Anaerobic digestion of food waste for volatile fatty acids (VFAs) production with different types of inoculum: Effect of pH
Introduction
Volatile fatty acids (VFAs) including fatty acids from C2 to C5 (acetic, propionic, butyric, etc.) are potentially renewable carbon sources. VFAs can be used for nutrients removal, production of biogas and biodiesel (Fontanille et al., 2012), generation of electricity (Chen et al., 2013c) and synthesis of biosurfactants, bioflocculants and PHAs (Li et al., 2011, Chen et al., 2013a). Additionally, Srikanth et al. (2009) demonstrated that VFAs are a potential substrate for hydrogen production via photo-fermentation by mixed microbial cultures. VFAs are usually derived from fossil fuels through chemical synthesis (Eggeman and Verser, 2005); however, anaerobic digestion (fermentation) of biomass has recently gained attention as a cost-effective and environmentally friendly alternative for VFAs production.
During the last decade, researchers have focused on waste activated sludge (WAS) fermentation to produce VFAs (Yuan et al., 2006). When compared with WAS, food waste contains higher levels of organic materials, such as starches, proteins and lipids; accordingly, it may become a plentiful source of inexpensive organic substrate for fermentative VFAs production as a result of reduction and stabilization of food waste. Moreover, food waste in China accounted for 40% of all MSW in 2011 (National Bureau of Statistics of China), and this value appears to be increasing. Owing to stringent environmental regulations, some methods for disposal of food waste such as landfill, ocean dumping, incineration, animal feed and fertilizers have been forbidden or become less desirable (Komemoto et al., 2009, Cuéllar and Webber, 2010). Anaerobic digestion is preferred as an efficient pathway for the recycling and reduction of food waste.
Hydrolysis, acidification, and methanogenesis are usually involved in anaerobic digestion, with VFAs produced in the first two steps. To induce the accumulation of higher levels of VFAs, the following strategies can be adopted: (a) improving the hydrolysis rate to produce more soluble substrates for acidification, (b) promoting the acidogenesis process, and (c) inhibiting the activity of methanogens (Yuan et al., 2006). Hydrolysis is the rate-determining step throughout the fermentation process (Li and Noike, 1992). pH, temperature, C/N ratio and hydraulic retention time (HRT) have been reported to be the key factors controlling the production of VFAs during fermentation (Chen et al., 2007, Chen et al., 2013a, Lee et al., 2014). Chen et al. (2013c) found that the optimal operating conditions for VFAs production during co-fermentation of food waste and WAS were pH 8, C/N 22, 37 °C, and a fermentation time of 6 d. However, the range of the parameters they used to determine this was limited.
To improve the production of VFAs, methanogens must be inhibited. To date, many investigations have been conducted to investigate methods of improving VFAs production from WAS by anaerobic digestion, especially under alkaline conditions (Yu et al., 2008). However, a large amount of NaOH must be added into reactors to generate such conditions, and the cost for acid recovery remains a challenge. The inhibition of methanogenesis is one of the primary effects of low pH during anaerobic fermentation (Yuan et al., 2006) because high methanogen activity requires a minimum pH of 6.5. Previous studies have reported that the optimal pH for effective hydrogenogenesis and inhibited methanogenesis was 5.0–6.5 (Fang and Liu, 2002).
Few studies have focused on VFAs production from food waste under acidic conditions. In addition, although type of inoculum is known to be one of the most important factors affecting the evolution of fermentative pathways (De Gioannis et al., 2013). Information regarding the effects of different inocula on anaerobic acidogenesis is still sparse. Accordingly, this study was conducted to efficiently produce VFAs from food waste by controlling anaerobic fermentation under acidic conditions while maintaining high hydrolysis and acidification rates without methane production. The effect of inocula on VFAs production was also evaluated, the composition of VFAs was examined, and the mechanism of VFAs production was discussed.
Section snippets
Food waste and inocula
Food waste was collected from a canteen on the campus of Zhejiang Gongshang University. The food waste, which mainly consisted of rice, vegetables and meat, was crushed with an electrical blender after sorting out animal bones and clamshells, after which the resulting slurry was sieved to less than 3 mm. Two types of inoculum were obtained, anaerobic activated sludge from an up-flow anaerobic sludge bed (UASB) reactor of the Xihu Brewery and aerobic activated sludge from the secondary settling
Effect of pH on food waste hydrolysis
Hydrolysis of food waste can be characterized by the changes in soluble chemical oxygen demand (SCOD) concentrations. During the initial 2 d, the products of VFAs and methane were relatively low; therefore, the observed SCOD concentrations, which primarily reflected fatty acids, soluble carbohydrates and protein, were regarded as the main hydrolysates (Zhang et al., 2009). Table 2 shows the solubilization rates of food waste on the second day under different conditions. During the first 2 days,
Conclusions
A high VFAs production could be obtained during anaerobic digestion at pH 6.0. VFAs were significantly enhanced by using anaerobic activated sludge to inoculate food waste for fermentation, with a concentration of 51.3 g-COD/L and a yield of 918 mg/g VSSremoval. Acetic and butyric acids were the dominant components of VFAs. During anaerobic digestion, carbohydrates were degraded rapidly within 4 days and NH4+-N release remained stable after 4 days. According to specific substrate utilization rates,
Acknowledgements
The authors appreciate the National Natural Science Foundation of China (Nos. 50908209, 20976162 and 51208464) for providing funding support for this project.
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