Volatile fatty acid production from co-fermentation of primary sludge and food waste without pH control

Abstract: The production of volatile fatty acids (VFAs) from waste stream is gaining high attention because of their high market value and wide range of applications. In this study, the production of VFA from co-fermentation of primary sludge from wastewater treatment plant and food waste without pH control was evaluated using a pilot-scale reactor in a semi-continuous mode of operation. In addition, the influence of substrate and inoculum on VFA production and composition was assessed using a batch fermentation experiment. The pilot-scale reactor was operated at a retention time of 7 days and 10 days in phase 1 (126 days) and phase 2 (25 days) respectively. A maximum VFA production of (687 mg COD/g VS) was obtained when the pilot-scale reactor was operated at a retention time of 7 days on day 107. The change in retention time from 7 to 10 days led to a higher hydrolysis rate; however, no improvement in VFA production was observed. The most abundant VFA produced after the reactor stabilized was caproic acid (50 %), followed by acetic acid (23%) and butyric acid (20%). Higher amount ammonium nitrogen (1.3 to 14.32 mg/g VS) compared to soluble phosphorus (0.69 to 7 mg/g VS) was released during the co-fermentation process. Furthermore, the loss of the VFA due to the production of methane was highly reduced because the pH of the reactor adjusted by itself in the range of (5 – 5.7). The batch fermentation experiment revealed that VFA production without pH control is highly influenced by the type of substrate and inoculum used. While the distribution of VFAs, is highly influenced by the inoculum type compared to the substrate used. Finding from this study indicates that there is a potential to produce VFA from co-fermentation of primary sludge and food waste without pH control.