Clarifiers

Purpose: The clarification step provides for the separation of the activated sludge mass from the wastewater. The pollutants that have been taken up and concentrated by the biological activity of the microorganisms are effectively removed in this process leaving the wastewater clear enough to discharge to Fountain Creek.

Description: The clarifiers at the Fountain WWTF are circular units 45 feet in diameter with a side water depth of 12 feet. Two clarifiers are provided, with one clarifier serving each aeration basin. The clarifier influent pipes are interconnected so that all plant flow from both aeration basins can pass through either clarifier if the other unit is out of service. Alternatively, if one aeration basin is out of service, flow can be fed to both clarifiers from the basin that is in service. The clarifiers are located in the southern half of the Operations Building. Each unit consists of a reinforced concrete tank fitted with mechanical equipment to collect the settled biomass (sludge) and floating material (scum). Mixed liquor flows by gravity from the aeration basins to the clarifiers in two 14-inch ductile iron pipelines, each one connecting one aeration basin effluent line to control the flow from each basin and one in the line interconnecting the two main lines to permit flow to be directed to the clarifiers as required by operating circumstances.

Flow enters the clarifiers at the center and rises through a vertical pipe, which also supports the mechanical equipment. The velocity of the entering flow is dissipated in a center feed well from which the flow passes into the main settling area of the tank where the liquid - solid separation takes place. Flow moves radially outward, passes over a V-notch weir into a concrete launder trough that collects the clarifier effluent and carries it to the clarifier effluent pipe connection point. The V-notch weir and launder run around the entire perimeter of the clarifier. A large number of V- shaped notches are spaced equally along the weir to produce a uniform rate of flow at all points within the clarifier.

Settled sludge is collected by a rotating collector mechanism driven at a slow rate by an electric motor through a multi-stage gear reduction.

The collector has two arms, one a suction pipe for rapid sludge collection and the other fitted with a series of flights to sweep any remaining material to the center. The floor of the clarifier tank slopes toward the center at a rate of 1 vertical to 12 horizontal to enhance the movement of sludge toward the center.

The collected sludge is removed from the clarifiers by the RAS pumping system. The sludge picked up by the rapid sludge collection arm moves through the suction pipe to a closed shroud at the base of the center column. The sludge collected by the rake arm moves along the floor of the clarifier and accumulates in a hopper formed in the clarifier floor near the base of the center column. Sludge is withdrawn through 8-inch ductile iron pipes. One pipe is connected to the space under the rapid sludge withdrawal shroud and a second pipe is connected to the sludge hopper. These two pipes run parallel to each other under the clarifier floor then, at a point beyond the clarifier tank perimeter, they join to become a single RAS suction pipe from each clarifier. These suction pipes enter the lower level pump room to become the headers that supply the RAS pumps. Just before the two sludge withdrawal lines join, manually actuated plug valves are installed to control RAS withdrawal. This arrangement allows the sludge withdrawal point to be switched between the rapid return shroud and the hopper. Normally, the valve on the line from the sludge hopper will be closed and the valve on the line from the rapid sludge return shroud will be fully open. This will draw sludge from the rapid sludge return system to enhance process performance by returning the freshest possible sludge to the aeration basins as quickly as possible. Nearly all of the settled sludge should be removed by the rapid return sludge system operating this way. The small amount of material not removed by the rapid return sludge system will be collected by the rake arm and accumulate in the hopper. This material should be renewed periodically by opening the valve on the line from the hopper and closing the valve on the line from the rapid return sludge shroud. Operation in this mode for 15 minutes once per week should be sufficient to remove any material accumulated in the hopper. Following this operation the valves should be returned to their normal operating position with the hopper closed and the rapid return shroud line full open. The sludge collector carries a skimmer arm that sweeps the surface of the clarifier to collect floating material. This floating material may be biomass from the activated sludge process that has the tendency to float rather than settle, or floatable inert material not removed by preliminary treatment that has passed through the aeration basins. The skimmer arm moves this material toward the outside of the clarifier and into the scum box from where the skimmings pass to a scum well. The accumulated material in the scum well is periodically removed by pumping to the aerobic digester.

A pneumatically actuated plug valve is located on each clarifier influent line just before it enters the clarifier. The purpose of these valves is to stop flow through the plant during a power failure to prevent the discharge of effluent that has not been disinfected because the U.V. disinfection system will not function during a power outage. Air for this system is provided by a small compressor located at the west end of the Clarifier Room.