12.2 Diatomaceous Earth Filtration (Subclass 3)

How the Process Works

Diatomaceous earth (DE) filtration, also called precoat filtration, is a physical straining process. Diatomite is the fossilized silica skeleton of diatoms, milled into grades of controlled permeability. The DEP definition of Subclassification 3 is precise: a precoat cake of diatomaceous earth filter media is deposited on a support membrane (the septum), and while the water is filtered by passing through the cake, additional filter media known as body feed is continuously added to the feed water to maintain the permeability of the filter cake.

The three components:

Precoat: Getting the Filter Into Service

1. Fill the vessel and establish recirculation from the filter back to the precoat tank.
2. Slurry the precoat DE and feed it into the recirculating stream. DEP's module specifies a precoat feed of 0.2 pounds per square foot of filter area, or enough to form a cake 1/8 to 1/5 inch thick.
3. Recirculate until the returning water meets finished-water goals. During recirculation the media bridges across the septum openings; once bridging starts, later particles are caught behind the bridges and the cake builds itself.
4. Switch slowly to service, opening the influent while maintaining flow so the cake is never unloaded.
5. Start body feed at the same instant service flow begins.

If the operator goes to service before the recirculated water clears, uncoated septum area passes raw water directly into the finished water. This is the most common cause of a DE plant turbidity failure.

Body Feed: The Run-Length Control

The captured solids from raw water are compressible. If they build up alone, the cake seals and the pressure differential climbs almost immediately. Body feed continuously mixes fresh, rigid diatomite into the accumulating solids so that the cake stays porous.

• DEP's published range is 1 to 10 mg/L of body feed media per 1 mg/L of suspended solids in the raw water, with pilot testing used to fix the number for a particular source.
• Too little body feed produces a pressure differential that rises steeply and early. The run ends in hours.
• Too much body feed wastes chemical and fills the available cake volume in the vessel, also shortening the run and increasing waste volume.
• A well-tuned plant shows a gradual, nearly linear pressure rise across the full run.

Generally, raising the body feed concentration increases the volume of filtrate produced before terminal headloss is reached, so the body feed cost is traded against precoat media cost and the labor of more frequent cleanings.

The filter is removed from service at terminal headloss. Pennsylvania's maximum allowable headloss is 30 psi for a pressure filter and 15 inches of mercury for a vacuum filter.

Media, Rate Control and Cake Removal

Two media are used. Diatomite is almost pure silica; perlite is derived from glassy volcanic rock. Finer grades capture smaller particles but produce a higher pressure drop. Wet DE has a specific gravity of about 2.3, while perlite is 32 to 70 percent less dense, and that lower density makes perlite less likely to migrate to the bottom of the septum during a run. Either medium can be bought pre-coated with an aluminum or iron coagulant or a cationic polymer, which studies show improves removal of small particles, turbidity, bacteria and viruses. Slurried media is stored at 12 percent concentration or less, kept continuously mixed, with mixers turning 40 to 60 rpm — fast enough to prevent settling, slow enough not to grind the media down.

Filtration rate is the other lever:

Rate changes must be made slowly and smoothly, using an effluent-side rate-of-flow control valve or a variable frequency drive. When system demand falls below the minimum, filtered water is recycled through the filter to hold flow above 0.1 gpm/sf rather than letting the unit idle.

Spent cake is removed by sluicing with high-pressure sprays that must cover the entire septum surface, by flow reversal with clean water, or by pressurizing the vessel with air and suddenly releasing it to create a high-velocity reverse flow. Incomplete removal is a recurring troubleshooting item: leftover media produces lumps and bare spots at the next precoat and an abnormally high initial headloss, and media left in the vessel resuspends and fouls the septum over successive cycles. Up to 90 percent of the media can be recovered, but recovered media may never be used for precoat because pathogens may remain in it; it must be disinfected before reuse as body feed.

The Continuous Flow Rule

The cake is held on the septum only by the differential pressure of flowing water. If flow stops or reverses even briefly, the cake falls off the septum. Recovery requires a complete shutdown, cleanout and fresh precoat, and any water produced during a momentary interruption is suspect. Practical implications:

• Filters are brought on and off line slowly using rate-of-flow controllers, never below 0.1 gpm/sf.
• Standby power or a controlled shutdown sequence protects against pump trips.
• Multiple smaller units are preferred over one large unit so that flow can be shifted without stopping.

Pennsylvania Performance Standards

DE filtration is one of the acceptable technologies under Pennsylvania's Surface Water Filtration Rule, but it is held to a different turbidity number than the coagulation-based processes:

Published testing puts measured Giardia removal by a properly operated DE filter between 2-log and 4-log, and DEP's LT2 rules may require additional Cryptosporidium treatment depending on source water occurrence.

Safety: Silica, Dust and Pressure

Media handling is a genuine hazard rather than a footnote. Crystalline silica, present in diatomaceous earth and in lesser amounts in perlite, is a probable carcinogen when inhaled, and the media is abrasive enough to cause irritation or injury on contact. Install dust collectors wherever dry media is loaded into hoppers or tanks, add media to moving water when making slurry to keep dust down, and require goggles, gloves and a respirator for anyone handling media, with an eyewash station nearby. On the equipment side, pressure vessels must carry a pressure relief device, and rupture disks need drain piping sized for a shattered disk and the initial pressure surge.

Where DE Filtration Fits in Pennsylvania

Because DE removes particles but not dissolved constituents, a Pennsylvania system using DE on a source with elevated total organic carbon still has to solve Stage 2 disinfection byproduct compliance separately, usually by moving the disinfectant application point or by changing disinfectant.