effects of iron materials on methane generation from food
EFFECTS OF IRON MATERIALS ON METHANE GENERATION FROM FOOD WASTE ANAEROBIC DIGESTION J.H. KO*, L.L. ZHOU*, AND Q.Y. XU* * School of Environment and Energy, Shenzhen Graduate School, Peking University, Shenzhen, China
SUMMARY: Food waste is the largest component of MSW in China, accounting for over 50%. Anaerobic digestion is a potential solution for food waste management and can be used to methane recovery. Laboratory experiments were conducted to investigate the effects of iron materials on methane generation from food waste anaerobic digestion, including iron oxides and element Fe. The result showed that the addition of iron materials could relieve acid inhibition and accelerate the generation rate of methane. The supplementation of iron material enhanced butyric-type and acetic-type fermentation but suppress propionate-type fermentation. The estimated performance parameters with the modified Gompertz model indicated that addition of iron oxides improved digester performance greatly. With the addition of iron oxides, the lag phase was shorten by 43% and the maximum methane production rate was increased by 48% compared to food waste digester without adding iron materials.
1. INTRODUCTION In China, municipal solid waste (MSW) generation has been increased in recent years. (Liu, 2014). The amount of MSW collected and transported in China was 191 million tons in 2015. Currently, Most most collected MSW being are landfilled or incinerated (Hu et al., 2012). Food waste is the largest portion among Chinese MSW components and ranges from 50% to 70% (Tai et al., 2011). However, another potential solution for food waste is anaerobic digestion that produces byproducts (methane and carbon dioxide) which can be used for generating electricity (Khanal et al., 2010). Iron is an essential nutrient for the growth of microorganism as it forms an important component of many of the enzymes involved in the metabolic pathways of the bacteria (Neilands, 1981). Methanogens have a specific growth requirement for iron (Patel et al., 1978). Recent studies have suggested that conductive iron oxide minerals or some conductive material like carbon can facilitate syntrophic metabolism of the methanogenic degradation of organic matter (Stams and Plugge, 2009;Jiang et al., 2013;Kouzuma et al., 2015;Dang et al., 2016). Kato et al. (2012) reported that the supplementation with iron oxides (hematite or magnetite) resulted in the acceleration of methanogenesis by shortening lag time and by increasing the production rate. This is because that the supplementation of soil microbes with (semi)conductive iron-oxide minerals creates unique interspecies interactions and facilitates methanogenesis. In addition, Yamada et al. (2015) proved that supplementation of conductive iron oxides (magnetite) accelerated methanogenesis from acetate and propionate under thermophilic conditions, while supplementation of ferrihydrite also accelerated methanogenesis from propionate. Li et al. (2015) reported that CH4 production was significantly accelerated in the Proceedings Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium/ 2 - 6 October 2017 S. Margherita di Pula, Cagliari, Italy / © 2017 by CISA Publisher, Italy
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
presence of nano Fe3O4 with syntrophic butyrate oxidation. Zero valent iron as a strong reducing agent has been often used for environmental remediation projects. The promotion for anaerobic digestion using Fe0 has also been documented (Liu et al., 2012;Meng et al., 2013;Xiao et al., 2013;Zhen et al., 2015). Meng et al (2013) observed that Fe0 powder dose into an acidogenic reactor (with propionate as only substrate) increased the propionate conversion rate comparing to a reactor without dosing Fe0. They also showed enzymatic activities of the acetogenesis were enhanced by the addition of Fe0. Zhen et al. (2015) concluded that zero valent scrap iron was responsible for stimulating the anaerobic sludge digestion because Fe0 acted as electron donors and provided favorable environment. Liu et al.(2012) reported that the hydrolysis/fermentation in an acidogenic reactor for wastewater treatment was accelerated by dosing Fe and by shortening hydraulic retention time (HRT) . However, nano zero valent iron (average size = 55 ±11 nm) has been proved to have suppressive effects on methanogenesis in anaerobic digestion by disrupting cell integrity and increasing H2 production (Yang et al., 2013). All of these studies provided a possibility for the achievement of improved operating anaerobic digestion of food waste system with high efficiency for removal of organics and generation of CH4 in presence of iron oxides or Fe0. However, many study have been done with pure culture and simple substrates or simplified microbial communities in the presence/absence of iron oxides or Fe0 (Kato et al., 2012;Zhou et al., 2014;Yamada et al., 2015;Zhuang et al., 2015) but limited research has been conducted in the engineered systems such as anaerobic digesters containing multiple microbial communities and/or mixed organic waste. Therefore, there is a need of studies about the impact of iron oxides or Fe0 addition in anaerobic digestion of food waste on methane production. The objective of this study was to investigate the effect of supplementation of iron oxides and Fe0 on food waste anaerobic digestion system. In addition, the possibility of utilizing iron scrap, a common byproduct from manufacturing processes, as a supplementary iron material for anaerobic digestion of food waste was examined in this study.
