noteworthy that the part of COD removed in the form of methane increased from 53.1% to 55.4% when temperature dropped from 35 to 25 °C, combined with an increase of CODVFAs from 13.6% to 16.9%. The mesophilic temperature (25 °C) did not seem to negatively influence the methane production of IAFMBR. Moreover, although more than 50% of the influent COD was converted into methane, only part of which was collected in the form of gas, about 21.6%, 28.6% and 45.2% of methane was dissolved into the water. In order to improve the utilization of methane in practical application, it is necessary to set a gas–water separator to recover methane dissolved in water. From a local viewpoint, the part of AFBR and AnMBR completed the transformation of organic carbon. The phase of practical domestic wastewater digestion is in the AFBR, and the result was good–there was an accumulation of VFAs in the effluent of AFBR at each temperature. With the step decrease of temperature, the accumulation of VFAs was more (Fig. 2). Therefore, there were enough VFAs flowing into AnMBR for methanogens, but the methanogenic process in AnMBR was not completed (Fig. 3). In fact, the phenomenon of the separation of acidogenic and methanogenic
33.3%
53.1%
27.7%
55.4%
35.8%
39.5%
13.6%
16.9%
24.7%
35
25
15
processes in an anaerobic process has been observed after longterm operation (Zhu et al., 2008). And the variety of VFA along a reactor makes it clear that the acidogenic and methanogenic are separated. An incomplete methanogenic process may lead to a harsh environment. In this study, a step dropped temperature is obviously negatively to methanogens activity. What’s more, attached growth in AnMBR systems may not be favorable for active microorganisms, especially at a low temperature. It is not difficult to speculate that the environmental conditions on cake layer are not good for attached growth. The methanogenic activity of biofilm is generally lower due to the shear force by cross flow on the cake layer, especially at a lower temperature. Another study showed the same conclusion, the microbial activity of suspended sludge continuously increased, while that of attached sludge gradually decreased at a lower temperature (Ho and Sung, 2010). 3.3. Membrane fouling control in IAFMBR The membrane biofouling of IAFMBR was conducted using transmembrane pressure (TMP) (Fig. 4). Granular active carbon (GAC) addition and temperature control was used for studying membrane fouling. According to the pervious researches, 40 g GAC was a proper dosage here (Gao et al., 2014), which was dosed into the inner tube (AnMBR) at each temperature. In order to examine the one-time operational duration of membrane biofouling, no backflushing or clean-up was employed. A cleaning cycle was operated on the day 24, 25 and 18 from 35, 25 to 15 °C. Fig. 4 shows the TMP profiles of three different temperatures. There was a sharp increase at the rate of TMP rise (dP/dt) at 35 °C. The rate of TMP rise was small in the first 17 day, after day 17, the TMP jump occurred. For the operation at 25 °C, the TMP jump occurred earlier (before day 7), and then the rate of TMP rise was little. Eventually, there was a similar transmembrane pollution cycle between 35 °C and 25 °C. When the temperature dropped to 15 °C, a steady rise of TMP was observed for the whole cycle. The faster growth rate of TMP actually produced acute membrane fouling. Therefore, the temperature shock in moderate temperature (from 35 to 25 °C) was not clear, when the temperature further dropped to 15 °C, the temperature shock was evident and membrane fouling became serious. The results showed that membrane fouling was significant in a low-temperature period, which was in accordance with some studies (Van den Brink et al., 2011; Wang et al., 2010). A rational explanation accounted for this may be the difference in the characteristics of dissolved organic matter between mesophilic and psychrophilic temperature (Gao et al., 2011a, 2013). The characteristic of organic matter in soluble microbial product (SMP) and extracellular polymeric substances (EPS) was analyzed to figure out how the dissolved organic matter impacts
30
Biomass and others Methane
VFAs
Fig. 3. Pathway of organic matter at different temperatures in IAFMBR. Ratios in each cake were calculated from COD data (in average). (‘‘VFAs’’ represents the COD concentrations of effluent VFAs; ‘‘Methane’’ is the part of organic matter that lost from IAFMBR in the form of methane including gaseous and dissolved; ‘‘Biomass & others’’ is the organic matter used by biomass formation and organic and inorganic compounds that are unbiodegradable but can be measured as a part of COD).
