Decolourization and removal of phenolic compounds from olive mill wastewater by electrocoagulation

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Abstract

The effective performance of electrocoagulation (EC) technique in the treatment of olive mill wastewater (OMW) has been investigated using sacrificial aluminium electrodes. The optimum working pH was found to be in the range 4–6, allowing OMW to be treated directly without pH adjustment. In addition, it is found that an increase in the current enhanced the speed of the treatment significantly. However, simultaneous increase of electrode and energy consumption was observed. The optimum current density allowing the quickest treatment with a low cost was found to be 75 mA cm−2. Therefore, a current density of 75 mA cm−2 was selected as an optimum that allows fast and low cost treatment.

Application of electrocoagulation procedure permitted high removal efficiencies of pollutants with both fresh and stored olive mill wastewater. The process produces a removal capacity of 76% of COD, 91% of polyphenols and 95% of dark colour, just after 25 min. The electrode consumption was found to be 2.11 kg m−3 of treated OMW. The results show that electrocoagulation could be considered as an effective alternative solution for the treatment of OMW or may be combined with a classical biological process to achieve a high quality effluent water.

Introduction

Olive mill wastewater (OMW) is a dark red to black effluent produced during olive oil extraction. It is estimated that around 30 million cubic meter of OMW are generated annually in the Mediterranean area [1], [2], during the seasonal extraction of olive oil. This huge amount of wastewater raised concerns about their possible negative effects on the environment. In fact, OMW is characterised by high concentrations of several organic compounds including sugars, tannins, pectins, lipids and phenolic substances [3], [4] which are responsible of their high COD and BOD. Their concentrations were generally in the range 80–200 g l−1 for COD [5], [6] and 12–63 g l−1 [7] for BOD. Specifically, it has been reported that polyphenolic components of OMW are responsible for its phytotoxicity [8], [9] and antibacterial activity [2], [10], [11]. Therefore, the practice of spreading OMW directly on agricultural soils must be accomplished with great vigilance, since it may result in more damage than beneficial effects on soil fertility. Moreover, OMW is characterised by its dark colour ascribed to polymerisation of tannins and low molecular weight phenolic compounds [12], [13]. Therefore, OMW must be treated to remove the phenolic fraction, before being discharged in receiving water bodies or used for irrigation purposes.

The simplest solution applied towards this direction consisted in constructing artificial big ponds into which OMW is stored, awaiting for its natural evaporation. However, this method, besides being very slow, causes subsequent unpleasant environmental pollution linked to generation of bad odours due to anaerobic activity [12]. Many studies exist on the biological treatment of OMW under aerobic [14], [15] or anaerobic conditions [3], [13]. However, high organic load specially phenolic compounds were found to inhibit the efficiency of anaerobic digestion and make OMW recalcitrant to biological degradation [3], [16]. Several other methods were used for OMW treatment, such as that proposed by Benitez et al. [17], [18] who studied the chemical treatment of wastewaters from olive industry by means of the ozone or Fenton’s reagent in the presence and absence of UV radiation. They showed that the combined action of UV radiation with the chemical reagent enhanced the removal efficiency to reach 76% of organic matter destruction, measured as COD. Other oxidising agents such as monosulphuric acid [19] and MnO2 [20] were also successfully used for polyphenols removal from OMW. Another method was recently applied to the treatment of OMW and consists of the application of an integrated centrifugation–ultrafiltration system [21] allowing an efficient reduction of pollution and a selective separation of some useful products.

In a recent study, Galiatsatou et al. [22] investigated the treatment of OMW using activated carbon produced from olive pulp. Treatments of OMW by activated clay [23] or lime [24] have also been reported to be effective for the removal of phenolic compounds.

On the other hand, purification of industrial and urban wastewaters by electrocoagulation (EC) is currently experiencing a renaissance. In this method, the coagulant is generated in situ by electrolytic oxidation of an appropriate anode material that leads, at appropriate pH, to insoluble metal hydroxide able to remove pollutants by surface complexation or electrostatic attraction. This technique characterised by its simple equipment, easy operation and decreased amount of sludge [25], was shown to be an effective and reliable technology that provides an environmentally compatible method for reducing a large variety of pollutants [25], [26], [27], [28].

In this study, electrocoagulation using aluminium electrodes was conducted to investigate the treatment of olive mill wastewater. The removal efficiency of the treatment was determined by monitoring the decrease of total phenol, COD and dark colour intensity.

Section snippets

Reagents and analytical procedures

Fresh olive mill wastewater was collected from a local olive extraction plant which uses a classical process. OMW was collected in a closed plastic container, diluted with tap water (1:1) and stored at 0 °C. Tap water was used for samples dilution because it is used in the extraction process. OMW from the same sample was also stored for 3 months at room temperature (>25 °C) in an aerated vessel in order to investigate the ageing effect. The main characteristics of fresh and stored olive mill

Results and discussion

Electrocoagulation is based on the in situ formation of the coagulant as the sacrificial anode corrodes due to an applied current, while the simultaneous evolution of hydrogen at the cathode allows for pollutant removal by flotation. This technique combines three main interdependent processes operating synergitically to remove pollutants: electrochemistry, coagulation and hydrodynamics. An examination of the chemical reactions occurring in the electrocoagulation process shows that the main

Conclusion

In this work it was shown that electrocoagulation treatment achieves a fast and effective reduction of pollutants (∼76% of COD, ∼91% of polyphenol and ∼95% of dark colour) present in fresh and stored olive mill wastewater. The optimum value of current density, allowing fast removal (25 min) of pollutants with low electrode consumption, was found to be 75 mA cm−2. In addition, it was demonstrated that the naturally occurring pH of OMW is appropriate to achieve an effective treatment. Moreover, the

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