Primary Clarifier | Activated Sludge Process | Wastewater Treatment Plant

Primary Clarifier

Biochemical Oxygen Demand (BOD)/Chemical Oxygen Demand (COD)

• Primary clarifiers that are well designed and operated should remove between 20 and 40% of BOD. Excess solids and BOD will be discharged to the secondary treatment system by poorly performing clarifiers, potentially resulting in effluent permit limit violations.
• Excessive hydraulic loads in primary clarifiers can sweep solids out and into the aeration tank, resulting in shorter detention times. Bring offline clarifiers online, if available, to lengthen the detention time.

 

To reduce the required detention time for effective settling, flocculants can be added to the primary clarifier influent.

Flocculants are chemicals that aid in the aggregation of smaller particles into larger, settleable particles. 

• Reduced clarifier performance may result from a shift in the size characteristics of the influent particles from heavy, rapidly settling particles to smaller particles. In this case, flocculants can be used to improve BOD and total suspended solids removal (TSS).
• Poor clarifier performance can be caused by faulty or inefficient sludge removal equipment, such as the sludge scraping mechanism and sludge draw off pump. If the sludge removal equipment fails, the sludge layer on the bottom of the clarifier may continue to thicken until solids are washed out of the clarifier through the effluent weir to the secondary treatment system. Regularly inspect the sludge scraping mechanism and sludge draw off pumps to ensure they are in good working order.

Total Suspended Solids (TSS)

• In general, well-designed and operated primary clarifiers should remove between 50 and 70% of TSS. The reasons for poor TSS removal in the primary clarifier, as well as suggestions for improvement, are similar to those for BOD.

Nutrients

• To function properly, activated sludge bacteria require an adequate carbon source as well as nutrients such as nitrogen and phosphorus. 
• The general rule of thumb is that the BOD-to-nitrogen-to-phosphorus ratio in primary clarifier effluent should be around 100:5:1.