Sanitisation of Faecal Sludge by Ammonia Treatment Technology for Safe Reuse in Agriculture

Faecal sludge contains valuable plant nutrients and can be used as a fertiliser in agriculture, instead of being emitted as a pollutant. As this involves a risk of pathogen transmission, it is crucial to inactivate the pathogens in faecal sludge. One treatment alternative is ammonia sanitisation, as uncharged ammonia (NH3) inactivates pathogens. The aim of this thesis was to study how the pathogen inactivation depends on treatment factors, mainly NH3 concentration, temperature and storage time, and based on this to make treatment recommendations that ensure pathogen inactivation. Salmonella inactivation was rapid and could be eliminated within a few days. Reovirus and adenovirus were inactivated more slowly than that, but more rapidly than bacteriophages PhiX174, 28B and MS2. Ascaris eggs were generally inactivated more slowly than the other studied organisms, especially at low temperatures (<20 °C). Ascaris egg inactivation was modelled as a function of NH3 concentration and temperature, which enabled the prediction of required treatment time. An assessment of health risk associated with consumption of crops eaten raw indicated that a 4.5 log10 reduction of Ascaris eggs and a 7.5 log10 reduction of rotavirus were required for unrestricted use of ammonia-treated faecal sludge as a fertiliser. Faecal sludge contains some ammonia mainly due to the ammonia in urine, but the concentrations can be low due to dilution with flushwater and losses to air. Mixing source-separated urine and faeces from urine-diverting dry toilets will give a high enough NH3 concentration for pathogen inactivation. Estimations of NH3 concentrations in faecal sludge from vacuum, pour-flush and low-flush toilets indicated that the ammonia concentrations required for stable pH may not be reached without the addition of ammonia. The addition can be urea, which is a common mineral fertiliser that hydrolyses to ammonia and carbonate through the enzyme urease found in faeces. Ammonia sanitisation of faecal sludge is a simple and robust technology enabling a high degree of pathogen inactivation. This can considerably reduce the health risk for farmers, food consumers and downstream populations. It is important to minimise flush water volumes in order to reduce the treatment costs.