## 3.3.4.7 Primary Clarifiers 

The primary clarifiers shall be designed for the removal of readily settleable solids and floating materials and, therefore, shall reduce the suspended solids content. The purpose of the primary clarifiers is to remove a substantial portion of the organic solids from the untreated wastewater, with a resulting decrease in the organic and solid loadings of the biological unit. 

Primary treatment efficiencies shall be based on the following assumptions 

* BOD$_{5}$ Reduction: ≤25% 

* COD Reduction: ≤25% 

* TSS Reduction: ≤50% 

* Total N Reduction: ≤10% 

* Total P Reduction: ≤11% 

The primary clarifiers shall be designed, constructed and equipped regarding the requirements set in Table 2-1. The maximum flow (Q$_{WW,h}$) of the related stage requirements shall be met for the unit. 

Primary Clarifiers shall be constructed in either circular or rectangular shape. The sludge shall be collected into sludge hoppers by scrapers that are mounted on the primary clarifiers’ bridge. Each primary clarifier shall have a separate bridge (i.e. each bridge should work individually). If the shape of clarifiers is circular, the bridge to be used shall be half bridge. 

The methodology of the sludge suction from the primary clarifier tanks shall be based on maintaining the separate sludge suction from hopper of the tanks. The instrumentation and automation system required for sludge suction shall be available from one hopper individually. 

In order to achieve this aim, each sludge line from each hopper of the primary clarifier tank should have one valve with electric actuator in accordance with 7.8.18. The operation status of the valves should be monitored and controlled by the SCADA system. 

The settled sludge shall be pumped via primary sludge pumps to the gravity thickeners. 

The surface scraper moving on the whole surface of the tank shall ensure collection and transport of floating sludge via screw conveyer system or chain track system or any other system which can remove floating materials by force. Surface scum and floating materials shall be removed from the tanks and should be conveyed to the drum sieve to be located on the distribution chamber of primary clarifier. The waste from drum sieve shall be disposed to a container with a capacity of 5m$^{3}$. Outlet water of the sieve shall be discharged into this distribution chamber tank. 

For emergency situations, a bypass line at downstream of the primary clarifiers shall be provided and this line shall be connected to treated effluent discharge line. The bypass of this unit shall be controlled by means of the weir type penstock. This penstock shall be channel type weir penstock with double spindle and fully watertight. It shall be proportioning and electrically driven actuators. The width of the overflow weir shall be designed and constructed by considering the second stage maximum flow rate. The hydraulic height of the overflow penstock shall be adjusted by considering the first stage maximum flow rate. Therefore this unit shall work as bypass when this penstock is open, otherwise it shall work as overflow. 

