Dry anaerobic digestion of the organic fraction of municipal solid waste

Anaerobic digestion is an attractive technology for solid waste management. This thesis describes the technological potentials of dry anaerobic digestion of the organic fraction of Municipal Solid Waste (MSW) using batch systems. In 1985 a research programme was started to develop the so- called BIOCEL system based on batchwise anaerobic digestion yielding biogas and compost. The research programme was financially supported by the Dutch National Programme for reuse of Waste (NOH), which is coordinated by NOVEM, the Dutch Organization for Energy and the Environment and RIVM, the Dutch Institute for Public Health and the Environment. The research was carried out on laboratory scale as well as on pilot-plant scale. This study presents the results of the experimental work.For start-up of the dry digestion of the organic fraction of MSW, the addition of a methanogenic inoculum appears to be essential. The best results are obtained with the digested residue as the methanogenic inoculum at start-up. Start-up of dry anaerobic digestion of Vegetable Fruit and Yard (VFY) waste, the source-separated organic fraction of Municipal Solid Waste, is also investigated. The total solids retention time (SRT) at an inoculum factor of 0.50 is 28 days.The influence of temperature and the total solids (TS) concentration on the rate of the anaerobic digestion process was investigated. Depending on the start-up procedure, anaerobic digestion proceeds in the range of 10 to 50 % T S at similar rate. The optimum temperature of the process is 40 °C. The acid formation rate shows less response to a temperature increase then does the methanogenesis.The effect of suboptimal transport of free liquid phase in the solid waste bed of solid waste digesters was studied. It has been found that dry anaerobic batch digestion of solid organic wastes can proceed at pH values as low as 5.2 and organic acid concentrations of 40-50 g CODA in the digester environment. Methanogenesis takes place in zones that are formed in the solid waste bed due to heterogenous mixing characteristics of the reactor contents. The phenomenon of methanogenesis in dry anaerobic digestion of solid wastes under extreme conditions, such as pH values below 6, and volatile fatty acid concentrations up to 40 g/I was studied. Methanogenesis is possible at an initial acetate concentration as high as 583 mM (38 g/1) and at pH = 7.0. Microscopic observations of enriched cultures show that the predominant organisms resemble the genus Methanosarcina sp.Pilot-scale experiments (5 m 3, 450 m 3) illustrated the technological potentials of the BIOCEL process. Based on the results of the experiments on several scales it can be concluded that the process is ready for full-scale application. Future research should deal with the microbial and kinetic aspects of the hydrolysis of particulate organic matter present in the organic fraction of Municipal Solid Waste