Waste Management xxx (2015) xxx–xxx
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Zero-valent iron enhanced methanogenic activity in anaerobic digestion of waste activated sludge after heat and alkali pretreatment Yaobin Zhang ⇑, Yinghong Feng ⇑, Xie Quan Dalian University of Technology, Key Laboratory of Industrial Ecology and Environmental Engineering, China
a r t i c l e
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Article history: Received 7 May 2014 Accepted 23 January 2015 Available online xxxx Keywords: Waste activated sludge Anaerobic digestion Zero-valent iron (ZVI) Heat pretreatment Alkali treatment
a b s t r a c t Heat or alkali pretreatment is the effective method to improve hydrolysis of waste sludge and then enhance anaerobic sludge digestion. However the pretreatment may inactivate the methanogens in the sludge. In the present work, zero-valent iron (ZVI) was used to enhance the methanogenic activity in anaerobic sludge digester under two methanogens-suppressing conditions, i.e. heat-pretreatment and alkali condition respectively. With the addition of ZVI, the lag time of methane production was shortened, and the methane yield increased by 91.5% compared to the control group. The consumption of VFA was accelerated by ZVI, especially for acetate, indicating that the acetoclastic methanogenesis was enhanced. In the alkali-condition experiment, the hydrogen produced decreased from 27.6 to 18.8 mL when increasing the ZVI dosage from 0 to 10 g/L. Correspondingly, the methane yield increased from 1.9 to 32.2 mL, which meant that the H2-utilizing methanogenes was enriched. These results suggested that the addition of ZVI into anaerobic digestion of sludge after pretreated by the heat or alkali process could efficiently recover the methanogenic activity and increase the methane production and sludge reduction. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Waste activated sludge generated from biological wastewater treatment processes has increased continuously in the recent decades due to increasing population and new construction of waste water treatment plants. Large amount of organic compounds containing in the waste sludge presents a potential threat to environment for example leaching of liquid and odor (Lee and Han, 2013). Anaerobic digestion is the most applied technique for waste activated sludge stabilization because of its high abilities to transform organic matters into biogas mixture of methane and carbon dioxide. However, the yield of methane is often limited by slow hydrolysis of sludge. To improve the hydrolysis of waste sludge, chemical, mechanical and biological methods (Lagerkvist and Morgan, 2012; Rani et al., 2012) have been applied to pretreat sludge to disintegrate sludge cells and release intracellular materials into the water phase. Heat treatment was one of prevalent method to improve the hydrolysis of waste sludge since its simple operation (Oh et al., 2003). Hydrolysis of sludge under alkali condition developed by Zhao et al. (2010) also gained attention in recent years due to its ⇑ Corresponding authors. Tel.: +86 411 8470 6140; fax: +86 411 8470 6263 (Y. Zhang). Tel.: +86 411 8470 6460; fax: +86 411 8470 6263 (Y. Feng). E-mail addresses: [email protected] (Y. Zhang), [email protected] (Y. Feng).
dramatic enhancement of the hydrolysis of sludge. Although improving the hydrolysis–acidification of sludge, the heat and alkali processes are harmful to methanogenesis because methanogens grow only under an appropriate temperature and a narrowly neutral pH range (Dunfield et al., 1993). Three temperature regimes can be used in anaerobic digesters: psychrophilic, mesophilic and thermophilic with varied optimum temperature ranges for the domination of different strains of methane-forming bacteria. Psychrophilic fermentors operate at about 25 °C, mesophilic ones at around 35 °C and thermophilic at around 55 °C. The heat treatment at 102 °C for approximately 30 min could kill most methanogens (Gallert and Winter, 1997). The alkali pH was also a factor in preventing methanogenic activity. Methanogenic bacteria are extremely sensitive to pH and prefer pH around 6.8–7.2 as the growth rate of methanogens is greatly reduced when pH out of this range (Zhang et al., 2013). Thus, after the pretreatment, adding extra seed sludge or inoculum (dosing ratio: 10–30%) into the digester was necessary to maintain the sustainable methane production (Li et al., 2012; Appels et al., 2010), which however not only might increase the operating cost, but also occupied the considerable digestion space to decrease waste sludge load. Zero-valent iron (ZVI), as a reductive material, has been widely applied in wastewater treatment, groundwater purification and soil remediation (Jiang et al., 2011). In previous study, it was found that ZVI could decline the oxidation–reduction potential (ORP) to
http://dx.doi.org/10.1016/j.wasman.2015.01.036 0956-053X/Ó 2015 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Zhang, Y., et al. Zero-valent iron enhanced methanogenic activity in anaerobic digestion of waste activated sludge after heat and alkali pretreatment. Waste Management (2015), http://dx.doi.org/10.1016/j.wasman.2015.01.036
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create a more favorable environment for anaerobic wastewater treatment when it was added into anaerobic systems (Liu et al., 2012). In another previous work, adding ZVI in an anaerobic digestion of waste sludge was confirmed to enhance methane production (Feng et al., 2013). Specifically, the methane production was raised by about 21% at 10 g/L of iron scrap added into the anaerobic sludge digester. Based on the consideration above, it was assumed that the addition of ZVI in anaerobic digestion was likely to improved the methanogenic activity to make the anaerobic system rapidly recovered from the heat and/or alkali inhibition. If this hypothesis is true, the sludge after the heat and/or alkali treatment might be directly processed for anaerobic digestion with no need of supplementing methanogens or seed sludge. The objectives of this work are (1) to investigate the effects of ZVI on the methanogenesis after the heat pretreatment and/or under alkali condition; and (2) to enhance the methane production from the suppressed sludge by adding ZVI. We expected to provide a new idea in enhancing the digestion of sludge suppressed by heat or alkali pretreatment. 2. Material and methods
digestion, the biogas produced from each bottle was collected into gasbag for analysis. After the digestion, the mixture was poured out, and their supernatant and remainder sludge were analyzed, respectively. 2.3. Influence of ZVI on methane production from sludge under alkali condition Another experiment was operated under alkali condition to compare the methane production from the sludge digestion with four dosage levels of ZVI (0, 1, 5, 10 g/L). Zhao et al. (2010) found that anaerobic fermentation under pH 10 could significantly enhance the hydrolysis–acidification and decrease the methanogenic activity. Although ZVI could alleviate the anaerobic acidity to help the system kept at a neutral pH through the reaction of Fe + 2H+ = Fe2+ + H2, it had no capacity to make the pH of digestion system up to 10. Therefore, 4 M NaOH was added to maintain the pH of all serum bottles at 10 during the digestion no matter how much ZVI was dosed. The digestion was lasted for 8 d. Other experimental conditions were the same as the sludge digestion with the heat-treated sludge. All the experiments were repeated twice, and their mean values were used as the experimental results.
