Sludge stabilisation and methanogenesis in a UTST septic tank (Thailand)

Humic acid (HA) may have an inhibitory effect on the different microbial processes occurring during anaerobic digestion. This research sought to investigate the extent of potential inhibition caused by accumulating humic matter acid in the upflow thermphilic septic tank (UTST) system. It also focused on the effect of HA on the hydrolytic and methanogenic organisms. Starch and sodium acetate were used as substrates for the experiments in which the impact on hydrolysis and methanogenesis was assessed, respectively. Experiments were performed in duplicates using anaerobic sludge as the inoculum. From the results obtained, it was concluded that the HA concentration in the UTST was about 1 g/L and was below the level that may cause toxicity in the system. Additionally, HA concentration of 5,000 mg/L caused up to 27% reduction in the methanogenic activity. It was also observed that while HA concentrations up to 250 mg/L did not have inhibitory effects on hydrolysis, the effect on hydrolysis at HA >250 mg/L could not be conclusively determined from the experiments. Though the HA content in the UTST did not present an immediate inhibition danger, it would be beneficial to periodically analyse the HA content of the UTST in order to establish the equilibrium concentration in the system.