Performance of an anaerobic biofilter for 2-Ethylhexanoic acid degradation

Abstract

A 20-litre anaerobic biofilter, packed with expanded clay pellets, was used to treat 2-ethylhexanoic acid (2-EHA), a component in a pharmaceutical plant effluent. The biofilter was operated for 425 days, at hydraulic retention times (HRT) from 20 to 0.83 days, with an influent 2-EHA concentration of 8200 mg/litre. The performance was optimal, in terms of COD removal rate, at 1.1 day HRT. COD removal efficiency was 92.8% and the biogas production rate averaged 128.7 litres/day with 83.0% of methane. The pH of the biofilter liquor remained between 6.9 and 7.5, right up to the time when the biofilter finally failed at 0.83 day HRT. At the point of failure, the COD removal efficiency was 20.1%. The high methane yield of 0.32 litre per g COD reduced, together with the low concentrations of volatile fatty acids, suggested that the biofilter failure was not due to 'souring'. The primary cause of failure was attributed to the gradual washout of attached biomass, starting at 2 days HRT until failure eventually occurred. Failure resulted in the accumulation of 2-EHA in the reactor liquor, which averaged 4407 mg/litre, and may have been inhibitory to certain components of the bacterial association.