A holistic approach to the recovery of valuable substances from the treatment sludge formed from chemical precipitation of fruit processing industry wastewater

In this study, recovery of phenolic substances with Soxhlet extraction, (SE) ultrasound-assisted extraction (UAS), and supercritical CO₂ (SC-CO₂) extraction methods from chemical sludge obtained with chemical precipitation (FeCl₃/PACS, Ca(OH)₂/PACS, perlite/PACS, FeCl₃/cationic polyelectrolyte) of lemon processing wastewater was investigated. The effect of used coagulants/flocculants and pH on COD and total phenolic substance content (TPC) removal was researched. Recovered phenolic substance profiles were also determined with HPLC-DAD. Additionally, response surface methodology was used to determine optimum treatment conditions. ANOVA analysis showed that pH is a more important variable than coagulant/flocculant doses for all chemical precipitation experimental sets. The highest removal efficiencies for COD and TPC was obtained in FeCl₃/PACS (COD: 72.0 %, TPC: 93.7 %). Optimum dose values were determined as pH: 4, FeCl₃: 3000 mg/L, PACS: 400 mg/L for FeCl₃/PACS, pH: 6.5, Ca(OH)₂: 1500 mg/L, PACS: 300 mg/L for Ca(OH)₂/PACS, pH: 5.5, PACS: 7000 mg/L, perlite: 50 g/L for perlite/PACS, pH: 4.5, FeCl₃: 500 mg/L, polyelectrolyte: 4 mg/L for FeCl₃/polyelectrolyte. TPC removal efficiencies were determined as 55 %, 35 %, 57 % and 58 % in these conditions, respectively. Maximum TPC in extracts was determined as 39.03 mg GAE/g extract, 8.81 mg GAE/g extract, and 4.34 mg GAE/g extract for SE, UAS, and SC-CO₂, respectively. TPC recovery efficiencies (R_TPC) for all chemical sludge were SE > UAS > SC-CO₂. Additionally, the TPC profile has shown a difference depending on the extraction method. According to the results of this study, it was concluded that the coagulation-flocculation process may be a suitable alternative for fruit juice processing industry wastewater in terms of both reducing environmental pollution and recovering polyphenolics from formed sludge. Consequently, this study presented a different perspective on the recovery from wastes with valuable substance recovery from chemical sludge.