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 (SF1) and Taiwan (SF2). The SF1 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 SF2 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 SF1, while abundance of Nitrobacter sp. was two orders of magnitude greater than Nitrospira sp. at the SF2. 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 SF1 and high DO concentration in anoxic tank of SF2 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.
Highlights
Increases in water pollution are usually related to growing urban populations
In earlier studies it has been shown that total nitrogen (TN) removal is improved with higher chemical oxygen demand (COD):N ratios, longer solids retention time (SRT), and low dissolved oxygen (DO) concentrations in anoxic zones
This work demonstrated that each variable is important in order to achieve adequate treatment in full-scale step-feed wastewater treatment plants (WWTPs)
Summary
Increases in water pollution are usually related to growing urban populations. Efficient removal of nitrogen in wastewater treatment plants (WWTPs) is essential to avoid downstream eutrophication which adversely affects animal and human health globally. Nitrogen is removed from wastewaters with physical methods (air stripping), chemical methods (ion exchange), biological treatment (nitrification and denitrification processes), and/. The most popular domestic wastewater treatment system for large communities is activated sludge process with plug flow configuration. With some sitespecific conditions, existing processes or equipment and demand for high biological nitrogen removal efficiency, a modification of plug flow with step-feed is recommended. Dividing a reactor tank into anoxic and aerobic zones and/or using step-feed configuration are commonly recommended for improving nitrogen removal [2].
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