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WAS Pumping/Thickening Overview
Purpose
Waste activated sludge (WAS) pumping addresses several needs of the secondary and anaerobic treatment processes. The term wasting refers to the action of removing solids or activated sludge to manipulate characteristics of the secondary treatment process. In the aerobic process or activated sludge process. wasting is used to manipulate factors such as sludge age, sludge retention time (SRT), mean cell residence time (MCRT), and the food to microorganism (F/M) ratio, along with total biomass production or yield in the secondary process.  Wasting activated sludge also contributes an organic source of food to the anaerobic digestion process for further stabilization and generation of methane gas.

The primary purpose of thickening WAS is to reduce the volume of WAS that must be handled after it is removed from the secondary clarifiers. The thickener can also be used to thicken digested sludge for either of the following purposes: (1) reducing the volume of digested sludge to be loaded in liquid form into trucks for disposal, or (2) increasing solids retention time in the digester by returning thickened sludge solids back to the digester.  The gravity belt thickener (GBT) removes water from conditioned sludge by passing it over a woven belt filter that retains the solids while filtrate passes through and is collected below.
Theory of Operation
The secondary treatment process converts primary effluent BOD to biological solids. More solids are generated than are needed in the secondary treatment system. The surplus biological solids are referred to as waste activated sludge (WAS). WAS pumping removes the surplus biological solids from the secondary treatment system to maintain the secondary treatment system solids inventory at the appropriate level. Activated sludge process control targets, primarily SRT, determine the amount of waste activated sludge to be removed from the system.

Thickening the WAS prior to anaerobic digestion reduces heating requirements and increases the detention time within the digester by increasing the solids concentration of the sludge.
Equipment

The WAS thickening system includes the following components:

  • WAS Pump
  • Polymer System
  • Washwater Booster Pump
  • Gravity Belt Thickener
  • Thickened WAS Pump
Description
WAS Pumping
The WAS pumping system consists of a dedicated heavy solids hopper, wet well and a single WAS pump. Clarifier solids collect in solids hoppers and flow to the wet well through a 6-inch pipe. The WAS pump conveys the sludge to the solids handling facility.

WAS Thickening
The WAS pump is variable speed and has a capacity of 400 gpm.  WAS pumping rates are operator determined and are based on the desired wasting quantity, WAS concentration, and solids through-put of the gravity belt thickener (GBT). WAS pumping and thickening is intermittent.  The WAS pump is powered by the MCC-2 located in the Blower Building.

The GBT is intended to normally be used to thicken WAS prior to feeding it into Digester 3. Sludge is fed to the GBT by the WAS pump during sludge wasting. The GBT is installed on the second floor of the Solids Handling Building. A raised platform around the thickeners allows visual inspection of the belt, plows and solids conditioning process.  Polymer solution is added to the WAS through an in-line polymer mixer located in the sludge feed line before the sludge enters a flash/mix tank and a flocculation tank. The flash mix tank aids in through mixing of the polymer and sludge. The flocculation tank provides solids conditioning prior to application on the belt. Once the solids are placed on the belt, plows (Chicanes) roll and separate the solid mass creating open lanes on top of the belt which enhances the flow of water from the conditioned solids, through the belt to the drain.  A hopper at the end of the GBT collects the thickened sludge.  Filtrate from the GBT drains back to the influent wet well for additional treatment.  The thickener is equipped with a high pressure wash water spraying system that is used to maintain drainage of the belt.  The GBT is a 2‑meter Ashbrook-Simon-Hartley Aquabelt with a hydraulic belt ten­sioning and belt alignment system. It is intended to receive WAS at a concentration of 0.5 to 1.0 percent solids and to discharge thickened sludge at a concentration of 4 to 6 percent.

Thickened sludge from the GBT is discharged into a thickened sludge hopper that directs it into the open throat of a progressing cavity thickened sludge pump. From there the thickened sludge is normally pumped to Digester 3.  The thickened sludge pump is driven by a variable frequency drive (VFD) located in the upper level of the solids building. Its flow capacity will range from 25 to 100 gpm, and will normally be controlled automatically in response to level in the thickened sludge hopper.

There are two control panels associated with the GBT, both located adjacent to the machine. The first was supplied by the supplier of the GBT and controls belt ten­sioning, belt alignment, the hydraulic power system, and other functions and alarms internal to the performance of the machine itself. Literature provided by the vendor should be consulted for information regarding the operation of this panel and the systems it controls.

A second control panel, LP‑8080, controls the interface between the GBT and associ­ated equipment, such as the washwater booster pumps, polymer pumps, thickened sludge pump, and WAS pump.

The GBT requires a supply of belt filter washwater at 40 gpm and 85 to 120 psi. Two washwater booster pumps have been provided. Pump 1 is dedicated to the GBT and has a capacity of 40 gpm at 120 psi. Pump 2 has a higher capacity and is dedicated to the belt filter press. If Pump 1 fails, a piping interconnection will permit the use of Pump 2 for the GBT, but manual control from panel LP‑8090 will be required. The automatic valve in the pump discharge line will con­tinue to shut the water supply on and off.
WAS Pump6

The WAS pump is located in the RAS/WAS Pump Station between the secondary clarifiers. 


The WAS pump is supplied from a dedicated sump in the bottom of each clarifier.  

WAS flow is pumped to the GBT. The pump is normally controlled automatically as part of the operation of the GBT. 

Polymer Feed Unit6
 

The polymer feed unit is located in the Solids Handling Building.

The polymer system mixes and feeds dry polymer solution to the sludge feed lines for the GBT and Belt Filter Press (BFP). The polymer solution mixes with the sludge and causes the solid particles in the sludge to agglomerate. This allows water to be removed more easily during sludge thickening and dewatering.
Washwater Booster Pump6
The GBT requires a supply of washwater of 40 gpm at 85 to 120 psi. Two washwater booster pumps have been provided.
 
Pump 1 is dedicated to the GBT and has a capacity of 40 gpm at 120 psi. Pump 2 has a higher capacity and is dedicated to the belt filter press. If Pump 1 fails, a piping interconnection will permit the use of the Pump 2 for the GBT, but manual control from panel LP‑8090 will be required. The automatic valve in the pump discharge line will con­tinue to shut the water supply on and off.
Gravity Belt Thickener6
  The GBT is a 2‑meter Ashbrook-Simon-Hartley Aquabelt with a hydraulic belt ten­sioning and belt alignment system.

It is intended to receive WAS at a concentration of 0.5 to 1.0 percent solids and to discharge thickened sludge at a concentration of 4 to 6 percent.

The thickener is equipped with a high pressure wash water spraying system that is used to maintain drainage of the belt.  
Thickened WAS Pump6
The gravity belt thickener discharges thickened waste activated sludge (TWAS) into a sump at the end of the thickener. A single pipe runs from the sump and into the suction of the TWAS pump. The variable speed, progressing cavity pump is equipped with inlet and discharge isolation valves, discharge check valves, and flow meter. The pump sends the TWAS to the digesters.

To protect the pump and piping, pressure switches have been provided. These switches trip the power to the pump off when the pressure in the discharge line raises above 50 psig.

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Last Updated: 9/16/2013 9:43:13 AM
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