40gGAC+35 40gGAC+25 40gGAC+15
25 TMP (KPa)
196
20 15 10 5 0 0
5
10
15
20
25
30
Time(day) Fig. 4. Transmembrane pressure (TMP) profile during three different temperatures in AnMBR.
(A)
Cconcentraon/Time (mg·L -1·day-1)
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Protein Polysaccharide
1.2 0.8
Acknowledgements
0.4
This research was supported by National Natural Science Foundation of China (No. 21177033), the Research Fund for the Doctoral Program of Higher Education, Ministry of Education of PR China (20092302110059).
0.0 SMP
SMP
SMP
SMP
SMP
SMP
35
25
15
35
25
15
References
Cconcentraon/Time (mg·g -1·day-1)
Mixed liquor
(B)
to VFAs, and about 53.1%, 55.4%, and 39.5% were removed as methane, correspondingly. Compared with a mesophilic temperature, a low temperature can accelerate membrane biofouling and the proteins were the main matters causing membrane fouling.
2.0 1.6
197
Cake layer
3.5 3.0
Protein
2.5
Polysaccharide
2.0 1.5 1.0 0.5 0.0 EPS
EPS
EPS
EPS
EPS
EPS
35
25
15
35
25
15
Mixed liquor
Cake layer
Fig. 5. The daily average concentration of EPS and SMP in mixed liquor and cake layer at different temperatures. (A) The daily average concentration of SMP; (B) The daily average concentration of EPS.
membrane fouling. Polysaccharides and proteins were studied as the major component of microbic metabolites (Fig. 5). In this case, proteins were the predominant component whatever in EPS or SMP at both mixed liquor and cake layer from 35, 25 to 15 °C. However, in some studies, polysaccharides were considered as the main matters causing membrane fouling at a low temperature (Ma et al., 2013; Miyoshi et al., 2009). Although the results don’t to correlate well across these different studies, it has generally been observed in the membrane fouling process. In the previous study, polysaccharides dominated metabolites at 30 and 20 °C, instead of protein at 10 °C (Gao et al., 2013). Similar results were recently reported by Martin Garcia et al. (2013), who found higher concentrations of protein in the SMP of AnMBR at a low temperature. In addition, Fig. 5 also showed the average accumulation rate of SMP (or EPS) in mixed-liquor (or cake-layer) when the temperature dropped down. The result showed that the content of SMP (or EPS) decreased at 25 °C and then increased at 15 °C, which was consistent with the variation of TMP. The protein in mixed-liquor (or cakelayer) was absorbed by GAC, and the reduction of protein improved the membrane filtration performance (Gao et al., 2014). The least membrane fouling was observed at 25 °C, which could be a result of temperature, microbial activities and activated carbon adsorption in IAFMBR. 4. Conclusions In this study, decreasing temperatures from 35, 25 to 15 °C were applied to an IAFMBR when treated practical domestic wastewater. The COD removal was 74.0 ± 3.7%, 67.1 ± 2.9% and 51.1 ± 2.6%, and methane production stayed at 0.19 ± 0.04, 0.19 ± 0.03, 0.14 ± 0.03 L/gCODremoval d at 35, 25 and 15 °C, respectively. About 13.6%, 16.9% and 24.7% of CODinfluent was transferred
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McCarty, P.L., Bae, J., Kim, J., 2011. Domestic wastewater treatment as a net energy producer-can this be achieved? Environ. Sci. Technol. 45, 7100–7106. McHugh, S., Carton, M., Collins, G., O’Flaherty, V., 2004. Reactor performance and microbial community dynamics during anaerobic biological treatment of wastewaters at 16–37 °C. FEMS Microbiol. Ecol. 48, 369–378. Meng, F., Chae, S.R., Drews, A., Kraume, M., Shin, H.S., Yang, F., 2009. Recent advances in membrane bioreactors (MBRs): membrane fouling and membrane material. Water Res. 43, 1489–1512. Miyoshi, T., Tsuyuhara, T., Ogyu, R., Kimura, K., Watanabe, Y., 2009. Seasonal variation in membrane fouling in membrane bioreactors (MBRs) treating municipal wastewater. Water Res. 43, 5109–5118. Perry, R.M., Chilton, C.M., 1973. Methane Solubility in Water. Table 3–138, Chemical Engineer’s Handbook, fifth ed. McGraw-Hill, New York. Seghezzo, L., Zeeman, G., Van Lier, J.B., Hamelers, H., Lettinga, G., 1998. A review: the anaerobic treatment of sewage in UASB and EGSB reactors. Bioresour. Technol. 65, 175–190.
