Virginia Law

A. At least two gravity filter units shall be provided in conventional filtration treatment plants and direct filtration treatment plants.

B. Filter loading rates shall not exceed 2.0 gpm/ft² of filter area for rapid rate filters and shall not exceed 4.0 gpm/ft² for high rate filters, during normal operation. Alternative loading rates may be approved by the department when effective filtration is demonstrated.

C. The filter structure shall be so designed as to comply with the following:

1. The walls within the filter shall be vertical;

2. The filter walls shall not protrude into the filter media;

3. There shall be no common wall between filtered or finished water and any lesser quality water;

4. The filter shall be covered by a superstructure if determined necessary under local climatic conditions;

5. There shall be head room to allow normal inspection and operation;

6. A curb at least four inches high shall surround each filter to prevent floor drainage into the filter;

7. The maximum velocity gradient of treated water in pipes and conduits to the filters shall not exceed that used in flocculation. Where velocity gradient is not used as a design parameter, the linear velocity in pipes and conduits from settling basins to filters shall not exceed 1.0 ft/sec;

8. Influent pipes or conduits, where solids loading is heavy, shall be straight and equipped with cleanouts;

9. Backwash water drain capacity shall be sufficient to carry the maximum flow;

10. Access in the form of walkways not less than 24 inches in width shall be provided to each filter; and

11. The normal operating water surface on a filter shall be at the same hydraulic grade level as the sedimentation basin, if no intermediate treatment process is provided.

D. Backwash water troughs shall be so designed as to provide:

1. Bottom elevation of the trough above the maximum level of expanded media during backwashing;

2. At least a two-inch freeboard inside the trough at the maximum rate of wash;

3. A level top or edge;

4. Spacing so that each trough serves an equal area of each filter; and

5. Maximum horizontal travel of suspended particles to reach the trough not to exceed 3.0 ft.

E. Filter media shall be free from detrimental chemical or bacterial contaminants. Acceptable filter media shall include anthracite coal, silica sand, garnet sand, and GAC. Other natural or synthetic media may be approved by the department when pilot-scale or full-scale demonstration studies demonstrate that the media is capable of meeting the filter effluent turbidity treatment technique requirements in Part II (12VAC5-590-395) of this chapter.

1. Filters may be of single media, dual media, or multimedia design depending upon the water to be treated and the specific filtration process employed. A total media depth of not less than 27 inches shall be provided after cleaning and scraping.

2. Types of filter media:

a. Anthracite coal. A sieve analysis shall be provided. Anthracite media shall have:

(1) An effective size from 0.45 to 0.55 mm with a uniformity coefficient not greater than 1.65 when used alone.

(2) An effective size from 0.8 to 1.2 mm with a uniformity coefficient not greater than 1.85 when used in dual or multimedia filters.

b. Silica sand. A sieve analysis shall be provided. The media shall be clean silica sand having an effective size from 0.35 to 0.55 mm and a uniformity coefficient not greater than 1.65.

c. Garnet sand. A sieve analysis shall be provided. The media shall have an effective size from 0.15 to 0.35 mm.

d. Granular activated carbon (GAC) may be used as a media for filtration. The department may require pilot studies where precursor or organics removal is a treatment objective. The design shall include the following:

(1) GAC media shall meet the basic specifications for filter media contained in this section, except the uniformity coefficient shall not be greater than 2.0. The department may allow larger size media based upon pilot-scale or full-scale demonstration testing. The department may require that a layer of sand media be placed below the GAC.

(2) Provisions shall be made for periodic treatment of GAC filter material for the control of bacteria and other growths.

(3) Provisions shall be made for GAC media replacement or regeneration.

(4) Only materials suitable for use with GAC media filters shall be utilized.

F. Support media.

1. Sand. A sieve analysis shall be provided. A three-inch layer of sand shall be used as a supporting media for the filter media where supporting gravel is used and shall have an effective size from 0.8 to 2.0 mm and a uniformity coefficient not greater than 1.7.

2. Gravel. When used as the supporting media, gravel shall consist of hard, rounded particles and shall not include flat or elongated particles. The coarsest gravel shall be 2-1/2 inches in size when the gravel rests directly on the strainer system and shall extend above the top of the perforated laterals or strainer nozzles. Not less than four layers of gravel shall be provided in accordance with the size and depth distribution specified in Table 874.1.

3. Changes of gravel depths and sizes may be considered by the department where proprietary filter bottoms are proposed.

G. Filter bottoms and strainer systems. The department may allow deviations from requirements of this subdivision for high rate filters and for proprietary filter bottoms. Porous plate bottoms shall not be used where iron, manganese, or hard water may result in clogging. The design of manifold-type collection systems shall:

1. Minimize loss of head in the manifold and laterals;

2. Assure even distribution of backwash water and an even rate of filtration over the entire area of the filter;

3. Provide a ratio of the area of the final openings of the strainer systems to the area of the filter of about 0.003;

4. Provide a total cross-sectional area of the laterals at about twice the total area at the final openings; and

5. