Nutrient recovery from source-diverted blackwater: Optimization for enhanced phosphorus recovery and reduced co-precipitation

High residual phosphorus in anaerobically digested source-diverted blackwater allows for increased phosphorus recovery through struvite (NH4MgPO4·6H2O) precipitation. Due to the complex matrix of blackwater, recovered products inevitably contain co-precipitates of other wastewater components that have not been well characterized. The potential hazards present include pathogens, antimicrobial resistance genes (ARGs), organic and inorganic compounds. In this study, several important struvite precipitation conditions (pH, Mg2+:PO4 3− molar ratio and MgCl2 dosing rate) were tested to determine their impact on phosphorus recovery, while taking into account the co-precipitation of enteric bacteria, ARGs and micro- and macronutrients. Results demonstrate that phosphorus recovery efficiency does not correlate to the degree of metal/microbial co-precipitation. Both PO4 3− recovery and co-precipitates were affected by the operation conditions examined, and the optimal condition for nutrient recovery from blackwater was at pH 9, a 1.5:1 Mg2+:PO4 3− molar ratio, and a dosing rate of 24 mM min−1. Although previous struvite recovery studies have identified optimal conditions for phosphorus recovery, limited information is available on the process optimization to minimize the potential risks. Downstream application of recovered struvite inevitably contains co-precipitated hazards that if not assessed can result in undesirable public health outcomes. It is necessary to evaluate struvite application and public health exposure to identify key steps that reduce public health impacts. Hence examination of struvite precipitation parameters on enhanced phosphorus recovery, along with co-precipitates, is necessary when considering the public health risk associated with efficiently recovering residual nutrients.