Anaerobic treatment of olive mill wastes in batch reactors
Introduction
Olive oil mills are small agro-industrial units located mainly around the Mediterranean, Aegean and Marmara 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 discontinuous press (classical process) or a solid/liquid centrifuge (centrifugal process). Both of these processes produce two waste streams, namely olive mill residual solids (OMRS or prina) which contain oil to be recovered 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 m3 per ton of olives processed, 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 concentrations reach 100 and 200 g l−1, respectively [1], [2], [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 m3 per year [4]. Olive mill wastes are a significant source of potential or existing environmental pollution in these countries [5], [6], [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) presence of organic compounds which are hard to biodegrade 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], [2], [3], [4], [6], [7], [8], [9], [10], [11], [12]. Table 1 summarizes the results of some anaerobic treatment studies of OMWWs.
However, even though anaerobic treatment techniques are considered to be feasible for the treatments of OMWWs, several difficulties were noted [10], [12], [13].
This study was conducted to investigate the anaerobic treatment and biogas production potential of OMWs originating from the Marmara region of Turkey. Furthermore, this study is the first attempt to examine the anaerobic treatability of OMRS (prina) that comprises a significant portion (30%) of the olive mill wastes [11].
Section snippets
Characterization of olive mill wastes
OMWs used were obtained from an olive oil extraction plant located near the city of Bursa, which uses a traditional discontinuous process. OMWWs and OMRSs were initially characterized and the results are tabulated in Table 2.
Biochemical methane potential tests
All three experiments conducted in this study were based on BMP tests [14] to determine anaerobic treatability. Sterile serum bottles with 125 ml (Experiments 1 and 2) and 250 ml (Experiment 3) total volumes were used as anaerobic batch reactors. The corresponding liquid
Anaerobic treatment and methane production potential of olive mill waste waters
This experiment was carried out to determine the anaerobic treatability of OMWW samples of varied COD concentrations and the associated methane production. For this purpose, four different COD concentrations, namely 2765, 8295, 13825 and 27650 mg l−1, were achieved in duplicate serum bottles. The VSS concentration in each bottle was adjusted to 5300 mg l−1. Thus, the initial COD to biomass ratios obtained in the reactors were 0.52, 1.57, 2.61 and 5.22 mg COD/mg VSS. The rationale for selecting
Conclusions
The following conclusions can be drawn from this study.
OMWW could be treated anaerobically with high efficiencies (85.4–93.4%) and treatment of 1 l OMWW by anaerobic methods resulted in production of 57.1±1.5 l of methane gas (i.e. 413 ml of methane gas was produced from degradation of 1 g of COD found in olive mill waste water).
To achieve anaerobic treatment of OMWW and conversion of the waste water to biogas, NH4Cl, (NH4)2HPO4 and KCl should be added to the reactors in addition to the
Acknowledgements
This study was funded by ATD Environmental Technologies Ltd. and the State Planning Organization of the Republic of Turkey.
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