Sludge Dewatering
Three Sludge Dewatering Units
The data presented on this page comes primarily from two sources as follows:
1. An excellent textbook entitled "Wastewater Sludge Processing." The complete reference for this textbook is: Turovskiy, Izrail S. and P.K. Mathai. "Wastewater Sludge Processing." Hoboken, New Jersey: John Wiley & Sons, Inc. 2006.
2. Another "set" of excellent textbooks from the Water Environment Federation were also referenced liberally, a three volume set entitled "Operation of Municipal Wastewater Treatment Plants." The complete reference, and I go to these books often, is: Water Environment Federation. “Operation of Municipal Wastewater Treatment Plants.” 5th ed. Manual of Practice—MOP 11. Alexandria, VA: WEF, 1996.
You should have these books in your personal library if you do any work with dewatering applications.
A variety of performance data is tabulated below for belt filter presses, centrifuges, and plate and frame presses. If you click on any table a full page version as a PDF file of that table will open up for better viewing or printing. There is a lot of data and pictures here so you'll need to patiently scroll down the page to find the information most relevant to your needs.
1. Belt Filter Press
I have the most experience operating belt filter presses (BFPs) so I tend to favor these units over the others profiled here. I like that you can make a change in your chemical feed or belt speed and immediately observe the impact of that change by watching the upper, gravity section of the press as well as the cake release at the discharge end of the press. For this reason alone I consider BFPs the easiest dewatering device to operate but many other operators who grew up on centrifuges would say they are the easiest to operate. Those operators also like that the sludge is completely contained within the centrifuge, reducing odors in and around the dewatering operation.
The image below is a well-known detailed drawing of a belt filter press. You'll see this exact drawing in several different textbooks.
2. Solid Bowl Centrifuge
The photograph of the centrifuge above was taken in September 2016 at an industrial plant in the United States midwest. This is a new centrifuge installation using Pieralisi centrifuges (www.pieralisi.com/us/). The installation was beautifully done and the centrifuges looked well-constructed and very rugged.
Struvite Scale
In the Water Environment Federation's Manual of Practice 8, "Design of Municipal Wastewater Treatment Plants," Fourth edition, the following statements are made:
For anaerobically digested sludge applications, struvite (magnesium ammonium phosphate scale) can build up in the centrate line; therefore, the design should provide for ferric chloride addition. The ferric chloride binds the phosphate ion to prevent struvite buildup.
The table below is a compilation from three tables shown in the WEF Operation of Municipal Wastewater Treatment Plants handbook, Volume 3, Chapter 33: Dewatering.
3. Plate and Frame Filter Press
Plate and frame filter presses (pressure filters) are often operated using ferric chloride and lime to condition the sludge for pressing. The ferric chloride would be added to the sludge at a dosage rate of 40 to 125 pounds per dry ton of feed solids. The dosage rate for lime can vary between 150 to 550 pounds per dry ton of feed solids.
The very large filter presses shown in the photo below are manufactured by JWI, now part of Evoqua, with a capacity of 243 cubic feet (6.9 cubic meters) per press.
This was an industrial site that used a 3% solution of Perlite as body feed to precoat the filter plates to improve cake release from the presses.
As you can see in the image below, the addition of Perlite slurry to precondition the presses before feeding chemically conditioned sludge resulted in excellent and complete release of the sludge cake from the filter plates.
The observant reader will notice the small size of the cake being held in the photo above. That is because this industrial site was well equipped, including a small test filter press to evaluate chemical dosage scenarios before operating the full size filter presses. The small test filter press can be seen in the photo below, to the right in the picture. This press proved to be extremely valuable in allowing us to experiment with many different chemical combinations before settling on our final chemical conditioning approach for the full-scale operation.
On a regular basis we would go back to this test press to recheck our formulations because the sludge being processed was being dredged from several ponds with different consistencies. So we would have to vary our chemical program in response to changes in the sludge feed.
Just one more photo, a close-up of the portable test filter press we should to such great advantage while processing highly variable sludge being dredged from several ponds.
