Impact of COD/N/S ratio on dentrification by the mixed cultures of sulfate reducing bacteria and sulfur denitrifying bacteria

This study proposed a new process, which can carry out denitrification and removal of organics simultaneously in the absence of an external carbon source or in a low COD/N ratio by taking advantage of the symbiotic relationship between sulfur denitrifying bacteria (SDB) and sulfate-reducing bacteria (SRB). The kinetic and stoichiometric coeficients for nitrate removal by SDB were as follows :k = 0.9 hr^-1, K_s = 7 mg NO₃-N/L, Y = 0.118 mgVSS/mgNO₃-N and K_d = 0.057 hr^-1. In the activity tests of the SRB culture, organic matter, acetic acid, was used by the heterotrophic denitrifying bacteria (HDB) as an electron donor in the denitrification reactions. When below the theoretical COD/N ratio, the formation of nitrogen gas did not occur because of nitrite accumulation. In the mixed cultures of SDB and SRB, most denitrification reactions were accomplished by SDB using the reduced sulfur compounds as a sulfur source for the removal of the nitrate. Denitrification was completed within 6 hours in all batch reactors with 60% of the produced gas being nitrogen. The rate of denitrification was unaffected by the COD/N ratios. However, the degradation rate of the organic matter was very low.