Grit Removal | Activated Sludge Process | Wastewater Treatment Plant

GRIT REMOVAL

Grit removal aims to remove heavy inorganics (as well as some organic components) from the waste stream so that they can't harm downstream facilities, such as wearing out raw wastewater pumps, overloading and breaking primary sludge flights (especially in plants treating combined wastewater and storm water), wearing out sludge pumps, and changing sludge characteristics. Grit is a serious issue at wastewater treatment plants, and grit removal methods are frequently ineffective.

Grit is made up of heavy items with a particular gravity that prevents them from being moved by the water flow when it is decreased to less than 2 feet per second. This contains small amounts of sand and gravel that haven't settled out in the collection system, as well as coffee grinds, egg shells, and other coarse, heavy things that settle quickly and easily when the stream flow is lowered quickly.

Grit chambers are intended for a peak hourly flow rate of around 1 minute and a detention time of roughly 1 minute at that rate. The tanks are typically shallow square units with forced aeration to keep organics suspended while heavier inorganics settle out. This aeration also agitates the material, allowing the particles to grind against each other, further cleansing the settleable elements and maintaining as much of the organic percentage in the waste stream as feasible. This reduces the putrescence of the materials removed and makes them easier to handle. The grit is usually dumped on the ground.

A basin with a flow rate of less than 2 ft./sec; a comparable basin that is aerated to provide a swirling effect that can help separate the grit from treated organic material that may be connected to it; or vortex-style degritters can all be used to remove grit. In all circumstances, the goal is to slow down the flow rate enough for the heavier particles to settle out while the lighter, organic fractions continue to flow through the treatment plant.