Minimizing solid wastes in an activated sludge system treating oil refinery wastewater

• Proposal of a novel sludge reduction technique.
• A new application for biosurfactants in wastewater treatment plants.
• 52% reduction in sludge disposal maintaining high COD removal.
• Area occupied by secondary clarifier can be reduced by 39–52%.

Biosurfactants are suitable for application in wastewater treatment systems due to their biodegradability, biocompatibility and low toxicity. In activated sludge systems, they reduce coalescence and disintegrate flakes, enabling more cells to have access to oxygen. At low concentrations, they may act as growth inhibitors. In this study, rhamnolipid was added to a bench scale sequential batch reactor operating in similar conditions as oil refinery wastewater treatment plants. Concentrations from 12 to 50 mg rhamnolipid/L were evaluated, the latter being the minimum concentration necessary to reduce sludge disposal. In this concentration, rhamnolipid reduces sludge disposal of up to 52%, maintaining COD removal of 81–97% and good sludge settling properties (SVI 120 mL/g) and could also reduce area occupied by secondary clarifier of 39–52%. However, biosurfactant application needs to be optimized, because its cost is even higher than the savings obtained with lower waste disposal.