Disruptive Effects of Cutting Oil Waste on Activated Sludge Performance and Dewaterability: Insights from Batch and Continuous Reactor Studies

One of the major environmental challenges today is the contamination of wastewater with oily substances, particularly those originating from metalworking industries. Cutting oil emulsions, widely employed in machining operations, are characterized by low biodegradability and pose significant toxicity risks to aquatic ecosystems. A substantial portion of these oils tends to accumulate within the sludge matrix during biological treatment, potentially impacting sludge handling processes such as dewatering. This study investigates the effect of cutting oil waste emulsions (COWEs) from the metallurgical industry on the centrifugal dewatering performance of activated sludge. Batch experiments were conducted to assess the influence of oil concentration, contact time, and oil type on sludge dewaterability. Three oil types were considered: raw cutting oil emulsions at 4% concentration (COWE1), used emulsions after machining operations (COWE2), and emulsions stored postuse (COWE3). COWE1 exhibited a high organic load (CODt=29.246 g L−1), which decreased by 27% and 41% in COWE2 and COWE3, respectively. Respirometric tests revealed a decline in biodegradability across the oil types, as indicated by maximum exogenous respiration rates of 24.6, 6.906, and 3.222 mg L−1 h−1 for COWE1, COWE2, and COWE3, respectively. The presence of cutting oils was found to impair the dewaterability of activated sludge, with COWE3 causing the most significant reduction in dehydration efficiency, as reflected by a minimum solid cake content of 21.44%. These findings highlight the detrimental impact of cutting oil contamination on sludge management and underscore the need for pretreatment or mitigation strategies to enhance sludge dewatering in industrial wastewater treatment systems.