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Treatment of domestic wastewater by an integrated anaerobic fluidized-bed membrane bioreactor under moderate to low temperature conditions Da-Wen Gao ⇑, Qi Hu, Chen Yao, Nan-Qi Ren State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology, Harbin 150090, China
h i g h l i g h t s The accumulation of VFAs was affected by temperature significantly. Low temperature accelerated membrane fouling process. Proteins were the dominant EPSs causing membrane fouling at low temperature. Granular active carbon can mitigate membrane fouling via protein absorption.
a r t i c l e
i n f o
Article history: Received 8 January 2014 Received in revised form 19 February 2014 Accepted 22 February 2014 Available online 3 March 2014 Keywords: Domestic wastewater Resource recovery Anaerobic membrane bioreactor (AnMBR) Temperature Membrane fouling
a b s t r a c t The performance of a novel integrated anaerobic fluidized-bed membrane bioreactor (IAFMBR) for treating practical domestic wastewater was investigated at a step dropped temperature from 35, 25, to 15 °C. The COD removal was 74.0 ± 3.7%, 67.1 ± 2.9% and 51.1 ± 2.6% at 35, 25 and 15 °C, respectively. The COD removal depended both on influent strength and operational temperature. The accumulation of VFAs (Volatile Fatty Acids) was affected by temperature, and acetic acid was the most sensitive one to the decrease of temperature. The methanogenic activity of the sludge decreased eventually and the methane yield was dropped from 0.17 ± 0.03, 0.15 ± 0.02 to 0.10 ± 0.01 L/L d. And as compared with a mesophilic temperature, a low temperature can accelerate membrane biofouling. Proteins were the dominant matters causing membrane fouling at low temperature and membrane fouling can be mitigated by granular active carbon (GAC) through protein absorption. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Domestic wastewater has been regarded more as a resource rather than a waste (McCarty et al., 2011; Wang et al., 2012), especially for current world that faces severe risks such as climate changes, energy crisis and water scarcity. Anaerobic process, which has been widely used to treat high-strength industrial wastewater for energy reclaiming, is currently recognized as a promising technology for domestic wastewater treatment (Foresti et al., 2006; Seghezzo et al., 1998). Many full-scale anaerobic treatment plants have been set up in tropical countries, such as India, Colombia, and Brazil (Seghezzo et al., 1998; Florencio et al., 2001). Given the common perception that anaerobic bioreactors can be operate efficiently under the mesophilic (30–40 °C) or thermophilic (50–60 °C) temperature. However, the temperatures of domestic wastewaters ⇑ Corresponding author. Tel./fax: +86 451 86289185. E-mail address: [email protected] (D.-W. Gao). http://dx.doi.org/10.1016/j.biortech.2014.02.086 0960-8524/Ó 2014 Elsevier Ltd. All rights reserved.