2. MATERIALS AND METHOD 2.1 Preparation of food waste and seed sludge The food waste (FW) used in this study was obtained from the canteen on the campus. The oil slick was first washed out and then FW was ground to reduce the particle size with a household disposer. The processed food waste was stored in a refrigerator at 4oC. Mesophilic anaerobic sewage sludge was collected from a wastewater treatment plant in Shenzhen Guangdong, China. The chemical characteristics of the collected FW and inoculum are shown in Table 1. Table 1. Characteristics of FW and seed sludge Parameter
Food wastes
Seed sludge
Total solids: TS (% wet weight)
20.86±0.05
2.51±0.01
Total volatile solids: TVS (% wet weight)
19.02±0.18
1.03±0.006
－ pH 7.06±0.07 After that the obtained material was kept under seal in cool and dry place for further use and
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
test for X-ray photoelectron spectroscopy (XPS). XPS was conducted on ESCALAB 250Xi (Thermo Fisher, UK) instrument with Al-Kα X-ray to investigate the chemical states of elements. Fig. 1 shows XPS spectra of Fe 2p region for rusted iron powder. The XPS spectra of the rusted iron powder showed a typical Fe2O3 XPS spectra (Wagner, 1979). 25000
Counts
20000
15000
10000
5000 740
730
720
710
700
bonding energy (eV)
Figure 1. XPS spectra of Fe 2p region for rusted iron powder
2.2 Preparation of Iron Iron powder (99% metal basis, 20 mesh) was purchased from Alfa Aesar. The iron powder was used for zero valent iron material without further purification. The purchased iron powder also was used to prepared iron oxide powder. Deionized water was added to the purchased iron powder and then the mixture was putted in an oven at 50 °C for 12 h in air. This process was repeated 5 times and then the mixture was dried in an oven at 100 °C for 5 h in air. 2.3 Reactor Setup and Operation The anaerobic digester was prepared in in 2.5 L glass reactors with (working volume: 1. 8 L). The substrate and inoculum (S/I) ratio used in this study was 3:1 (g-VS substrate/ g-VS inoculum). Weighed inoculum, food waste, and iron material were added into a the glass reactor which was purged with nitrogen gas. Each reactor was tightly sealed with a rubber stopper connected to a 3-L aluminum gas pack (China Dalian delin Delin gas packing co., ltd, China). The reactors were operated in a mesophilic temperature range (35 ±0.2 °C). 2.4 Sample collection and analysis Leachate was sampled regularly and analyzed for pH (Sartorius, PB-10, German), soluble chemical oxygen demand (soluble COD), total dissolved iron, and volatile fatty acids (VFAs) of digestion samples were measured over time. COD was measured by the fast digestionspectrophotometric method (the Environmental Protection Industry Standard of the People's Republic of China, HJ/T 399-2007). VFAs (acetic acid, propionic acid, butyric acid, and valeric acid) were measured on a gas chromatograph (Agilent, 7890A, United States) equipped with a capillary column (HP INNOWAX 15 m×0.530 mm ×1.00 μm) and flame ionization detector.
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
Detector temperature: 300˚C; make-up: N2 at 30 mL•min−1; delay: 2.5 min; Injected volume: 1.0 μL; Injector temperature: 250˚C; split: 10; Carrier gas (H2): 3.2 mL•min−1; Oven temperature: 80˚C (1 min) 8˚C/min 150˚C (1 min) 10˚C/min 200˚C (2 min); 14.3 min of chromatographic run. The collected biogas was measured for the volume and gas composition. Methane and carbon dioxide were determined by a gas chromatograph (FULI, 9790, China) equipped with a packed column and thermal conductivity detector. The injector and detector temperatures were maintained at 50 °C and 100 °C, respectively. The column temperature was gradually increased from 30 °C to 70 °C. Hydrogen was used as the carrier gas.
3. RESULTS AND DISCUSSION 3.1 Methane production 3.1.1 Title title title Fig.2 shows daily methane production and cumulative biogas production from each reactors. A large amount of biogas production was observed within the first 3 days of each digester indicating some readily useable substrates were used by methanogens directly at the beginning of the operation. With a sudden pH drop, the lag phase begun. After the lag phase, the peak methane generation appeared in each reactor. The peak methane generation rate and the time of the peak appearance also varied among reactors as shown in Fig. 2. The peak generation of RIO appeared earliest between day 16 to day 21.
2500
Methane production (mL/d)
2000
1500
1000
500
Total methane production (mL)
0 20000
15000
10000
RCont
5000
RIO RZVI
0 0
10
20
30
40
Time (day)
Figure 2. Daily methane production and cumulative methane volume of each reactor
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
The average daily methane production rate was 1099 (±73) mL/day. The peak generation of RZVI was observed between 21 days and 27 days with the average production rate of 938 (±126) mL/day. In contrast, the methane generation peak of RCont was observed between 26 days and 34 days with the average production rate of 861 (±116) mL/day. For estimate performance parameters, the Modified Gompertz equation has been used with gas production data(DonosoBravo et al., 2010;Lo et al., 2010). In this study, the measured methane generation rate data were used curve fitting with the Modified Gompertz equation. Because of unexpected methane production in the beginning of the operation, A constant parameter (y0) was added to the Modified Gompertz equation
where y is accumulative methane volume (mL/g VS) at time t (day), A is the methane production potential (mL/g VS). μ_m is the maximal methane production rate (mL/g VS/day), and λ is the lag phase (day) and e is equal to 2.7. The summary of performance parameters from the curve fitting with Modified Gompertz equation are given in Table 2. In the curve fitting, methane generation data of the first 3 days was not included because Gompertz equation is for a sigmoidal shape curve.
