This study evaluates the removal of nitrogen compounds from wastewater in modified, small diameter gravity slope (SDGS) pipes during its conveyance. A 13-meter long, closed loop, wastewater collection network was designed and built at laboratory scale. The modified SDGS consists of Polyvinyl Chloride (PVC) tubes with perforated plastic netting fixed to the inner surfaces, to enhance biofilm attachment and growth under gravity flow. The system was operated at constant temperature using synthetic wastewater similar to municipal wastewater. The efficiency of ammoniacal nitrogen (NH3-N) removal at initial chemical oxygen demand (COD) concentrations of 340, 570, and 860 mg/L was studied. The NH3-N batch concentrations tested were 4.58, 6.32, and 9.48 mg/L, respectively. The results showed that nitrogen loss under aerobic conditions may have been due to simultaneous nitrification and denitrification, which began to operate when the biofilm was between 2.5 and 5.5 mm thick. A maximum NH3-N removal efficiency of 75% was achieved following 10 hours' circulation period, at a COD concentration of 570 mg/L.

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