Improvement of biological process using biocoagulation–flocculation pretreatment aid in olive mill wastewater detoxification

Abstract

Olive mill wastewaters (OMWW) are a significant source of potential or existing environmental pollution in the Mediterranean countries. Treatment and reuse of OMWW present significant challenges, both due to the nature of olive oil production (seasonal and concentration in a small scale) and due to the characteristics of the wastewater (high chemical oxygen demand (COD) and high phenolic content). In the present work, experiments of different pretreatment methods were undertaken and compared in order to select the most adequate process to be applied before biological treatment of OMWW. In fact, three different pretreatments were tested and compared: two different coagulants (alginate and Kim2120) and electrocoagulation (EC) process. At the second stage, biological treatment (using white-rot fungus) was investigated. Tests were conducted with raw OMWW and with OMWW diluted in 50% tap water and adjusted to a neutral pH with lime. The raw effluent presents high COD (178 g O2 l−1), about 6.4g l−1 of total suspended solids (TSS), a high conductivity (29 mS cm−1), acidic pH 4.8, and turbidity of 90 NTU. Results show that after 90 min of EC process, using 15 V/m2 voltage, a maximum of 49% of organic matter removals was achieved. The optimum removals of COD and turbidity obtained were 39 and 75%, respectively when 20 mg l−1 dose of alginate is used, especially at pH 7. Coagulation using Kim2120 at 10 mg l−1 applied on diluted OMWW showed reduction reaching 52.3% of COD and 85% of turbidity. The maximum removal of total polyphenols, tyrosol and hydroxytyrosol obtained were 60.1, 23.1, and 59.1% with lime/alginate and 64.2, 67.8, and 76.3% with lime/kim2120, respectively. Only 57% of polyphenols was removed by EC. The pretreated effluents by different physicochemical processes tested showed that the best yields were obtained with lime/kim2120. The pretreated olive mill wastewater was then treated aerobically with white-rot fungus. The results showed that the white-rot fungus was capable of reducing 68% and 75% of COD, respectively for the diluted OMWW and pretreated by lime or lime/Kim2120, after only 5 days of growth. The best biological treatment was obtained with neutral pH.