Table.2. Parameters and goodness fit obtained with the Modified Gompertz model
Reactor
RCont
RIO
RZVI
A
281.1
251.9
278.9
19.1
28.2
22
23.2
13.3
16.1
106 0.991
112.1 0.995
105.8 0.993
λ R
2
As the results of the model fitting, the lag phase varied with different types of iron supplements. The shortest lag phase was shown by RIO as 13.3 days and by RZVI followed as 16.1 days. RCont had the longest lag phase (23.2 days) among the tree reactors. of RIO also showed the largest value as 28.2 mL/g VS/day. of RZVI (22.0 mL/g VS/day) was higher than that of RCont (19.1 mL/g VS/day) but lower than RIO. The estimated performance parameters indicated that addition of iron oxides improved digester performance greatly by shortening the lag phase by 43 % and enhancing the maximum methane production rate by 48% compared to food waste digester with no iron material. Performance improvement by the food digester with rusted iron powder are consistent with the results with the supplementation with conductive iron oxides (hematite or magnetite) (Kato et al., 2012;Zhuang et al., 2015;Baek et al., 2016). It implies that the supplementation of rusted iron played an important role in extracellular electron transfer by providing readily available bridges for interspecies electron transfer (IET) and/or by cooperating with other IET systems (e.g., pili, electron shuttle, etc), otherwise, the digestion system required additional time to build structurally complicate and coast electron transfer system (Liu et al., 2015). Meanwhile, the methane production from the reactor with Fe0 was also accelerated to some extent compared to the control. The methane enrichment effect of zero valent iron has been
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reported elsewhere. In this study, RZVI showed slightly higher cumulative methane volume as 382.5 mL/g VS at STP than the others. However, the difference was not significant. The cumulative methane volumes of RIO and RCont was 369.1 mL/g VS and 363.9 mL/g VS, respectively. It has been reported that some element metals, like zero valent iron, as electron sources can be used to produce methane from CO2 (Belay and Daniels, 1990). The methane enrichment effect of zero valent iron was not considerable in this study. This is because the iron powder used in this study was relatively coarse grain size (20 mesh). In both RCont and RZVI, the methane percentage during active methane production phase was 77% in average. 3.2 pH, total dissolved iron and COD pH, total dissolved iron, and COD are presented in Fig. 3. The initial pH value in RCont , RIO and RZVI were 7.02, 7.16 and 7.00, respectively. In all reactors, sharp drops in pH to around 6.0 appeared in the first day (control 5.99, iron oxides 6.08, Fe0 6.09). The dropped pH values recovered quickly, as massive methane production, and then gradually increased after another slight pH drop. pH remained at stable levels with small fluctuation between 7.7 and 7.9 at the end of digestion process. 8.5
pH
8.0 7.5 7.0 RCont RIO RZVI
6.5 6.0 60 50
Iron (mg/L)
40 30 20 10 0 30000
COD (mg/L)
25000 20000 15000 10000 5000 0 0
10
20
30
Time (day)
Figure 3. pH, total dissolved iron, and COD of each reactor.
40
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The concentration of total soluble iron (Fig. 3) in all conditions reached their peak value at the first day due to the lowest pH in the environments (6.2 to 6.3). RIO showed the highest concentration (56.2 mg/L) of total dissolved iron at day 1. RZVI and RCont presented 26.6 mg/L and 11.6 mg/L of dissolved iron at the same day, respectively. Freshly precipitated or amorphous Fe(III) oxide is relatively soluble compared to other iron oxide minerals. Also, reducing environment created by organic matter decomposition is responsible for the increased solubility and availability of Fe (Lindsay, 1991). After the peak iron concentration, the soluble iron concentration of RCont fluctuated around 1-3 mg/L range in the most of time. The dissolved iron concentrations of RIO was not different considerably with those of RZVI after the peak concentrations at the first day. The average iron concentration in both of RIO and RZVI remained 4.7 mg/L for the rest of period. Because the difference of iron concentration is small among the reactors in the most operation period, it is considered that the trace nutrient effects of dissolved iron was not critical to accelerate methanogenesis in these reactors. COD in all conditions showed a first increased due to the organic compounds of food wastes initially disintegrated into soluble matters. As shown in Fig. 3, COD in these three reactors reached maximum values of 24,000 mg/L at the 12th day and the maximum COD values from the reactors were not varied. Since the COD peak appeared, COD reduced drastically in all reactors. The COD reduction coincided with gas generation. RIO showed the fastest COD reduction and followed by RZVI and then RCont, which implied a good performance in the facultative syntrophic interactions. 3.3 Volatile fatty acids As shown in Fig. 4, a large amount of acetic acid (around 1500 mg/L) and butyric acid (1400 mg/L – 1800 mg/L) were measured in each reactor at the beginning of the experiment. These readily useable acids reduced quickly corresponding to the formation of methane production peaks in the beginning of operation. When acetic acid was used up, the lag phase of methane production started in each reactor. And then acetic acid concentration increased until methane production restarted. The beginning of acetic acid concentration reduction in RIO and RZVI coincided with the beginning of its methane production. Comparing VFA concentrations of each reactor, the time of the peak formation did not vary. For example, the highest acetic acid concentration in all reactors appeared at 10 days. However, acetic acid concentration increased faster in RIO and RZVI than in RCont. and also decreased quicker in in RIO and RZVI. In contrast, the propionic acid concentrations in increased in RIO and RZVI slower than in RCont. These results suggested that the addition of iron material promoted the formation of acetic acids but suppressed the formation of propionic acid. Feng et al. (2014) also observed both the enhancement of acetic acid formation and the reduction of propionic acid formation by increasing zero valent iron dosage. They explained that the addition of zero valent iron led to create a more reductive environment which could enhance butyric-type and acetic-type fermentation but suppress propionate-type fermentation. The difference of the acid concentration reduction rate was more obvious. These results suggested that iron material could influence considerably on the VFA consumption processes than the VFA formation processes. With increasing methane production in each reactor, the acetic acid concentration decreased but maintained around 900 mg/L temporarily with the peak methane production rates. The maximum methane generation indicates the maximum consumption rate of acetic acid. Therefore, it is considered that the large amount acetic acid was generated additionally during the maximum consumption period. The formation of acetic acid in this period is also supported by the fast decrease of propionic acid concentrations in each reactor. The change of propionic acid concentration showed a similar pattern to acetic acid. Propionic or butyric acid degradation in reactor in presence of iron oxides or Fe0
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
proceeded to completion more rapidly than control. The oxidation of propionate and butyrate is thermodynamically unfavorable with high H2 partial pressure. So, the concentrations of H2 must be maintained low as possible (Thauer et al., 1977). It is only possible with the cooperation with hydrogen-utilizing methanogens (Stams and Plugge, 2009). It was thought that propionate and butyrate oxidations were substantially affected by the presence of iron oxides or Fe0 by boosting the syntrophic between fermentative bacterial species and hydrogen-consuming methanogen in anaerobic digestion of food waste.
