Mechanisms and modeling of pathogen fate in pilot scale nutrient recovery reactors

Due to its high nutrient content, source-separated urine can be processed into marketable liquid or solid fertilizers. The Valorisation of Urine Nutrients in Africa (VUNA) project developed and tested pilot-scale nitrification/distillation and struvite (MgNH4PO4•6H2O) production reactors in Durban, South Africa, where over 80,000 urine diverting dry toilets (UDDTs) are in place in rural and peri-urban communities. Source-separated urine collected from UDDTs at a community scale contains pathogens due to faecal contamination of the urine. The greatest public health threat of urine nutrient recovery in this context thus depends on the fate of pathogens during fertilizer production and potential transmission of pathogens to humans. This work brings together several separate studies to evaluate the fate of bacteria and viral pathogen surrogates through the production of VUNA’s liquid and solid fertilizers. Limited inactivation of hydrophobic viruses was observed during nitrification due to inactivation at the solid-liquid-air interface. Otherwise, conversion of ammonia to nitrate and reduction of pH during nitrification reduces the natural inactivation capacity of urine. High temperature treatment via distillation is thus needed following nitrification. Microbial inactivation during struvite production was correlated with moisture content reduction during product drying. However, initial retention of moisture by drying at elevated humidity enhanced bacterial inactivation, suggesting practical strategies to improve treatment without costly energy input in remote field conditions. Finally, adsorption of viruses to human skin following contact with urine, as well as residual urine on the hands of reactor operators, was evaluated as a potential source of human health risk. A quantitative microbial risk assessment of the struvite production process was conducted to model the transfer of pathogens from urine to skin and the potential for pathogen ingestion during hand-to-mouth contacts.