The Primary treatment stage at a wastewater treatment plant serves the purpose of removing organic and inorganic suspended and floating solids from the wastewater. Conventional primary treatment typically consists of screening, grit removal, flow equalization and primary clarification.

Bar screens are typically the first layer of treatment at a wastewater plant. These bar screens, composed of bars spaced one to six inches apart, are placed in the influent channels where wastewater passes through the bar screens at a velocity of approximately one to three feet per second. The suspended and floating (large) solids are trapped by the screen. Accumulated solids can be removed manually by the plant operator or automatically by mechanical raking. The wastewater then moves to the grit chamber which removes the dense inorganic solids such as gravel and sand which have made their way into the sewers. If not removed, grit can cause operational difficulties in downstream processes and damage pumps.


Flow equalization will typically occur next. Wastewater enters the plant at varying flow rates from hour to hour. However, the primary and secondary wastewater treatment processes operate best when provided a constant flow rate, thus the importance of flow equalization. Flow equalization is usually performed using large basins, which are designed to receive and store wastewater so that it can then be pumped to downstream processes at a constant rate.

Primary clarification is the most common and well known aspect of primary treatment. Primary clarifiers typically remove 20-30% of the BOD and 50-60% of the TSS from the wastewater. As wastewater flows through the primary clarifiers, either circular or rectangular, sedimentation by gravity is used to remove the suspended material from the wastewater, allowing it to settle at the bottom of the tank. In rectangular primary clarifiers, rotating scrapers move the solids from the bottom of the tank to a hopper where they are removed as primary sludge. Smaller floatables, oil, and grease are also skimmed from the surface of the wastewater in the primary clarifiers through the use of troughs.


ClearCove’s EPT technology performs screening, grit removal, flow equalization and primary clarification all in a single tank while at the same time achieving superior results compared to existing primary treatment processes.