acetic acid (mg/L)
3000 2500 2000 1500 1000 500
propionic acid (mg/L)
0 3000 2500 2000 1500 1000 500 0
butyric acid (mg/L)
3000
Day
2500 2000 1500 1000 500
valeric acid (mg/L)
0 3000
Day
2500
RCont RIO
2000
RZVI
1500 1000 500 0 0
10
20
30
40
50
day
Fig. 4. Change of VFA component (acetic acid, propionic acid, butyric acid, and valeric acid) concentration. 4. CONCLUSION Laboratory experiments were conducted to inverstigate the effects of iron oxides and element iron on methane generation from food waste anaerobic digestion. The result showed that the addition of iron materials could reduce the lag phase and accelerate methane generation. The cumulative methane production was about 370 mL/g VS in all reactors. However, compared to the reactor without any additional iron materials, the lag phase was shorten by 43% and the maximum methane production rate was increased by 48% in the iron reactors. With the addition of iron materials, the dissolved iron concentrations and increased promoted the formation of acetic acids, which accelerates methanogenesis in these reactors. The reactor
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
with iron additon showed the fastest COD reduction, followed by the reactor with element iron and control. The COD reduction coincided with methane generation. The results indicated that the addition of iron materials can be used as an effective way to promote methane generation from food waste anaerobic digestion.
ACKNOWLEDGEMENTS This research was supported by the Shenzhen government of China with Grant No. JCYJ20150616145013931 and CXZZ20151117141320317.
REFERENCES Baek, G., Kim, J., Lee, C., 2016. A long-term study on the effect of magnetite supplementation in continuous anaerobic digestion of dairy effluent – Enhancement in process performance and stability. Bioresource Technology 222, 344-354. Belay, N., Daniels, L., 1990. Elemental metals as electron sources for biological methane formation from CO2. Antonie van Leeuwenhoek 57, 1-7. Caporaso, J.G., Lauber, C.L., Walters, W.A., Berg-Lyons, D., Huntley, J., Fierer, N., Owens, S.M., Betley, J., Fraser, L., Bauer, M., Gormley, N., Gilbert, J.A., Smith, G., Knight, R., 2012. Ultra-high-throughput microbial community analysis on the Illumina HiSeq and MiSeq platforms. ISME J 6, 1621-1624. Caporaso, J.G., Lauber, C.L., Walters, W.A., Berg-Lyons, D., Lozupone, C.A., Turnbaugh, P.J., Fierer, N., Knight, R., 2011. Global patterns of 16S rRNA diversity at a depth of millions of sequences per sample. Proceedings of the National Academy of Sciences of the United States of America 108, 4516-4522. Dang, Y., Holmes, D.E., Zhao, Z., Woodard, T.L., Zhang, Y., Sun, D., Wang, L.-Y., Nevin, K.P., Lovley, D.R., 2016. Enhancing anaerobic digestion of complex organic waste with carbon-based conductive materials. Bioresource Technology 220, 516-522. Donoso-Bravo, A., Pérez-Elvira, S.I., Fdz-Polanco, F., 2010. Application of simplified models for anaerobic biodegradability tests. Evaluation of pre-treatment processes. Chemical Engineering Journal 160, 607-614. Feng, Y., Zhang, Y., Quan, X., Chen, S., 2014. Enhanced anaerobic digestion of waste activated sludge digestion by the addition of zero valent iron. Water Research 52, 242-250. Hu, X., Zhang, M., Yu, J., Zhang, G., 2012. Food waste management in China: status, problems and solutions. Shengtai Xuebao/Acta Ecologica Sinica 32, 4575-4584. Jiang, S., Park, S., Yoon, Y., Lee, J.H., Wu, W.M., Phuoc Dan, N., Sadowsky, M.J., Hur, H.G., 2013. Methanogenesis facilitated by geobiochemical iron cycle in a novel syntrophic methanogenic microbial community. Environ Sci Technol 47, 10078-10084. Kato, S., Hashimoto, K., Watanabe, K., 2012. Methanogenesis facilitated by electric syntrophy via (semi)conductive iron-oxide minerals. Environ Microbiol 14, 1646-1654. Khanal, S.K., Surampalli, R.Y., Zhang, T.C., Lamsal, B.P., Tyagi, R., Kao, C., 2010. Bioenergy and biofuel from biowastes and biomass, American Society of Civil Engineers (ASCE). Kouzuma, A., Kato, S., Watanabe, K., 2015. Microbial interspecies interactions: recent findings in syntrophic consortia. Frontiers in Microbiology 6. Li, H., Chang, J., Liu, P., Fu, L., Ding, D., Lu, Y., 2015. Direct interspecies electron transfer
