FILTRATION:

PROCESS: There are several different types of filter processes that may be employed in potable water treatment. They are Diatomaceous Earth (DE) Filtration, Gravity Filtration, Pressure Filtration, and Slow Sand Filtration.

Diatomaceous Earth (DE) filtration uses a vessel with plates or internal septums that are precoated with diatomaceous earth (DE) under gravity pressure, pump pressure, or vacuum. Filter to waste until the precoat is established, then raw water introduced. Body feed DE to extend filter run. 1.0 to 1.5 gal/min/ft2. The plate type shown is a GNT Filter Co. filter. They are ideal for small water systems, with finsihed water turbidities less than 0.2 NTUs 95% of the time. They have NO backwash water.... the expendable filter media, the incorporated de, and the captured particles are removed all at once. The new media is placed in the plates, and operation begins again. (Contact GNT at (707) 426-0926)

DE filters may be of a plate type [the above picture of the red filter], or a septum (tube of very fine mesh screen) type (the picture of the blue filters to the left). Each of these blue DE septum filters shown, has 745 septums, and has to be backwashed with filtered water.

Gravity filtration: single (sand), dual (anthracite and sand), and multi-media (anthracite coal, sand, garnet, pea gravel) with 2 to 10 gal/min/sq ft hydraulic loadings. May be limited to 2 to 3 gal/min/ft2 or less by DOHS, etc.

Pressure filtration: 2 to 3 gal/min/sqft in a pressure tank, usually smaller package plants. Not as reliable as gravity filter system.

Slow sand filtration: limited use in this country. Very low loading rates: 0.015 to 0.15 gal/min/sqft. with biological action, adsorption, and straining on top sand layer. Large amounts of land required.

FILTRATION PLANT TERMS: there are two terms that one should be aware of, the first is "Conventional Filtration" and the other is "Direct Filtration."

Conventional filtration plant: coagulation, flocculation, and sedimentation preceding filtration process. Direct filtration plant: no sedimentation process, coagulation, flocculation, then direct to filtration due to lower turbidities and suspended particles in the source (raw) water supply.

Filter Media Classification

The filter media is classified by it's effective size (refers to the size of the sieve opening which allows passage of 10% of the media, and holds 90% on the media); specific gravity ( what its mass is in ratio to water); hardness (strength); uniformity coefficient (ratio of 60% of the particle diameters and 10% of the media weight <smaller number means particles are closer to the same size>)

OPERATION:
Turbidity is reduced, suspended matter is reduced; affected by raw water quality, type and number of particles to be removed; filter type; upstream process type and efficiency; filter operation. Proper chemical dosage and upstream process control minimizes filtration problems and cost to operate. Desirable to have some particle penetration into the filter media, but not break through; desire "bed loading" to maximize filter run length. Dual media: desire to have solids in anthracite coal layer (on top) with sand (lower layer) to polish out finer particles and prevent break through. As filter loads up with pounds of particles, increases "head loss"; (caused by resistance to water flowing through the filter voids that are filling with turbidity causing particles). We want to backwash filter: just before break through (filter clogged up and turbidity broke through) and to coincide with filter head loss time, to maximize best operational cost and design. Backwash to clean filter, with 10 to 30 gpm/sqft; using 2 to 4 % of filtered water to do so. Polymer dosages to filter range from 0.02 to 0.2 mg/L for nonionic polymers; 0.2 to 2 mg/l for cationic polymers.

INTERACTION:
Must control upstream processes: coagulation, flocculation, and sedimentation (if conventional plant). Must control filter operational parameters of coagulant, filtration rate, rate of head loss, filter run time, filter effluent turbidity, proper backwash. Do not want larger particles to hit filters as they coat the filter surface and cause short filter runs and can also breakup and increase filter effluent turbidity. The downstream disinfection process is not capable of removing particles! This is the last physical process to remove particles and turbidity, must make it count!

INTERACTION:
Must control upstream processes: primary sedimentation, the secondary treatment process; any upstream coagulation, flocculation, step prior to filtration. Must control filter operational parameters of coagulant dosage and flocculation, filtration rate, rate of head loss, filter run time, filter effluent turbidity (suspended solids), & proper backwash. Do not want larger particles to hit filters as they coat the filter surface and cause short filter runs and can also breakup and increase filter effluent turbidity. The downstream disinfection process is better served not having to disinfect large/numerous particles! Filter backwash water is best when the backwash water and the solids it is carrying, are not "shock loaded" hydraulically or solids loading rate, back to the plant headworks for treatment.

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