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Secondary Clarifiers/Scum and RAS Pumping Overview
Purpose
The secondary clarifiers provide gravity separation of mixed liquor solids, producing a clear effluent suitable for disinfection. In addition, the clarifiers thicken the settled mixed liquor suspended solids (MLSS) into return activated solids (RAS). The RAS pumps convey the thickened solids from the clarifier to the primary effluent pipe immediately upstream of Aeration Basins Splitter Box. Floatable materials (scum) collected on the surface of the secondary clarifiers are pumped to anaerobic digestion or rerun back to the influent for removal.
Theory of Operation
Secondary clarification determines the effluent quality of the activated solids process. A critical factors in secondary clarifier design is the surface overflow rate. The surface overflow rate is the rate of flow leaving the clarifier divided by the clarifier surface area. The overflow rate is thus the average upward velocity of effluent leaving the clarifier. Suspended solids will be captured in the clarifier if their settling velocity is greater than the average overflow rate.
 
The secondary clarifier design operating conditions are as follows:
Secondary Clarifier Design Operating Conditions 
Criteria Units Design Annual Average Design Maximum Day Design Peak Hour
Units each 2 2
Diameter ft 80 80 80
Sidewater Depth ft 12 12 12
Area (total) sf 10,048 10,048 10,048
Volume (each) gal 450,954 450,954 450,954
Hydraulic Loading Rate gpd/sf 550 1,095 2,190
Secondary Clarifier Description
The secondary clarifiers consist of two circular clarifiers with spiral type mechanisms, scum handling system, and algae sweep system.

From the mixed liquor (ML) splitting box at the aeration basins, ML is conveyed to each clarifier by gravity through a 30-inch pipe. Flow enters the circular clarifiers through the center column of the secondary clarifier mechanisms. Solids settle to the bottom of the clarifiers and clarified effluent flows over and through V-notch weirs and scum baffles into the collection launders located around the perimeter of the clarifiers. Secondary effluent from both clarifiers exits the launders through two separate 30” pipes that convey flow to the UV Disinfection Facility. Manual slide gates at each effluent pipe entrance to the UV influent channel can be closed to isolate the clarifier for maintenance and prevent backwater from the UV Disinfection Facility from entering the clarifier launder.

The circular clarifiers are 80-feet in diameter with a 12-foot sidewater depth, a bottom slope of 1 foot per 12 feet, outboard launders, and a spiral mechanism. The clarifiers have a common scum pit. Each clarifier is capable of handling 50% of the peak plant flow at an overflow rate of 1,095 gpd/ft2. The capacity allows one unit to be taken out of service for maintenance without adversely impacting effluent quality during flow events less than the maximum month flow of 5.5 mgd.

The spiral type clarifier mechanisms replaced the original rapid sludge withdrawal “organ pipe” RAS and ML pipe arrangement at the center of the clarifier to allow the spiral type clarifier sludge collection ring to interface with the existing piping. The spiral clarifier mechanism contains a sludge removal header and 2 scraper arms. The mechanism rotates within the clarifier. The scraper arm moves settled solids on the floor of the basin towards the center of the clarifier and into a solids hopper near the center of the clarifier. A sludge header from the hopper conveys solids to the WAS wet well. A separate sludge removal header conveys settled solids to the RAS wet well from a sludge collection ring in the center of the clarifier. Once the RAS leaves the clarifier it is controlled by a motorized downward activating weir gate. As the RAS spills over the top of the weir gate it is combined with the RAS from the other clarifier in the RAS wet well. From the wet well the RAS is conveyed to the anoxic zone of each aeration basin by two adjustable speed pumps and piping.

Each clarifier has a rotating scum skimmer and scum trough which removes scum and other floatable material from the secondary clarifier water surfaces. A 6” pipe conveys skimmed material from the troughs to a scum pit. A progressive cavity type scum pump conveys scum from the pit to the Solids Handling Facility. This pump is a replacement of plunger type pump originally installed when the clarifiers were first constructed.

