Article taken from Service Industry New July edition
Diatomaceous earth is the fossilized skeletal remains of marine life know as diatoms.
Diatoms are a type of algae known as phytoplankton with porous silica cell walls that makes them useful in industrial applications. Diatomaceous earth is used extensively as filters, fillers, insulation and abrasives.
The pool industry began incorporating D.E as a filter medium in the 1940's because its minute porous structure enable extremely fine filtration. In fact, today, it is among the best types of filter media, capable of filtering particles as small a 3 microns.
In a D.E. filter, the filter media is held against a support such as a cloth-covered grid by the movement of water.
The filter is designed such that a rigid structure, called a grid, holds and supports a flexible septum, which can be composed of variety of materials.
In most cases, the septum, which can be composed of a variety of materials. In most cases, the septa made of woven polyester cloths, but they can also be made from porous stone, or stainless steel hardware cloth.
When the D.E is added, the septa acts as a screen to hold it in place. D.E produces fantastic water clarity. However, the main disadvantage is that the medium is temporary. This mean that it must be replaced, or recharged, frequently D.E filters must be recharged after every backwashing or cleaning. Another disadvantage lies in how to dispose of the used and dirty D.E. The U.S Environmental Protection Agency prohibits releasing spent D.E into a drinking water source or into storm drains.
It is recommended that backwashed water be sent into a separation tank. The tank has a bag that collects the D.E from the water, which can then be disposed of according to local regulations.
Finally, care must be taken when handling D.E, which may pose a further disadvantage, While it might seem like a harmless substance, D.E is very dangerous if inhaled because it is composed of microscopically sharp silica. A protective mask should be worn to avoid inhaling.
D.E filters at a rate of 2 gallons per minute per square foot. There are three main types of D.E filtration systems and these include the pressure D.E filter, which uses a closed pressurized tank; the regenerative D.E filter, which is also a pressure system but uses an interrupt filter step to redistribute partially soiled D.E; and the vacuum D.E filter, which has an open tank that is usually fed water by gravity.
The pressure D.E filter is comprised of a closed pressure tank containing the septa and grids and is places after the pump in the water path. In this arrangement, the water is forced through the tank by the outward pump pressure.
Water generally comes in from the bottom of the tank and forces the D.E up to the grid. As the water passes through the septa, any suspended debris is collected by the D.E These types of filters are cleaned by backwashing, which results in D.E loss, that must be replaced.
The time to backwash is determined by a pressure increase on the influent (incoming) gauge of 8-to-10 psi over the start-up reading. When recoating the grid elements, the new coating of D.E should be applied slowly and evenly. A slurry of D.E and water is fed into the filter, and care should be taken when mixing the D.E to avoid pulverizing it and there fore reducing its efficacy.
A regenerative DE filter is also a pressure filter in which water id forced into the filter by the pump. It differs from a standard D.E filter in that it uses an interrupt filtration step called "bumping" in which the pump stops briefly, allowing the D.E filter to drift away from the septa, presenting a different, cleaner surface to the water.
Bumping the D.E preserves the life of the filter and associated D.E because it recycles the D.E in the tank, extending the filter cycles before backwash becomes necessary. Bumping can regenerate the D.E to near original flow rates. The filters can be bumped multiple times prior to back washing. By using regenerative filters, backwashing is not necessary nearly as often, which obviously saves a lot of time, water and D.E.
While regenerative D.E filters automatically bump, other types of D.E filters can be manually bumped. Here, we comply close the valves to and from the pool, and turn off the pump for 3 minutes or so. Gravity will cause the D.E to fall away from the septa, but when the pump is turned back on, it will recoat the elements with new surface, thus maximizing the use of the same D.E
In a vacuum D.E filterm the filter is in an open tank that uses gravity to feed water to the filter. The tank will contain at least one spetum with an element to hold the D.E. Because the pump is located after the filter, suction pull the water through the D.E- coated septa.
Unlike pressure D.E filters, these are no influent and effluent gauges. Instead, a vacuum gauge is installed between the pump and the filter and as the media collects pollutants from the water, there is an increase in vacuum on the pump side. The reading on the vacuum gauge, in inches of mercury, gets higher as the filter gets dirty. Eventually, when the suction becomes too great (around 17 inches of mercury), alarms and switches are used for safe shutdown.
Gravity vacuum D.E filtration has become common in the design of exceptionally large pools. Here, the pool has a below deck, gravity-fed surge pit, with a very large D.E element suspended inside. Maintenance of these systems is facilitated by walkways inside the surge pit so that service technicians can observe the entire operation of the filter.
While the excavation required is costly, a configuration such as this is easy to inspect, and requires infrequent cleaning. Incases where the filter is higher than the pool, a lift pump is used to pull water to the filter. This configuration has the obvious disadvantage that the two pumps are now required to maintain the pool: both a lift pump as well as a circulation pump, doubling the electrical costs, and adding to future maintenance concerns. This would be an installation option when excavation of a surge pit is impossible.
One of the main differences between vacuum filter, and pressure filters is in maintenance. Technicians do not backwash a vacuum filter, as the water flow is not reversed to clean the septa. Instead, the tank must be drained, and the dirty D.E washed away. This can be done with a hose and some systems are equipped with water jets to a completed this.
And as with all D.E filters the drained water associated D.E must be disposed of according to local regulations.
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