It has been known wastewater with low COD/TKN ratio produces higher effluent NO3-N and adversely affects poly P microbes returning it to an anaerobic stage in BNR systems. Nightsoil applicability to return activated sludge line to minimize NO3-N effect to poly P microbes in anaerobic stage was examined with laboratory BNR systems operated at 20°C. The study results indicated nightsoil application could improve nitrogen and phosphorus removal efficiencies both with low strength settled municipal and piggery wastes presenting low COD/TKN ratios of 6 and 2.2, respectively. Even organic loading rates increased to 20 to 60% due to nightsoil application, the effluent COD increased only 10 to 20%. This would suggest nightsoil can be used as another carbon source and nightsoil application to return activated sludge line can be another alternative modification to improve BNR systems.

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