Transformation and fate of urea in pit-toilet blackwater after discharge to environment.

Abstract

Studies on urea transformation reactions in blackwater are limited as urea rapidly hydrolyses under anaerobic condition. Since ammonium content of blackwater mainly originates from urine-urea, studying urea hydrolysis reactions is important to predict potential nitrate loads in aquifers from on-site sanitation facilities. In this study, urea spiked blackwater samples from pour flush pit toilet are used to examine the urea-ammonium pathways at varying initial urea concentrations and temperature. Based on laboratory results, the annual nitrate load imposed by the urea constituent of blackwater in a hard rock aquifer is predicted. Laboratory results illustrated that experimental temperature of 37 °C and pH range of 6.7 to 8.1 facilitated optimum urease enzyme activity at the initial substrate concentration of 500 mg/L. The Q10 value for urea transformation reactions indicated that increase in temperature has positive influence on enzyme activity. The reduction in urea concentration with time followed first-order kinetics. Part of ammonium ions in blackwater oxidises as nitrate ions that travel to the aquifer. Upon mixing and dilution, the nitrate concentration in 1 km2 of hard rock aquifer would annually increase by 0.004 mg/L due to blackwater infiltration from single household pour flush toilet.