The Primary Settling Tank are designed to reduce the velocity of the wastewater flow for organic solids (called raw sludge) to settle. Scrapers present in the tank move continuously along the floor of the tank to deposit the raw sludge in hoppers for removal. The scum which floats to the surface is directed by water jets to the sludge sump. The raw sludge is then pumped to the sludge digester for biological treatment. Approximately 60% of suspended solids and 35% of BOD removal efficiency can be achieved at this stage. From the primary settling tank, the liquid goes through an aeration and or bio filtration process before entering the secondary tank.either bio filtration or an activated sludge process.
In the sedimentation tank the solids settle out to be removed by mechanically driven scrapers for further processing. In the case of circular tanks the scraper blades scrape the settled sludge to a central sludge removal hopper and in rectangular tanks to a hopper at one end of the tank. Scraper mechanisms for primary tanks will also be fitted with surface scum removal equipment to remove floating matter, scum and grease etc. Approximately 65% of organic solids and 35% of the BOD in the sewage is removed in the primary process.
From the primary settling tank, the liquid goes through an aeration and or bio filtration process before entering the secondary tank.