Integrated resource recovery from co-fermentation of food waste and sludge: Balancing denitrification potential and biogas recovery

Replacing fossil-based carbon sources with sustainable alternatives for denitrification supports nitrogen compliance, lowers indirect emissions, and mitigates cost increases in WWTPs. This study investigated the pilot-scale fermentation of food waste (FW) and primary sludge (PS) at different ratios to produce volatile fatty acids (VFA) and lactic acid, for use as carbon source in denitrification. Four substrate ratios (FW:PS) were tested (100:0, 50:50, 25:75, and 0:100) under mesophilic conditions (34 ± 0.4 °C) without pH control. Fermentates from the tests produced a wide range of sCOD (6-82 g COD/L) and VFA concentrations (3-56 g VFA_COD/L). No statistically significant differences in VFA yields between reactors were detected, whereas a significant difference in COD yields were observed. Higher FW proportion increased VFA concentrations and shifted the VFA composition towards lactic and propionic acid. Despite these differences, denitrification rates remained consistent (7-9 mg NO₃^--N/g VSS∙h), suggesting the microbial community's functional flexibility in utilising varying VFA profiles. Methane potential analysis of the solid residues after fermentation and VFA removal indicated that FW-fermentate maintained biogas yields, whilst PS-fermentate showed a 50 % reduction compared to untreated substrates. Estimated carbon source volume demands in large-scale applications varied significantly due to differences in sCOD and nutrient concentrations. Co-fermentation of PS with 25 % and 50 % FW reduced volume requirements by a factor of 8 and 12, respectively, compared to fermenting PS alone. These findings demonstrate the feasibility of fermenting FW and PS to balance denitrification efficiency, biogas recovery, and resource utilisation at WWTPs.