D.E. acts as a microscreen that strains out insoluble particles.
Slurry, or body, feeding takes place when additional D.E. is fed during the filtering process to keep the cake porous. After a period of time when the filter cake becomes clogged with dirt, it must be removed and replaced with new D.E. The principle of surface filtration is involved in the typical D.E. filter. Nearly all of the solids are trapped before they have an opportunity to penetrate the surface of the filter medium. Thus, if the dirt load is heavy, the filter will become impervious in a short time. As a means of overcoming the problem, body or slurry feeding is used to “dilute” the dirt. This process involves continuously feeding diatomaceous earth, mixed with water, into the filter influent flow in a concentration equal to or greater than the concentration of dirt being removed.
An acceptable flow rate through a D.E. filter is 1.25 GPM/ft2 to 1.5 GPM/ft2. Using low flow rates greatly extends filter runs, thus helping reduce maintenance costs.
The elements, called septums, in D.E. filters can be discs, flat leaves, curved plates or cylinders.
Cleaning methods for D.E. filters range from hand-rinsing the elements with a hose to pressure backwashing and jet spray cleaning.
Pressure diatomaceous earth systems have a lower backwash water volume than a pressurized sand backwash. But there is not a significant advantage over a pressure sand system with the exception that D.E. filters can produce a slightly clearer (polished) water.
Vacuum diatomaceous earth filtration with 1-1.5 GPM/ft2 of filter area is an option to consider. The backwash discharge from the open top filter tank is by gravity and the filter elements are cleaned by water jet sprays and/or by manual labor. As a result, only a little more than the volume of the filter tank including the spent D.E. needs to be discharged via the sanitary sewer system.
Many jurisdictional authorities require a reclamation tank between the D.E. filter tank and the backwash outfall so that the spent D.E. is captured and not discharged into the sanitary sewer. The captured D.E. is then hauled to an approved dump site.
The one requirement with a vacuum diatomaceous earth system is that the top of the tank must be slightly above the water level of the pool. The pumps and motors must be below water level for a flooded suction situation. The lower tank can also serve as the surge tank.
Increase consideration should be given to D.E. on heavy demand pools, both indoor year round and outdoor seasonal pools (Pirate’s Cove).
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