To fulfil the UN millennium goals for sustainable development, there is an urgent need for alternatives to conventional water based sanitation. Faeces and urine contain valuable plant nutrients and should be considered as resources rather than wastes. Collection with efficient water usage enables faeces and urine to be reused and environmental pollution may be better prevented. When using human excreta as plant fertilisers, it is important to prevent disease transmission by reducing the content of gastrointestinal pathogens. Nitrogen based treatment of faeces and manure with ammonium hydroxide and urea has been shown to be an efficient method for inactivating bacterial pathogens, nematode eggs and protozoan cysts. The substance responsible for microbial inactivation is uncharged ammonia, NH3 (aq). As ammonia is a weak base, the ammonia equilibrium can be controlled by additions of alkaline agents such as lime. A pH above 8 is needed to produce substantial amounts of ammonia in the form of NH3.
The objective of this study was to examine the potential of nitrogen based treatment for reduction of human bacterial and parasitic pathogens in faeces collected separately from a dry sanitation system. The faeces samples were inoculated with Enterococcus faecalis, Salmonella Typhimurium, Escherichia coli O157:H7 and Ascaris suum eggs prior to treatment. Treatments were performed at 14°C and consisted of ammonia (1% w/w), urea (0.5, 1 and 2% w/w) and/or slaked lime and storage. Inactivation of bacteria was monitored by plate count methods and viability of ascaris eggs by microscopy. Ammonia content was determined by distillation and titration and NH3 concentration calculations based on pH.
Addition of ammonia, urea and/or lime resulted in an increase in pH from the initial 7.14, whereas the pH in the storage treatment decreased. The 1% ammonia treatment with equimolar addition of total ammonia as 2% urea resulted in a higher pH (10.2) than 2% urea (9.2). Addition of urea gave a more stable pH over time compared to addition of lime, although pH declined with time in all treatments, thus affecting NH3 concentration. On day 21, 1% ammonia and 2% urea were the only treatments with sufficiently high pH to produce a substantial amount of NH3, although reductions in bacteria fitted well to an exponential function even after day 21. No viable A. suum eggs were observed after 41 days in the 0.5% urea, 2% urea and storage treatments. On day 12, no significant differences in viability could be observed between the different treatments. However a tendency for reduced A. suum viability according to the urea gradient could be observed. E. faecalis was less sensitive to the treatments than any of the pathogenic bacteria studied and E. coli was more sensitive than S. Typhimurium, although the differences were small. The 1% ammonia and 2% urea treatments were the most efficient at reducing bacteria, resulting in a decimal reduction time of 0.13-5 days for 1% ammonia treatment and 0.2-41 days for 2% urea treatment. Regression analysis of the coefficients k for the bacterial reduction function and the concentrations of NH3 in the treatments revealed a significant linear correlation for all bacteria studied. However, pH was only significantly correlated to k for E. faecalis, though weaker than to NH3. The relationship between NH3 concentration and reduction coefficient gave a change in k of 0.022 units per mM NH3 for E. coli and 0.014 units for S. Typhimurium. The pathogenic bacteria were more sensitive to NH3 concentration than E. faecalis, with a change in k of 0.00054 units per mM NH3. As E. faecalis seemed to have a higher threshold concentration for inactivation by ammonia based treatments and its reduction time exceeded that of the path...
