Optimisation of biodegradation conditions for the treatment of olive mill wastewater
Introduction
The liquid effluent from olive presses, left after removal of the olive oil, causes a serious environmental hazard in olive producing countries, especially around the Mediterranean basin. It has been estimated that 30 × 106 m3 are produced per year (Merchichi and Sayadi, 2005). The problems mainly arise from the effluent’s high level of pollutants leading to a high chemical oxygen demand (>200 g/L) (Della Greca et al., 2004, Skerratt and Ammar, 1999). In particular, the pollutants include carbohydrates, proteins, and lipids but mono and polyaromatic compounds such as polyphenols are also present (Borja et al., 2005). The polyphenols, which are refractory to biodegradation, are the principal cause of the toxicity of the effluent (D’Annibale et al., 2006, Di Gioia et al., 2001, El Alami, 2000, El Hajjouji et al., 2007a, Merchichi and Sayadi, 2005). However, pollution form mill waste is closely related to the extraction system used. Effluent production is minimal in mills that extract the olive oil by pressing, compared to systems that use extraction by centrifugation: Raggi et al. (2000) reported 40% effluent for the presses and 95% effluent for centrifugation. Accordingly, Skerratt and Ammar (1999) reported that pressing 100 kg of olives in continuous extraction processes, such as those used in modern mills, produces 20 L of olive oil and 70–150 L of liquid effluent. The effluent from traditional presses is more concentrated, containing a larger proportion of organic matter than the effluent from modern mills. In Morocco the problem becomes particularly significant, olive mills being amongst the greatest sources of industrial pollution (El Hajjouji et al., 2007b, Zenjari et al., 1999, Zenjari et al., 2006). The extraction is generally by press in the traditional units while the modern plants are based on three-phase centrifugation (Ait Baddi et al., 2003a, Fakharedine et al., 2006, Hafidi et al., 2004). In Morocco, the effluent produced by the plants in one season of harvest is estimated at 250,000 m3 (IOM, 2003/2004). With a view to protecting surface and underground water, several types of physical, chemical and biological processes have been proposed to treat the effluent (Balis et al., 1996, Carmona et al., 2000, Fountoulakis et al., 2002, Mantzavinos and Kalogerakis, 2005). But, owing to the high cost of the treatments on an industrial scale, few have actually been applied in commercial olive mills. However, the richness of the effluent in terms of fertilizing elements and organic matter could favour its use as a soil amendment in particular in Morocco where the soils are poor in organic matter and very liable to erosion. However, the agronomic quality and hence the economic value of the final product are directly dependent on the degree of maturity reached during treatment.
To determine the parameters affecting the quality of the final product, with the intention to optimise the process during further work, the set of factors influencing the treatment of the effluent was studied. A full factorial matrix was established and processing the results with the Nemrodw software (New Efficient Methodology for Research using Optimal Design) enabled us to quantify the direct effects and interactions of the various factors on the responses chosen and to identify those that will require fine-tuning during the optimisation phase.
Section snippets
Origin and physical–chemical characteristics of the olive mill wastewater
The wastewater was taken from a modern three-phase centrifugation olive mill in Marrakech, Morocco. The wastewater samples were taken at different places in the storage lagoon and a homogeneous and representative sample was taken to the laboratory and stored at 4 °C for subsequent experimentation and physico-chemical analyses. The wastewater was homogenised before experiments.
The pH was measured at ambient temperature following the recommendations of Rodier (1971). The total organic carbon (TOC)
First series of experiments
The responses noted for the 16 experiments (drying time, pH, C/N, drop in polyphenols) are presented in Table 3. It can be seen that the time taken for the wastewater to dry is dependent on the treatment received. For the last eight treatments, drying only required seven days. This can be attributed to the high temperatures (40 °C) maintained during these experiments. For the others, the effluent took from 20 to 48 days to dry. In the same way, Ait Baddi (2005) attributed the maintenance of high
Conclusion
The treatment of olive mill wastewater by aerobic degradation using optimisation by experimental design, allowed us to determine the optimal conditions of treatment. Optimisation of pH (with lime or sodium hydroxide) and of the C/N ratio (urea or ammonium nitrate) improved microbial activity very significantly. This led to reductions in the levels of polyphenols of 51 and 76%. The lowering of the levels of polyphenols, which are the direct cause of the toxicity of the olive mill effluent, is
Acknowledgement
This work was financially supported in part by an Eiffel grant from the French government.
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