Nutrient recovery from anaerobic digester supernatant using a fluidized-bed reactor

This study investigated nutrient recovery by struvite precipitation from anaerobic digester supernatant containing nitrogen and phosphorus at high concentrations using a fluidized-bed reactor. In the first phase, hydraulic retention time (HRT) in the system was reduced from 15 min to 2.5 min at molar Mg²⁺/NH₄⁺/PO₄³⁻ ratio of 1.5/16.6/1. No significant differences were detected and struvite could be precipitated at high performances even at an HRT of 2.5 min. In the second stage, the impact of Mg/P molar ratio on struvite precipitation performance was evaluated at the molar ratios of 1/1, 1.5/1, 2/1 and 2.5 min HRT. The highest NH₄-N and PO₄-P precipitation performances were around 38 % and 99.7 %, respectively, at an Mg/P molar ratio of 2. In the last stage, the impact of volatile fatty acid presence on the system performance was investigated by adding 1000 mg/L acetate to the synthetic wastewater at an Mg/P molar ratio of 1.5/1. Observing above 99 % PO₄-P removal efficiency both in the presence and absence of acetate confirmed that VFA may not have adverse impact on struvite precipitation performance. According to XRD results, struvite precipitates had a crystal structure with only a small amount of impurities.