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Solids Residual Handling
Dewatering ( the removal of water from sludge ) is a very important process. The more water one removes from sludge the less it costs to burn in an incinerator or trucked off site to be landfilled. Trucking costs are based on wet tons thus the more water in the sludge the more water one pays to truck. In incineration, wet sludges reduce capacity, decrease incinerator "bed" temperature and increase fuel costs.
Once the sludge is collected in the hoppers in the bottom of the primary clarifiers, it is pumped to cyclone degritters for further removal of grit. Degritted primary sludge passes through a fine mechanical bar screen for further removal of rags and flows to the primary sludge gravity thickener. The grit from the cyclone degritters is landfilled on site and the rags are incinerated.
Gravity thickeners are the first and least expensive method of dewatering sludge. Gravity thickeners hold sludge and allow the sludge to settle down to the bottom and thicken by gravity. Primary clarifier sludge is sent to one thickener and secondary clarifier sludge is sent to the other gravity thickener. Primary sludge mixed only with effluent water for conditioning naturally settles easily in one of the two gravity thickeners. A polymer is added to the secondary sludge before it enters the other gravity thickener to aid in settling. This is because secondary sludge is mostly microorganisms and does not settle easily.
The primary and secondary sludge are combined immediately after the gravity thickeners. Potassium Permanganate is occasionally added to the conditioned sludge for odor control. The sludge mixture is mixed with a polymer (to help remove water), then pumped into one of four plate and frame presses where excess water is "squeezed" out of the sludge. The remaining product is a sludge cake which is approximately twenty to thirty percent (20-30%) solids.
The next step in the dewatering process is to squeeze the water out of the sludge with diaphragm filter presses. The thickened sludge from the gravity thickeners are blended together then mixed with a polymer to help coagulate particles together which aids in the dewatering process. This mixture is feed into the diaphragm filter press under high pressure and the water in the sludge is forced out through a filter cloth. The press then squeezes the sludge & polymer mix to dry the sludge further. The dry sludge "sludge cake" then fall down into a bin where it is pumped with cement pumps "schwing pumps" to a bin for the incinerators or directly to the incinerators weigh belts or into a truck to be hauled off site to a landfill.