Effect of baling on the behaviour of domestic wastes: laboratory study on the role of pH in biodegradation

This study was carried out using real domestic waste, which was fractioned and then reconstituted. It was incubated after having been compacted or not in airtight containers placed in a room thermoregulated at 28°C. In one series of experiments the waste was initially incubated at 40% humidity to simulate the average moisture of real domestic waste bales. Some of these assays were not modified over 16 months of incubation, whereas others were saturated with water after several weeks of incubation to allow pH measurements and neutralization of the waste. In another series of experiments, the waste was incubated under watersaturated conditions from the beginning. The results show very low biodegradation for the assays carried out under non-saturated conditions simulating the actual moisture of domestic wastes in real bales. Acidoic biodegradation, which was limited by the small amount of O2 in the containers, was insignificant as shown by the absence of heat production in the containers, the temperature of the wastes never being higher than the room temperature. Anaerobic biodegradation was also very low as shown by the low gas production (between 8 and 16 1 of gas/kg of dry organic matter) and the absence of methane and H2S in the biogas. The observed inhibition of biological activity under unsaturated conditions was caused by acidification of the medium. This acidification was due to production of volatile fatty acids during the first days of incubation by facultative anaerobic micro-organisms, under anoxic conditions. The biological evolution was, therefore, inhibited at the acid production phase. The biodegradation was very significantly increased when the waste was saturated with water and the pH adjusted above 6.5 whether the waste was compacted or not. In this case, the total production of biogas was between 250 (compacted waste) and 290 (non-compacted waste) litres of gas/kg of dry organic matter, with a methane concentration of 60%. A complete anaerobic biodegradation of the organic matter would have led to a biogas production of about 1000 1 of gas/kg of dry organic matter.