2.1. Characteristic of waste sludge 2.4. Analysis Waste activated sludge used in this study was obtained from the secondary sedimentation tank of a municipal wastewater treatment plant in Dalian, China. The sludge was concentrated by settling for 24 h, and stored at 4 °C before use. It was reported that most methanogens could be killed at 102 °C for 30 min (Gallert and Winter, 1997). Therefore the raw sludge was heated at 102 °C for 30 min to become the heat-treated sludge. The characteristics of the waste activated sludge and heat-treated sludge are listed in Table 1. 2.2. Effects of ZVI on methane production from heat pretreated sludge Adding inoculum sludge into waste sludge is a common method to proceed a sludge digestion (the inoculum seed ratio ranges from 10% to 30%). This model had been operated in our previous works to investigate the effects of ZVI (Feng et al., 2013). In the present study, in order to more directly observe the function of ZVI in recovering the methanogenic activity from the pretreatment, the sludge digestion was operated under no addition of inoculums but with whole pretreated sludge. After the sludge was heated and cooled down to the room temperature, 200 mL of the sludge was added into five 250 mL serum bottles, respectively. Five dosage levels of ZVI powder (0, 1, 2, 5 and 10 g/L, 0.2 mm diameter, 0.05 m2/g BET surface area, purity >98%) were added into the five serum bottles above, respectively. All serum bottles were capped with rubber stoppers and flushed with nitrogen gas to remove oxygen before the anaerobic digestion. The bottles were placed in an air-bath shaker (120 rpm) at 35 ± 1 °C for 20 d. During the
Table 1 Characteristics of the raw sludge and heat-treated sludge. Parameters
Raw sludge
Heat-treated sludge
pH 7.0 ± 0.2 7.0 ± 0.2 TSS (total suspended solids, mg/L) 11,832 ± 749 10,885 ± 868 VSS (volatile suspended solids, mg/L) 7219 ± 401 6497 ± 286 TCOD (total chemical oxygen demand, mg/L) 12,194 ± 1345 10,730 ± 1171 SCOD (soluble chemical oxygen demand, mg/L) 384 ± 34 662 ± 61 Total protein (mg/L) 3511 ± 212 3230 ± 335 Total polysaccharide (mg/L) 868 ± 39 781 ± 71 Standard deviation obtained from triplicate tests.
Sludge samples from the reactors were analyzed for total suspended solid (TSS), volatile suspended solids (VSS), total protein and total polysaccharide. Then the samples were centrifuged at 8000 rpm for 10 min and immediately filtered through a cellulose membrane with a pore size of 0.45 lm for analysis of soluble COD (SCOD), soluble protein, soluble polysaccharide and VFAs. TSS, VSS and SCOD were determined according to Standard Methods for the Examination of Water and Wastewater (APHA., 1998). Proteins were measured with Lowry’s method using bovine serum albumin as a standard solution (Fr et al., 1995). Polysaccharide was measured with phenol–sulfuric acid method using glucose as a standard solution (Masuko et al., 2005). The equivalent relationships between COD and substrates were as follows: 1.5 g-COD/g protein, 1.06 g-COD/g carbohydrate, 1.07 g-COD/g acetate, 1.51 g-COD/g propionate, 1.82 g-COD/g butyrate, and 2.04 g-COD/g valerate (Lu et al., 2012). The ORP was measured using an ORP combination glass-body redox electrode (Sartorius PY-R01, Germany). The components of biogas (including methane, hydrogen and carbon dioxide) were analyzed with a gas chromatograph (Shimadzu, GC-14C) equipped with a thermal conductivity detector and a 1.5 m stainless-steel column (Molecular Sieve, 80/100 mesh). The temperatures of injector, detector and column were kept at 100 °C, 105 °C and 60 °C according to Zhao and Yu (2008). Nitrogen was used as the carrier gas at a flow rate of 30 ml/min. VFAs (acetate, propionate, butyrate and valerate) were measured in another gas chromatograph (Shimadzu, GC2010) with GC-flame ionization detector, FID (Shimadzu, Model 14B) and a 30 m 0.25 mm 0.25 lm fused-silica capillary column (DB-FFAP). The operating temperatures for the injection port and the FID were 170 °C. The temperature in the oven was gradually increased from 100 to 130 °C at a rate of 5 °C/min according to Fan et al. (2006). 3. Results and discussion 3.1. Effects of ZVI on methane production from heat-pretreated sludge 3.1.1. Biogas production It was reported that heat pretreatment could ruptures the cell wall and cell membranes of bacteria in the waste sludge (Carrère et al., 2010). It caused the complex organic molecules such as
Please cite this article in press as: Zhang, Y., et al. Zero-valent iron enhanced methanogenic activity in anaerobic digestion of waste activated sludge after heat and alkali pretreatment. Waste Management (2015), http://dx.doi.org/10.1016/j.wasman.2015.01.036
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3.1.2. VFA production Sludge acidification is the result that the soluble organics with low molecular weight were decomposed by acidogenic bacteria. As the main products in the acidification of sludge, VFA would be consumed by methanogens. Thus, the methanogenic activity could also be evaluated by the content of VFA in the digesters (Ahring et al., 1995). As shown in Fig. 2a, the initial concentration of VFA was 351 COD-mg/L, and then increased rapidly. After 8-day fermentation, the production of VFA reached the maximum for all groups and their concentrations were as follows: the control group (2339.4 mg-COD/L) > 1 g/L (2080.0) > 2 g/L (1555.3) > 5 g/L (1405.2) 10 g/L (1461.1). At 20 d, the VFA concentration decreased, and almost the same order of ZVI dosage was observed, i.e., the control (805.4 mg-COD/L) > 1 g/L (696.4) > 2 g/L (295.9) 5 g/L (318.7) > 10 g/L (262.1). The cumulative VFA yield in the control reactor was higher than the other groups during the whole digestion, in agreement with the low methanogenic activity. The concentration and percentage of individual VFA after 20 day fermentation are shown in Table 2. The VFA produced from waste sludge mainly included acetate, propionate, butyrate and valerate. After the fermentation, acetate was the most prevalent VFA for all groups, accounting for 30–45%. The concentration of acetate was 379.2, 311.4, 103.0, 90.8 and 78.6 mg-COD/L for 0, 1, 2, 5 and 10 g/L of ZVI, respectively. It is well-known that acetate was degraded into CH4 and CO2 directly by methanogens, resulting in the acetate decrease. This result indicated that the addition of
2500
(a)
400
(a) 2000
Control 1 g/L 2 g/L 5 g/L 10 g/L
300
200
100
0
25.9%, 48.4%, 52.5% and 57.2% for ZVI of 1, 2, 5 and 10 g/L. Although the methanogenic activity was recovered over time in the control reactor, the concentration of methane was still lower than the reactor with ZVI (58.4% for the control and 64% for 10 g/L ZVI at day 20).
TVFA (mg-COD/L)
Cumulative methane yield (mL)
proteins, carbohydrates, lipids and nucleic acids to be released from the cells and be broken down. On the other hand, the heat treatment of sludge inactivated methanogenic microorganisms, but had little effect on spore-forming anaerobic bacteria in the hydrolysis–acidification process (Lay and Fan, 2003). The biogas of the sludge after the heat treatment is recorded in Fig. 1. From Fig. 1a, the methane production increased with the increase of ZVI dosage. For the control group (without ZVI added), there was little methane generated in the first 13 days, and then the methane production increased over the fermentation time. This meant that the methanogenic activity was quite low after the heat pretreatment, and its activity was not recovered in a short period. At the ZVI dosage of 1 g/L, there was a little increase in the methane production. When further increasing the ZVI dosage, the increment of the methane production was more obvious. For 10 g/L of ZVI, the methane production was observed in the initial fourth day. This result indicated that the ZVI accelerated the recovery of the methanogenic activity which was suppressed by the heat pretreatment. After 20 d, at ZVI of 0, 1, 2, 5 and 10 g/L, the cumulative methane production was 201.5, 215.4, 259.5, 301.7 and 385.9 mL, respectively, or 155.0, 165, 199.6, 232 and 296.8 mL/g-VSS, respectively. In other words, the methane productivity at the dosage of 10 g/L increased by 91.5% compared to the no-ZVI dosage. The methane production at 10 g ZVI approached to references (Heo et al., 2003). From Fig. 1b, when increasing ZVI from 0 to 10 g/L, the CO2 production rose from 129.9 to 188.5 mL. It was because CO2 was a byproduct of the anaerobic digestion and higher CO2 indicated more substrates were mineralized. This result was in agreement with the methane production. Also, the percentage of methane in the biogas increased significantly with the addition of ZVI (data no shown). At day 14, the methane percentage in the control reactor was only 11.4%, whereas this percentage reached
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(b)
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Fermentation time (days) Fig. 1. Biogas production (a) methane and (b) carbon dioxide.
