Effect of COD:N ratio on biological nitrogen removal using full-scale step-feed in municipal wastewater treatment plants

This study investigated the effect of low and high chemical oxygen demand (COD):N ratios on biological nitrogen removal and microbial distributions in full-scale step-feed (SF) municipal wastewater treatment plants (WWTPs) in Thailand (SF1) and Taiwan (SF2). The SF1WWTP had a low COD:N (4:1) ratio, a long solids retention time (SRT) (> 60 d), and low dissolved oxygen (DO) conditions (0.2 mg L(- 1)in anoxic tank and 0.9 mg L(- 1)in aerobic tank). The total nitrogen (TN) removal efficiency was 48%. The SF2WWTP had a high COD:N (10:1) ratio, a short SRT (7 d), and high DO (0.6 mg L(- 1)in anoxic tank and 1.8 mg L(- 1)in aerobic tank). The TN removal efficiency was 61%. The nitrification and denitrification rates from these two plants were inadequate. Using a quantitative polymerase chain reaction (qPCR) technique, the populations of ammonium oxidizing bacteria (AOB) and ammonium oxidizing archaea were quantified. Measurement of ammonia monooxygenase (amoA) gene abundances identified these AOB:Nitrosomonassp.,Nitrosospirasp.,Nitrosoccussp. andZoogloeasp. Higher amounts of the archaeal-amoAgene were found with long SRT, lower DO and COD:N ratios. Abundance ofNitrobactersp. was slightly higher thanNitrospirasp. at the SF1, while abundance ofNitrobactersp. was two orders of magnitude greater thanNitrospirasp. at the SF2. More denitrifying bacteria were of thenirS-type than thenirK-type, especially at higher COD:N ratio. Most bacteria belong to the phyla Acidobacteria, Actinobacteria Bacteroidetes, Chloroflexi, Proteobacteria. The results from this work showed that insufficient carbon sources at the SF(1)and high DO concentration in anoxic tank of SF(2)adversely affected nitrogen removal efficiencies. In further research work, advanced techniques on the next generation sequencing with different variable regions should be recommended in full-scale WWTPs.