Providing for the efficiency of contact clarifiers under conditions of reduced design filtration rate

Summary

An analysis of methods of providing for the efficiency of contact clarifiers under conditions of reduced design filtration rates is presented. Under actual operating conditions of contact clarifiers with a lower than design capacity after the introduction of a coagulant (aluminum sulfate), the estimated residence time of the coagulant in the water is 5.3 minutes before contact filtration starts. Furthermore, the motion speed of the flow being two to three times lower than the design value, is approximately 0.02 m/s, which is typical for the floc formation conditions. As a result, an undesirable coagulation process occurs in the free volume, so that previously formed aluminum hydroxide flocs disperse to form colloidal aluminum hydroxide. The formation of colloidal aluminum hydroxide in the free volume under contact coagulation conditions may worsen rather than improve water purification processes; in particular, the rate of pressure loss in the granular filter bed may increase. The results of pilot tests at the Kolpino Water Treatment Plant operated by SUE «Vodokanal of St. Petersburg» are presented, followed by the introduction of the best technical solutions.

Key words

contact clarifier , contact coagulation , particle size distribution of the filter media , mixed chemicals