Effect of COD:N ratio on biological nitrogen removal using full-scale step-feed in municipal wastewater treatment plants
Supaporn Phanwilai, Pongsak Noophan, Chi‐Wang Li, Kwang‐Ho Choo
Abstract
Abstract 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 (SF 1 ) and Taiwan (SF 2 ). The SF 1 WWTP 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 SF 2 WWTP 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: Nitrosomonas sp., Nitrosospira sp., Nitrosoccus sp. and Zoogloea sp. Higher amounts of the archaeal- amoA gene were found with long SRT, lower DO and COD:N ratios. Abundance of Nitrobacter sp. was slightly higher than Nitrospira sp. at the SF 1 , while abundance of Nitrobacter sp. was two orders of magnitude greater than Nitrospira sp. at the SF 2 . More denitrifying bacteria were of the nirS -type than the nirK -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.