Modelling Nitrogen Removal in the Kibendera Wastewater Stabilization Ponds in Ruiru, Kenya

Abstract

Application of waste stabilization ponds (WSP) in wastewater treatment in the tropical regions is primarily due to their affordability and relatively high treatment performance. Monitoring of 2-year nitrogen removal behavior in Kibendera WSP in Ruiru, Kenya, was undertaken between January 2021 and December 2022. The experimental work determined the concentrations and removal efficiencies of Ammonia, Nitrate, Nitrite, Organic Nitrogen and Total Nitrogen. Standard Methods for the examination of water and wastewater determined Nitrogen and Dissolved Oxygen (DO) concentrations. Based on the experimental data obtained, mass balance reaction rate models characterized the nitrogen transformation and removal behavior in the WSP. Whereas model calibration was achieved using observed data from January to December 2021, model validation was achieved using observed data from January to December 2022. Ammonia volatilization, sedimentation, mineralization, nitrification, denitrification and microbial ammonia uptake were the possible transformation and removal pathways. Whereas ammonia volatilization contributed the least to the overall nitrogen removal (0.01-0.02 mg/L.d), denitrification contributed the most (2.12-14.67 mg/L.d). Low DO levels and high ammonia concentrations were responsible for low nitrification rates and high microbial ammonia uptake respectively. Comparison between experimental and modelled effluent concentrations yielded correlation coefficients (<i>r</i>) of 0.77 and 0.69 for ammonia and organic nitrogen respectively during the calibration period. The corresponding model validation <i>r</i> values were 0.74 and 0.93 respectively. The good agreement between the model output and observed effluent concentrations implies that nitrogen removal prediction and optimization is possible. External aeration to spike DO concentration levels is necessary to enhance the long-term nitrification rates.