The solid materials that are removed from the wastewater and are collected throughout the treatment process need to be dewatered, or thickened, prior to their disposal.

The solids coming from the Primary Clarifiers, called Primary Sludge, are thickened in the Gravity Thickeners.

The solids from the Final Clarifiers, called Waste Activated Sludge, are thickened on the Gravity Belt Thickeners.

The solids collected at our Septage site are already quite concentrated on their arrival.

These thickened Primary and Waste Activated Sludges, along with the Septage, need to be further dewatered before they can be incinerated. This is done using four Belt Filter Press (BFP) units. Only two of the four BFPs are in operation at a time, with the remaining two in stand-by. We alternate which two BFPs are in use as part of our preventative maintenance program, to allow for routine cleaning and inspection.

The sludges are combined in Sludge Holding Tanks and mixed to keep them well blended. The blended sludges are conditioned with polymer before being fed onto the Belt Filter Presses. Just as in the Gravity Belt Thickeners (GBTs), the polymer aids in the separation of the solids and the water.

The sludge is moved along the BFPs on a porous cloth belt, which allows the water to drain through. The solids remain on top of the belt, quickly concentrating as the water drains away, much like the GBTs. However, unlike the GBTs, the Belt Filter Presses have a second belt at the end of the first where the solids are squeezed between the two. The belts move through a series of rollers applying more than 500 pounds of pressure, to squeeze out as much water as possible. The solids coming off of the BFPs are referred to as dewatered "cake" and are conveyed to the incinerators.

Efficient and economical operation of the incinerators depends on the BFPs producing the driest cake possible. The wetter the cake going into the incinerator, the more auxiliary fuel must be used within the furnace to reduce the cake to ash, and stay within stack emission limits required by regulations.