The term “Sedimentation”, or “precipitation by gravity”, denotes the separation of suspended particles having a specific weight higher than water.
Sedimentation is used to remove sand, suspended solids in the primary sedimentation tanks, the flakes of biological sludge in sedimentation tanks, located downstream of the activated sludge unit, and flakes of chemical products in the processes of coagulation. It is also adopted to increase the solids concentration in the unit of sludge thickening.
In this case, we want to deepen the sedimentation of biological sludge, which has per main objective the production of a clarified effluent.
The clarifier is a tank where the stillness conditions are created, in order to allow the settlement of sludge flakes. Here, it is installed a bridge equipped with bottom scrapers, which, thanks to its slow movement, forwards the settled sludge to a central sump. The most part of this sludge is re-circulated in the head to the biological basin, while the excess sludge is sent to following treatments of thickening and dehydration. Instead, in the upper part of the tank remains the decanted water that comes out of the clarifier by overflow.
Where the available space does not permit the installation of a circular clarifier, a lamella clarifier can be used, which bases its efficiency of sedimentation on the clarification area instead of the retention time. The system consists of a series of baffles or tubular elements that greatly increase the total surface and that reduce the speed of the water to be treated. These elements have an appropriate angle of inclination to ensure the maximum removal efficiency and simultaneously prevent sedimentation of sludge inside the elements themselves.
The water to be treated enters in the lower part of the basin below the lamellar pack, while the clarified water is collected at the top and comes out by overflow along the parallel channels, that divide the basin surface and ensure a very low final outlet speed.
Also in this case, a scraper bridge pushes the settled sludge on the bottom sump, in order to be re-circulated or treated in the next stages. With lamellar clarifier, it is necessary to pay particular attention to the level of the sludge under the packs, in order to avoid that it obstructs the lamellar channels or, even, reaches the area above the packs themselves.
The needed space for a lamellar clarifier, compared to the one of a conventional clarifier of equal flow, is about 7 - 8 times smaller. If you also consider that (in the case of circular clarifier) the area close to the circumference is not suitable for other purposes, the saved space can reach 9 - 10 times.
This technology is an integral part of the treatment process of a Biological plant.