in regions without hot climates are relatively low (average of 16 °C in the U.S.) (Smith et al., 2013), thus it’s quite practical to study anaerobic technologies in a lower temperature (<20 °C). Now the performance of anaerobic treatment for domestic wastewater at low temperatures is being closely focused (Bandara et al., 2012; Donoso-Bravo et al., 2013; Elmitwalli et al., 2002; Gao et al., 2011a). A technology for domestic wastewater treatment now actively being pursued is anaerobic membrane bioreactor (AnMBR), which allows high mixed liquor suspended solids (MLSS), enables high removal of organic matter and low production of excess sludge. The membrane leads to nearly absolute biomass retention, with the potential to generate a high quality effluent. Studies have been focused on assessing AnMBR performance for domestic wastewater treatment, which evaluated AnMBR performance at psychrophilic temperatures (Ho and Sung, 2010; Smith et al., 2013). Another alternative for anaerobic treatment of domestic wastewater at low temperatures could be the two-stage system, which consists of two high-rate reactors for allowing an increase in the
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methanogenic activity (Elmitwalli et al., 2002; Alvarez et al., 2008). These studies examined the performances of different anaerobic reactor combination for domestic wastewater treatment at low temperatures. In view of the advantages of AnMBR, it makes good sense to try using membrane bioreactor as one of the combination reactors. In fact, several studies have used compact systems containing AnMBR to meet the more stringent environment regulation and recover resources (An et al., 2009; Kim et al., 2011). However, membrane bioreactor technology is still facing another serious problem, membrane fouling, which hindered the large-scale application of MBR (Gao et al., 2010; Meng et al., 2009). Membrane fouling can mainly be attributed to an accumulation of cells, extracellular polymeric substances (EPS) and soluble microbial product (SMP) on the membrane surface (Meng et al., 2009). Previous studies reported that granular activated carbon (GAC) or powder activated carbon (PAC) was adopted to reduce or alleviate the membrane fouling as they can effectively adsorb microbial metabolic products (Choo et al., 2000; Akram and Stuckey, 2008; Hu and Stuckey, 2007). Recently, fluidized GAC was used to alleviate the membrane fouling by scouring action on the membrane surface (Kim et al., 2011; McCarty et al., 2011). Based on the concept of AFBR and AnMBR, an integrated anaerobic fluidized bed membrane bioreactor (IAFMBR) was proposed for domestic wastewater treatment. In the previous study, the treatment efficiency of IAFMBR for practical domestic wastewater under different hydraulic retention times (HRTs) was discussed (Gao et al., 2014). As temperature was a vital factor in an anaerobic process, the IAFMBR application under moderate to low temperature was worth substantial attention. On this basis, a study was continued conducted to assess the feasibility of actual domestic wastewater treatment by an IAFMBR. The primary objectives of this study were to investigate the effect of temperature on the performance under relatively moderate to lower temperature. A mass balance on COD at different temperatures was conducted to assess the pathway of the organic. In addition, the membrane fouling at different temperatures was also studied in order to provide references for the practical project.
2. Methods 2.1. Reactor design A laboratory-scale integrated anaerobic fluidized bed membrane bioreactor (IAFMBR) which was made of 8 mm thick plexiglas plate and the total volume was 7.6 L with effective volume 5.8 L, consisted of three parts, i.e. outer tube, middle tube and inner tube (Gao et al., 2014). The outer tube was served as AFBR with granular activated carbon (200–300 g) as a carrier, and the inner tube performed as anaerobic membrane bioreactor (AnMBR) which installed hollow fiber membrane (Mitsubishi Rayon Co., Ltd. Tokyo, Japan). The designed membrane flux was 11.3 L/(m2 h) with a total area of 0.19 m2 and an average pore diameter of 0.4 lm. The operating flux was 7.1 L/(m2 h) at the a hydraulic retention time (HRT) of 6 h.
2.2. Feed stock The reactor was fed with synthetic wastewater containing acetate as a substrate at start-up period and then gradually fed with domestic wastewater (Gao et al., 2014). The actual domestic wastewater was daily collected from a septic tank located within a community near university campus (Harbin, China), with a pH of 7.18–7.99.
