ADSW2005 Conference Proceedings – Volume 1 – Session 6b: Process engineering 527 Anaerobic Digestion of Coffee waste Neves, L. 1 , Ribeiro, R. 1 , Oliveira, R. 1 , Alves, M.M 1* 1 Centro de Engenharia Biológica, Universidade do Minho, 4700 Braga, Portugal ( 1 *) Author for correspondence Centro de Engenharia Biológica - Universidade do Minho, 4710-057 Braga Portugal Fax: +351 253 678 986; Telephone: +351 253 604 402, e-mail: Madalena.alves@deb.uminho.pt Abstract The anaerobic co-digestion of five different by-products from instant coffee substitutes production was studied in mesophilic conditions. The co-substrate was the excess of sewage sludge from the wastewater treatment plant located in the same coffee factory. Four of the tested wastes produced methane in the range of 0.24-0.28 m 3 CH 4(STP) /kgVS initial . Reduction of 50-73% in total solids and 75-80% in volatile solids were obtained and the hydrolysis rate constants were in the range of 0.035-0.063 d -1 . After 40 days, one waste, composed of 100% barley, achieved a methane yield as low as 0.02 m 3 CH 4(STP) /kgVS initial and 31% and 40% total and volatile solids reduction, respectively,. Two different strategies were applied to enhance the biodegradability of this waste. An alkaline hydrolysis pre-treatment, that increased the methane production up to 0,22 m 3 CH 4(STP)/ kgVS initial and the total and volatile solids reductions up to 67 and 84%, respectively. A co-digestion with kitchen waste, that increased the methane production up to 0,36 m 3 CH 4(STP)/ kgVS initial and the total and volatile solids reductions up to 61 and 67%, respectively. Keywords: alkaline hydrolysis pre-treatment, biogas, hydrolysis rate constant, instant coffee substitutes, kitchen waste. Introduction The EU legislation through the Council Directive 1999/31/EC imposes that the amount of biodegradable organic waste that is disposed in landfills should be decreased by 65%, relatively to the total amount of organic fraction of municipal solid waste produced in 1995, by July 2016. Anaerobic technology is placed as one of the best available technologies to face the problem of organics disposal (Mata-Alvarez, 2003). Nevertheless, some organic solid wastes present a low biodegradability in spite of the high COD content and, therefore, studies to enhance the biomethanation process of such wastes are still required. Coffee waste is a typical example of such kind of wastes. Instant coffee production process comprises roasting the beans and extracting the soluble fraction with hot water, giving rise to the generation of large amounts of a dark coloured liquid waste containing about 20% of insoluble solids. When instant coffee substitutes are produced the raw material contains barley, rye, malted barley, chicory and coffee, the relative amount of each depending on the specific substitute to be produced. Due to the different raw matter used to produce the different substitutes, the waste composition changes sequentially, being important to evaluate their individual performance as far as the anaerobic digestion (AD) process is concerned. Whatever the raw material used, the waste is mainly composed of carbohydrate fibbers such as cellulose, hemi-cellulose and also lignin (Dinsdale et al., 1996). Lignin is highly recalcitrant and its degradation is considered the limiting step in the decomposition of lignocellulosic substrates (Pavlosthatis and Girald-Gomez, 1991). The aim of this work was to study the anaerobic biodegradation of five wastes from the instant coffee substitute production, under mesophilic conditions, in co-digestion with the excess of activated sludge from a wastewater treatment plant located in the same coffee factory. With the objective of enhancing the methane production from the waste composed by 100% barley, two different approaches were used: an alkaline pre-treatment before the co-digestion with sewage sludge and the co-digestion with kitchen waste. The alkaline hydrolysis at ambient temperatures has been proposed as the chemical pre- treatment more compatibly with the AD process, since the bioconversion generally requires an adjustment of pH by increasing alkalinity (Pavlosthatis and Gosset, 1985). On the other hand,