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Fermentation time (days) Fig. 2. Effect of ZVI on TVFA (a) and SCOD (b).
Please cite this article in press as: Zhang, Y., et al. Zero-valent iron enhanced methanogenic activity in anaerobic digestion of waste activated sludge after heat and alkali pretreatment. Waste Management (2015), http://dx.doi.org/10.1016/j.wasman.2015.01.036
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Table 2 Individual VFA concentration and percentage. ZVI dosage
Acetate Propionate Butyrate Valerate
Control
1
2
5
379.2 (47) 40.3 (5) 135.5 (17) 250.2 (31)
311.4 (45) 37.3 (5) 116.3 (17) 231.2 (33)
103.0 (35) 25.1 (8) 88.5 (30) 79.3 (27)
90.8 22.5 78.8 47.0
10 (38) (9) (33) (20)
78.6 25.4 81.1 77.0
(30) (10) (31) (29)
concentration of VSS was 6497 mg/L before the digestion, and after the digestion its contents decreased to 4562, 4374, 4178, 3870 and 3743 mg/L at the ZVI of 0, 1, 2, 5 and 10 g/L, respectively. It indicated that the sludge reduction increased from 29.8% to 42.4% as increasing ZVI from 0 to 10 g/L. After the digestion, the TCOD in the sludge decreased correspondingly. As increasing ZVI from 0 to 10 g/L, the concentration of TCOD decreased from 8300 to 6796 mg/L, and the removal ratio increased from 22.6% to 37.0%.
The concentrations are expressed in mg/L with the percentage in bracket.
3.2. Influence of ZVI on methane production under alkali condition
3.1.3. Sludge reduction Sludge reduction was another parameter to evaluate the performance of anaerobic digestion. Most of organic matters from the sludge were decomposed and even mineralized after anaerobic digestion. VSS and TCOD are frequently used to characterize the sludge reduction rate. From Fig. 3, the VSS and TCOD after the fermentation decreased with the increase of ZVI dosage. The
It was reported that most methanogens could only grow at a narrow pH range from 6.8 to 7.2. Alkali or acidic pH significantly suppressed the activity of methanogens (Zhang et al., 2013). The methanogenic activity was investigated under alkali condition (pH 10) with the addition of ZVI in this study. The total biogas production during fermentation is shown in Fig. 4a. For the control group (no ZVI added), hydrogen was the main compound in the biogas, accounting for 78.2% of the biogas (Fig. 4b). As an intermediate in anaerobic digestion, hydrogen would be accumulated when the hydrogen consumers (mainly methanogens) were inhibited. This indicated that the methanogenic activity was very low for the control group. Also, the solubility of CO2 would increase under pH 10 which might decrease the concentration of CO2 in the biogas. This result was in agreement with the report of Zhao et al. (2010), who found the yield of hydrogen increased significantly under pH 10 (yield about 27 mL/g-VSS). With the increase of ZVI dosage from 0 to 10 g/L, the hydrogen production decreased and the methane production increased. For 0, 1, 5 and 10 g/L of ZVI the hydrogen yield was 27.6, 28.3, 23.7 and 18.8 mL, accounting for 78.2%, 76.0%, 53.0% and 28.9% of total biogas, respectively. The methane yield was 1.9, 2.6, 13.1 and 32.2 mL for 0, 1, 5 and 10 g/L of ZVI, respectively. It suggested that ZVI enhanced the methanogenic activity under the alkali condition.
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(a) 60
Yield (mL)
ZVI enhanced the methanogenic activity and accelerated the consumption of VFA. SCOD, as the production of the hydrolysis and acidification, mainly included soluble protein, soluble polysaccharide and VFA. Generally, SCOD increased in the beginning of the fermentation. Afterwards, SCOD would decrease when they were decomposed to VFA and further mineralized to CH4 and CO2. Fig. 2b shows the changes of SCOD during the digestion. With the increase of ZVI dosage, the SCOD decreased, which was in accordance with the enhancement of methanogenesis by ZVI. Previous study proved that the addition of ZVI enhanced the formation of VFA, especially the formation of acetate (Feng et al., 2013). It was because ZVI could maintain ORP at a relatively lower ORP level to enhance the acetic-type fermentation and weaken the propionic-type fermentation (Meng et al., 2013). On the other hand, the addition of ZVI promoted a number of key enzymes activities in the acetogenesis process (Feng et al., 2013). In this study, an opposite phenomenon was observed that the concentration of acetate was decreased with the increase of ZVI dosage. The reason might be attributed to the enrichment of acetoclastic methanogens. In anaerobic digestion process, methanogenic archaea are generally divided into two main groups based on their available substrates (Karri et al., 2006), i.e., acetoclastic methanogenes responsible for conversion of acetate into CH4 and hydrogenotrophic methanogenes capable of converting H2/CO2 into CH4. Among them, acetoclastic methanogenes have a major role in CH4 production (approximately 70% of CH4 is formed from acetate) (Koster et al., 1986). This result was in agreement with the report by Yang et al. (2013), who found that the addition of nano-ZVI enhanced the population of acetoclastic methanogens.
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Fig. 3. Concentration of VSS and TCOD after the digestion.
Fig. 4. The total yield and composition of the biogas from fermentation (a) biogas concentration and (b) percentage.
Please cite this article in press as: Zhang, Y., et al. Zero-valent iron enhanced methanogenic activity in anaerobic digestion of waste activated sludge after heat and alkali pretreatment. Waste Management (2015), http://dx.doi.org/10.1016/j.wasman.2015.01.036
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H2 concentration is considered as an indicative parameter to evaluate anaerobic digester performance. The decrease of hydrogen concentration and increment of methane concentration were the results from the acceleration of CO2 utilization by hydrogen-utilizing microorganisms according to the reaction (1) and (2).
CO2 þ 4H2 ¼ CH4 þ 2H2 O
ð1Þ
CO2 þ 4Fe0 þ 8Hþ ¼ CH4 þ 4Fe2þ þ 2H2 O
ð2Þ
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The reaction (1) was one of main pathways for the methane generation driven by hydrogenotrophic methanogens, accounting for about 30–40% of the total methane production (Kotsyurbenko et al., 2004). In this process, hydrogen played a predominant role to serve as the electron carrier between acidification bacteria and hydrogenotrophic methanogens. The lack of hydrogenotrophic methanogens would result in the accumulation of hydrogen to impede the decomposition of organic acids because hydrogen was considered as a thermodynamically unfavorable intermediate (Siriwongrungson et al., 2007; Fukuzaki et al., 1990). Schmidt and Ahring (1993) reported that the addition of H2-utilizing methanogenes into disintegrating granules increased the degradation rate of both propionate and butyrate. Accordingly, the enrichment of hydrogenotrophic methanogens would enhance the degradation efficiency of intermediate VFA in an anaerobic system. It was also proved that the ZVI could directly serve as an electron donor for hydrogenotrophic methanogens to enhance the growth of hydrogenotrophic methanogens according to the reaction (2) (Daniels et al., 1987). It might decrease the partial pressure of hydrogen to drive the anaerobic digestion especially methanogenesis to process. It was reported that the waste sludge fermentation under alkali condition enhanced the hydrolysis–acidification of sludge (Yuan et al., 2006). However, the drawback of this process was low-rate methanogenic activity, impeding the complete mineralization of organic matters. To address this drawback, Zhang et al. (2010) proposed a second fermentation using the alkali– fermented sludge with pH adjustment and methanogens addition. Although a high methane yield was achieved, the complex operation and additional chemicals addition made it difficult to be established in the full-scale application. A better strategy perhaps was that both hydrolysis–acidification and methanogenesis were enhanced simultaneously under the alkali condition. Fig. 5 shows the VSS concentration and reduction ratio after the fermentation. The enhancement of hydrolysis–acidification of sludge under the alkali condition was significant. The VSS removal ratio of the control group was 44.4%. As increasing ZVI from 0 to 10 g/L,
30
Fig. 5. VSS concentration and reduction efficiency after the digestion (the concentration of VSS was expressed by pillar and its removal efficiency was expressed by diamond).
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the concentration of VSS decreased from 5.02 to 3.56 g/L, and the removal ratio increased from 44.4% to 60.06%. These results indicated that addition of ZVI could decrease the alkali inhibition on the methane production to accelerate sludge digestion. 4. Conclusion The methanogens was sensitive to the environment, and the variation of environment factors generally made against methanogenesis and thus reduced the methane yield in anaerobic sludge digestion. In this study, the addition of ZVI into anaerobic sludge digester could efficiently recover the methanogenic activity suppressed by heat-pretreatment and alkali condition, respectively. ZVI shortened the lag time of methane production, and increase the methane yield by 91.5%. The decomposition of acetate and hydrogen were accelerated by ZVI, indicating that the acetoclastic methanogenesis and H2-utilizing methanogenesis were enhanced. Also the VSS removal increased from 44.4% to 60.06% after the digestion at pH 10. This study suggested that ZVI could efficiently enhance methanogenenesis to improve the anaerobic sludge digestion even if methanogens were removed or totally inhibited in the pretreatment. Possibly, ZVI made the sludge digestion directly proceed with no need of adding seed sludge after the heat or alkali pretreatment, which not only might decrease the operating cost, but also could save a considerable digester space to have a higher sludge treatment load. Acknowledgment The authors acknowledge the financial support from the National Natural Scientific Foundation of China (51378087, 21177015). References Ahring, B.K., Sandberg, M., Angelidaki, I., 1995. Volatile fatty acids as indicators of process imbalance in anaerobic digestors. Appl. Microbiol. Biotechnol. 43, 559– 565. A.P.H.A., 1998. Standard Methods for the Examination of Water and Wastewater. Appels, L., Degrève, J., Van der Bruggen, B., Van Impe, J., Dewil, R., 2010. Influence of low temperature thermal pre-treatment on sludge solubilisation, heavy metal release and anaerobic digestion. Bioresour. Technol. 101, 5743–5748. Carrère, H., Dumas, C., Battimelli, A., Batstone, D., Delgenes, J., Steyer, J., Ferrer, I., 2010. Pretreatment methods to improve sludge anaerobic degradability: a review. J. Hazard. Mater. 183, 1–15. Daniels, L., Belay, N., Rajagopal, B.S., Weimer, P.J., 1987. Bacterial methanogenesis and growth from CO2 with elemental iron as the sole source of electrons. Science 237, 509–511. Dunfield, P., Dumont, R., Moore, T.R., 1993. Methane production and consumption in temperate and subarctic peat soils: response to temperature and pH. Soil Biol. Biochem. 25, 321–326. Fan, K.S., Kan, N.r., Lay, J.j., 2006. Effect of hydraulic retention time on anaerobic hydrogenesis in CSTR. Bioresour. Technol. 97, 84–89. Feng, Y., Zhang, Y., Quan, X., Chen, S., 2013. Enhanced anaerobic digestion of waste activated sludge digestion by the addition of zero valent iron. Water Res. 51, 242–250. Fr, B., Griebe, T., Nielsen, P., 1995. Enzymatic activity in the activated-sludge floc matrix. Appl. Microbiol. Biotechnol. 43, 755–761. Fukuzaki, S., Nishio, N., Shobayashi, M., Nagai, S., 1990. Inhibition of the fermentation of propionate to methane by hydrogen, acetate, and propionate. Appl. Environ. Microbiol. 56, 719–723. Gallert, C., Winter, J., 1997. Mesophilic and thermophilic anaerobic digestion of source-sorted organic wastes: effect of ammonia on glucose degradation and methane production. Appl. Microbiol. Biotechnol. 48, 405–410. Heo, N., Park, S., Lee, J., Kang, H., 2003. Solubilization of waste activated sludge by alkaline pretreatment and biochemical methane potential (BMP) tests for anaerobic co-digestion of municipal organic waste. Water Sci. Technol. 48 (8), 211–219. Jiang, Z., Lv, L., Zhang, W., Du, Q., Pan, B., Yang, L., Zhang, Q., 2011. Nitrate reduction using nanosized zero-valent iron supported by polystyrene resins: role of surface functional groups. Water Res. 45, 2191–2198. Karri, S., Sierra-Alvarez, R., Field, J.A., 2006. Toxicity of copper to acetoclastic and hydrogenotrophic activities of methanogens and sulfate reducers in anaerobic sludge. Chemosphere 62, 121–127.
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Koster, I., Rinzema, A., De Vegt, A., Lettinga, G., 1986. Sulfide inhibition of the methanogenic activity of granular sludge at various pH-levels. Water Res. 20, 1561–1567. Kotsyurbenko, O.R., Chin, K.J., Glagolev, M.V., Stubner, S., Simankova, M.V., Nozhevnikova, A.N., Conrad, R., 2004. Acetoclastic and hydrogenotrophic methane production and methanogenic populations in an acidic WestSiberian peat bog. Environ. Microbiol. 6, 1159–1173. Lagerkvist, A., Morgan, S.F., 2012. The effects of substrate pre-treatment on anaerobic digestion systems: a review. Waste Manage. 32, 1634–1650. Lay, J.-J., Fan, K.S., 2003. Influence of chemical nature of organic wastes on their conversion to hydrogen by heat-shock digested sludge. Int. J. Hydrogen Energy 28, 1361–1367. Lee, I., Han, J.I., 2013. The effects of waste-activated sludge pretreatment using hydrodynamic cavitation for methane production. Ultrason. Sonochem. 20, 1450–1455. Li, H., Li, C., Liu, W., Zou, S., 2012. Optimized alkaline pretreatment of sludge before anaerobic digestion. Bioresour. Technol. 123, 189–194. Liu, Y., Zhang, Y., Quan, X., Li, Y., Zhao, Z., Meng, X., Chen, S., 2012. Optimization of anaerobic acidogenesis by adding Fe0 powder to enhance anaerobic wastewater treatment. Chem. Eng. J. 192, 179–185. Lu, L., Xing, D., Liu, B., Ren, N., 2012. Enhanced hydrogen production from waste activated sludge by cascade utilization of organic matter in microbial electrolysis cells. Water Res. 46, 1015–1026. Masuko, T., Minami, A., Iwasaki, N., Majima, T., Nishimura, S.I., Lee, Y.C., 2005. Carbohydrate analysis by a phenol–sulfuric acid method in microplate format. Anal. Biochem. 339, 69–72. Meng, X., Zhang, Y., Li, Q., Quan, X., 2013. Adding Fe0 powder to enhance the anaerobic conversion of propionate to acetate. Biochem. Eng. J. 73, 80–85. Oh, S.-E., Van Ginkel, S., Logan, B.E., 2003. The relative effectiveness of pH control and heat treatment for enhancing biohydrogen gas production. Environ. Sci. Technol. 37, 5186–5190.
Rani, R.U., Kaliappan, S., Kumar, S.A., Banu, J.R., 2012. Combined treatment of alkaline and disperser for improving solubilization and anaerobic biodegradability of dairy waste activated sludge. Bioresour. Technol. 126, 107–116. Schmidt, J.E., Ahring, B.K., 1993. Effects of hydrogen and formate on the degradation of propionate and butyrate in thermophilic granules from an upflow anaerobic sludge blanket reactor. Appl. Environ. Microbiol. 59, 2546–2551. Siriwongrungson, V., Zeng, R.J., Angelidaki, I., 2007. Homoacetogenesis as the alternative pathway for H2 sink during thermophilic anaerobic degradation of butyrate under suppressed methanogenesis. Water Res. 41, 4204–4210. Yang, Y., Guo, J., Hu, Z., 2013. Impact of nano zero valent iron (NZVI) on methanogenic activity and population dynamics in anaerobic digestion. Water Res. 47, 6790–6800. Yuan, H., Chen, Y., Zhang, H., Jiang, S., Zhou, Q., Gu, G., 2006. Improved bioproduction of short-chain fatty acids (SCFAs) from excess sludge under alkaline conditions. Environ. Sci. Technol. 40, 2025–2029. Zhang, D., Chen, Y., Zhao, Y., Zhu, X., 2010. New sludge pretreatment method to improve methane production in waste activated sludge digestion. Environ. Sci. Technol. 44, 4802–4808. Zhang, C., Su, H., Tan, T., 2013. Batch and semi-continuous anaerobic digestion of food waste in a dual solid–liquid system. Bioresour. Technol. 145, 10–16. Zhao, Q.B., Yu, H.Q., 2008. Fermentative H2 production in an upflow anaerobic sludge blanket reactor at various pH values. Bioresour. Technol. 99, 1353–1358. Zhao, Y., Chen, Y., Zhang, D., Zhu, X., 2010. Waste activated sludge fermentation for hydrogen production enhanced by anaerobic process improvement and acetobacteria inhibition: the role of fermentation pH. Environ. Sci. Technol. 44, 3317–3323.
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Zero-valent iron enhanced methanogenic activity in anaerobic digestion of waste activated sludge after heat and alkali pretreatment Yaobin Zhang ⇑, Yinghong Feng ⇑, Xie Quan Dalian University of Technology, Key Laboratory of Industrial Ecology and Environmental Engineering, China
a r t i c l e
i n f o
Article history: Received 7 May 2014 Accepted 23 January 2015 Available online xxxx Keywords: Waste activated sludge Anaerobic digestion Zero-valent iron (ZVI) Heat pretreatment Alkali treatment
a b s t r a c t Heat or alkali pretreatment is the effective method to improve hydrolysis of waste sludge and then enhance anaerobic sludge digestion. However the pretreatment may inactivate the methanogens in the sludge. In the present work, zero-valent iron (ZVI) was used to enhance the methanogenic activity in anaerobic sludge digester under two methanogens-suppressing conditions, i.e. heat-pretreatment and alkali condition respectively. With the addition of ZVI, the lag time of methane production was shortened, and the methane yield increased by 91.5% compared to the control group. The consumption of VFA was accelerated by ZVI, especially for acetate, indicating that the acetoclastic methanogenesis was enhanced. In the alkali-condition experiment, the hydrogen produced decreased from 27.6 to 18.8 mL when increasing the ZVI dosage from 0 to 10 g/L. Correspondingly, the methane yield increased from 1.9 to 32.2 mL, which meant that the H2-utilizing methanogenes was enriched. These results suggested that the addition of ZVI into anaerobic digestion of sludge after pretreated by the heat or alkali process could efficiently recover the methanogenic activity and increase the methane production and sludge reduction. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Waste activated sludge generated from biological wastewater treatment processes has increased continuously in the recent decades due to increasing population and new construction of waste water treatment plants. Large amount of organic compounds containing in the waste sludge presents a potential threat to environment for example leaching of liquid and odor (Lee and Han, 2013). Anaerobic digestion is the most applied technique for waste activated sludge stabilization because of its high abilities to transform organic matters into biogas mixture of methane and carbon dioxide. However, the yield of methane is often limited by slow hydrolysis of sludge. To improve the hydrolysis of waste sludge, chemical, mechanical and biological methods (Lagerkvist and Morgan, 2012; Rani et al., 2012) have been applied to pretreat sludge to disintegrate sludge cells and release intracellular materials into the water phase. Heat treatment was one of prevalent method to improve the hydrolysis of waste sludge since its simple operation (Oh et al., 2003). Hydrolysis of sludge under alkali condition developed by Zhao et al. (2010) also gained attention in recent years due to its ⇑ Corresponding authors. Tel.: +86 411 8470 6140; fax: +86 411 8470 6263 (Y. Zhang). Tel.: +86 411 8470 6460; fax: +86 411 8470 6263 (Y. Feng). E-mail addresses: [email protected] (Y. Zhang), [email protected] (Y. Feng).
dramatic enhancement of the hydrolysis of sludge. Although improving the hydrolysis–acidification of sludge, the heat and alkali processes are harmful to methanogenesis because methanogens grow only under an appropriate temperature and a narrowly neutral pH range (Dunfield et al., 1993). Three temperature regimes can be used in anaerobic digesters: psychrophilic, mesophilic and thermophilic with varied optimum temperature ranges for the domination of different strains of methane-forming bacteria. Psychrophilic fermentors operate at about 25 °C, mesophilic ones at around 35 °C and thermophilic at around 55 °C. The heat treatment at 102 °C for approximately 30 min could kill most methanogens (Gallert and Winter, 1997). The alkali pH was also a factor in preventing methanogenic activity. Methanogenic bacteria are extremely sensitive to pH and prefer pH around 6.8–7.2 as the growth rate of methanogens is greatly reduced when pH out of this range (Zhang et al., 2013). Thus, after the pretreatment, adding extra seed sludge or inoculum (dosing ratio: 10–30%) into the digester was necessary to maintain the sustainable methane production (Li et al., 2012; Appels et al., 2010), which however not only might increase the operating cost, but also occupied the considerable digestion space to decrease waste sludge load. Zero-valent iron (ZVI), as a reductive material, has been widely applied in wastewater treatment, groundwater purification and soil remediation (Jiang et al., 2011). In previous study, it was found that ZVI could decline the oxidation–reduction potential (ORP) to
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Please cite this article in press as: Zhang, Y., et al. Zero-valent iron enhanced methanogenic activity in anaerobic digestion of waste activated sludge after heat and alkali pretreatment. Waste Management (2015), http://dx.doi.org/10.1016/j.wasman.2015.01.036
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create a more favorable environment for anaerobic wastewater treatment when it was added into anaerobic systems (Liu et al., 2012). In another previous work, adding ZVI in an anaerobic digestion of waste sludge was confirmed to enhance methane production (Feng et al., 2013). Specifically, the methane production was raised by about 21% at 10 g/L of iron scrap added into the anaerobic sludge digester. Based on the consideration above, it was assumed that the addition of ZVI in anaerobic digestion was likely to improved the methanogenic activity to make the anaerobic system rapidly recovered from the heat and/or alkali inhibition. If this hypothesis is true, the sludge after the heat and/or alkali treatment might be directly processed for anaerobic digestion with no need of supplementing methanogens or seed sludge. The objectives of this work are (1) to investigate the effects of ZVI on the methanogenesis after the heat pretreatment and/or under alkali condition; and (2) to enhance the methane production from the suppressed sludge by adding ZVI. We expected to provide a new idea in enhancing the digestion of sludge suppressed by heat or alkali pretreatment. 2. Material and methods
digestion, the biogas produced from each bottle was collected into gasbag for analysis. After the digestion, the mixture was poured out, and their supernatant and remainder sludge were analyzed, respectively. 2.3. Influence of ZVI on methane production from sludge under alkali condition Another experiment was operated under alkali condition to compare the methane production from the sludge digestion with four dosage levels of ZVI (0, 1, 5, 10 g/L). Zhao et al. (2010) found that anaerobic fermentation under pH 10 could significantly enhance the hydrolysis–acidification and decrease the methanogenic activity. Although ZVI could alleviate the anaerobic acidity to help the system kept at a neutral pH through the reaction of Fe + 2H+ = Fe2+ + H2, it had no capacity to make the pH of digestion system up to 10. Therefore, 4 M NaOH was added to maintain the pH of all serum bottles at 10 during the digestion no matter how much ZVI was dosed. The digestion was lasted for 8 d. Other experimental conditions were the same as the sludge digestion with the heat-treated sludge. All the experiments were repeated twice, and their mean values were used as the experimental results.
