As the water slowly flows through the tanks, oil and grease float to the surface and are scraped off. The fine solids, called sludge, settle to the bottom, and great mechanical blades scrape the sediment to where it is pumped away for further treatment.
पण जैव पदार्थ अर्थात गाळ तळाशी जमतो आणि यंत्रसंचलित पट्ट्यांनी तो गोळा करून पुढे अधिक प्रक्रिया करण्यासाठी पाठवला जातो.