The main goal of the thickeners is to increase the solids concentration of the sludge. In doing this, the total volume of sludge sent to the dewatering phase of the sludge handling process is reduced. This will reduce the time and energy spent during the dewatering step.
Each thickener contains a thickening mechanism that slowly rotates. The mechanism stirs the solids in the tank without breaking them apart. This allows the water to escape to the top of the thickener. The sludge settles and thickens at the bottom of the thickeners and moves towards the center of the thickener where it is removed. The sludge in the thickeners has increased to 8 percent solids when it is sent through the dewatering process.
Dewatering Process -Conditioning
American Bottoms conditions the sludge with the use of a chemical called a polymer. The polymer comes in a granular form that is mixed with water before being used. The addition of the polymer conditions the sludge in several different ways. It aids in better flocculation, or clumping, of the solids. It also allows the solids to more easily separate from the water.
The conditioning tanks are very important in the polymer addition process. There are three 100 gallon conditioning tanks, one for each belt filter press. As the sludge enters the tank, it begins to mix with the polymer. Once the sludge and polymer have been thoroughly mixed, they overflow onto the belt filter press.
Dewatering Process – Belt Filter Press
The belt filter press is a mechanically operated system made of 16 rollers and a fabric belt filter. The filter is constantly moving through the rollers. As the sludge flows out of the conditioning tanks, it flows onto the belt filter. Initially, the water freely falls from the recently conditioned sludge. After several feet along the belt filter press, the belts begin to squeeze the sludge. The sludge is squeezed harder and harder as the belt filter moves through the rollers. The dewatered sludge, also called filter cake, is scraped onto a conveyor belt. It is either stored in a sludge storage bin to await pick-up or conveyed directly to a truck to be hauled away to a landfill.