Treatment of sludges from on-site sanitation - low-cost options

An overview of the current literature-based knowledge regarding faecal sludge (FS) treatment along with the results and conclusions from own field research are presented. Issues for further faecal sludge treatment studies are also addressed. The article focuses firstly on the characteristics of the various types of faecal sludge. A set of variables is proposed for minimum faecal sludge determination and faecal sludge treatment plant (FSTP) design and control. Priority treatment options particularly relevant to developing and newly industrialising countries are listed. They include solids-liquid separation by settling/thickening processes, sludge dewatering and drying in drying lagoons or drying beds, stabilisation ponds, and co-composting with refuse or other bulking/organic material of high carbon content. The results of in-depth monitoring of a faecal sludge treatment plant in Accra, Ghana, are reported. The plant receives septage and public toilet sludge and comprises solids-liquid separation by settling/thickening followed by a series of four ponds for the treatment of the liquid fraction. The four ponds in series all function as anaerobic ponds. Facultative pond conditions do not develop. This appears to be due to the suppression of algal growth through high levels of ammonia (NH3). Final effluent BOD is 300 mg/l as against 2,000 mg/l in the raw sludge mixture and 1,600 mg/l in the sedimentation tank effluent.