Process Biochemistry 36 (2000) 243 – 248 Anaerobic treatment of olive mill wastes in batch reactors T.H. Ergu ¨der, E. Gu ¨ ven, G.N. Demirer * Department of Enironmental Engineering, Middle East Technical Uniersity, Inonu Bulari, 06531 Ankara, Turkey Received 20 December 1999; received in revised form 18 May 2000; accepted 3 June 2000 Abstract Anaerobic treatment of olive oil mill wastes, namely black water and prina, was investigated in batch reactors. Biochemical methane potential (BMP) tests were conducted to determine the anaerobic biodegradability of black water and/or prina. With these BMP tests the biodegradability of olive mill wastes (OMWs) at different initial chemical oxygen demand (COD) concentrations and corresponding methane gas productions were investigated. Furthermore, a screening study was performed to determine the most important nutrients for the anaerobic digestion of black water. The results indicated that OMWW could be treated anaerobically with high efficiencies (85.4 – 93.4%) and treatment of 1 l olive mill waste waters (OMWW) by anaerobic methods resulted in production of 57.1 1.5 l of methane gas. Anaerobic treatment of the olive mill residual solids (OMRS) alone was poor; however, when OMRS was mixed with OMWW in certain ratios, OMRS could be treated efficiently under anaerobic conditions. Anaerobic cultures needed an adaptation period of 15 – 25 days for treatment of OMRS with and without OMWW. © 2000 Elsevier Science Ltd. All rights reserved. Keywords: Olive-mill wastes; Prina; Black water; Anaerobic treatment; Biochemical methane potential www.elsevier.com/locate/procbio 1. Introduction Olive oil mills are small agro-industrial units located mainly around the Mediterranean, Aegean and Mar- mara seas that account for approximately 95% of the worldwide olive oil production. They process olives for the extraction of olive oil either by means of a discon- tinuous press (classical process) or a solid/liquid cen- trifuge (centrifugal process). Both of these processes produce two waste streams, namely olive mill residual solids (OMRS or prina) which contain oil to be recov- ered by means of solvent extraction, and olive mill waste waters (OMWW or black water). The mean waste stream volumes from classical and centrifugal processes are 1.18 and 1.68 m 3 per ton of olives pro- cessed, respectively. The corresponding mean organic loads in terms of COD are 79.2 and 121.7 kg COD per ton of olives processed, respectively. The maximum biological oxygen demand (BOD) and COD concentra- tions reach 100 and 200 g l -1 , respectively [1–3]. In the olive growing countries of the Mediterranean area (Greece, Italy, Lebanon, Portugal, Spain, Syria, Tunisia and Turkey) olive oil mill effluent production is more than 30 million m 3 per year [4]. Olive mill wastes are a significant source of potential or existing environ- mental pollution in these countries [5–7]. The difficulties of treatment of olive mill effluents are mainly related to (a) high organic loading, (b) seasonal operation, (c) high territorial scattering, and (d) pres- ence of organic compounds which are hard to biode- grade such as long-chain fatty acids and phenolic compounds. The seasonal production and high organic loading of OMWs make anaerobic treatment a very attractive option for these wastes. Furthermore, production of much less biosolids (sludge) and biogas as a valuable end product, which may offset the associated treatment costs, further add to the positive aspects of anaerobic treatment. Thus, anaerobic treatment of OMWWs (black water) has been the subject of several studies [1–4,6–12]. Table 1 summarizes the results of some anaerobic treatment studies of OMWWs. However, even though anaerobic treatment tech- niques are considered to be feasible for the treatments of OMWWs, several difficulties were noted [10,12,13]. * Corresponding author. Tel.: +90-3122105861; fax: +90- 3122101260. E-mail address: goksel@metu.edu.tr (G.N. Demirer). 0032-9592/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved. PII: S0032-9592(00)00205-3