The effectiveness of pH adjustment and controlled oxygen injection to enhance acidogenic performance in two stage anaerobic digestion

Abstract

The use of two stage anaerobic reactors is intended to separate the hydrolysis process and the acidogenesis process from the methanogenesis process, thus allowing optimization of different operating conditions in each of these processes. With such optimization, this reactor has some advantages in processing the complex substrate compared to a single stage anaerobic reactor, especially when the substrate has high fat content, such as palm oil mill effluent (POME). The purpose of this research is to get the strategy of separating the acidogenesis process from methanogenesis process, with pH adjustment and air injection. The experiments were run in 8 batch reactors with each working volume of 500mL. Each reactor was filled with 10mL aqueous glucose solution with the concentration of 1.5g/L as the ideal substrate, 5mL of aqueous Na 2 CO 3 solution with 1.5g/L concentration as buffer, and 400mL of inoculum. Four of the reactors were adjusted to pH 4, while the other four reactors were set on pH 5. In each pH variation, two reactors used biodiesel waste inoculum, while the other two used cow manure inoculum. Prior to data collection, all reactors were flushed using a mixture of 85% v/v N 2 and 15% v/v CO 2 to ensure zero oxygen environment. For every inoculum type, one of the reactors was run as a completely anaerobic reactor, the other was run with periodic injection of air 10ml/day. The results showed that separation of acidogenic/acetogenic and methanogenic stages in anaerobic digestion could be conducted effectively by pH adjustment and controlled air injection. With air injection, pH value was not significantly essential for suppressing methanogenic activity as oxygen presence will inhibit methanogenic microbes and the pH will automatically drop as VFA start accumulating. The information presented in this paper is quite useful to define further study in manipulating the performance of anaerobic microbial consortium. Although this consortium is widely known as syntrophic community, which will be difficult to isolate each species in the community, it is still possible to enhance the dominance of one stage (acidogenic/acetogenic) over the other (methanogenic) by controlled air injection.