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The solids handling portion of the Amherst Water Pollution Control Facility (WPCF) is a multi-step process that separates solids from wastewater. Components of the process are as follows:
1. Equalization Tanks
The solids treatment facility treats approximately 93,600 gallons per day of primary solids that are wasted from the Equalization Tanks. Solids that settle in the four Equalization Tanks (3-1, 3-2, 40-1, and 40-2) are pumped by four primary feed pumps to Building 4 for subsequent processing. The settled solids (sludge) consist of a mix of primary solids and cosettled waste activated sludge.
2. Sludge Strainers
Two Strainpress ® Sludge Strainers are designed to remove large solids from the sludge to prevent the solids from causing problems further downstream. The strainers screen out the large solids and press them into a relatively dry mass that is discharged to a roll off container for later removal to a landfill. The remaining strained sludge passes on to one of the four 86,600 gallon tanks, which are normally used to hold screened solids before they pass on to the Gravity Belt Thickener. However, in the event of an extended plant shutdown digested solids can also be stored in these tanks.
3. DAF Tanks 1, 2, 3, 4, and South and North Sludge Holding Tanks
Four 86,600-gallon DAF tanks are used between processes. Normally, the DAF tanks are used to hold screened solids before they pass on to the Gravity Belt Thickener. However, in the event of an extended plant shutdown digested solids can also be stored in the DAF tanks.
The South (Blending) Tank holds up to 217,000 gallons of sludge from the Gravity Belt Thickener. The sludge is thoroughly mixed in the South Tank so as to provide a homogenous feed to the anaerobic digesters. The North (Storage) Tank holds up to 217,000 gallons of digested sludge from the Digester before being pumped to the centrifuge process. Both the South Tank and the North Tank have mixers running at approximately 3 rpm.
Digested solids are dewatered using a centrifuge. The Dewatering Centrifuge Feed Pumps transfer digested solids from the North Tank to one of the three centrifuges. Generally, only the large centrifuge and its corresponding feed pumps are used for this purpose. However, the other two smaller centrifuges are available for backup.
Polymer is added to thicken the sludge and aid in the centrifuging process. The Polymer pump feeds polymer at a rate of 26-28 gal/min. The feed sludge is approximately 4.5% solids. The centrifuge is cylindrical at one end and conical at the outlet end. The centrifugal force pushes the greater density solids portion of sludge to the shell wall where a screw conveyor pushes it out the conical discharge end. The centrate (liquid portion) flows out the opposite end where it is pumped back to the Stage I influent. The remaining sludge, at approximately 25% solids is transported to the drying area via conveyor.
Note that presently only one of the four available centrifuge feed pumps is generally used. Centrifuge feed pump is piped to the large centrifuge but is not used, and the two small centrifuges and their corresponding pumps are only exercised monthly to prevent them from freezing up. Also note that when necessary, undigested sludge is fed from the DAF Tanks or the South Tank to the Centrifuge and the dewatered sludge is sent directly to the landfill.