2.3. Seed sludge The reactor was inoculated with 5 L of waste sludge from a municipal wastewater treatment plant in Harbin, China, with an initial MLSS and MLVSS concentration of approximately 20,500 mg/L and 13,300 mg/L, respectively (MLVSS/MLSS = 0.65). 2.4. Operation of IAFMBR The reactor was equipped with a temperature sensor and a water-heating system for temperature control. The experiment was conducted at different temperatures, reducing from 35 °C, 25 °C to 15 °C. The reactor was operated at 35 °C for 31 days, for the following 33 days at 25 °C, and for 37 days at 15 °C. The water permeation was kept at about 23.2 L/d in IAFMBR, which corresponded to a hydraulic retention time (HRT) of 6 h. And 40 g GAC (10 30 mesh) was added into the inner tube (AnMBR) of IAFMBR to prevent membrane surface from the formation of biomass cake and clogging. The membrane module was cleaned before a new cycle each time (Gao et al., 2011b). 2.5. Analytical methods COD and MLSS were tested according to the standard methods (APHA, 2001). DO and pH were monitored by Handheld MultiParameter Instruments (pH/Oxi 340i, WTW, Germany). The volume of biogas production was measured daily at room temperature using a wet gas meter. The concentration of acetic acid, propionic acid, butyric acid and valeric acid in effluent samples was determined by gas chromatography (HP7890 Agilent Technologies, Palo Alto, CA) equipped with a flame ionization detector (GC-FID) (Gao et al., 2011a). Biogas content (methane, carbon dioxide and hydrogen) was determined by gas chromatography according to the literature (Gao et al., 2011a). For the methane dissolved in effluent, a calculation based on the Henry’s Law was considered to compensate under estimation total methane production, assuming that methane in effluent was saturated (Gao et al., 2011a). The solubility constant in each temperature period was dependent on methane content in biogas and experience data obtained from the literature (Perry and Chilton, 1973). The extracellular polymeric substance (EPS) and soluble microbial product (SMP) samples were collected according to the previous study (Gao et al., 2011b), and then extracted based on the reference (Malamis and Andreadakis, 2009). The content of polysaccharides was tested by the phenol–sulphuric acid method (Dubois et al., 1956). Proteins’ concentration was measured by the Modified BCA kit (Sangon, China). 3. Results and discussion 3.1. The performance of IAFMBR 3.1.1. COD removal The influent concentration fluctuated greatly, in the range of 247–449 mg/L for COD (Fig. 1). In general, there was no significant decrease in the COD removal for IAFMBR when temperature dropped from 35 °C to 25 °C. When temperature dropped further to 15 °C, the COD removal decreased obviously (Fig. 1). The performance of IAFMBR at different temperatures was summarized in Table 1. The respective COD removal was 74.0 ± 3.7%, 67.1 ± 2.9% and 51.1 ± 2.6% when temperature decreasing from 35, 25 to 15 °C. The volumetric COD removal rate was 0.95, 0.81 and 0.73 gCOD/L d at 35, 25 and 15 °C, respectively. The COD removal efficiency was the highest at 35 °C, and then dropped with the decreasing temperature. In contrast, one point should be noted,
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set, a portion of refractory organics could be further removed biologically by microorganism on the membrane surface (Gao et al., 2014).
40
400
30
300 20 200 10
100
0
0 0
20
40 60 Time (day)
80
100
Fig. 1. COD concentrations of influent and effluent of AFBR and the effluent of AnMBR at different temperatures.
there was an obvious increase (from 1.21 to 1.44 gCOD/L d) for volumetric COD loading from 25 to 15 °C simultaneously. There was a lower microbial activity at 15 °C compared with 25 °C. At this point, the increase in volumetric loading was negative on COD removal, and therefore, COD removal efficiency decreased. Individually, the role of outer tube (AFBR) and inner tube (AnMBR) of IAFMBR on COD removal was also studied when temperature dropped (Fig. 1). The COD removal efficiency of AFBR was 61.2 ± 2.9%, 58.0 ± 4.3% and 44.2 ± 2.4% while the average CODeffluent was 124.9 ± 13.4, 126.9 ± 22.2, and 200.7 ± 22.1 mg/L at 35, 25, and 15 °C, respectively (Fig. 1). For AnMBR, COD removal declined from 32.9 ± 9.5% to 21.1 ± 7.8% when temperature dropped from 35 to 25 °C. At 15 °C, the COD removal was 12.3 ± 2.7%. The result showed a better performance at a moderate temperature than low temperature for both AFBR and AnMBR. At the same temperature, AFBR played a significant role in COD removal but AnMBR was limited. The results can be explained from the difference between physical and biological mechanisms of membrane in COD removal (Baek and Pagilla, 2006). When the mixed liquor is passed through the membrane filter, some suspended organic fractions can be rejected by smaller pore, and soluble organic substances can be adsorbed on the membrane surface. These are the physical mechanisms in membrane system. The biological mechanism is the biological COD removal occurring in the bioreactor. Generally, there was a physical barrier against the soluble and insoluble organic-carbon for membrane filter, and the soluble organics was adsorbed on the membrane surface. Then the biological COD removal occurs in the membrane surface. In this study, as COD removal was mostly carried out in AFBR, the physical-COD removal in AnMBR was reduced and the organic on membrane surface was limited. Therefore, it probably happened that there was no abound biomass on the membrane surface. When temperature dropped below 15 °C, the biological-COD removal on membrane surface was weaker. However, once a longer HRT was
