Filter Underdrain Homemade Laterals

Rate of backwash =

minutes for water in filter to rise 1 ft

Lower water level to sand, slowly open backwash valve, 20 to 25 gpm/ft2 minimum (32 to 40 in. rise/min).

Orifice area = 0.25 to 0.30% of filter area

Lateral area = 2 x Orifice area

Manifold area = 1.5 to 2 x Total area of laterals

The rate controller is omitted in a declining-rate filter. A gate valve or orifice is used in its place. An acceptable rate of flow (filtration) is set at the start of a run but decreases as the head loss increases. The use of this concept is recommended in developing countries and other areas where skilled operators are not generally available.52 See also "Slow Sand Filter" (filtration rate control) in this chapter and Table 2.2.

In a combination anthracite over sand bed filter (dual media), use is made of the known specific gravity of crushed anthracite of 1.35 to 1.75 and the specific gravity of sand of 2.5 to 2.65. The relative weight of sand in water is three times that of anthracite. The anthracite effective size is 0.9 to 1.1 and uniformity coefficient 1.6 to 1.8. The sand effective size is 0.45 to 0.55 and uniformity coefficient 1.5 to 1.7 (see Figure 2.4). Anthracite grains can be twice as large as sand grains; after backwashing, the sand will settle in place before the anthracite in two separate layers.53 High-rate sand-anthracite filters require careful operating attention and usually use of a filter conditioner to prevent floc passing through while at the same time obtaining a more uniform distribution of suspended solids throughout the media depth. Longer filter runs, such as two to three times the conventional filter, at a rate of 4 to 6 gpm/ft2 and up to 8 or 10 gpm/ft2 and less washwater are reported. A mixed or multimedia may consist of anthracite on top, effective size 0.95 to 1.0 mm and uniformity coefficient 1.55 to 1.75; silica sand in the middle, effective size 0.45 to 0.55 and uniformity coefficient 1.5 to 1.65; and garnet sand* on the bottom, effective size 0.2 to 0.35 mm and uniformity coefficient 1.6 to 2.0. Total media depth is 24 to 48 in. Under the same conditions, filter runs will increase with dual media and increase further with mixed media over single-media sand. The turbidity of the effluent can be expected to be less than 0.5 NTU in a well-operated plant. Granular activated-carbon media (GAC) in place of sand and anthracite can function both as an adsorbent of organics and as a filter medium, but backwash rates will have to be reduced and carefully controlled. If the organics are synthetic chemicals, frequent regeneration will be required. If the organics are taste- and odor-producing compounds, activated carbon may remain effective for several years. The GAC will dechlorinate water that has been chlorinated.

Treatment of the raw water by coagulation, flocculation, and settling to remove as much as possible of the pollution is usually necessary and an important preliminary step in the rapid sand filtration of water. Without coagulation, virus removal is very low. The settled water, in passing to the filter, carries with it some flocculated suspended solids, color, and microorganisms. This material forms a mat on top of the sand that aids greatly, together with adsorption on the

* Garnet sand has a specific gravity of 4.0-4.2.

Declining Rate Filtration
FIGURE 2.4 Essential parts of a rapid sand filter. The minimum total depth is 8.5 ft, 12 ft preferred.

bed granular material, in the straining and removal of other suspended matter, color, and microorganisms, but this also causes rapid clogging of the sand. Special arrangement is therefore made in the design for washing the filter (every 48 to 72 hours, depending on the head loss) by forcing water backward up through the filter at a rate that will provide a sand expansion of 40 to 50 percent based on the backwash rate, water temperature, and sand effective size. For example, with a 0.4-mm-effective-size sand, a 40 percent sand expansion requires a washwater rate rise of 21 in./min with 32°F (0°C) water and a rise of 32.5 inches with water at 70°F (21°C).54 The dirty water is carried off to waste by troughs built in above the sand bed 5 to 6 feet apart. Separate air laterals in the underdrain system can increase the backwash efficiency. A system of water jets or rakes or a 1.5- to 2-in. pressure line at 45 to 75 psi with a hose connection (including vacuum breaker) should be provided to scour the surface of the sand to assist in loosening and removing the material adhering to it, in the pores of the sand on the surface, and in the filter depth. Air scour or wash is also very effective for cleaning the entire bed depth, especially in beds 4 to 6 feet deep. A backwash rate of 10 gpm/ft2 may be acceptable with an anthracite or granular activated-carbon bed. Effective washing of the sand is essential. Backwash should start when the turbidity begins to rise above 0.1 to 0.2 NTU, not after the turbidity reaches 1.0 NTU. Before being placed back into service the filter effluent should be sent to waste for 5

to 20 minutes to reestablish filtration efficiency. Aluminum in the filtered water should be less than 0.15 mg/l. Filters that have been out of service should be backwashed before being returned to service. The filter sand should be inspected periodically. Its condition can be observed by lowering the water level below the sand and looking for mounds and craters, debris on the surface, or cracks on the surface or along the walls. Depth samples of the sand may also be taken for laboratory observation. During the washing operation the samples should be inspected for uneven turbulence and the presence of small lumps known as mud balls. Any of these conditions requires further investigation and correction. A conventional rapid sand filter plant flow diagram and unit processes are shown in Figure 2.5.

When properly operated, a filtration plant, including coagulation, flocculation, and settling, can be expected to remove 90 to 99.9 percent of the bacteria, protozoa, and viruses,55 a great deal of the odor and color, and practically all of the suspended solids. Adequate pretreatment is essential. Nevertheless, chlorination must be used to ensure that the water leaving the plant is safe to drink. Construction of a rapid sand filter should not be attempted unless it is designed and supervised by a competent environmental engineer. Pilot plant studies, including preliminary treatment for heavily polluted water, may be required to ensure the proposed treatment will produce a water meeting drinking water standards at all times. Adequate coagulation, flocculation, and settling, in addition to granular media filtration and disinfection, are necessary to ensure the removal of bacteria, protozoa (Giardia and E. histolytica cyst), and viruses. Improper pH control can result in weak floc and passage of dissolved coagulant.

A flow diagram of a typical treatment plant is shown in Figure 2.6.

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