Methane Production from Coffee Pulp by Microorganism of Rumen Fluid and Cow Dung in Co-digestion

Abstract

Coffee pulp is abundantly available in Indonesia every year, about 1.65 × 106 t of coffee pulp were produced during the 2011 - 2015. However, they have been poorly utilized or dumped in the environment. Meanwhile, utilizing coffee pulp for production of methane (CH4) is one of the most demanding technologies for production of environmentally friendly energy. This study aims to investigate the methane production from coffee pulp by some variables; microorganism existed in cow dung, rumen-fluid, and cow dung/rumen fluid mixture in anaerobic batch reactor with 3.6 L working volume at mesophilic temperature (30 - 35 °C). Some parameters such as Total Solid (TS), Volatile Solid (VS), Volatile Fatty Acids (VFAs), Chemical Oxygen Demand (COD) conversion and CH4 generated as a function of digestion time were analysed and compared with each variable and another substrate: rice straw. The result showed that the rate of TS, VS conversion of almost all digesters was low during 30 d of digestion. Consequently, a low of CH4 generated was detected, except in cow dung digester which had a higher concentration of CH4 after 20 d of digestion. The highest COD value decreasing was from cow dung reactor of 78.05 % that was converted to VFAs. The VFAs value was represented by acetic acid as the main substrate for generating CH4. The digestion rate was limited by bacteria growth because of the presence of toxic compounds in coffee pulp such as caffeine, tannin, and free phenol. Nevertheless, the conversion of TS, VS, COD and methane production still continued, though with a slower rate. At the final digestion time, the optimum accumulation of CH4 was generated from cow dung digester of 85.1 Ndm3/kgCOD removal, but it was approximately ten times as small as optimum accumulation of CH4 from rice straw. This shows that different substrate source can affect the amount of generated biogas.