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accelerates syntrophic oxidation of butyrate in paddy soil enrichments. Environ Microbiol 17, 1533-1547. Lindsay, W., 1991. Iron oxide solubilization by organic matter and its effect on iron availability. Iron nutrition and interactions in plants, Springer: 29-36. Liu, F., Rotaru, A.-E., Shrestha, P.M., Malvankar, N.S., Nevin, K.P., Lovley, D.R., 2015. Magnetite compensates for the lack of a pilin-associated c-type cytochrome in extracellular electron exchange. Environmental microbiology 17, 648-655. Liu, G., 2014. Food Losses and Food Waste in China. Journal. Liu, Y.W., Zhang, Y.B., Quan, X., Li, Y., Zhao, Z.Q., Meng, X.S., Chen, S., 2012. Optimization of anaerobic acidogenesis by adding Fe-0 powder to enhance anaerobic wastewater treatment. Chemical Engineering Journal 192, 179-185. Lo, H.M., Kurniawan, T.A., Sillanpää, M.E.T., Pai, T.Y., Chiang, C.F., Chao, K.P., Liu, M.H., Chuang, S.H., Banks, C.J., Wang, S.C., Lin, K.C., Lin, C.Y., Liu, W.F., Cheng, P.H., Chen, C.K., Chiu, H.Y., Wu, H.Y., 2010. Modeling biogas production from organic fraction of MSW codigested with MSWI ashes in anaerobic bioreactors. Bioresource Technology 101, 6329-6335. Meng, X.S., Zhang, Y.B., Li, Q., Quan, X., 2013. Adding Fe-0 powder to enhance the anaerobic conversion of propionate to acetate. Biochemical Engineering Journal 73, 80-85. Neilands, J.B., 1981. Microbial iron compounds. Annu Rev Biochem 50, 715-731. Patel, G.B., Khan, A.W., Roth, L.A., 1978. Optimum Levels of Sulfate and Iron for the Cultivation of Pure Cultures of Methanogens in Synthetic Media. Journal of Applied Bacteriology 45, 347356. Stams, A.J., Plugge, C.M., 2009. Electron transfer in syntrophic communities of anaerobic bacteria and archaea. Nat Rev Microbiol 7, 568-577. Tai, J., Zhang, W., Che, Y., Feng, D., 2011. Municipal solid waste source-separated collection in China: A comparative analysis. Waste management 31, 1673-1682. Thauer, R.K., Jungermann, K., Decker, K., 1977. Energy conservation in chemotrophic anaerobic bacteria. Bacteriol Rev 41, 100-180. Wagner, C.D., 1979. Handbook of x-ray photoelectron spectroscopy: a reference book of standard data for use in x-ray photoelectron spectroscopy, Physical Electronics Division, PerkinElmer Corp. Xiao, X., Sheng, G.-P., Mu, Y., Yu, H.-Q., 2013. A modeling approach to describe ZVI-based anaerobic system. Water Research 47, 6007-6013. Yamada, C., Kato, S., Ueno, Y., Ishii, M., Igarashi, Y., 2015. Conductive iron oxides accelerate thermophilic methanogenesis from acetate and propionate. Journal of bioscience and bioengineering 119, 678-682. 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. Zhen, G.Y., Lu, X.Q., Li, Y.Y., Liu, Y., Zhao, Y.C., 2015. Influence of zero valent scrap iron (ZVSI) supply on methane production from waste activated sludge. Chemical Engineering Journal 263, 461-470. Zhou, S., Xu, J., Yang, G., Zhuang, L., 2014. Methanogenesis affected by the co-occurrence of iron(III) oxides and humic substances. FEMS Microbiol Ecol 88, 107-120. Zhuang, L., Tang, J., Wang, Y., Hu, M., Zhou, S., 2015. Conductive iron oxide minerals accelerate syntrophic cooperation in methanogenic benzoate degradation. Journal of Hazardous Materials 293, 37-45.
SUMMARY: Food waste is the largest component of MSW in China, accounting for over 50%. Anaerobic digestion is a potential solution for food waste management and can be used to methane recovery. Laboratory experiments were conducted to investigate the effects of iron materials on methane generation from food waste anaerobic digestion, including iron oxides and element Fe. The result showed that the addition of iron materials could relieve acid inhibition and accelerate the generation rate of methane. The supplementation of iron material enhanced butyric-type and acetic-type fermentation but suppress propionate-type fermentation. The estimated performance parameters with the modified Gompertz model indicated that addition of iron oxides improved digester performance greatly. With the addition of iron oxides, the lag phase was shorten by 43% and the maximum methane production rate was increased by 48% compared to food waste digester without adding iron materials.