2.1. Characteristic of waste sludge 2.4. Analysis Waste activated sludge used in this study was obtained from the secondary sedimentation tank of a municipal wastewater treatment plant in Dalian, China. The sludge was concentrated by settling for 24 h, and stored at 4 °C before use. It was reported that most methanogens could be killed at 102 °C for 30 min (Gallert and Winter, 1997). Therefore the raw sludge was heated at 102 °C for 30 min to become the heat-treated sludge. The characteristics of the waste activated sludge and heat-treated sludge are listed in Table 1. 2.2. Effects of ZVI on methane production from heat pretreated sludge Adding inoculum sludge into waste sludge is a common method to proceed a sludge digestion (the inoculum seed ratio ranges from 10% to 30%). This model had been operated in our previous works to investigate the effects of ZVI (Feng et al., 2013). In the present study, in order to more directly observe the function of ZVI in recovering the methanogenic activity from the pretreatment, the sludge digestion was operated under no addition of inoculums but with whole pretreated sludge. After the sludge was heated and cooled down to the room temperature, 200 mL of the sludge was added into five 250 mL serum bottles, respectively. Five dosage levels of ZVI powder (0, 1, 2, 5 and 10 g/L, 0.2 mm diameter, 0.05 m2/g BET surface area, purity >98%) were added into the five serum bottles above, respectively. All serum bottles were capped with rubber stoppers and flushed with nitrogen gas to remove oxygen before the anaerobic digestion. The bottles were placed in an air-bath shaker (120 rpm) at 35 ± 1 °C for 20 d. During the
Table 1 Characteristics of the raw sludge and heat-treated sludge. Parameters
Raw sludge
Heat-treated sludge
pH 7.0 ± 0.2 7.0 ± 0.2 TSS (total suspended solids, mg/L) 11,832 ± 749 10,885 ± 868 VSS (volatile suspended solids, mg/L) 7219 ± 401 6497 ± 286 TCOD (total chemical oxygen demand, mg/L) 12,194 ± 1345 10,730 ± 1171 SCOD (soluble chemical oxygen demand, mg/L) 384 ± 34 662 ± 61 Total protein (mg/L) 3511 ± 212 3230 ± 335 Total polysaccharide (mg/L) 868 ± 39 781 ± 71 Standard deviation obtained from triplicate tests.
Sludge samples from the reactors were analyzed for total suspended solid (TSS), volatile suspended solids (VSS), total protein and total polysaccharide. Then the samples were centrifuged at 8000 rpm for 10 min and immediately filtered through a cellulose membrane with a pore size of 0.45 lm for analysis of soluble COD (SCOD), soluble protein, soluble polysaccharide and VFAs. TSS, VSS and SCOD were determined according to Standard Methods for the Examination of Water and Wastewater (APHA., 1998). Proteins were measured with Lowry’s method using bovine serum albumin as a standard solution (Fr et al., 1995). Polysaccharide was measured with phenol–sulfuric acid method using glucose as a standard solution (Masuko et al., 2005). The equivalent relationships between COD and substrates were as follows: 1.5 g-COD/g protein, 1.06 g-COD/g carbohydrate, 1.07 g-COD/g acetate, 1.51 g-COD/g propionate, 1.82 g-COD/g butyrate, and 2.04 g-COD/g valerate (Lu et al., 2012). The ORP was measured using an ORP combination glass-body redox electrode (Sartorius PY-R01, Germany). The components of biogas (including methane, hydrogen and carbon dioxide) were analyzed with a gas chromatograph (Shimadzu, GC-14C) equipped with a thermal conductivity detector and a 1.5 m stainless-steel column (Molecular Sieve, 80/100 mesh). The temperatures of injector, detector and column were kept at 100 °C, 105 °C and 60 °C according to Zhao and Yu (2008). Nitrogen was used as the carrier gas at a flow rate of 30 ml/min. VFAs (acetate, propionate, butyrate and valerate) were measured in another gas chromatograph (Shimadzu, GC2010) with GC-flame ionization detector, FID (Shimadzu, Model 14B) and a 30 m 0.25 mm 0.25 lm fused-silica capillary column (DB-FFAP). The operating temperatures for the injection port and the FID were 170 °C. The temperature in the oven was gradually increased from 100 to 130 °C at a rate of 5 °C/min according to Fan et al. (2006). 3. Results and discussion 3.1. Effects of ZVI on methane production from heat-pretreated sludge 3.1.1. Biogas production It was reported that heat pretreatment could ruptures the cell wall and cell membranes of bacteria in the waste sludge (Carrère et al., 2010). It caused the complex organic molecules such as
Please cite this article in press as: Zhang, Y., et al. Zero-valent iron enhanced methanogenic activity in anaerobic digestion of waste activated sludge after heat and alkali pretreatment. Waste Management (2015), http://dx.doi.org/10.1016/j.wasman.2015.01.036
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3.1.2. VFA production Sludge acidification is the result that the soluble organics with low molecular weight were decomposed by acidogenic bacteria. As the main products in the acidification of sludge, VFA would be consumed by methanogens. Thus, the methanogenic activity could also be evaluated by the content of VFA in the digesters (Ahring et al., 1995). As shown in Fig. 2a, the initial concentration of VFA was 351 COD-mg/L, and then increased rapidly. After 8-day fermentation, the production of VFA reached the maximum for all groups and their concentrations were as follows: the control group (2339.4 mg-COD/L) > 1 g/L (2080.0) > 2 g/L (1555.3) > 5 g/L (1405.2) 10 g/L (1461.1). At 20 d, the VFA concentration decreased, and almost the same order of ZVI dosage was observed, i.e., the control (805.4 mg-COD/L) > 1 g/L (696.4) > 2 g/L (295.9) 5 g/L (318.7) > 10 g/L (262.1). The cumulative VFA yield in the control reactor was higher than the other groups during the whole digestion, in agreement with the low methanogenic activity. The concentration and percentage of individual VFA after 20 day fermentation are shown in Table 2. The VFA produced from waste sludge mainly included acetate, propionate, butyrate and valerate. After the fermentation, acetate was the most prevalent VFA for all groups, accounting for 30–45%. The concentration of acetate was 379.2, 311.4, 103.0, 90.8 and 78.6 mg-COD/L for 0, 1, 2, 5 and 10 g/L of ZVI, respectively. It is well-known that acetate was degraded into CH4 and CO2 directly by methanogens, resulting in the acetate decrease. This result indicated that the addition of
2500
(a)
400
(a) 2000
Control 1 g/L 2 g/L 5 g/L 10 g/L
300
200
100
0
25.9%, 48.4%, 52.5% and 57.2% for ZVI of 1, 2, 5 and 10 g/L. Although the methanogenic activity was recovered over time in the control reactor, the concentration of methane was still lower than the reactor with ZVI (58.4% for the control and 64% for 10 g/L ZVI at day 20).