The secondary scum piping can be valved to discharge scum to the solids process (normal condition when WAS thickening on the gravity belt thickeners is occurring). When the WAS thickening process is off, the secondary scum discharge piping can be valved to discharge scum back to the mixed liquor junction box and is re-circulated through the aeration basin and clarifier. This allows the scum removal to continue during unattended operation periods and prevents excessive accumulation of secondary scum on the clarifier water surface.

The algae sweep system consists of plastic brushes mounted on stainless steel arms that are attached to the scum skimmer. The system cleans the inner and outer baffles and weirs, top spillway surface, inner launder wall, launder bottom, and the outer launder wall. Surfaces are cleaned as the skimmer arm rotates in the clarifier.

The secondary clarifier mechanism and scum pumps are controlled by the PCS, and powered from the MCC-2 located in the Blower Building.
Secondary Clarifier Design Criteria
Click on the link to obtain information on the Secondary Clarifier Design Criteria.
Secondary Scum Pumping Description
Scum accumulated on the clarifier is collected via a scum skimmer and scum trough arrangement and flows to the scum well. The scum pump takes its suction from this well and discharges to the gravity belt thickener or to the influent. The scum pump is controlled by a HOA switch in the recirculation pump station. In auto mode the pump starts and stops at predetermined set points on the plant computer. Those set points are monitored by a pressure transducer in the scum pit.
Return Activated Sludge Pumping Description
The RAS pumping system consists of a 12-inch RAS collection pipe installed under each clarifier, a downward acting slide gates with electric actuators and RAS pumps with variable frequency drives (VFDs). The collection pipe conveys RAS from the center of the clarifier to a small well upstream of the RAS flow control weir gate. The RAS rate is controlled by operating the RAS weir gate for each clarifier. RAS from both clarifiers is combined in the RAS wet well and then is pumped back to the anoxic zone of each aeration basin.  Each adjustable speed RAS pump has a maximum capacity of 1,900 gpm, which approximately equals 50% of the plant influent flow under maximum month conditions with one RAS pump unit out of service.
RAS is pumped into a common 12-inch pipe and through a magnetic flow meter. The meter measures and transmits RAS flow to the flow controller of the PCS.  RAS flow is adjusted based on the measured plant effluent flow rate.  The RAS pumps are powered by VFDs in MCC-2, located in the Blower Building.
Return Activated Sludge Pumping Criteria
Click on the link to obtain information on the Return Activated Sludge Pumping Design Criteria.
Components
The Secondary Clarifier and RAS Pump systems include the following components: 
  • Secondary Clarifiers
  • Algae Sweep System
  • Secondary Clarifier Scum Removal System
  • RAS Pumps and VFDs
Secondary Clarifier & Drives Units6
 
The spiral clarifier mechanism contains a sludge removal header and 2 scraper arms. The mechanism rotates within the clarifier. The scraper arm moves settled solids on the floor of the basin towards the center of the clarifier and into a heavy solids hopper near the center of the clarifier. 
 
A density current baffle is attached to the bottom of the effluent launder to minimize the possibility of short circuiting. 
Algae Sweep System6 
The algae sweep system consists of plastic brushes mounted on stainless steel arms that are attached to the scum skimmer. The system cleans the inner and outer baffles and weirs, top spillway surface, inner launder wall, launder bottom, and the outer launder wall. Surfaces are cleaned as the skimmer arm rotates in the clarifier.
Secondary Clarifier Scum Removal System 6
 
Each clarifier has a rotating scum skimmer and scum trough which remove scum and other floatable material from the secondary clarifier water surfaces. A 6” pipe conveys skimmed material from the troughs to a scum pit. A progressive cavity type scum pump conveys scum from the pit to the Solids Handling Facility.
RAS Pumps & VFDs6

Each adjustable speed RAS pump has a maximum capacity of 1,900 gpm which approximately equals 50% of the plant influent flow under maximum month conditions with one RAS pump unit out of service. 


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Last Updated: 9/16/2013 9:28:48 AM
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