Decolourization and removal of some organic compounds from olive mill wastewater by advanced oxidation processes and lime treatment

Background Olive mill wastewater (OMW) generated by the olive oil extracting industry is a major pollutant, because of its high organic load and phytotoxic and antibacterial phenolic compounds which resist biological degradation. Mediterranean countries are mostly affected by this serious environmental problem since they are responsible for 95% of the worldwide oliveoil production. There are many methods used for OMW treatment, such as adsorption, electro coagulation, electro-oxidation, biological degradation, advanced oxidation processes (AOPs), chemical coagulation, flocculation, filtration, lagoons of evaporation and burning systems, etc. Currently, there is no such economical and easy solution. The aim of this study was to evaluate the feasibility of decolourization and removal of phenol, lignin, TOC and TIC in OMW by UV/H₂O₂ (AOPs). The operating parameters, such as hydrogen peroxide dosage, times, pH, effect of UV and natural sunlight were determined to find the suitable operating conditions for the best removal. Moreover, there is no study reported in the literature related to the use of UV/H₂O₂ and lime together in OMW treatment. Methods OMW was obtained from an olive-oil producing plant (Muğla area of Turkey) which uses a modern production process. No chemical additives are used during olive oil production. This study was realised by using two different UV sources, while taking the time and energy consumption into consideration. These two sources were mercury lamps and natural sunlight. Before starting AOPs experiments, one litre of OMW was treated by adding lime until a pH of 7.00. Then, 100 ml was taken from each sample, and 1 to 10 ml of a 30% H₂O₂ (Riedel-deHaen) solution was added. These solutions in closed vessels were laid in the natural sunlight for a week and their compositions and colour changes were analysed daily by UV-Vis spectrophotometer. At the end of the one-week period, they were treated with lime. In this study, the effect of changes in the initial pH, times and H₂O₂ concentrations on removal was investigated. At the end of all experiments, changes in colour, phenol, lignin, TOC and TIC concentrations were analysed according to standard methods. Results and Discussion In the samples exposed to natural sunlight and having an H₂O₂/OMW ratio of 3 ml/100 ml, a significant colour removal was achieved approximately 90% of the time at the end of 7 days. When the same samples were treated with lime (pH: up to 7), 99% efficiency was achieved. When phenol and lignin removals were examined in the same concentration, phenol and lignin removal were found 99.5%, 35%, respectively. However, for maximum lignin removal, more use of H₂O₂ (10 ml H₂O₂/100 ml OMW) was found to be necessary. Under these conditions, it was found that lignin can be removed by 70%, but to 90% with lime, at the end of a seven-day period. Rate constants obtained in the experiments performed with direct UV were found to be much higher than those of the samples exposed to natural sunlight (k^a _lignin = 0.3883 ≫ k^b _lignin = 0.0078; k^a _phenol = 0.5187 ≫ k^b _phenol = 0.0146). Moreover, it should be remembered in this process that energy consumption may induce extra financial burden for organisations. Conclusions It was found, in general, that colour, lignin, total organic carbon and phenol were removed more efficiently from OMW by using H₂O₂ UV and lime OMW. Moreover, in the study, lime was found to contribute, both initially and after radical reactions, to the efficiency to a great extent. Recommendations and Perspectives Another result obtained from the study is that pre-purification carried out with hydrogen peroxide and lime may constitute an important step for further purification processes such as adsorption, membrane processes, etc.