The objective of primary wastewater treatment is to remove the readily settleable and floatable solids from the raw wastewater stream.
Primary treatment at the Petawawa WPCP includes chemical addition, screening, grit removal, preaeration, scum removal (skimming) and sedimentation (or primary clarification). The purpose of these processes are outlined below:
Chemical Addition - To improve settling performance in sedimentation tanks by increasing floc formation. Also used to precipitate and remove phosphorous and to control odors.
Screening - To remove larger solids which could interfere with other components of the wastewater treatment system.
Grit Removal - To remove discrete granular particles such as sand, cinders, rocks and other substances which could interfere with other components of the wastewater treatment system.
Pre-aeration - To provide a more uniform distribution of suspended and floating solids to sedimentation facilities and to a lesser extent to provide grease separation, odour control, and increase BOD removal rates.
Scum Removal - To remove grease and other floatable materials from the wastewater. Typically carried out in sedimentation tanks.
Sedimentation - To remove flocculent particles and hence remove a portion of the BOD and SS from the wastewater. This is typically accomplished in sedimentation tanks or clarifiers which allow settleable materials to accumulate at the bottom of the tank and supernatant to overflow into a collection system. Solids thicken at the bottom and are drawn off for further treatment and disposal. Supernatant is conveyed to further treatment processes and/or discharged to a receiving water body.