The Anaerobic Digestion of Organic Solid Wastes of Variable Composition

Abstract Every year in Canada approximately 8 million tonnes of organic solid waste is placed in landfills where it decomposes anaerobically over decades, produces large volumes of leachate requiring treatment, and releases 20 million tonnes of greenhouse gas emissions as CO2eq. Anaerobically digesting this waste prior to landfill would obviously be beneficial, but this is difficult to achieve because solid waste is a complex, heterogeneous and variable mixture, making any form of processing much more expensive than landfill. This thesis investigates the capabilities of a new approach to the anaerobic digestion of solid waste designed to overcome these obstacles. Most of the costly separation and pretreatment steps common in European anaerobic digesters are eliminated. The waste remains stationary, the leachate is recirculated through it, and the resulting digestate is aerobically cured. The biogas generated is recovered for the generation of electricity or the production of renewable natural gas. A laboratory scale system comprising six sequentially batch fed leach beds and an upflow anaerobic sludge blanket reactor was constructed, and operated continuously for 616 days. The feedstock consisted of a mixture of cardboard, boxboard, newsprint, and fine paper, to which varying amounts of food waste were added (from 0% to 29% on a COD basis). The digester accommodated these and other changes without any signs of process upset or instability. It was found that the addition of food waste increased biogas production from the fibre mixture from 101 L.kg-1CODfibreadded to 330 L. kg-1CODfibreadded an increase of 225%. A substrate destruction efficiency of 65% (on a COD basis) and a methane yield of 225 L.kg-1 CODadded were achieved, at a solids retention time of 42 days. This performance was similar to that of a CSTR digesting similar wastes. A financial analysis showed that the technology can be competitive with landfill.Ph.D