Inactivation kinetics of indicator microorganisms during urea treatment for sanitizing finished compost from composting toilet

Abstract

This study aimed at estimating the sanitizing effectiveness of urea treatment by studying the inactivation kinetics of selected indicator microorganisms. Finished composts from a composting toilet were inoculated with indicator microorganisms and subjected to different urea concentrations (0.5–2% w/w) and temperatures (22, 32 and 42°C). The inactivation kinetics parameters were determined in relation to pH, ammonia content and temperature during treatment time. The results show that urea addition to compost enhanced inactivation of microorganisms. The decline in number of E. coli and Enterococus followed a linear reduction, while that of Ascaris lumbricoides eggs followed a linear reduction plus shoulder. The inactivation rate constants of all microorganisms tested were positively correlated to the increase of NH3(aq) concentration and temperature. The relationship between the inactivation rate of microorganisms, ammonia through urea concentration and temperature were established. Therefore, the best decimal decay of E. coli, Enterococus and A. lumbricoides eggs occurred with 2% w/w urea concentration at 42°C within 0.9, 1.1 and 1.4 days, respectively. E. coli was the most sensitive microorganism to urea treatment, while Enterococcus and A. lumbricoides eggs showed resistance, especially at lower temperatures. Urea treatment has proved to be an efficient option for safe reuse of compost from composting toilets.