TVFA (mg-COD/L)
Cumulative methane yield (mL)
proteins, carbohydrates, lipids and nucleic acids to be released from the cells and be broken down. On the other hand, the heat treatment of sludge inactivated methanogenic microorganisms, but had little effect on spore-forming anaerobic bacteria in the hydrolysis–acidification process (Lay and Fan, 2003). The biogas of the sludge after the heat treatment is recorded in Fig. 1. From Fig. 1a, the methane production increased with the increase of ZVI dosage. For the control group (without ZVI added), there was little methane generated in the first 13 days, and then the methane production increased over the fermentation time. This meant that the methanogenic activity was quite low after the heat pretreatment, and its activity was not recovered in a short period. At the ZVI dosage of 1 g/L, there was a little increase in the methane production. When further increasing the ZVI dosage, the increment of the methane production was more obvious. For 10 g/L of ZVI, the methane production was observed in the initial fourth day. This result indicated that the ZVI accelerated the recovery of the methanogenic activity which was suppressed by the heat pretreatment. After 20 d, at ZVI of 0, 1, 2, 5 and 10 g/L, the cumulative methane production was 201.5, 215.4, 259.5, 301.7 and 385.9 mL, respectively, or 155.0, 165, 199.6, 232 and 296.8 mL/g-VSS, respectively. In other words, the methane productivity at the dosage of 10 g/L increased by 91.5% compared to the no-ZVI dosage. The methane production at 10 g ZVI approached to references (Heo et al., 2003). From Fig. 1b, when increasing ZVI from 0 to 10 g/L, the CO2 production rose from 129.9 to 188.5 mL. It was because CO2 was a byproduct of the anaerobic digestion and higher CO2 indicated more substrates were mineralized. This result was in agreement with the methane production. Also, the percentage of methane in the biogas increased significantly with the addition of ZVI (data no shown). At day 14, the methane percentage in the control reactor was only 11.4%, whereas this percentage reached
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(b)
3000
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Fermentation time (days) Fig. 1. Biogas production (a) methane and (b) carbon dioxide.
0
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Fermentation time (days) Fig. 2. Effect of ZVI on TVFA (a) and SCOD (b).
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Table 2 Individual VFA concentration and percentage. ZVI dosage
Acetate Propionate Butyrate Valerate
Control
1
2
5
379.2 (47) 40.3 (5) 135.5 (17) 250.2 (31)
311.4 (45) 37.3 (5) 116.3 (17) 231.2 (33)
103.0 (35) 25.1 (8) 88.5 (30) 79.3 (27)
90.8 22.5 78.8 47.0
10 (38) (9) (33) (20)
78.6 25.4 81.1 77.0
(30) (10) (31) (29)
concentration of VSS was 6497 mg/L before the digestion, and after the digestion its contents decreased to 4562, 4374, 4178, 3870 and 3743 mg/L at the ZVI of 0, 1, 2, 5 and 10 g/L, respectively. It indicated that the sludge reduction increased from 29.8% to 42.4% as increasing ZVI from 0 to 10 g/L. After the digestion, the TCOD in the sludge decreased correspondingly. As increasing ZVI from 0 to 10 g/L, the concentration of TCOD decreased from 8300 to 6796 mg/L, and the removal ratio increased from 22.6% to 37.0%.
The concentrations are expressed in mg/L with the percentage in bracket.
3.2. Influence of ZVI on methane production under alkali condition
3.1.3. Sludge reduction Sludge reduction was another parameter to evaluate the performance of anaerobic digestion. Most of organic matters from the sludge were decomposed and even mineralized after anaerobic digestion. VSS and TCOD are frequently used to characterize the sludge reduction rate. From Fig. 3, the VSS and TCOD after the fermentation decreased with the increase of ZVI dosage. The
It was reported that most methanogens could only grow at a narrow pH range from 6.8 to 7.2. Alkali or acidic pH significantly suppressed the activity of methanogens (Zhang et al., 2013). The methanogenic activity was investigated under alkali condition (pH 10) with the addition of ZVI in this study. The total biogas production during fermentation is shown in Fig. 4a. For the control group (no ZVI added), hydrogen was the main compound in the biogas, accounting for 78.2% of the biogas (Fig. 4b). As an intermediate in anaerobic digestion, hydrogen would be accumulated when the hydrogen consumers (mainly methanogens) were inhibited. This indicated that the methanogenic activity was very low for the control group. Also, the solubility of CO2 would increase under pH 10 which might decrease the concentration of CO2 in the biogas. This result was in agreement with the report of Zhao et al. (2010), who found the yield of hydrogen increased significantly under pH 10 (yield about 27 mL/g-VSS). With the increase of ZVI dosage from 0 to 10 g/L, the hydrogen production decreased and the methane production increased. For 0, 1, 5 and 10 g/L of ZVI the hydrogen yield was 27.6, 28.3, 23.7 and 18.8 mL, accounting for 78.2%, 76.0%, 53.0% and 28.9% of total biogas, respectively. The methane yield was 1.9, 2.6, 13.1 and 32.2 mL for 0, 1, 5 and 10 g/L of ZVI, respectively. It suggested that ZVI enhanced the methanogenic activity under the alkali condition.
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(a) 60
Yield (mL)
ZVI enhanced the methanogenic activity and accelerated the consumption of VFA. SCOD, as the production of the hydrolysis and acidification, mainly included soluble protein, soluble polysaccharide and VFA. Generally, SCOD increased in the beginning of the fermentation. Afterwards, SCOD would decrease when they were decomposed to VFA and further mineralized to CH4 and CO2. Fig. 2b shows the changes of SCOD during the digestion. With the increase of ZVI dosage, the SCOD decreased, which was in accordance with the enhancement of methanogenesis by ZVI. Previous study proved that the addition of ZVI enhanced the formation of VFA, especially the formation of acetate (Feng et al., 2013). It was because ZVI could maintain ORP at a relatively lower ORP level to enhance the acetic-type fermentation and weaken the propionic-type fermentation (Meng et al., 2013). On the other hand, the addition of ZVI promoted a number of key enzymes activities in the acetogenesis process (Feng et al., 2013). In this study, an opposite phenomenon was observed that the concentration of acetate was decreased with the increase of ZVI dosage. The reason might be attributed to the enrichment of acetoclastic methanogens. In anaerobic digestion process, methanogenic archaea are generally divided into two main groups based on their available substrates (Karri et al., 2006), i.e., acetoclastic methanogenes responsible for conversion of acetate into CH4 and hydrogenotrophic methanogenes capable of converting H2/CO2 into CH4. Among them, acetoclastic methanogenes have a major role in CH4 production (approximately 70% of CH4 is formed from acetate) (Koster et al., 1986). This result was in agreement with the report by Yang et al. (2013), who found that the addition of nano-ZVI enhanced the population of acetoclastic methanogens.