3.1.2. Biogas production The composition of biogas was methane, carbon dioxide, hydrogen, hydrogen sulfide, etc. During the whole trial, the biogas was collected after absorption by sodium hydroxide solution, so the gas measured by wet gas meter can be seen gaseous methane yield. Table 2 summarized the methane yield when temperature dropped from moderate to low temperature. The methane yield was 0.19 ± 0.04, 0.19 ± 0.03 to 0.14 ± 0.03 L/gCODremoval d at 35, 25 and 15 °C, respectively. Generally, the biogas yield was increased with increasing organic loading at the same temperature (An et al., 2009; Elmitwalli et al., 2002). In this study, as the influent was practical domestic wastewater, the organic load changed randomly. The volumetric organic loading was 1.29 ± 0.15, 1.21 ± 0.14 to 1.44 ± 0.15 gCOD/L/d at 35, 25 and 15 °C, respectively. 3.1.3. VFAs accumulation The VFAs accumulated more with the stepwise drop of temperature from 35, 25 to 15 °C, and the total VFAs production in the effluent for each temperature was about 32.83, 41.51, and 71.99 mg/L. The concentrations of acetic acid, propanoic acid, butyric acid and valeric acid at a different temperature were shown in Fig. 2. Acetic acid was the most sensitive one to the temperature decrease, whose concentration at 15 °C was nearly three times as that at 35 °C in the effluent of both AFMR and AnMBR. Acetic acid took up 57.1%, 70.7%, and 73.0% of the total VFAs in AFBR effluent at 35, 25 and 15 °C, separately. For AnMBR, the ratio was 56.2%, 69.7% and 72.7%, correspondingly. A conclusion can be drawn that the low-temperature had a strong negative effect on the metabolic
VFAs concentraon (mg/L)
COD (mg/L)
Effluent of AFBR Temperature
Temperature ( º C)
Influent of AFBR Effluent of AnMBR
500
90 80 70 60 50 40 30 20 10 0
Valeric acid
Butyric acid
Propionic acid
Acec acid
AFBR AnMBR AFBR AnMBR AFBR AnMBR 35
25
15
Fig. 2. The variation of VFAs at different temperatures (average value) in AFBR and IAFMBR.
Table 1 COD removal efficiency of IAFMBR at different temperatures. Temperatures (°C)
HRT (h)
Water inflow (L/d)
Influent (COD g/d)
Effluent (COD g/d)
Volumetric COD loading (gCOD/L/d)
Volumetric COD removal (gCOD/L/d)
Removal efficiency per unit volume (%)
35 25 15
6 6 6
23.2 23.2 23.2
7.49 ± 0.88 7.00 ± 0.79 8.35 ± 0.86
1.96 ± 0.41 2.31 ± 0.39 4.09 ± 0.49
1.29 ± 0.15 1.21 ± 0.14 1.44 ± 0.15
0.95 ± 0.08 0.81 ± 0.07 0.73 ± 0.06
74.0 ± 3.7 67.1 ± 2.9 51.1 ± 2.6
Table 2 The methane production at different temperatures. Temperatures (°C)
COD removal (COD g/d)
Methane yield (L/d)
Volumetric methane yield (L/L/d)
Methane conversion amount (L/gCODremoval/d)
35 25 15
5.51 ± 0.63 4.62 ± 0.36 4.22 ± 0.49
1.01 ± 0.16 0.88 ± 0.13 0.58 ± 0.07
0.17 ± 0.03 0.15 ± 0.02 0.10 ± 0.01
0.19 ± 0.04 0.19 ± 0.03 0.14 ± 0.03
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activity of mesophilic methanogens. The concentrations of propionic acid in effluent also gradually increased, with slight variation compared with acetic acid. Both butyric acid and valeric acid in the effluent were comparatively low, and the concentrations were nearly unchanged. The same acetic acid-accumulating phenomenon was also pointed in other anaerobic reactors (Gao et al., 2011a; McHugh et al., 2004). 3.2. Pathway of organic matter in IAFMBR The pathways of CODinfluent in IAFMBR can be summarized as CODVFAs, CODmethane and CODbiomass&others. CODVFAs included the COD of acetic acid, propionic acid, butyric acid and valeric acid in the effluent. CODmethane was the part of organic matter that both measured in biogas and dissolved in the effluent. CODbiomass&others was the COD of other patterns, which included the organic that utilized for biomass formation, converted to trace amounts of CO2, H2 or other types of VFAs, and unbiodegradable organic matter but can be measured as a part of COD. The pathway of the organic in IAFMBR was obviously changed with temperature (Fig. 3). About 13.6%, 16.9% and 24.7% of CODinfluent were transferred to VFAs at 35, 25 and 15 °C, separately, and about 53.1%, 55.4% and 39.5% were removed in the form of methane, respectively. The part of CODVFAs in the effluent gradually increased as the temperature was reduced. While lowering operational