Upgrading cola wastewater-derived organics as an effective carbon source in biological denitrification

Cola wastewater, generated during the production of sugar-containing beverages, is a low-toxicity effluent rich in organic matter and may serve as a promising external carbon source for biological denitrification. However, its phosphate content often exceeds the relevant quality limit, and its low carbon bioavailability limits its suitability as an effective carbon source for denitrification. To address these limitations, an integrated pretreatment strategy combining selective phosphate removal using a polymeric ion-exchange resin with UV/H₂O₂ was developed to improve substrate quality. The process reduced the phosphate concentration from 132.0 to 16.1 mg/L and increased the BOD₅/COD from 0.22 to 0.56, indicating enhanced carbon bioavailability. In denitrification tests at carbon-to-nitrogen ratios ≥ 6, the pretreated wastewater achieved nitrate removal efficiencies of 87.5%-91.5%, comparable to those achieved with glucose (88.8%-90.9%) and considerably higher than those achieved with untreated wastewater (67.8%-78.2%). Effluent chemical oxygen demand was lower and more stable, indicating improved substrate utilisation and process stability. Mechanistically, pretreatment increased the availability of readily biodegradable small-molecule organic compounds, thereby enhancing microbial access to substrates and denitrification efficiency. Collectively, these findings demonstrate a feasible approach that integrates phosphate control with organic matter transformation to valorise cola wastewater-derived organic carbon as a sustainable carbon source for biological nitrogen removal.