Abstract
Water shortage is increasing worldwide and becoming a social and environmental problem. To address the increasing need of water human population is currently facing, the use of advanced oxidation processes (AOPs) after a biological treatment seem to be a right approach for the treatment of industrial wastewaters. AOPs such as ozone (O3), ultrasound (US), ultraviolet light (UV) or electrochemical oxidation produce in situ powerful oxidants (i.e. OH radicals) that can easily degrade recalcitrant organic matter and inactivate microorganisms. Among different industries, current industrialised livestock agriculture has one of the highest consumptions of water and produces up to ten times more polluted wastewaters in comparison to domestic sewage. Therefore, the present study looks into the use of single and combined AOPs for the treatment of real slaughterhouse wastewaters. O3 proved to be a powerful oxidant, reducing significantly chemical oxygen demand (COD) and biological oxygen demand (BOD), as well as lowering colour and total suspended solids (TSS) to minimum values. O3 also acted as a strong disinfectant, inactivating most of the microorganisms present in the wastewater. After an activated sludge process, a complete inactivation of total coliforms (TC) was obtained with 17 min of ozonation at an O3 inlet concentration of 71 mg O3/Lgas. For total viable counts (TVC), a drastic reduction was observed after 30 min of ozonation (5 log inactivation). The addition of US only increased ozonation performance slightly (44% COD and 74% BOD reduction), principally due to the high inlet O3 concentration supplied to the system. US alone showed a modest effect in organic matter removal, reducing 18% COD, 50% BOD and 25% TSS with 300 kHz at an applied power of 40 W. A minor removal was achieved for TC (< 1 log), while an insignificant reduction in TVC was measured. The application of UVC light (254 nm) and hydrogen peroxide (H2O2) individually did not reduce any dissolved organic carbon (DOC), while the combination of UVC and H2O2 led to a large synergy. The coupled system reached 26% DOC abatement with an optimised H2O2 concentration after 3 h of treatment, principally due to the attack of OH radicals formed through the cleavage of H2O2 molecules by UVC. DOC removal was further increased to 41% when O3 was used as a pre-treatment. This was attributed to the reduction in colour, turbidity and TSS by O3 pre-treatment, which lowered UV inner filtering effects and hence, increased the performance of the subsequent UVC/H2O2 process. The application of a pre-ozonation step also improved organics removal by converting original organic compounds into easily oxidisable compounds. O3, UVC and H2O2 were later combined in a single process, but no synergy was observed with a DOC reduction of 43% and the COD was reduced down to 61 mg O2/L. However, less O3 (mg O3/Lwastewater) was used with the combined O3/UVC/H2O2 system compared to that supplied during O3 pre-treatment (O3+UVC/H2O2), increasing O3 performance. Electrochemical oxidation alone reduced DOC values by 15%, while the use of an electrochemical cell along with UVC and H2O2 led to a DOC removal of ~81%. The significant production of OH radicals and electrochemically generated oxidants during the combined process would be responsible for such a high removal. A similar removal percentage was achieved when O3 was used as a pre-treatment prior to EO and UVC combined. Colour values below 25 mg Pt-Co/L were achieved, typical acceptable colour limit values for treated wastewaters leaving wastewater treatment plants. O3 pre-treatment also reduced the production of hazardous compounds such as ClO4- in subsequent treatments. The individual application of AOPs did not show high efficiency in organic matter removal, while the combination of different AOPs reached direct discharge limits set by the European Union and showed potential to be used for water treatment intended for agricultural irrigation. Particularly, the coupling of UVC and H2O2 showed a high synergy, with and without O3 pre-treatment, as well as the combination of EO, UVC and H2O2, where the highest DOC removal was observed.