temperature from 35 to 15 °C, the accumulation of acetic acid in the effluent increased sharply (the contribution of acetic acid for CODVFAs was 56.2% and 72.7%). A logical explanation to acetic acid accumulation should be the decrease of methanogens activity. However, it was noteworthy that the part of COD removed in the form of methane increased from 53.1% to 55.4% when temperature dropped from 35 to 25 °C, combined with an increase of CODVFAs from 13.6% to 16.9%. The mesophilic temperature (25 °C) did not seem to negatively influence the methane production of IAFMBR. Moreover, although more than 50% of the influent COD was converted into methane, only part of which was collected in the form of gas, about 21.6%, 28.6% and 45.2% of methane was dissolved into the water. In order to improve the utilization of methane in practical application, it is necessary to set a gas–water separator to recover methane dissolved in water. From a local viewpoint, the part of AFBR and AnMBR completed the transformation of organic carbon. The phase of practical domestic wastewater digestion is in the AFBR, and the result was good–there was an accumulation of VFAs in the effluent of AFBR at each temperature. With the step decrease of temperature, the accumulation of VFAs was more (Fig. 2). Therefore, there were enough VFAs flowing into AnMBR for methanogens, but the methanogenic process in AnMBR was not completed (Fig. 3). In fact, the phenomenon of the separation of acidogenic and methanogenic
33.3%
53.1%
27.7%
55.4%
35.8%
39.5%
13.6%
16.9%
24.7%
35
25
15
processes in an anaerobic process has been observed after longterm operation (Zhu et al., 2008). And the variety of VFA along a reactor makes it clear that the acidogenic and methanogenic are separated. An incomplete methanogenic process may lead to a harsh environment. In this study, a step dropped temperature is obviously negatively to methanogens activity. What’s more, attached growth in AnMBR systems may not be favorable for active microorganisms, especially at a low temperature. It is not difficult to speculate that the environmental conditions on cake layer are not good for attached growth. The methanogenic activity of biofilm is generally lower due to the shear force by cross flow on the cake layer, especially at a lower temperature. Another study showed the same conclusion, the microbial activity of suspended sludge continuously increased, while that of attached sludge gradually decreased at a lower temperature (Ho and Sung, 2010). 3.3. Membrane fouling control in IAFMBR The membrane biofouling of IAFMBR was conducted using transmembrane pressure (TMP) (Fig. 4). Granular active carbon (GAC) addition and temperature control was used for studying membrane fouling. According to the pervious researches, 40 g GAC was a proper dosage here (Gao et al., 2014), which was dosed into the inner tube (AnMBR) at each temperature. In order to examine the one-time operational duration of membrane biofouling, no backflushing or clean-up was employed. A cleaning cycle was operated on the day 24, 25 and 18 from 35, 25 to 15 °C. Fig. 4 shows the TMP profiles of three different temperatures. There was a sharp increase at the rate of TMP rise (dP/dt) at 35 °C. The rate of TMP rise was small in the first 17 day, after day 17, the TMP jump occurred. For the operation at 25 °C, the TMP jump occurred earlier (before day 7), and then the rate of TMP rise was little. Eventually, there was a similar transmembrane pollution cycle between 35 °C and 25 °C. When the temperature dropped to 15 °C, a steady rise of TMP was observed for the whole cycle. The faster growth rate of TMP actually produced acute membrane fouling. Therefore, the temperature shock in moderate temperature (from 35 to 25 °C) was not clear, when the temperature further dropped to 15 °C, the temperature shock was evident and membrane fouling became serious. The results showed that membrane fouling was significant in a low-temperature period, which was in accordance with some studies (Van den Brink et al., 2011; Wang et al., 2010). A rational explanation accounted for this may be the difference in the characteristics of dissolved organic matter between mesophilic and psychrophilic temperature (Gao et al., 2011a, 2013). The characteristic of organic matter in soluble microbial product (SMP) and extracellular polymeric substances (EPS) was analyzed to figure out how the dissolved organic matter impacts
30
Biomass and others Methane
VFAs
Fig. 3. Pathway of organic matter at different temperatures in IAFMBR. Ratios in each cake were calculated from COD data (in average). (‘‘VFAs’’ represents the COD concentrations of effluent VFAs; ‘‘Methane’’ is the part of organic matter that lost from IAFMBR in the form of methane including gaseous and dissolved; ‘‘Biomass & others’’ is the organic matter used by biomass formation and organic and inorganic compounds that are unbiodegradable but can be measured as a part of COD).