1. INTRODUCTION In China, municipal solid waste (MSW) generation has been increased in recent years. (Liu, 2014). The amount of MSW collected and transported in China was 191 million tons in 2015. Currently, Most most collected MSW being are landfilled or incinerated (Hu et al., 2012). Food waste is the largest portion among Chinese MSW components and ranges from 50% to 70% (Tai et al., 2011). However, another potential solution for food waste is anaerobic digestion that produces byproducts (methane and carbon dioxide) which can be used for generating electricity (Khanal et al., 2010). Iron is an essential nutrient for the growth of microorganism as it forms an important component of many of the enzymes involved in the metabolic pathways of the bacteria (Neilands, 1981). Methanogens have a specific growth requirement for iron (Patel et al., 1978). Recent studies have suggested that conductive iron oxide minerals or some conductive material like carbon can facilitate syntrophic metabolism of the methanogenic degradation of organic matter (Stams and Plugge, 2009;Jiang et al., 2013;Kouzuma et al., 2015;Dang et al., 2016). Kato et al. (2012) reported that the supplementation with iron oxides (hematite or magnetite) resulted in the acceleration of methanogenesis by shortening lag time and by increasing the production rate. This is because that the supplementation of soil microbes with (semi)conductive iron-oxide minerals creates unique interspecies interactions and facilitates methanogenesis. In addition, Yamada et al. (2015) proved that supplementation of conductive iron oxides (magnetite) accelerated methanogenesis from acetate and propionate under thermophilic conditions, while supplementation of ferrihydrite also accelerated methanogenesis from propionate. Li et al. (2015) reported that CH4 production was significantly accelerated in the Proceedings Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium/ 2 - 6 October 2017 S. Margherita di Pula, Cagliari, Italy / © 2017 by CISA Publisher, Italy
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
presence of nano Fe3O4 with syntrophic butyrate oxidation. Zero valent iron as a strong reducing agent has been often used for environmental remediation projects. The promotion for anaerobic digestion using Fe0 has also been documented (Liu et al., 2012;Meng et al., 2013;Xiao et al., 2013;Zhen et al., 2015). Meng et al (2013) observed that Fe0 powder dose into an acidogenic reactor (with propionate as only substrate) increased the propionate conversion rate comparing to a reactor without dosing Fe0. They also showed enzymatic activities of the acetogenesis were enhanced by the addition of Fe0. Zhen et al. (2015) concluded that zero valent scrap iron was responsible for stimulating the anaerobic sludge digestion because Fe0 acted as electron donors and provided favorable environment. Liu et al.(2012) reported that the hydrolysis/fermentation in an acidogenic reactor for wastewater treatment was accelerated by dosing Fe and by shortening hydraulic retention time (HRT) . However, nano zero valent iron (average size = 55 ±11 nm) has been proved to have suppressive effects on methanogenesis in anaerobic digestion by disrupting cell integrity and increasing H2 production (Yang et al., 2013). All of these studies provided a possibility for the achievement of improved operating anaerobic digestion of food waste system with high efficiency for removal of organics and generation of CH4 in presence of iron oxides or Fe0. However, many study have been done with pure culture and simple substrates or simplified microbial communities in the presence/absence of iron oxides or Fe0 (Kato et al., 2012;Zhou et al., 2014;Yamada et al., 2015;Zhuang et al., 2015) but limited research has been conducted in the engineered systems such as anaerobic digesters containing multiple microbial communities and/or mixed organic waste. Therefore, there is a need of studies about the impact of iron oxides or Fe0 addition in anaerobic digestion of food waste on methane production. The objective of this study was to investigate the effect of supplementation of iron oxides and Fe0 on food waste anaerobic digestion system. In addition, the possibility of utilizing iron scrap, a common byproduct from manufacturing processes, as a supplementary iron material for anaerobic digestion of food waste was examined in this study.
2. MATERIALS AND METHOD 2.1 Preparation of food waste and seed sludge The food waste (FW) used in this study was obtained from the canteen on the campus. The oil slick was first washed out and then FW was ground to reduce the particle size with a household disposer. The processed food waste was stored in a refrigerator at 4oC. Mesophilic anaerobic sewage sludge was collected from a wastewater treatment plant in Shenzhen Guangdong, China. The chemical characteristics of the collected FW and inoculum are shown in Table 1. Table 1. Characteristics of FW and seed sludge Parameter
Food wastes
Seed sludge
Total solids: TS (% wet weight)
20.86±0.05
2.51±0.01
Total volatile solids: TVS (% wet weight)
19.02±0.18
1.03±0.006
－ pH 7.06±0.07 After that the obtained material was kept under seal in cool and dry place for further use and
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
test for X-ray photoelectron spectroscopy (XPS). XPS was conducted on ESCALAB 250Xi (Thermo Fisher, UK) instrument with Al-Kα X-ray to investigate the chemical states of elements. Fig. 1 shows XPS spectra of Fe 2p region for rusted iron powder. The XPS spectra of the rusted iron powder showed a typical Fe2O3 XPS spectra (Wagner, 1979). 25000
Counts
20000
15000
10000
5000 740
730
720
710
700
bonding energy (eV)
Figure 1. XPS spectra of Fe 2p region for rusted iron powder
2.2 Preparation of Iron Iron powder (99% metal basis, 20 mesh) was purchased from Alfa Aesar. The iron powder was used for zero valent iron material without further purification. The purchased iron powder also was used to prepared iron oxide powder. Deionized water was added to the purchased iron powder and then the mixture was putted in an oven at 50 °C for 12 h in air. This process was repeated 5 times and then the mixture was dried in an oven at 100 °C for 5 h in air. 2.3 Reactor Setup and Operation The anaerobic digester was prepared in in 2.5 L glass reactors with (working volume: 1. 8 L). The substrate and inoculum (S/I) ratio used in this study was 3:1 (g-VS substrate/ g-VS inoculum). Weighed inoculum, food waste, and iron material were added into a the glass reactor which was purged with nitrogen gas. Each reactor was tightly sealed with a rubber stopper connected to a 3-L aluminum gas pack (China Dalian delin Delin gas packing co., ltd, China). The reactors were operated in a mesophilic temperature range (35 ±0.2 °C). 2.4 Sample collection and analysis Leachate was sampled regularly and analyzed for pH (Sartorius, PB-10, German), soluble chemical oxygen demand (soluble COD), total dissolved iron, and volatile fatty acids (VFAs) of digestion samples were measured over time. COD was measured by the fast digestionspectrophotometric method (the Environmental Protection Industry Standard of the People's Republic of China, HJ/T 399-2007). VFAs (acetic acid, propionic acid, butyric acid, and valeric acid) were measured on a gas chromatograph (Agilent, 7890A, United States) equipped with a capillary column (HP INNOWAX 15 m×0.530 mm ×1.00 μm) and flame ionization detector.
