The Water Environment Research Foundation sponsored a one-year study quantifying the effect of certain wastewater characteristics and their variability on the performance of primary clarifiers. Eight municipal agencies participated and tested eleven different primary clarifiers. Several different tests were conducted to quantify the magnitude and variance of the following factors, all characteristics of the wastewater being treated, believed to affect primary clarifier performance:

• the nonsettleable TSS concentration,
• the influent TSS concentration,
• the settling characteristics of the settleable solids,
• the surface overflow rate,
• the soluble COD concentration,
• the ratio of particulate COD (or BOD5) to TSS in the primary effluent.