Estimation of Ascaris lumbricoides egg inactivation by free ammonia treatment of ash-amended UDDT vault products using stored urine in Uganda

Urine-diverting dry toilets (UDDTs) are designed to recover nutrients and organic matter from human excreta for use as agricultural amendments, and have been promoted in many developing countries, including Uganda. Wider UDDT implementation could help address problems in areas where water scarcity limits sanitation coverage and/or declining soil fertility jeopardizes growing populations’ nutritional security. However, concerns have been raised regarding the safety of recovered UDDT vault products, which may contain persistent pathogens such as Ascaris lumbricoides eggs. A. lumbricoides eggs can be inactivated through elevation of free ammonia levels. This study assessed the feasibility of a secondary ammonia treatment strategy for UDDT ash-amended vault products using urine. Treatment parameters were measured in mixtures of urine, ash-amended vault products, and wood ash, a model was developed to account for temperature fluctuations, and A. lumbricoides egg inactivation times were estimated using a previously published model. A mixture containing two parts urine and one part ash-amended vault products was estimated to provide 2-log10 inactivation after 3 months of indoor storage (daily mean temperatures: 22.8 ± 0.3 °C) or 2 months of outdoor storage (25.9 ± 1.3 °C). This strategy could improve the safety of recovered products for agricultural use to improve the nutritional security of vulnerable populations.