Abstract
Electrochemical oxidation (EO) and EO related processes, either alone or in combination with pre-ozonation, were investigated as a polishing step for slaughterhouse wastewater treatment. The wastewater had previously been subjected to grit removal, degreasing, biological treatment and settling, but failed to comply with European emission limits for treated urban wastewaters in regards to organic compounds, suspended solids and colour. Besides EO alone, the following processes were applied: EO with hydrogen peroxide (EO/H₂O₂), EO with ultraviolet C light (EO/UVC) and EO with ultraviolet C light and hydrogen peroxide (EO/UVC/H₂O₂). Without pre-ozonation, electrochemical processes could be arranged in the following order according to their ability to mineralisation and colour removal: EO ˂ EO/H₂O₂ ˂ EO/UVC ˂ EO/UVC/H₂O₂. To reach a colour of 25 mg Pt-Co/L, it took more than 480 min for EO, ῀400 min for EO/H₂O₂, ῀260 min for EO/UVC and ῀120 min for EO/UVC/H₂O₂. At this treatment time, chemical oxygen demand and suspended solids were below the European emission limit values. The pre-ozonation step improved organics removal by EO and all related processes by converting the original organic compounds into easily oxidisable compounds. Beyond that, ozonation itself led to suspended solids and colour abatement to values in agreement with the legislated/permissible discharge limits.