Sludge, or the accumulation of waste products that end up in the sewage, contains a considerable amount of water. Before any form of sewage treatment or disposal of waste takes place, water must first be removed from the sludge. Thus, a dewatering management plan is key to the effective treatment of sewage and production of biosolids.
There are different ways to reduce the water content of sludge. Some dewatering processes are more effective than others. Here are a few methods employed by wastewater treatment plants in handling sludge.
Vacuum Filters
In Ontario, rotary vacuum filters were at a time the systems most commonly used in treating sludge. But the use of vacuum filters started to decline when cheaper and more effective dewatering systems were developed.
The method is suitable for slurries, and liquids with higher proportion of solids, which could clog other types of filter. A drum rotates through the sludge and the vacuum sucks the waste through the filter media. The solids stick to the outside of the drum, which then goes through a blade, cutting off the solids.
The dewatering performance of vacuum filters are affected by sludge type characteristics, such as solids concentration, nature of sludge solids, chemical composition, sludge compressibility, age, temperature and filtrate viscosity. Also, higher solid concentrations in the sludge feed result in lower filtrate solids.
Centrifuges
With the use of a centrifuge, the sewage is spun at high speed to break down the sludge into its most basic components. The part of the sludge with the least density tends to rise in the mixture. Taking into account the weight of the sludge, anywhere from 25 percent to 35 percent of solids may be recovered with the use of centrifuges.
In considering an effective dewatering management plan, it must be recognized that a centrifuge is best suited only in situations where the recovery of oil is the main concern because the quality of residual solids it produces leaves much to be desired.
The continuous countercurrent solid bowl centrifuge is the type most frequently used in sewage sludge dewatering. Other types are the basket and disc centrifuges.
Belt Filter Presses
Another method requires the use of a belt filter press (BFP). It is often used when high moisture content is not much of a variable in your dewatering management plan because it produces a low-quality filter cake. Taking the weight of the sludge into account, a BFP normally produces between 18 to 25 percent of solids from it.
In using a belt filter press, you’ll have a continuous dewatering process perfect for high volume waste streams such as those that accumulate in municipal wastewater treatment plants.
A typical BFP is made up of two separate belts: a press belt and a filter belt. Sludge passes between the two belts, with the press belt exerting pressure on the filter belt to squeeze the water out of the sludge.
A BFP does away with sludge pickup problems often encountered with other dewatering methods while it only uses up a third of the power requirements of vacuum filters. Also, BFPs may be used to further dewater the sludge cake from vacuum filters.
Plate and Frame Filter Press
This system has the capacity to process all types of sludge and produce very high concentrations of solids. Recent changes in the design of filter presses, including the elimination of leakage problems, improvement of filter media, greater unit capacities and development of high molecular weight polymers and compatible polymer feed systems, have resulted in renewed interest in this method of sludge dewatering.
It involves the use of recessed-chamber filter plates to isolate the liquids from the solids. Two plates are combined to create a chamber to pressurize the sludge and squeeze the filtrate out through the filter cloth lining in the chambers. Filter cake is left on the surface of the filter plate cloths and released onto a horizontal conveyor. It can come up with concentrations as high as 45 to 50 percent with properly conditioned sludges. As a result, filter presses can produce relatively clear filtrate.
One must consider the disadvantage of this system when developing a dewatering management plan. The frame filter press process is not continuous and must be undertaken in batches. Also, cake removal still requires some manual work while large quantities of conditioning agents are necessary for the process to be effective. Noise caused by feed pumps also tend to be very loud.
In coming up with an effective dewatering management plan, plant managers must take into account operating costs, including human resources, equipment, chemicals and energy for each process. Studies must be undertaken on the outcome that each procedure has on sewage treatment, sludge management and ultimate disposal method.
If you are a municipality in Ontario and in need of a biosolids management solution, please feel free to contact us at 1 (877) 479-1388.
Source:
https://www.ontario.ca