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Fig. 3. Concentration of VSS and TCOD after the digestion.
Fig. 4. The total yield and composition of the biogas from fermentation (a) biogas concentration and (b) percentage.
Please cite this article in press as: Zhang, Y., et al. Zero-valent iron enhanced methanogenic activity in anaerobic digestion of waste activated sludge after heat and alkali pretreatment. Waste Management (2015), http://dx.doi.org/10.1016/j.wasman.2015.01.036
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H2 concentration is considered as an indicative parameter to evaluate anaerobic digester performance. The decrease of hydrogen concentration and increment of methane concentration were the results from the acceleration of CO2 utilization by hydrogen-utilizing microorganisms according to the reaction (1) and (2).
CO2 þ 4H2 ¼ CH4 þ 2H2 O
ð1Þ
CO2 þ 4Fe0 þ 8Hþ ¼ CH4 þ 4Fe2þ þ 2H2 O
ð2Þ
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The reaction (1) was one of main pathways for the methane generation driven by hydrogenotrophic methanogens, accounting for about 30–40% of the total methane production (Kotsyurbenko et al., 2004). In this process, hydrogen played a predominant role to serve as the electron carrier between acidification bacteria and hydrogenotrophic methanogens. The lack of hydrogenotrophic methanogens would result in the accumulation of hydrogen to impede the decomposition of organic acids because hydrogen was considered as a thermodynamically unfavorable intermediate (Siriwongrungson et al., 2007; Fukuzaki et al., 1990). Schmidt and Ahring (1993) reported that the addition of H2-utilizing methanogenes into disintegrating granules increased the degradation rate of both propionate and butyrate. Accordingly, the enrichment of hydrogenotrophic methanogens would enhance the degradation efficiency of intermediate VFA in an anaerobic system. It was also proved that the ZVI could directly serve as an electron donor for hydrogenotrophic methanogens to enhance the growth of hydrogenotrophic methanogens according to the reaction (2) (Daniels et al., 1987). It might decrease the partial pressure of hydrogen to drive the anaerobic digestion especially methanogenesis to process. It was reported that the waste sludge fermentation under alkali condition enhanced the hydrolysis–acidification of sludge (Yuan et al., 2006). However, the drawback of this process was low-rate methanogenic activity, impeding the complete mineralization of organic matters. To address this drawback, Zhang et al. (2010) proposed a second fermentation using the alkali– fermented sludge with pH adjustment and methanogens addition. Although a high methane yield was achieved, the complex operation and additional chemicals addition made it difficult to be established in the full-scale application. A better strategy perhaps was that both hydrolysis–acidification and methanogenesis were enhanced simultaneously under the alkali condition. Fig. 5 shows the VSS concentration and reduction ratio after the fermentation. The enhancement of hydrolysis–acidification of sludge under the alkali condition was significant. The VSS removal ratio of the control group was 44.4%. As increasing ZVI from 0 to 10 g/L,
30
Fig. 5. VSS concentration and reduction efficiency after the digestion (the concentration of VSS was expressed by pillar and its removal efficiency was expressed by diamond).
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the concentration of VSS decreased from 5.02 to 3.56 g/L, and the removal ratio increased from 44.4% to 60.06%. These results indicated that addition of ZVI could decrease the alkali inhibition on the methane production to accelerate sludge digestion. 4. Conclusion The methanogens was sensitive to the environment, and the variation of environment factors generally made against methanogenesis and thus reduced the methane yield in anaerobic sludge digestion. In this study, the addition of ZVI into anaerobic sludge digester could efficiently recover the methanogenic activity suppressed by heat-pretreatment and alkali condition, respectively. ZVI shortened the lag time of methane production, and increase the methane yield by 91.5%. The decomposition of acetate and hydrogen were accelerated by ZVI, indicating that the acetoclastic methanogenesis and H2-utilizing methanogenesis were enhanced. Also the VSS removal increased from 44.4% to 60.06% after the digestion at pH 10. This study suggested that ZVI could efficiently enhance methanogenenesis to improve the anaerobic sludge digestion even if methanogens were removed or totally inhibited in the pretreatment. Possibly, ZVI made the sludge digestion directly proceed with no need of adding seed sludge after the heat or alkali pretreatment, which not only might decrease the operating cost, but also could save a considerable digester space to have a higher sludge treatment load. Acknowledgment The authors acknowledge the financial support from the National Natural Scientific Foundation of China (51378087, 21177015). References Ahring, B.K., Sandberg, M., Angelidaki, I., 1995. Volatile fatty acids as indicators of process imbalance in anaerobic digestors. Appl. Microbiol. Biotechnol. 43, 559– 565. A.P.H.A., 1998. Standard Methods for the Examination of Water and Wastewater. Appels, L., Degrève, J., Van der Bruggen, B., Van Impe, J., Dewil, R., 2010. Influence of low temperature thermal pre-treatment on sludge solubilisation, heavy metal release and anaerobic digestion. Bioresour. Technol. 101, 5743–5748. Carrère, H., Dumas, C., Battimelli, A., Batstone, D., Delgenes, J., Steyer, J., Ferrer, I., 2010. Pretreatment methods to improve sludge anaerobic degradability: a review. J. Hazard. Mater. 183, 1–15. Daniels, L., Belay, N., Rajagopal, B.S., Weimer, P.J., 1987. Bacterial methanogenesis and growth from CO2 with elemental iron as the sole source of electrons. Science 237, 509–511. Dunfield, P., Dumont, R., Moore, T.R., 1993. Methane production and consumption in temperate and subarctic peat soils: response to temperature and pH. Soil Biol. Biochem. 25, 321–326. Fan, K.S., Kan, N.r., Lay, J.j., 2006. Effect of hydraulic retention time on anaerobic hydrogenesis in CSTR. Bioresour. Technol. 97, 84–89. Feng, Y., Zhang, Y., Quan, X., Chen, S., 2013. Enhanced anaerobic digestion of waste activated sludge digestion by the addition of zero valent iron. Water Res. 51, 242–250. Fr, B., Griebe, T., Nielsen, P., 1995. Enzymatic activity in the activated-sludge floc matrix. Appl. Microbiol. Biotechnol. 43, 755–761. Fukuzaki, S., Nishio, N., Shobayashi, M., Nagai, S., 1990. Inhibition of the fermentation of propionate to methane by hydrogen, acetate, and propionate. Appl. Environ. Microbiol. 56, 719–723. Gallert, C., Winter, J., 1997. Mesophilic and thermophilic anaerobic digestion of source-sorted organic wastes: effect of ammonia on glucose degradation and methane production. Appl. Microbiol. Biotechnol. 48, 405–410. Heo, N., Park, S., Lee, J., Kang, H., 2003. Solubilization of waste activated sludge by alkaline pretreatment and biochemical methane potential (BMP) tests for anaerobic co-digestion of municipal organic waste. Water Sci. Technol. 48 (8), 211–219. Jiang, Z., Lv, L., Zhang, W., Du, Q., Pan, B., Yang, L., Zhang, Q., 2011. Nitrate reduction using nanosized zero-valent iron supported by polystyrene resins: role of surface functional groups. Water Res. 45, 2191–2198. Karri, S., Sierra-Alvarez, R., Field, J.A., 2006. Toxicity of copper to acetoclastic and hydrogenotrophic activities of methanogens and sulfate reducers in anaerobic sludge. Chemosphere 62, 121–127.
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