Solids Handling Presentation
The solids handling portion of the Amherst Water Pollution Control Facility (WPCF) is a multi-step process that separates solids from wastewater in order to produce fertilizer pellets. The fertilizer pellets can then be sold to offset the cost of water treatment, while alleviating the environmental strain and cost of producing excess solid waste. 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) Gravity Belt Thickener System
Two variable speed Thickener Feed Pumps pump the screened sludge from the tanks to the Komline-Sanderson Gravabelt Thickener (GBT). A dilute polymer mixture (0.33 % wt/wt) is added to the sludge to help dewater it before going through the GBT. A four-point injection ring is used to add polymer to the system. Immediately following the injection ring is a variable-orifice mixing valve that thoroughly mixes the polymer into the sludge due to the turbulence and shear forces created. The GBT can process approximately 385-584 gpm of sludge at 3.7% solids, producing a thickened sludge that contains approximately 7.5% solids. Water is removed from the sludge via gravity utilizing cones positioned along the width of the belt to "hold up" the sludge while allowing free water to drain through the belt. Multiple coned stages are used to maximize total drainage. The remaining sludge flows to the South (Blending) Tank where it is mixed so that a homogenous mixture is fed to the digesters. Water removed from the thickened sludge is returned to the Stage I Influent.
4) Anaerobic Digester System
Two Digester Feed Pumps pump sludge from the South (Blending) Tank to the two Digesters. The sludge is then anaerobically digested using biological degradation. Digestion of organic material releases carbon dioxide, methane and water and uses up much of the available energy rendering the remaining solids stable. Using this technique 25 to 50 percent of the raw sludge can be destroyed. Close monitoring of the internal digester environment is necessary to prevent upsets because the bacteria used in the digestion are extremely sensitive to changes in temperature, pH and other environmental factors. After digestion is complete one of two corresponding Digested Sludge Pumps pump the digested sludge from the digesters to the North (Storage) Tank where it is held before being pumped to the centrifuge process.
5) 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.
6) Centrifuge
Digested solids are dewatered using a centrifuge prior to processing of the solids through the pellet plant. 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.
7) Drum Drying System (DDS)
Centrifuged dewatered sludge is stored in the Live Bottom Bin and then in the Wet Material Bin before going to the Mixer. In the mixer the dewatered sludge (25% solids) is mixed with dry recycled material (97% solids) from the Recycle Material Bin prior to being fed to the Andritz Drum Drying System. The drum dryer is a triple pass rotating drum that uses high speed hot air (700 ºF) to dry the sludge until it is light enough to be pneumatically conveyed out of the dryer. The sludge is approximately 97% solids after it leaves the drum drying apparatus. The drying apparatus is designed to process approximately 1,800 pounds of pellets per hour and 5,000 pounds of water per hour while running for 60 hours per week.
After exiting the Drum Dryer the high-speed air is carrying evaporated moisture and dried sludge. During the first step of a two-step separation process the high-speed air enters the Preseparator and impacts a plate that is perpendicular to the direction of airflow. As the air enters the chamber it is forced to abruptly change direction separating the product from the air. From here the air enters the Polycyclone, which is a group of small high efficiency cyclones where the smaller sludge particles are removed from the air. Once the product is removed from the air the product is sent to the Screen for crushing and sizing. The hot air passes through a water scrubber (saturator condenser) to remove remaining dust before returning to the inlet of the DDS furnace.
In the screen, three screens are used to classify the dried pellets by size. The top screen separates out the large oversized particles that are greater than 10 mm in diameter and are too large to be used. They are removed and sent to the landfill. The next screen separates the oversized particles, which are sent through the Crusher where they are crushed before being returned to the Recycled Material Bin to be mixed with incoming wet sludge. The next screen separates out the correctly sized product. Product pellets are sent to a pellet cooler, to cool them to below 100 ºF to avoid fires, and then to one of three storage silos where they are stored until being transported offsite. The undersized materials that pass through the product screen (3rd screen) are also returned to the Recycle Material Bin to be added to incoming wet sludge before the drying process.
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