Anaerobic digestion and acidification monitoring parameters for Korean food waste

Abstract

As direct disposal of food wastes into the ocean has been prohibited in Republic of Korea ever since 2012, alternative disposal and treatment method is imperative. Anaerobic digestion, which can degrade organic materials to methane and carbon dioxide, has been considered as an attractive eco-friendly waste treatment process. For its high biodegradability and organic content, food waste is considered as an ideal subject for anaerobic digestion. The anaerobic digestion process is conventionally operated under mesophilic condition, in which a steady state of degradation and stabilization of organic materials is maintained. However, the process under thermophilic condition is known to offer several advantages over the conventional mesophilic anaerobic digestion. These advantages include, but are not limited to, a high degree of waste stabilization, decreased detention time, improved solids-liquid separation, and increased destruction of viral and bacterial pathogens. In spite of these benefits, however, lack of thorough understanding and poor operational stability still prevents thermophilic anaerobic digestion from being widely commercialized. Anaerobic digestion process instability is generally caused by feeding problems, temperature variations, lack of necessary trace elements, and the presence of inhibitory or toxic substances. A number of parameters that can monitor biological stability within anaerobic reactors exist, but the potential of a specific monitoring parameter to act as an indicator depends on the process configuration and the characteristics of the waste. Therefore, to determine the possible early indicator of system acidification and its applicability to full-scale food waste anaerobic digestion facilities in Republic of Korea, lab-scale anaerobic reactors were operated with and without additional buffer. Actual food waste generated from G-district of Seoul, Republic of Korea and standard food waste proposed by the Korean Ministry of Environment were used as the substrates for this study. Results revealed that anaerobic digestion of Korean food waste is not feasible under mesophilic and thermophilic conditions in the absence of additional buffer due to system acidification. Depression in reactor pH due to VFA accumulation and alkalinity consumption led to subsequent biogas yield reduction and process failure. Therefore, maintenance of alkalinity was found to be imperative for stable anaerobic digestion operation. With supplementary alkalinity, not only pH depression but VFA accumulation could also be prevented. In the absence of buffer addition, accumulation of VFAs and consumption of alkalinity led to constant increase in FOS/TAC ratio while P/A ratio remained relatively constant. Therefore, the FOS/TAC ratio, which takes both VFA concentration and alkalinity into consideration, was found to be more sensitive than the P/A ratio to system acidification in the anaerobic digestion process for Korean food waste. While accumulation of VFAs and consumption of alkalinity (i.e., increased FOS/TAC ratio) ultimately lead to system acidification, accumulation of both acetic acid and propionic acid does not necessarily raise the P/A ratio. In sum, this study revealed that proper monitoring of the FOS/TAC ratio can effectively prevent system acidification without having to regulate the input organic load.