Research Article Effect of operational parameters on anaerobic co-digestion of dairy cattle manure and agricultural residues: A case study for the Kahramanmaras - region in Turkey The aim of this study was to investigate the effect of temperature and hydraulic retention time (HRT) on anaerobic co-digestion/biomethanation of cattle manure and agricultural residues (clover, grass and wheat straw). For this purpose, 12 semi-continuous reactors, fed with/without agricultural residues, were operated under varied temperature (101, 201 and 35711C) and HRT (20 and 30 days) conditions. During the experimental study, all reactors were fed once on a daily basis and operated with an organic loading rate of 3 g volatile solids (VS)/L  d. Daily biogas production, pH, biogas composition, volatile fatty acids, chemical oxygen demand and solids’ (dry matter andVS) concentrations were analyzed. Results indicate that the effect of agricultural residue addition did not influence the rate and extent of biomethanation of cattle manure. An effect of temperature was clearly observed on reactor performance for both operational HRTs of 20 and 30 days. At 35711C, reactors produced 299–324 mL biogas/g VS added, whereas this value remained between 87–138 mL biogas/g VS for the reactors run at 201 1C. The results were comparable to the studies performed on anaerobic digestion of cattle manure in terms of both methane production yield (39–182 mL CH 4 /g VS added) and dry matter reduction efficiencies (33–51%). Keywords: Agricultural residues / Anaerobic co-digestion / Biogas / Cattle manure / Turkey Received: March 5, 2010; revised: August 11, 2010; accepted: October 6, 2010 DOI: 10.1002/elsc.201000037 1 Introduction In the last few decades, cattle breeding came to forefront with increasing number of farms all around the world. As a result of animal husbandry operations especially due to large-scale cattle farms, serious ecological damage is caused where proper environmental management is not practiced. The damage is mostly in the form of soil and ground water contamination resulted from unsanitary disposal of cattle manure produced in huge amounts. An average-sized dairy cattle (635 kg) produces 22 805 kg of manure annually. This amount corresponds to 2429 kg of chemical oxygen demand (COD) and 2208 kg of volatile solids (VS) to be treated and/or properly disposed. In Turkey, proper management of cattle manure is not established. In most of the regions of Turkey, manure is either applied to the soil as fertilizer, without taking any precautions against groundwater/soil pollution, or it is burned for heating purposes. There are also other ecological issues related to mismanagement of cattle manure including eutro- fication of surface waters, pathogen-sourced contamination and associated health effects [1]. The data obtained from General Directorate of Electrical Power Resources Survey and Development Administration of Turkey indicate that 40 million ton of cattle manure is produced annually in Turkey [2] (http://www.eie.gov.tr). According to Kaya et al. [3] (www.enerji.gov.tr) in Turkey animal manure production is 84 million ton/year which accounts for an energy equivalent of 1.8 million ton of oil equivalent (TOE) when converted to biogas. This value is more than two times of the national natural gas production of Turkey in 2007, which corresponds to 827 000 TOE [4]. There are many methods applicable for the management of cattle manure. Soil application [5, 6], lagoons [7], composting Emrah Alkaya Tuba Hande Erguder Go ¨ksel N. Demirer Department of Environmental Engineering, Middle East Technical University, Ankara, Turkey Abbreviations: COD, chemical oxygen demand; DM, dry matter; GC, gas chromatograph; HRT , hydraulic retention time; tVFA, total VFA; VFA, volatile fatty acids; VS, volatile solids Correspondence: Professor Go ¨ksel N. Demirer (goksel@metu.edu.tr), Department of Environmental Engineering, Middle East Technical University, Inonu Bulvari, 06531, Ankara, Turkey & 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim http://www.els-journal.com 552 Eng. Life Sci. 2010, 10, No. 6, 552–559