Sonoma Valley Treatment Plant Detailed Virtual Tour

The District is pursuing projects to minimize loss of particulate grease and floatable solids from the secondary clarifiers into the chlorine contact tanks. Since SVTP is an extended aeration activated sludge plant with no primary clarifiers or preliminary grease removal, it is vital to continually capture and recycle floatable solids until the activated sludge bacteria completely consume it. The District is also installing additional surface skimmers within the chlorine contact tanks to enhance this recycle.

SVTP has two secondary clarifiers that are 140' diameter with an average depth of 15'. They have WesTec spiral blade sludge removal and full radius ducking skimmers with rotating scum troughs. Fiberglass launder shade-covers are used to keep the launder and weir free of algae. They are each designed to treat 8MGD peak wet weather flow.

As seen on the far side of this empty clarifier, the concrete launders hang inboard from the clarifier walls. The fiberglass scum baffle, inboard from the weirs, is approximately 16" tall. The District believes that extending this scum baffle down an additional 16-20" would enhance floatable solids recovery within the clarifiers.

Before proceeding with the scum baffle extension, dye testing was performed to eliminate the possibility that strong density currents might carry floatable solids up under the concrete launder where they would eventually migrate up the inboard face of the launder and weir, outside of the scum baffle.

Hany Gerges, Shannon Haley, and Kevin Kennedy of HDR Engineering performed the dye testing with support from SVTP staff. HDR is the clarifier engineering design firm.

Hany dosed a full gallon of Rotamine dye to the mixed liquor splitter box, which feeds the clarifier. This location was chosen for maximum mixing prior to entering the clarifier.

After several minutes the dyed mixed liquor entered the clarifier.

After nearly a full half-hour, plumes of dye could be detected several feet below the surface, about 1/3-diameter inboard of the weirs, and moving inward. These plumes were very subtle and disperse and cannot be distinguished in these photos.

After 45 minutes these plumes could be seen clearly, still below the surface, 2/3-diameter inboard, and still moving inward to the center ring.

The weirs were among the last places to see the effects of the dye which took well over one hour to permeate the tank volume.

This dye testing clearly showed that the SVTP clarifiers are definitely not subject to density current induced problems. The District is likely to proceed with the scum baffle extension project and/or a reduction in scum sweep speed, but will first attempt some underwater video analysis at the scum baffle. A visual check of how floatable solids might migrate under the existing scum baffle is the only good way to qualify the decision to extend those baffles. One initial attempt at video was unsuccessful and the District is investigating better equipment for this task.