AERATED GRIT REMOVAL DESIGN

 

 Normal operation consists of maintaining hydraulic capacity, design and peak flows, through each chamber and the desired mixing energy to keep the organics in suspension and allow the heavier grit particles to settle out. Grit can be removed at regular intervals (one or two times per hour), from the grit hopper depending on the equipment and also the grit loading. The grit removed from the basin should be on an intermittent basis.

12” grit classifier with hydrocyclone

 

 As with any process basin, it is best to maintain a state of equilibrium in the grit basin. As grit enters the basin and settles out in the hopper bottom it should be periodically removed because this combination of materials can potentially solidify if left for extended periods in the hopper bottom, plugging pipes and possibly ruining equipment. It is best to remove the settled grit several times per hour in small increments rather than remove a days worth all at once. This can be achieved using two timers, one determines the number of pumping cycles per hour and a second timer sets the duration for each pumping cycle. The timer either energizes a pump motor or solenoid valve or opens an electric operated plug valve depending on whether the system is a pumped or gravity design. If the system is not automatic, the operator has to turn on the pump and classifier at regular intervals to eliminate potential clogging problems.

 When abnormal operating conditions exist due to high flows or heavy solids loadings, adjust air flow to the tank to maintain the desired grit removal at the operating conditions experienced. If the solids are not removed, they may carry over to the primary clarifiers or aeration basins. It is best to maintain the air mixing intensity level on the low side. Too high a velocity and the mixing energy and resulting roll pattern will keep some of the grit particles in suspension and they will carry over the weir to downstream basins. A byproduct of an aerated grit removal system is the stripping of hydrogen sulfide gases from the influent wastewater. This isolates the odor caused by these gases to the grit chamber area. Some customers consider covering the grit chamber to capture and eliminate these odors.

 The grit slurry is introduced into the classifier settling pool where the grit particles settle out and are removed with a conveyor screw up to the discharge chute and into the Owners receptacle. The excess liquid flows over a weir at the lower end of the classifier and is returned to the grit basin or into the plant drain system and carried back to the lift station.

 Shutdown procedures consist of diverting wastewater flows around the aerated grit chamber, stopping the air supply and draining the basin. A bypass channel is typically provided to allow bypassing the grit basin. Draining the basin is not necessary during temporary shutdown, such as taking a unit out of service during low flows. It is best to maintain airflow to the grit basin at all times.

 If the grit slurry pumps are located at the bottom of the structure, with a lot of head over the pumps, the piping should be arranged to keep a siphon from forming and draining thru the piping even when the pumps are not running.