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Detector temperature: 300˚C; make-up: N2 at 30 mL•min−1; delay: 2.5 min; Injected volume: 1.0 μL; Injector temperature: 250˚C; split: 10; Carrier gas (H2): 3.2 mL•min−1; Oven temperature: 80˚C (1 min) 8˚C/min 150˚C (1 min) 10˚C/min 200˚C (2 min); 14.3 min of chromatographic run. The collected biogas was measured for the volume and gas composition. Methane and carbon dioxide were determined by a gas chromatograph (FULI, 9790, China) equipped with a packed column and thermal conductivity detector. The injector and detector temperatures were maintained at 50 °C and 100 °C, respectively. The column temperature was gradually increased from 30 °C to 70 °C. Hydrogen was used as the carrier gas.
3. RESULTS AND DISCUSSION 3.1 Methane production 3.1.1 Title title title Fig.2 shows daily methane production and cumulative biogas production from each reactors. A large amount of biogas production was observed within the first 3 days of each digester indicating some readily useable substrates were used by methanogens directly at the beginning of the operation. With a sudden pH drop, the lag phase begun. After the lag phase, the peak methane generation appeared in each reactor. The peak methane generation rate and the time of the peak appearance also varied among reactors as shown in Fig. 2. The peak generation of RIO appeared earliest between day 16 to day 21.
2500
Methane production (mL/d)
2000
1500
1000
500
Total methane production (mL)
0 20000
15000
10000
RCont
5000
RIO RZVI
0 0
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20
30
40
Time (day)
Figure 2. Daily methane production and cumulative methane volume of each reactor
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The average daily methane production rate was 1099 (±73) mL/day. The peak generation of RZVI was observed between 21 days and 27 days with the average production rate of 938 (±126) mL/day. In contrast, the methane generation peak of RCont was observed between 26 days and 34 days with the average production rate of 861 (±116) mL/day. For estimate performance parameters, the Modified Gompertz equation has been used with gas production data(DonosoBravo et al., 2010;Lo et al., 2010). In this study, the measured methane generation rate data were used curve fitting with the Modified Gompertz equation. Because of unexpected methane production in the beginning of the operation, A constant parameter (y0) was added to the Modified Gompertz equation
where y is accumulative methane volume (mL/g VS) at time t (day), A is the methane production potential (mL/g VS). μ_m is the maximal methane production rate (mL/g VS/day), and λ is the lag phase (day) and e is equal to 2.7. The summary of performance parameters from the curve fitting with Modified Gompertz equation are given in Table 2. In the curve fitting, methane generation data of the first 3 days was not included because Gompertz equation is for a sigmoidal shape curve.
Table.2. Parameters and goodness fit obtained with the Modified Gompertz model
Reactor
RCont
RIO
RZVI
A
281.1
251.9
278.9
19.1
28.2
22
23.2
13.3
16.1
106 0.991
112.1 0.995
105.8 0.993
λ R
2
As the results of the model fitting, the lag phase varied with different types of iron supplements. The shortest lag phase was shown by RIO as 13.3 days and by RZVI followed as 16.1 days. RCont had the longest lag phase (23.2 days) among the tree reactors. of RIO also showed the largest value as 28.2 mL/g VS/day. of RZVI (22.0 mL/g VS/day) was higher than that of RCont (19.1 mL/g VS/day) but lower than RIO. The estimated performance parameters indicated that addition of iron oxides improved digester performance greatly by shortening the lag phase by 43 % and enhancing the maximum methane production rate by 48% compared to food waste digester with no iron material. Performance improvement by the food digester with rusted iron powder are consistent with the results with the supplementation with conductive iron oxides (hematite or magnetite) (Kato et al., 2012;Zhuang et al., 2015;Baek et al., 2016). It implies that the supplementation of rusted iron played an important role in extracellular electron transfer by providing readily available bridges for interspecies electron transfer (IET) and/or by cooperating with other IET systems (e.g., pili, electron shuttle, etc), otherwise, the digestion system required additional time to build structurally complicate and coast electron transfer system (Liu et al., 2015). Meanwhile, the methane production from the reactor with Fe0 was also accelerated to some extent compared to the control. The methane enrichment effect of zero valent iron has been
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reported elsewhere. In this study, RZVI showed slightly higher cumulative methane volume as 382.5 mL/g VS at STP than the others. However, the difference was not significant. The cumulative methane volumes of RIO and RCont was 369.1 mL/g VS and 363.9 mL/g VS, respectively. It has been reported that some element metals, like zero valent iron, as electron sources can be used to produce methane from CO2 (Belay and Daniels, 1990). The methane enrichment effect of zero valent iron was not considerable in this study. This is because the iron powder used in this study was relatively coarse grain size (20 mesh). In both RCont and RZVI, the methane percentage during active methane production phase was 77% in average. 3.2 pH, total dissolved iron and COD pH, total dissolved iron, and COD are presented in Fig. 3. The initial pH value in RCont , RIO and RZVI were 7.02, 7.16 and 7.00, respectively. In all reactors, sharp drops in pH to around 6.0 appeared in the first day (control 5.99, iron oxides 6.08, Fe0 6.09). The dropped pH values recovered quickly, as massive methane production, and then gradually increased after another slight pH drop. pH remained at stable levels with small fluctuation between 7.7 and 7.9 at the end of digestion process. 8.5
pH
8.0 7.5 7.0 RCont RIO RZVI
6.5 6.0 60 50
Iron (mg/L)
40 30 20 10 0 30000
COD (mg/L)
25000 20000 15000 10000 5000 0 0
10
20
30
Time (day)
Figure 3. pH, total dissolved iron, and COD of each reactor.
