The Occurrence of Residual Biological and Chemical Hazards in Recovered Struvite from Blackwater

Abstract

Phosphorus is an essential element in several industries such as agriculture and a requirement for growth. Dwindling phosphate rock sources thus requires the security of new phosphorus sources to maintain future populations. On the other hand, wastewater contains a considerable amount of nutrients that are often released into the receiving environment. To reduce the eutrophication that results from effluent release and secure a new phosphorus source, nutrient recovery from treated wastewater can be an attractive option. Phosphorus and nitrogen recovery in the form of struvite (MgNH4PO4·6H2O) has been identified as a product easily precipitated from wastewater effluent and could help alleviate phosphorus pressures by acting as a slow release fertilizer. More specifically, anaerobic digestion of source-diverted blackwater has been shown to provide a good source for energy and nutrient recovery. However, since source-diverted blackwater lacks the dilution typically associated with municipal wastewater, the concentration of hazards would be expected to be much higher. Potential hazards of concern include microbial pathogens, antimicrobial resistance genes (ARGs), pharmaceuticals and heavy metals. Though struvite precipitation from wastewater is not a new process, the level of risk associated with applying recovered struvite to agriculture has not been well characterized. A full analysis of pilot and full scale struvite samples in comparison to lab produced struvite shows that all samples can be identified as struvite regardless of their source. However, further examination into the co-precipitation that occurs shows that a more complex matrix such as blackwater would produce a product with reduced purity as bacteria, viruses, ARGs and other macro- and micronutrients can be detected. Additionally, many water chemistry conditions have been identified as optimal conditions for enhancing the precipitation process, but the consideration of hazard co-precipitation has not been taken into account. This study shows that pH of 9, 1.5:1 Mg2+:PO43- molar ratio and MgCl2 dosing rate of 24 mL•min-1 are most optimal for enhanced phosphorus recovery and reduced co-precipitation from blackwater. Furthermore, it is shown that the post-processing methods, such as drying would be required to further reduce the risk presented by viable bacterial cells.