Effectiveness of H₂O₂/UV and O₃/H₂O₂/UV for metaldehyde removal in drinking water and full-scale costing analysis

Metaldehyde is a widely used pesticide in the UK and has been reported to produce drinking water quality failures across the national water utilities. Advanced Oxidation Processes (AOPs) are effective in the removal of recalcitrant organic constituents but pose (i) high energy and chemical costs and (ii) by- products formation upon disinfection stage. In this context, UV/H₂O₂ and O₃/H₂O₂

• H₂O₂/UV were tested at pilot-scale concurrently at Draycote water treatment work (WTW) (UK) for metaldehyde degradation and disinfection by products formation potential (DBPFP) at different energy demands (Eᴅ), H₂O₂ and O₃ doses; and pesticide concentrations. Whole-life costing analysis (WLCA) was undertaken to assess which AOP was most economic for metaldehyde removal at a Draycote sized WTW. Sensitivity analysis considered potential costs of each AOP. Metaldehyde was removed up to 98% (UV/H2O2) and 97% (O₃/H₂O₂ + H₂O₂/UV) successfully fulfilling European Directive 98/83/EC below both 1.0 kWh m⁻³ and 2.14 µg L⁻¹ pesticide inlet water level. Net DBPFP contribution ranged -283 µg L⁻¹ to +255 µg L⁻¹ for trihalomethanes formation potential (THMFP), which are UK regulated. DBPFP was related to organic background characteristics rather than the operating conditions tested in both AOPs. Cost estimating the AOPs for a 0.72-log metaldehyde removal enabled the total expenditures to be determined indicating that energy consumption accounted for 50% (UV/H₂O₂), 49 % (O₃/H₂O₂+ H₂O₂/UV) and 31% (O₃/H₂O₂) of their operating costs. Similar £14.7 million capital costs were appraised for O₃/H₂O₂ and UV/H₂O₂. Overall, O₃/H₂O₂ sub process outperformed the rest of AOPs when considering both metaldehyde degradation (≤96%) and net DBPFP contribution (≤51 µg L⁻¹ THMFP) at the most economically operating costs for 5-month yearly treatment (£0.07 m⁻³). Sensitivity analysis showed the O₃/H₂O₂ more efficient process requiring less inputs than UV/H₂O₂ for the same metaldehyde removal. However, the O₃/H₂O₂ was cost effective at an ideal stoichiometric mass ratio of 0.7 H₂O₂:O₃. Therefore, this would require validation at full-scale. The AOPs were robust for metaldehyde degradation with reasonable costs.