Control of accumulated volatile fatty acids by recycling nitrified effluent

Volatile fatty acids (VFA) often accumulate in anaerobic digestion systems, decreasing pH levels and causing unstable operational performance and poor biogas production. The aim of this study is to improve anaerobic digestion efficiency by controlling/reducing the accumulation of VFAs in a continuous anaerobic digestion system.

NO₃ ^--N was added to the digester and its effects on VFAs were investigated. When the system reached an unstable condition with the accumulation of VFAs, the digester was fed at an organic loading rate of 6 kg COD (chemical oxygen demand)/m³∙d and 0.5Q of aeration tank effluent (1500 mg/L of NO₃ ^--N) was recirculated.

With the addition of NO₃ ^--N, VFAs were utilized during denitrification, after which methane production started. Furthermore, the accumulated VFAs could be used as a carbon substrate by denitrifying bacteria. After 56 d, a normal VFA concentration could be achieved. Methane production was 0.02-0.03 L CH₄/g VS higher with NO₃ ^--N recirculation and feeding than that without feeding.

The results show that the addition of NO₃ ^--N is a potentially feasible method to control VFAs. Combined with recirculation and feeding, the method can be used to effectively prevent the inhibition of methanogenic microbial activities caused by accumulated VFAs and enhance denitrification and methane production in anaerobic digesters.