Enhanced methane production by semi-continuous mesophilic co-digestion of potato waste and cabbage waste: Performance and microbial characteristics analysis
Introduction
According to China Statistical Yearbook, it is estimated that almost 9 × 107 tons of fruit and vegetable wastes (FVW) has been produced in 2015. Potato waste (PW) and cabbage waste (CW) are the two important wastes among them, especially PW occupied a large proportion. As the fourth largest food crop behind rice, wheat and corn, China’s potato harvest is over 95 million tons cultivated in a massive scale of 5.6 million hectares in 2015 (SCPRC, 2015). A large amount of PW and CW is generated in the process of transportation, storage, marketing, machining, and consumption (Marwaha et al., 2010, Mayer, 1998). Sometimes, potatoes and cabbages are abandoned in the field due to overproduction and inefficient marketing. Thus, the disposal of PW and CW is becoming an urgent issue in China.
Anaerobic digestion of FVW for biogas production has become a part of the national strategy of energy-saving and energy-producing in China (Jia et al., 2014). Recently, some researchers begun to focused on utilizing PW for biogas production given its readily digestible characteristic (Bayr et al., 2014, Liang and McDonald, 2015, Jacob and Banerjee, 2016). There are also a few reports on the biogas production from CW (Kafle et al., 2014b). Specific methane yields of 239 and 273 L/kg-VSfed were produced from raw and pre-fermented PW, respectively (Liang and McDonald, 2015). The low methane yield was mainly owing to the quick acidogenesis and poor methanogenesis. PW is mainly composed of 84% water (w/w) and 45% carbohydrate (Jacob and Banerjee, 2016), thus anaerobic digestion of such easily degradable waste produces a large amount of volatile fatty acids (VFAs) and CO2 during the acidogenesis. Most of the CO2 escapes from the reactors but the accumulated VFAs decreases pH and thereby inhibits the methanogenesis. CW contains more than 90% water and also decomposes readily. Thus, there is a need to develop methods to disinhibit the methanogenesis caused by the accumulated VFAs generated from PW and CW digestion.
The obvious strategy to overcome these difficulties is to separate the acidogenesis and methanogenesis stages by a two-phase system, which could protect methanogens from VFAs, low pH and high ammonium (Bouallagui et al., 2004). Mata-Alvarez et al. (1993) reported that the acidogenesis could not work effectively in the absence of pH control. Moreover, there are other problems with the complicated reactor which cannot be operated continuously. The technology of co-digestion has been successfully conducted to digest PW with other cellulose/nitrogen-rich materials such as Pistia stratiotes (Jacob and Banerjee, 2016), sugar beet leaves, rendering wastes (Bayr et al., 2014, Kryvoruchko et al., 2009), and pig manure (Kaparaju and Rintala, 2005). It is inescapably clear that the methanogenesis disinhibition was attributed to the increased hydraulic retention times (HRTs, 20 days or more) induced by the addition of those obstinate co-substrates. CW has low carbon/nitrogen (C/N) of 9–14, and be noted for rapid acidogenesis (Kafle et al., 2014a). Thus, it is very possible that the co-digestion of PW and CW improves the methane generation without increasing the HRT.
The inoculum is vital for the start-up of co-digester of PW and CW. Because the quick decrease of pH (4–5) resulted from the accumulated VFAs would not allow methanogens to survive under such physiological conditions when there are not enough methanogens in the inoculum to consume VFAs (Kim et al., 2016). Generally, anaerobically digested sludge is used as inoculum, whereas its availability and insufficient quantity (at a particular place and duration) poses a limitation for commercial production of methane. Thus, an improved methanogen inoculum should be employed for readily degradable wastes (Jacob and Banerjee, 2016). In this study, the acclimated anaerobic granular sludge (AGS) was used as a replacement for the conventional inoculum to enhance the methanogenesis (Van Lier, 2008, Zhang et al., 2010). To our knowledge, no worthwhile research has so far been carried out on biomethanation of PW and CW. Digestion of carbohydrate-rich PW (C/N:29) with nitrogen-rich CW (C/N:12) could be a promising approach to improve methane yields.
This study aimed to boost methane-rich biogas generation and shorten the HRT by co-digestion of PW and CW with inoculum of AGS. Batch tests were conducted at different mixing ratios of PW and CW to define an optimum mixing ratio. The semi-continuous co-digestion was conducted with the recommended optimum mixing ratio of PW/CW and a stepwise increase of organic load rate (OLR) to enhance microbial adaptability and methane generation. The emphasis was made to the effect of the co-digestion on the methane generation, digestate quality and the dynamics of bacterial and archaeal communities.
Section snippets
Substrates and inoculum
PW and CW were collected from three local markets in the city of Jinan (China), and then chopped into small pieces of less than 2 cm in size for batch and semi-continuous experiments. The prepared waste samples was stored at 4 °C for a few days for further use. Prior to each daily feeding during the semi-continuous experiments, some parameters of the wastes were measured again to guarantee the stored wastes not been spoiled. Otherwise, the new wastes must be freshly prepared. The physicochemical
AGS inoculum
The use of AGS as the inoculum for treating readily digestible substrates like FVW enhanced the microbial community diversity without the need for a lengthy acclimatization, and consequently accelerated the biodegradation (Mu et al., 2017). A small quantity of AGS could be equivalent to a large amount of the conventional inoculum due to the granular appearance and its high methanogen abundance. For example, a higher ratio of inoculum (S/I of 1:3) was reported to be suitable for co-digesting
Conclusions
PW and CW (1:1, VS basis) was co-digested successfully in batch and semi-continuous modes. The batch co-digestion produced the highest methane yield of 295 mL/g-VS with substrate concentration of 3.0 g VS/L. The highest methane yield of 360 mL/g-VS was observed at an OLR of 5.0 kg VS/m3·d during the semi-continuous co-digestion, which respectively represented 18.4% and 24.1% higher as compared to the mono-digestion of PW and CW. Further microbial community analysis revealed that the enhanced methane
Acknowledgements
This work was financially supported by Natural Science Foundation of China (51608314 and 51408348), Shandong Province Natural Science Foundation (ZR2014EEQ023), Shandong Province Key Research and Development Program (2016GSF117037).
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