Thermal wet oxidation improves anaerobic biodegradability of raw and digested biowaste.

Anaerobic digestion of solid biowaste generally results in relatively low methane yields of 50-60% of the theoretical maximum. Increased methane recovery from organic waste would lead to reduced handling of digested solids, lower methane emissions to the environment, and higher green energy profits. The objective of this research was to enhance the anaerobic biodegradability and methane yields from different biowastes (food waste, yard waste, and digested biowaste already treated in a full-scale biogas plant (DRANCO, Belgium)) by assessing thermal wet oxidation. The biodegradability of the waste was evaluated by using biochemical methane potential assays and continuous 3-L methane reactors. Wet oxidation temperature and oxygen pressure (T, 185-220 degrees C; O2 pressure, 0-12 bar; t, 15 min) were varied for their effect on total methane yield and digestion kinetics of digested biowaste. Measured methane yields for raw yard waste, wet oxidized yard waste, raw food waste, and wet oxidized food waste were 345, 685, 536, and 571 mL of CH/g of volatile suspended solids, respectively. Higher oxygen pressure during wet oxidation of digested biowaste considerably increased the total methane yield and digestion kinetics and permitted lignin utilization during a subsequent second digestion. The increase of the specific methane yield for the full-scale biogas plant by applying thermal wet oxidation was 35-40%, showing that there is still a considerable amount of methane that can be harvested from anaerobic digested biowaste.

Novel trends in anaerobic digestion of municipal solid waste.

Anaerobic digestion capacity has been installed on a large scale for the treatment of biowaste coming from municipal solid waste in the 90's. However, in recent years, a new trend has developed in which anaerobic digestion is applied more and more for the treatment of mixed or grey waste. It is expected that the installed capacity for grey/mixed waste will surpass the capacity installed for biowaste digestion. Five years ago, more than 85% of the treatment capacity was for biowaste digestion and only 15% for grey/mired waste digestion, derived from two old plants that were constructed prior to 1990. By the end of 2004, a digestion capacity of 1,285,000 ton per year will be available in Europe for the treatment of grey or mixed MSW, while digestion capacity for biowaste will only amount to 1,270,000 ton per year. Especially dry digestion offers new perspectives. Dry digestion is particularly suited for the treatment of grey/mixed waste due to its insensitivity to the presence of heavy inerts and light materials. Heavy inerts such as sand, glass and stones cause sedimentation and the light materials cause floatation and scum formation in the more conventional wet and semi-dry systems. Due to the high initial dry solids content, the digestate coming from dry digestion can be treated in a variety of ways. Besides the conventional mechanical dewatering, drying with waste heat or aerobic drying by addition of a fresh waste can be utilized for the production of a high-quality compost in case of the treatment of biowaste. Digestate from mixed or grey waste is not immediately suitable for the production of a high-quality compost. However, integration with incineration plants and landfills can be optimized easily with such a dry digestate and offers various interesting alternatives. In case a maximum of recyclables is pursued, the dry digestion can be followed by a wet separation in order to produce marketable endproducts such as sand and fibers.