Limits of UV disinfection: UV/electrolysis hybrid technology as a promising alternative for direct reuse of biologically treated wastewater

Although UV irradiation represents an energy efficient disinfection method, bacterial regrowth in UV irradiated secondary effluents is a serious problem for their direct reuse (e.g., for domestic or irrigation purposes). The decrease of the lethal UV fluence caused by coverings (scaling and biofouling) on quartz sleeves of UV lamps and/or fluctuating specific water parameters (suspended solids, transmittance or turbidity) results in a reversible inactivation of faecal coliforms (Escherichia coli). The reactivation of E. coli is increased with rising light intensity (≥470 Lux) and rising temperature (≥20 °C). The supplementation of UV disinfection with an electrolysis compartment ensures a reliable, sustained bacterial reduction and prevents reactivation of E. coli in UV irradiated (H = 138–140 J m⁻²) wastewater at a concentration of total oxidants of 0.5 mg L⁻¹. The electric charge input of 0.012 Ah L⁻¹ was lowest on MOX (mixed oxide) electrodes compared to particle BDD (boron-doped diamond) and thin film BDD electrodes. The formation of organic by-products (adsorbable organically bound halogens, trihalomethanes) ranged from marginal to moderate. In contrast to BDD electrodes, no chlorite, chlorate and perchlorate were observed on MOX electrodes. The energy consumption of the UV/electrolysis hybrid reactor was 0.17 kWh m⁻³.