Deterministic modelling of removal kinetics of the chemical oxygen demand and ammonia nitrogen in pilot-scale waste stabilisation ponds treating leachate from unplanted faecal sludge drying beds at the University of Dar es Salaam, Tanzania

ABSTRACT Graphical abstract showing treatment of faecal sludge drying-bed leachate through two sequential facultative ponds (FP1 and FP2), producing improved treated water for potential reuse or discharge. First-order plug-flow kinetics were identified as the best fit, with COD and NH₃-N removal rate constants of 1.31 and 1.28 d⁻¹, respectively, supporting leachate-specific pond treatment design and evidence-based faecal sludge management policy. Faecal sludge (FS) dewatering through unplanted drying beds generates leachate which requires further treatment before safe discharge or reuse. One of the options for leachate treatment is waste stabilisation ponds (WSPs). However, existing WSP design approaches are largely based on conventional municipal wastewater kinetics removal which may not adequately represent FS leachate, characterised by high ammoniacal nitrogen (NH3–N) and organic content represented by chemical oxygen demand (COD). This study evaluated deterministic removal kinetics of COD and NH3–N in pilot-scale WSPs treating leachate from unplanted FS drying beds at the University of Dar es Salaam, Tanzania. Five kinetic models incorporating plug-flow and completely mixed flow assumptions and using different kinetic orders were assessed using the coefficient of determination (R2). Based on 27 samples collected from three experimental batches, the system achieved mean removals of 70% for COD and 78% for NH3–N. The combined first-order plug-flow model (Model type 2) best described the removal kinetics, with R2 = 0.98 for both parameters. Corresponding first-order rate constants were 1.31 and 1.28 d−1 for COD and NH3–N, respectively. Across all models, COD removal kinetics ranged from 0.78 to 9.04 d−1 and NH3–N from 0.57 to 5.20 d−1. The study provides, for the first time, validated leachate-specific removal kinetics, thereby supporting the refinement of WSP design guidelines for faecal sludge management systems in low- and middle-income countries.