The treatment of phenolic contaminants from shale gas drilling wastewater: a comparison with UV-Fenton and modified UV-Fenton processes at neutral pH

In a shale gas field, wastewater from drilling processes contains many recalcitrant and toxic pollutants. These pollutants are difficult to dispose in a classical wastewater treatment system. In this study, a comparative experiment was conducted to check the treatment efficiency of a UV-Fenton process at pH = 3 and a modified UV-Fenton process at neutral pH with real drilling wastewater, which was collected by a coagulative effluent in a sewage treatment plant in a shale gas field, Chongqing. The consumption of Fenton reagents and the rate of removal of COD and BOD5 during three processes were evaluated in order to obtain an optimal process. In addition, phenolic contaminants in the effluent were measured by GC-MS and 7 of them were singled out to serve as an evaluation standard for effective treatment, which has not been reported in previous experiments. The results showed that three UV-Fenton processes had the ability to treat drilling wastewater and a modified UV-Fenton process with carboxymethyl-beta-cyclodextrin (CMCD) had positive effects on the treatment of phenolic contaminants at neutral pH with minimal reagent consumption (Fe2+ = 14 mg L-1 and H2O2 = 768 mg L-1). It was demonstrated that modified UV-Fenton with CMCD could extend the pH range in the treatment process and avoid negative effects in UV-Fenton process at acidic pH.