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Diatomaceous Earth Filtration

Overview
 
Figure 1. Typical diatomaceous earth vacuum filtration system flow diagram.
Figure 1. Typical diatomaceous earth vacuum filtration system flow diagram.576
Figure 2. Typical diatomaceous earth pressurized filtration system flow diagram.
Figure 2. Typical diatomaceous earth pressurized filtration system flow diagram.576

Diatomaceous earth (DE) is a filtration method in which diatomaceous earth is applied to precoat a mesh screen (called a septum) prior to each filter run. DE consists of a powdered media, made of almost pure silica manufactured from diatoms - the fossilized skeletons (frustules) of fresh water, unicellular algae. DE filters are most suitable to treat waters with low bacterial counts and low turbidities. Coagulant and filter aid may be required to improve filtrate quality. Effluent water quality of DE filtration depends on influent water quality and quality (or grade) of DE used.

DE filtration can be described as a three-step process. First a filter cake or precoat of about 1/8-in. deposited on the septum, a porous material designed to strain-out and support the DE material. Second, filtration begins, while a small amount of DE is feed to maintain the porosity of the precoat. Straining is the primary mechanism of particle removal which occurs largely within the filter cake formed on the septum. Third, when maximum headloss is reached the filter cake is removed from the septa through backwash. Thus, diatomaceous earth filter media is flushed and wasted at the end of each filter run.

There are three main types of DE filters: vacuum, pressure, and horizontal plate filters. All three types operate on the same basic principle of pre-coating the septum before to each filter run. The differences in equipment are due to the location of the filter pump and the disposal methods of spent diatomaceous earth.

Vacuum DE filters are configured such that they are able to contain a large amount of surface area in a proportionally small process unit. As such, vacuum DE filters are typically used for larger, higher flow installations. In vacuum DE filters, the filter vessel consists of an open tank with multiple flat filter elements submerged in the tank. A septum is wrapped around each filter element. Water passes through the septum on both sides of the filter element into the filter element core and is removed from the tank by a filter pump that sucks water from filter elements through a manifold system. The vacuum created by water passing through the filter septa holds the DE on the filter elements.

Pressure DE filters are limited by the size of the pressure vessel used in this configuration and are typically used for smaller installations. The filter consists of an enclosed pressure vessel with flat plates or cylinders connected to a manifold. A pump supplies pressurize raw water to the vessel. The DE is held onto the filter septa by the pressure of the water passing through the each filter element.

Horizontal plate DE filtration is a type of pressure filtration typically used in smaller applications where disposal of wash water is problematic. In this configuration the filter elements are oriented in horizontal plates. Raw water is pumped through one side of the filter plate. When terminal headloss is reached, the raw water supply is turned off and compressed air is blown through the filter plates to force the remaining water from the system. The dewatered filter cake is then removed from the filter plates.

There is no theoretical limitation to what pressure may be applied to a pressurized system. However, most systems are designed for a 25 psi maximum head loss. The total head available for vacuum systems is limited by the suction capacity of the pump (generally, about 18 in of mercury). Typically, flow rate does not affect filtrate quality, however, it is normally limited to approximately up to 3 gpm/sf for pressurized systems and 1 gpm/sf for vacuum systems. Filter runs can last from 1 - 30 days depending on water quality.

In terms of advantages, DE filters typically require lower capital costs and require less space than conventional filters, and they are simple to operate and effective for the removal of cycts, algae and asbestos fibers. Limitations of DE filters include that their operation is more labor intensive than conventional filters and that DE filters are not suitable to deal with wide variations in influent water quality.



576 Washington State Department of Health; 2003; Slow Sand Filtration and Diatomaceous Earth Filtration for Small Water Systems; Slow Sand Filtration and Diatomaceous Earth Filtration for Small Water Systems; DOH PUB. #331-204, pp.50-52, Training and Outreach Section, Division of Drinking Water, Department of Health, Olympia, WA.



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