The Importance of Primary Clarification: A Key to System Efficiency
Well-functioning primary clarifiers are essential for wastewater treatment efficiency. However, they are often overlooked when diagnosing the causes of declining effluent quality. Their primary function is to remove insoluble BOD (such as fiber) and inorganic solids (like lime and ash), thereby reducing the load on the biological treatment stage.
From a cost-saving perspective, primary clarifiers remove more BOD and TSS for less operational expense than any other treatment process, as rejected fiber from the process can contribute up to 0.4 pounds of BOD per pound of TSS.
The diagram shows the difference between what enters the primary clarifier versus what leaves.
Green bars represent the TSS and BOD concentrations coming into the Unsettled Primary Clarifier Feed from the mill. If operators bypass the clarifier, this feed will go directly into the ASB.
Purple bars in the diagram represent the TSS and BOD concentrations in a typical Settled Primary Clarifier Effluent. These concentrations typically enter the ASB when the primary clarifier is functioning correctly.
Understanding Phantom BOD
Phantom BOD represents the insoluble portion of BOD that will not show up in the 5- day BOD test. This fraction will settle out somewhere in the ASB and break down to soluble BOD, creating additional oxygen demand later on. The primary clarifier significantly reduces the phantom BOD load to the ASB in this example. As seen in this diagram, a primary clarifier serves not only to reduce TSS but also a significant portion of BOD. Since more solids enter the ASB when bypassing the clarifier, bacteria will need time to acclimate to the higher BOD loading. There will likely be carryover of BOD and TSS into the effluent. Also, since more phantom BOD will be added to the ASB, the consequences of clarifier bypass may be seen weeks later in the treated effluent.
Our experienced wastewater consultants help you closely monitor trends in primary clarifier performance, providing early detection of issues before they escalate. We also help ensure the biological portion of your treatment system remains resilient, even during periods when primary clarifier performance fluctuates!