Hygienic quality of sewage sludge compost

The numerous bacterial, viral, protozoal, helminthic and fungal pathogens in sewage are concentrated in raw sewage sludge during primary sedimentation. Anaerobic digestion, air-drying and activated sludge treatment may reduce the levels of these organisms, but a risk to human health still remains. Although the magnitude of this risk cannot be quantified or even predicted at this stage, the probability of infection must be minimised to protect the community. Composting reduces the health hazard by decreasing the virulence (numbers and viability) of pathogenic organisms. Results of experiments conducted by the NIWR showed that total coliforms, E. coli, coliphage, Salmonella and Ascaris ova can be eliminated in less than a week in a pilot-scale composter utilising dewatered raw and air-dried digested sludges with wood chips and refuse as filler material. In full-scale plants elimination of indicator organisms took much longer, depending on the degree of control exercised.