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Grit Removal

Grit is a problem in all combined sewer systems.  A majority of the equipment maintenance and operational problems at the treatment plant can be associated with excess grit being transported through the system.  Studies were done to evaluate various alternative grit collection technologies for use at the plant.

HDR Engineering, Inc. recommended that WWMA consider the use of a vortex separator for grit removal at the plant.  The most significant reason for poor grit removal is variable flow, especially during high flow periods.  The vortex system is designed to maintain a relatively constant design flow throughout the process regardless of flow variation.  This allows the system to be operated at constant and efficient rates which, in turn, results in a higher level of grit removal.  Studies have shown that grit systems that do not have flow control are 70% less effective at removing grit and separating organics from incoming wastewater during peak or highly variable flows.  A grit concentrator was recommended for additional separation of organics that may not have been separated during the initial vortex process.  Separated organics are returned to the head of the vortex system and allowed to cycle through again without being removed with the grit.  This is extremely helpful in assuring a much higher level of organics separation from the discharged grit.

Raw sewage flow that enters the plant is initially pumped up to the vortex grit tank.  This mechanism uses a cyclonic swirling action that separates material such as sand, small rocks, dirt, and other heavy particles from the raw water waste flow.  The flow then exits the grit tank and is transported to the primary diversion structure.  Grit removal early in the process is very important because grit wears down the plant equipment and can accumulate in the tanks and reduce treatment                                                                                 efficiency.


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Last modified: July 8, 2021