40
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The concentration of total soluble iron (Fig. 3) in all conditions reached their peak value at the first day due to the lowest pH in the environments (6.2 to 6.3). RIO showed the highest concentration (56.2 mg/L) of total dissolved iron at day 1. RZVI and RCont presented 26.6 mg/L and 11.6 mg/L of dissolved iron at the same day, respectively. Freshly precipitated or amorphous Fe(III) oxide is relatively soluble compared to other iron oxide minerals. Also, reducing environment created by organic matter decomposition is responsible for the increased solubility and availability of Fe (Lindsay, 1991). After the peak iron concentration, the soluble iron concentration of RCont fluctuated around 1-3 mg/L range in the most of time. The dissolved iron concentrations of RIO was not different considerably with those of RZVI after the peak concentrations at the first day. The average iron concentration in both of RIO and RZVI remained 4.7 mg/L for the rest of period. Because the difference of iron concentration is small among the reactors in the most operation period, it is considered that the trace nutrient effects of dissolved iron was not critical to accelerate methanogenesis in these reactors. COD in all conditions showed a first increased due to the organic compounds of food wastes initially disintegrated into soluble matters. As shown in Fig. 3, COD in these three reactors reached maximum values of 24,000 mg/L at the 12th day and the maximum COD values from the reactors were not varied. Since the COD peak appeared, COD reduced drastically in all reactors. The COD reduction coincided with gas generation. RIO showed the fastest COD reduction and followed by RZVI and then RCont, which implied a good performance in the facultative syntrophic interactions. 3.3 Volatile fatty acids As shown in Fig. 4, a large amount of acetic acid (around 1500 mg/L) and butyric acid (1400 mg/L – 1800 mg/L) were measured in each reactor at the beginning of the experiment. These readily useable acids reduced quickly corresponding to the formation of methane production peaks in the beginning of operation. When acetic acid was used up, the lag phase of methane production started in each reactor. And then acetic acid concentration increased until methane production restarted. The beginning of acetic acid concentration reduction in RIO and RZVI coincided with the beginning of its methane production. Comparing VFA concentrations of each reactor, the time of the peak formation did not vary. For example, the highest acetic acid concentration in all reactors appeared at 10 days. However, acetic acid concentration increased faster in RIO and RZVI than in RCont. and also decreased quicker in in RIO and RZVI. In contrast, the propionic acid concentrations in increased in RIO and RZVI slower than in RCont. These results suggested that the addition of iron material promoted the formation of acetic acids but suppressed the formation of propionic acid. Feng et al. (2014) also observed both the enhancement of acetic acid formation and the reduction of propionic acid formation by increasing zero valent iron dosage. They explained that the addition of zero valent iron led to create a more reductive environment which could enhance butyric-type and acetic-type fermentation but suppress propionate-type fermentation. The difference of the acid concentration reduction rate was more obvious. These results suggested that iron material could influence considerably on the VFA consumption processes than the VFA formation processes. With increasing methane production in each reactor, the acetic acid concentration decreased but maintained around 900 mg/L temporarily with the peak methane production rates. The maximum methane generation indicates the maximum consumption rate of acetic acid. Therefore, it is considered that the large amount acetic acid was generated additionally during the maximum consumption period. The formation of acetic acid in this period is also supported by the fast decrease of propionic acid concentrations in each reactor. The change of propionic acid concentration showed a similar pattern to acetic acid. Propionic or butyric acid degradation in reactor in presence of iron oxides or Fe0
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
proceeded to completion more rapidly than control. The oxidation of propionate and butyrate is thermodynamically unfavorable with high H2 partial pressure. So, the concentrations of H2 must be maintained low as possible (Thauer et al., 1977). It is only possible with the cooperation with hydrogen-utilizing methanogens (Stams and Plugge, 2009). It was thought that propionate and butyrate oxidations were substantially affected by the presence of iron oxides or Fe0 by boosting the syntrophic between fermentative bacterial species and hydrogen-consuming methanogen in anaerobic digestion of food waste.
acetic acid (mg/L)
3000 2500 2000 1500 1000 500
propionic acid (mg/L)
0 3000 2500 2000 1500 1000 500 0
butyric acid (mg/L)
3000
Day
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valeric acid (mg/L)
0 3000
Day
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RCont RIO
2000
RZVI
1500 1000 500 0 0
10
20
30
40
50
day
Fig. 4. Change of VFA component (acetic acid, propionic acid, butyric acid, and valeric acid) concentration. 4. CONCLUSION Laboratory experiments were conducted to inverstigate the effects of iron oxides and element iron on methane generation from food waste anaerobic digestion. The result showed that the addition of iron materials could reduce the lag phase and accelerate methane generation. The cumulative methane production was about 370 mL/g VS in all reactors. However, compared to the reactor without any additional iron materials, the lag phase was shorten by 43% and the maximum methane production rate was increased by 48% in the iron reactors. With the addition of iron materials, the dissolved iron concentrations and increased promoted the formation of acetic acids, which accelerates methanogenesis in these reactors. The reactor
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with iron additon showed the fastest COD reduction, followed by the reactor with element iron and control. The COD reduction coincided with methane generation. The results indicated that the addition of iron materials can be used as an effective way to promote methane generation from food waste anaerobic digestion.
ACKNOWLEDGEMENTS This research was supported by the Shenzhen government of China with Grant No. JCYJ20150616145013931 and CXZZ20151117141320317.
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