Oxidation of organics in retentates from reverse osmosis wastewater reuse facilities

The use of membrane processes for wastewater treatment and reuse is rapidly expanding. Organic, inorganic, and biological constituents are effectively removed by reverse osmosis (RO) membrane processes, but concentrate in membrane retentates Disposal of membrane concentrates is a growing concern. Applying advanced oxidation processes (AOPs) to RO retentate is logical because extensive treatment and energy inputs were expended to concentrate the organics, and it is cheaper to treat smaller flowstreams. AOPs (e.g., UV irradiation in the presence of titanium dioxide; UV/TiO₂) can remove a high percentage of organic matter from RO retentates. The combination of AOPs and a simple biological system (e.g., sand filter) can remove higher levels of organic matter at lower UV dosages because AOPs produce biologically degradable material (e.g., organic acids) that have low hydroxyl radical rate constants, meaning that their oxidation, rather than that of the primary organic matter in the RO retentate, dictates the required UV energy inputs. At the highest applied UV dose (10 kWh m^-3), the dissolved organic carbon (DOC) in the RO retentate decreased from ∼40 to 8 mg L^-1, of which approximately 6 mg L^-1 were readily biologically degradable. Therefore, after combined UV treatment and biodegradation, the final DOC concentration was 2 mg L^-1, representing a 91% removal. These results suggest that UV/TiO₂ plus biodegradation of RO retentates is feasible and would significantly reduce the organic pollutant loading into the environment from wastewater reuse facilities.