Optimization of mesophilic anaerobic digestion of a conventional activated sludge plant for sustainability

Potential of nutrient recovery as struvite (NH4MgPO4·6H2O) with sole waste activated sludge (WAS) of a conventional activated sludge treatment plant has been investigated using computational modelling supported by laboratory experiments. Predicted key outcomes using Bio-Win modelling tool indicated that anaerobic digestion of WAS could produce a supernatant consisting of 300–420 mg/L ammonia , 100–200 mg/L phosphate and 12–30 mg/L magnesium and laboratory experiments confirmed that struvite can be precipitated using these nutrient concentrations. Study of sludge retention time (SRT 15–30 days) and temperature (34–38 °C) on the nutrient concentrations of digester supernatant revealed that ammonia and phosphate concentrations of supernatant increased with SRT and temperature. The removal efficiencies of magnesium and phosphate in struvite precipitation reactions were 91% and 50% respectively, indicating that magnesium addition is required for extra phosphate removal in the digester supernatant. It was found that a substantial excess ammoniac nitrogen would remain and need alternative method to remove and recover, if sustainability is to be achieved. The outcome of this study confirms that WAS from conventional wastewater treatment plants such as Loganholme can be successfully upgraded towards a nutrient recovery facility by optimizing anaerobic digester parameters. Modelling is a suitable first approach for planning and decision-making