Optimization of Ozonation Process to Remove Psychoactive Drugs from Two Municipal Wastewater Treatment Plants

The increasing presence of pharmaceutical products in municipality wastewaters has raised serious environmental concerns. Ozonation is one of the advanced oxidation processes that can degrade these substances, instead of simply transferring them from aqueous to another phase. Treatability studies applying ozonation to degrade benzodiazepine drugs are scarce. The aim of this investigation was to apply design of experiments (DoE) to optimize ozonation processes for removal of bromazepam, clonazepam, and diazepam from two effluents of municipal wastewater treatment plants (WWTP1 and WWTP2) both located in Rio de Janeiro City, Brazil. During a preliminary study using ultrapure water, OHº radicals were more efficient (removal efficiency greater than 99% (< LOQ) at pH = 10, 15 min) than O3 in degrading all psychoactive drugs (removal efficiency from 46 to 85% at pH = 4, 15 min). The optimum values for the tested variables were pH = 8.03; O3 specific dose = 4.8 mgO3.mgDOC−1; and O3 mass flow rate = 22.07 mgO3.min−1. The WWTP1 and WWTP2 effluents were treated under optimized conditions by ozonation in two effluent volumes (5L and 8L). When 8 L effluent was treated, the degradation of all benzodiazepine drugs was improved, due to the higher water column used, which favors the mass transfer of O3 from the gas phase to the liquid phase, promoting greater contact between oxidant and target compounds. Using a specific dose of 1.8 mgO3.mgDOC−1, more than 93% of all anxiolytic drugs were removed from 8 L of WWTP2 effluent, whereas for the WWTP1, more than 90% were removed using 2.7 times higher specific dose (4.8 mgO3.mgDOC−1).