Mechanical-Biological Pre-Treatment of Waste: State of the Art and Potentials of Biotechnology

Abstract

Mechanical-biological treatment of waste (MBP) is the processing or conversion of waste from human settlements with biologically degradable components via a combination of mechanical, other physical processes and biological processes. It is a technological alternative to waste incineration. It is applicable for the treatment of waste prior to depositing, but also for the production of refuse derived fuels (RDF). A capacity of about 2 million tons has been established in Europe over the last ten years, and a broad technological variety including aerobic and anaerobic bioprocesses is available. The process design is mainly empirical, but biotechnological information is widely used. The following figures characterise the process results: Up to 95% of the degradable TOC and 94% resp. 86% of non-cellulosic carbohydrates resp. cellulose are metabolised. The stability of the treated waste is defined by a respiratory coefficient (AT4) or a gas production coefficient (GB 21); typical process results on the technical scale are 5 mg/g dry matter for AT4 and 20 l/kg dry matter for GB 21. Emissions from the processes include organic compounds metabolised or generated by bioprocesses, such as methane and carbon dioxide, as well as volatile organics, which are stripped out from the waste. A treatment by biofilters results in a 20% to 50% reduction; after a further treatment by incineration a value <55 g TOC per ton of waste is achieved. The level of contaminants in both leachate and gas emissions from the landfill is reduced up to 95% compared with untreated waste. One kilogram of treated waste potentially releases a total load of 1–3 g COD, 0.5–1.5 g TOC and 0.1–0.2 g NH4–N into the leachate.