40gGAC+35 40gGAC+25 40gGAC+15
25 TMP (KPa)
196
20 15 10 5 0 0
5
10
15
20
25
30
Time(day) Fig. 4. Transmembrane pressure (TMP) profile during three different temperatures in AnMBR.
(A)
Cconcentraon/Time (mg·L -1·day-1)
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Protein Polysaccharide
1.2 0.8
Acknowledgements
0.4
This research was supported by National Natural Science Foundation of China (No. 21177033), the Research Fund for the Doctoral Program of Higher Education, Ministry of Education of PR China (20092302110059).
0.0 SMP
SMP
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SMP
SMP
SMP
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25
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35
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15
References
Cconcentraon/Time (mg·g -1·day-1)
Mixed liquor
(B)
to VFAs, and about 53.1%, 55.4%, and 39.5% were removed as methane, correspondingly. Compared with a mesophilic temperature, a low temperature can accelerate membrane biofouling and the proteins were the main matters causing membrane fouling.
2.0 1.6
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Cake layer
3.5 3.0
Protein
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Polysaccharide
2.0 1.5 1.0 0.5 0.0 EPS
EPS
EPS
EPS
EPS
EPS
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Mixed liquor
Cake layer
Fig. 5. The daily average concentration of EPS and SMP in mixed liquor and cake layer at different temperatures. (A) The daily average concentration of SMP; (B) The daily average concentration of EPS.
membrane fouling. Polysaccharides and proteins were studied as the major component of microbic metabolites (Fig. 5). In this case, proteins were the predominant component whatever in EPS or SMP at both mixed liquor and cake layer from 35, 25 to 15 °C. However, in some studies, polysaccharides were considered as the main matters causing membrane fouling at a low temperature (Ma et al., 2013; Miyoshi et al., 2009). Although the results don’t to correlate well across these different studies, it has generally been observed in the membrane fouling process. In the previous study, polysaccharides dominated metabolites at 30 and 20 °C, instead of protein at 10 °C (Gao et al., 2013). Similar results were recently reported by Martin Garcia et al. (2013), who found higher concentrations of protein in the SMP of AnMBR at a low temperature. In addition, Fig. 5 also showed the average accumulation rate of SMP (or EPS) in mixed-liquor (or cake-layer) when the temperature dropped down. The result showed that the content of SMP (or EPS) decreased at 25 °C and then increased at 15 °C, which was consistent with the variation of TMP. The protein in mixed-liquor (or cakelayer) was absorbed by GAC, and the reduction of protein improved the membrane filtration performance (Gao et al., 2014). The least membrane fouling was observed at 25 °C, which could be a result of temperature, microbial activities and activated carbon adsorption in IAFMBR. 4. Conclusions In this study, decreasing temperatures from 35, 25 to 15 °C were applied to an IAFMBR when treated practical domestic wastewater. The COD removal was 74.0 ± 3.7%, 67.1 ± 2.9% and 51.1 ± 2.6%, and methane production stayed at 0.19 ± 0.04, 0.19 ± 0.03, 0.14 ± 0.03 L/gCODremoval d at 35, 25 and 15 °C, respectively. About 13.6%, 16.9% and 24.7% of CODinfluent was transferred
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