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Secondary Sedimentation Tanks

The Secondary Sedimentation Tanks are circular tanks equipped with rotating mechanical sludge and scum collectors. Appurtenant systems include spray systems for moving scum and for odor control, and pumps for draining tanks. Sedimentation Tank No. 1 is 75 feet in diameter with side water depth of 9 feet. Sedimentation Tank No. 2, is located just south of Sedimentation Tank No. 1, is 65 feet in diameter with a side water depth of 12 feet. The walls of both tanks extend above the predicted 100 year flood-stage elevation. A flap gate in each tank’s wall allows floodwater inflow so the structure will not become buoyant and prevents wastewater outflow. The effluent from the Aeration Structure enters each tank through the bottom, rises up through the center column, and then is distributed into the sedimentation zone. Settled sludge is removed through collecting pipes located on the submerged collecting rake arms and by means of hydraulic differential flows to the sludge collection chamber near the top of the center column. Adjusting valves on the upper end of each collecting pipe can be set so the sludge removal is equalized throughout the tank. The sludge then flows back to the wet well in the Aeration Structure, where it is pumped and divided into either RAS or WAS. Scum is collected from the surface of the wastewater in each tank and returned to a sump in the Aeration Structure, from which it is then pumped to either the Headworks or the Sludge Thickener. The sludge and scum collectors are driven by ¾ horsepower motors with torque overload protection. The drain pumps are self-priming centrifugal pumps driven by 7 – ½ horsepower motors.

Sewage flow into the Secondary Sedimentation Tanks will continue during power outages. The tanks are on the standby power supply and will continue to operate during power outages.

Secondary Sedimentation Tank No 1 has been designed with a nominal surface loading of approximately 450 gallons per square foot per day, while Secondary Sedimentation Tank No. 2 has been designed with a nominal surface loading of approximately 600 gallons per square foot per day. Secondary Sedimentation Tank No. 2 provides additional secondary sedimentation capacity during peak flows when the design maximum peak overflow rate of Tank No. 1 is exceeded, and provides a standby tank should Secondary Sedimentation Tank No. 1 need servicing.

As outlined above, sewage from the aeration structure flows into each secondary sedimentation tank up through the center column and is distributed into the sedimentation zone. As the sewage passes through the sedimentation zone, the activated sludge settles out and the clarified sewage flows over the weirs of the circumferential launder, and then through the outlet pipe to the chlorination structure.