Assessment of an integrated coagulation–DAF and biological treatment system for olive mill wastewater at bench and pilot scales

This study presents a pilot-scale investigation into the treatment of olive mill wastewater (OMW), a high-strength effluent characterized by high organic load and phenolic content. An integrated system combining physicochemical treatment, mainly coagulation/flocculation followed by Dissolved Air Flotation (DAF), and biological processes using activated sludge were evaluated. The coagulation/flocculation stage was optimized at the bench scale using Response Surface Methodology (RSM), focusing on key operational parameters: pH, lime dose, mixing rate, and mixing time. Optimal conditions, pH 10, lime dose of approximately 30,000 mg L⁻¹, mixing rate of 250 rpm, and mixing time of 5 minutes, yielded removal efficiencies of 21% for Chemical Oxygen Demand (COD), 90% for phenol, and 100% for turbidity. The biological treatment was further optimized by varying the volumetric percentage of return activated sludge (RAS) in the feed. A 50% RAS ratio resulted in 65% COD and 91% phenol removal over a 12-day retention time. A 20% RAS ratio also demonstrated high phenol removal efficiency, suggesting its suitability where shorter retention or lower sludge generation is preferred. Overall, the integrated coagulation–DAF and biological treatment pilot-scale system achieved combined removal efficiencies of 89.5% for COD and 87.7% for phenol. These results highlight the potential of the proposed setup as a sustainable first-stage treatment for recalcitrant industrial wastewater, enabling significant pollutant reduction prior to discharge or further polishing in municipal treatment systems.