Preliminary & Primary Treatment

The headworks include the influent channel, coarse screens and aerated grit chambers where preliminary treatment occurs. Flow measurement, screening, pumping, and grit removal are the typical steps in preliminary treatment. Wastewater enters the influent channel into the course screens. The screens remove large debris that enters the sewage collection system such as rags, tramp metal, sticks, broken glass, rocks, sand and the vast variety of other materials. Screens are utilized early in the wastewater treatment process to minimize pump and equipment damage within the facility. In many wastewater treatment plants, fine screens are utilized to remove smaller debris. All screened debris is removed and disposed as landfill. The wastewater is then pumped into grit removal chambers. Air is introduced into the chamber to scour the organic materials from the grit before the grit settles to the bottom of the chamber. The settled grit or sand is delivered by a screw conveyor to a pit at one end of the chamber. From there, it is pumped by a grit pump to a grit/water separator. This debris is also disposed as landfill. Liquid separated from the grit is returned to the grit chamber. Wastewater from the grit chamber then flows to the primary clarifiers.

The primary treatment process reduces the solids content of wastewater through sedimentation.  Wastewater slowly flows into large tanks called primary clarifiers, where heavier particles are allowed to settle at the bottom of the clarifier.  Scrapers move the settled solids (primary sludge) to sumps at one end of the clarifier. From there, the primary sludge is pumped into a holding tank where solids processing commences.