Multi-response optimization of resource oriented coagulation/flocculation of fecal sludge from Ethiopia using Moringa stenopetala and alum by response surface modelling

Resource recovery based fecal sludge (FS) treatment is crucial for developing countries to align with Sustainable Development Goals 6 and 7, which target clean water and sanitation for all and affordable and clean energy, respectively. This study, therefore, compared the coagulation-flocculation potential of aqueous Moringa stenopetala seed extract (MSE) to that of alum from a resource recovery perspective. A response surface central composite design was employed, using pH and coagulant dose as independent variables for each coagulant, with five levels resulting in 13 total runs for each coagulant. Multi-response optimization and modelling, which was not addressed in previous similar studies on FS, was employed to maximize chemical oxygen demand (COD) removal and energy content of the sludge, increase total solid concentration and minimize sludge volume. The optimum working conditions for MSE were a dose of 216 mg/g TS of FS at pH 2.2, while for alum, the optimum dose was 98 mg/g TS of FS at pH 6. At these optimum conditions, MSE achieved 94 % COD removal, a calorific value of 18.4 MJ/kg, a sludge volume of 259 mL/L and a TS concentration of 48.8 g/L. In comparison, alum achieved 92 % COD removal, a calorific value of 17.3 MJ/kg, a sludge volume of 370 mL/L and a TS concentration of 32.7 g/L. These results emphasize that MSE has greater potential as a coagulant than alum from a resource recovery perspective.