Pilot-scale evaluation of sequential ozone dosing in UV/O3 treatment of distillery effluent: Process performance and molecular transformation.

Although biological treatment substantially reduces the organic load of distillery effluent, the remaining dissolved organic matter is still sufficiently refractory to hinder ultra-low polishing treatment and may sustain residual biological effects. In this pilot-scale study, a sequential ozone dosing strategy was evaluated in a UV/O3 system to achieve ultra-low discharge while elucidating DOM and halogenated organic transformation. Through systematic optimization, a strategy involving sequential ozone dosing with intensified dosage in later stages and targeted placement of UV irradiation achieved consistent compliance with the stringent COD target. Molecular-level analysis revealed that while the treatment was dominated by oxygenation and fragmentation of aromatic and unsaturated compounds, concurrent polymerization reactions led to an increase in the weighted average molecular weight. Moreover, halogenated organic compounds (OXCs, X = Cl or Br) were more recalcitrant than their non-halogenated counterparts, with brominated species (OBrCs) exhibiting greater persistence than chlorinated ones (OClCs). This persistence may partly explain the limited reduction in acute toxicity despite excellent COD removal, indicating that compliance with COD standards did not necessarily translate into a proportional reduction in biological effects. An energy consumption of 6.00 kWh/m3 was required, underscoring a performance-energy trade-off. This study provides a comprehensive technical and mechanistic framework for segmented UV/O3 in distillery wastewater treatment, highlighting its promise and trade-offs on the path to ultra-low COD effluent.