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Many developing countries, mostly situated in the tropical region, have incorporated a biological nitrogen removal process into their wastewater treatment plants (WWTPs). Existing wastewater characteristic data suggested that the soluble chemical oxygen demand (COD) in tropical wastewater is not sufficient for denitrification. Warm wastewater temperature (30 °C) in the tropical region may accelerate the hydrolysis of particulate settleable solids (PSS) to provide slowly-biodegradable COD (sbCOD) for denitrification. This study aimed to characterize the different fractions of COD in several sources of low COD-to-nitrogen (COD/N) tropical wastewater. We characterized the wastewater samples from six WWTPs in Malaysia for 22 months. We determined the fractions of COD in the wastewater by nitrate uptake rate experiments. The PSS hydrolysis kinetic coefficients were determined at tropical temperature using an oxygen uptake rate experiment. The wastewater samples were low in readily-biodegradable COD (rbCOD), which made up 3–40% of total COD (TCOD). Most of the biodegradable organics were in the form of sbCOD (15–60% of TCOD), which was sufficient for complete denitrification. The PSS hydrolysis rate was two times higher than that at 20 °C. The high PSS hydrolysis rate may provide sufficient sbCOD to achieve effective biological nitrogen removal at WWTPs in the tropical region.

Intensive aeration for nitrification is a major energy consumer in sewage treatment plants (STPs). Low-dissolved-oxygen (low-DO) nitrification has the potential to lower the aeration demand. However, the applicability of low-DO nitrification in the tropical climate is not well-understood. In this study, the potential of low-DO nitrification in tropical setting was first examined using batch kinetic experiments. Subsequently, the performance of low-DO nitrification was investigated in a laboratory-scale sequential batch reactor (SBR) for 42 days using real tropical sewage. The batch kinetic experiments showed that the seed sludge has a relatively high oxygen affinity. Thus, the rate of nitrification was not significantly reduced at low DO concentrations (0.5 mg/L). During the operation of the low-DO nitrification SBR, 90% of NH-N was removed. The active low-DO nitrification was mainly attributed to the limited biodegradable organics in the sewage. Fluorescence in-situ hybridisation and 16S rRNA amplicon sequencing revealed the nitrifiers were related to Nitrospira genus and Nitrosomonadaceae family. Phylogenetic analysis suggests 47% of the operational taxonomic units in Nitrospira genus are closely related to a comammox bacteria. This study has demonstrated active low-DO nitrification in tropical setting, which is a more sustainable process that could significantly reduce the energy footprint of STPs.

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Conventionally, nitrification in biological nitrogen removal (BNR) requires high dissolved oxygen (DO) concentrations (>2 mg L−1), making the process energy intensive. Recent studies have shown that efficient ammonium removal and energy reduction can be realized by operating the nitrification at low DO concentrations (<1 mg L−1). In this study, the low-DO oxic anoxic (low-DO OA) process was operated in a pilot-scale sequencing batch reactor (SBR) over 218 days to evaluate the feasibility of nitrogen removal from low chemical oxygen demand-to-nitrogen ratio (COD/N) tropical municipal wastewater. The results revealed that the low-DO OA process attained high removal efficiency for ammonium (97%) and total nitrogen (TN) (80%) under an average DO concentration of 0.6 mg L−1. The effective TN removal efficiency is attributed to the occurrence of simultaneous nitrification–denitrification (SND) under low DO conditions. Further batch tests revealed that slowly biodegradable COD (sbCOD) in tropical wastewater can support denitrification in the post-anoxic phase, resulting in a high TN removal rate. Compared with high DO concentrations (2 mg L−1), low DO conditions achieved 10% higher TN removal efficiency, with similar ammonium and COD removal efficiency. This study is crucial in promoting the energy efficiency and sustainability of wastewater treatment plants treating low COD/N wastewater.