Albert Camus (1913-1960) Abstract Co-disposal of septic tank sludge had a positive effect on the municipal solid waste (MSW) stabilisation process in bioreactor landfill simulators. Codisposal experiments were carried out using the bioreactor landfill approach aiming to solve the environmental problems caused by indiscriminate and inadequate disposal of MSW and especially of septic tank sludge. The simulator receiving septic tank sludge exhibited a 200 days shorter lag-phase as compared to the 350 days required by the control simulator to start the exponential biogas production. Additionally, the simulator with septic sludge apparently retained more moisture (>60% w/w), which enhanced the overall conversion of organic matter hence increasing the biogas production (0.60 m3 biogas kg-1 VS converted) and removal efficiency of 60% VS from the simulator. Alkaline pH values (pH>8.5) did not inhibit the biogas production; moreover it contributed to reduce partially the negative effects

of NH4+ (>2 g L-1) due to NH3 volatilisation thus reducing the nitrogen content of the residues. Associated risks and hazards with septage disposal were practically eliminated as total coliform and faecal coliform content were reduced by 99% and 100%, respectively at the end of the experiment. These results indicate that co-disposal has two direct benefits, including the safe and environmentally sound disposal of septic tank sludge and an improvement of the overall performance of the bioreactor landfill by increasing moisture retention and supplying a more acclimatised bacterial population. 1. Introduction

Urban and rural areas without sewage coverage rely heavily on on-site

sanitation systems such as latrines and septic tanks for temporary treatment and disposal of human excreta (Chaggu et al., 2002). The objective of the septic tank is to provide initial treatment to household wastewater by intercepting and separating solid faecal matter from the liquid part. However, these systems, which require to be ideally emptied every 2-3 years, are not handled properly and usually after 5 to 10 years are overloaded reducing their treatment efficiency (NDUDP, 2001). Available methods of septic tank sludge treatment and disposal like composting, anaerobic digestion, chemical treatment are often not feasible for developing countries due to high handling and operational costs. Therefore, these rich moisture separated solids are often employed in agriculture or aquaculture or discharged indiscriminately into lanes, drainage ditches, sewer networks, onto open urban spaces, and into fresh and marine water bodies, causing serious environmental and health impacts (Bradley, 1981; Chen, 1988; NDUDP, 2001).