Conventional Gravity Filters provide 3 stage water treatment consisting of flocculation, clarification and filtration. Conventional filters are recommended for raw waters with turbidity greater than 5 NTU and/or colour greater than 20 TCU.
First, chemicals are rapidly mixed into the raw water and then gently agitated in a detention tank called the flocculation chamber. The main group of chemicals added are referred to as coagulants. Coagulants when mixed with the raw water form precipitates that trap the suspended solids (dirt, bacteria, viruses and other microorganisms).The detention time in the flocculator allows the precipitates to form and remove the contaminates. The colder the raw water the longer the detention time must be due to slower chemical reaction rates. Agitation of the water is required to ensure collisions between the forming precipitates and the remaining contaminant particles. In the gravity water treatment plant, the agitation is provided by mechanical means usually consisting of a gear motor and vertical propeller blade mixer. Variable speed control is often provided to allow mixing velocity adjustment to provide maximum mixing without damaging the already formed particulate matter (floc).
After flocculation, the water flows into the clarifier. The clarifier is a large chamber where the flow velocity of water is minimized. The low velocities allow the particulate matter (floc reminiscent of snowflakes) to settle out of suspension by gravity. This process is often aided by the addition of tube or plate settlers. Settlers are simple angled plates or tubes spaced closely together. This allows a settling particle to contact a surface very quickly. After a floc particle has contacted a surface, or other floc particles on that surface, it is unlikely that the particle will be resuspended. The angle of the plates or tubes (usually 55 60 degrees off horizontal) allows the collected floc particles (called sludge) to flow by gravity to the bottom of the clarifier. The sludge that is periodically removed from the bottom of the clarifier is called desludging. The particulate removal by the clarifier reduces the solids loading on the filter media thereby extending the filter run time between backwashes.