What is sludge dewatering?
Sludge dewatering: Mechanical removal of water from sewage sludge
In short
Sludge dewatering is the mechanical removal of water from sewage sludge, turning a liquid of 2 to 5 percent solids into a cake of 15 to 35 percent that can be hauled, landfilled, land-applied, dried or incinerated. The main machines are belt filter presses, centrifuges, screw presses and plate-and-frame presses, all fed with sludge conditioned with polymer.
Hauling and disposal are priced by the wet ton, so dewatering is where a plant controls its solids budget. Klarifi tracks 341 biosolids and dewatering projects in published US city capital improvement plans, and 1,539 plants with anaerobic digesters upstream of their dewatering.
Thickening, dewatering, drying: the difference
Thickening takes sludge from about 1 percent solids to 4 to 6 percent, and it still flows like a liquid. Dewatering takes it to a cake that is handled as a solid. Drying evaporates most of the remaining water, to 90 percent solids or more. Each step cuts volume sharply: going from 3 to 25 percent solids removes almost 90 percent of the mass that has to be hauled.
The main dewatering technologies
Belt filter presses squeeze sludge between two porous belts. They are simple and use little power, and produce a cake of 15 to 25 percent solids, at the cost of wash water, odor and operator attention. Centrifuges spin sludge at up to 3,000 times gravity; they are enclosed and compact and reach 20 to 30 percent, but use more power and polymer. Screw presses turn slowly inside a screen, run unattended with low energy use, and have taken much of the market at small and medium plants. Plate-and-frame presses work in batches and give the driest cake, 30 percent and above. Drying beds and geotextile tubes serve small plants and lagoons.
All of them depend on polymer, which binds fine particles into flocs so that the water can escape. After disposal, polymer is usually the largest operating cost of dewatering.
What drives dewatering projects
Equipment reaches the end of its life after 20 to 25 years, disposal costs rise every year, and PFAS rules are closing land application outlets in some states, which pushes plants towards drier cake, dryers and incineration. Capital plans therefore carry a steady stream of press and centrifuge replacements and new solids-handling buildings. The water removed, called centrate or filtrate, returns to the head of the plant loaded with ammonia and phosphorus, a design issue of its own at plants with nutrient limits.