Environ. Sci. Technol. 2004, 38, 3418-3424
Downloaded via IOWA STATE UNIV on January 29, 2019 at 00:10:08 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
Thermal Wet Oxidation Improves Anaerobic Biodegradability of Raw and Digested Biowaste GEERT LISSENS,† ANNE BELINDA THOMSEN,‡ LUC DE BAERE,§ WILLY VERSTRAETE,| AND B I R G I T T E K . A H R I N G * ,† Environmental Microbiology and Biotechnology, BioCentrum-DTU, Technical University of Denmark, DK-2800 Lyngby, Denmark, Plant Research Department, Risø National Laboratory, P.O. Box 49, DK-4000 Roskilde, Denmark, Organic Waste Systems, Dok Noord 4, B-9000 Gent, Belgium, and Laboratory of Microbial Ecology and Technology (LabMET), UGent, Coupure L 653, B-9000 Ghent, Belgium
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 °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 CH4/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.
Introduction Recent estimates show that about 1.3 billion t of organic waste and 700 million t of agricultural wastes are produced annually within the European Union (EU). This represents a yearly biodegradable fraction of municipal solid waste (MSW) production of 107 million t of dry matter or ap* Corresponding author phone: +45 4525 6183; fax: +45 4588 3276; e-mail:
[email protected]. † Technical University of Denmark. ‡ Risø National Laboratory. § Organic Waste Systems. | UGent. 3418
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 38, NO. 12, 2004
proximately 2.2 t of dry organic matter per European citizen, of which more than 60% is still landfilled (1). In recent years, the EU policy is diverting the disposal of organic waste away from disposal routes such as landfilling because of the production of (toxic) leachates and greenhouse gas emissions (e.g., methane) (2) and because organic waste is increasingly regarded as a potentially valuable resource for renewable and green electricity production (3, 4). It is generally recognized that anaerobic digestion is a more controlled and sustainable way of treating organic waste as compared to other disposal routes (i.e., landfilling or composting) (5). Despite the higher investment and treatment costs, anaerobic digestion is expected to gain considerable importance soon due to its valuable energy recovery in the form of biogas (6). So far, full-scale anaerobic digestion facilities have often relied upon a 15-20 d digestion phase transforming the readily biodegradable fraction, followed by a post-digestion stabilization of the remaining lignocellulosic solids (7-10). Hence, post-treatments (typically composting) are necessary to obtain a high-quality stable digestion product that can be stored and reused for agricultural purposes (6). Cellulose and hemicellulose (holocellulose) are the principal biodegradable components of biowaste and are found together with lignin in rigid hemicellulose complexes (11). The degradation of these lignocellulose complexes, which can make up to 80% of the fiber content for some refuse components (e.g., paper), is however limited to yields of at most 50% (
