Detritors are a set of equipment that collectively remove fine grits from the influent stream. The equipment are typically part of the Pre-Treatment Unit (PTU), an integral part of most Sewage Treatment Plants (STP), irrespective of the secondary treatment process. 

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The detritor is placed after a fine screen. In this stage, the heavy suspended solids such as silt, gravel, and other such items are forced to settle in the basin by dropping the velocity below the critical flow velocity. While the grits settle to the floor of the basin, the water runs off into the collection launder over a control weir. The flow into this settling basin is distributed evenly over the entire breadth with the help of deflection baffles so as to allow for even distribution of silt. 

 

The settling basin has a scraping mechanism mounted with collection bins at the periphery of each arm. This raking mechanism is guided powered by a central drive head which essentially contains an overload protection device. The settled grits are slowly pushed outward and into a trap which directly leads into the classifier chamber. 

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The classifier chamber is provided with a screw type classifier, that carries out the collected grits out of the water and into the discharge bin while leaving behind all the water and other organic materials that require further treatment. The Classifier chamber also contains a device called an organic return pump.

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The Organic Return Pump is a type of axial turbine which pushes water from the water from the classifier chamber into the grit settling basin through a pipe thus emptying the water from classifier chamber.

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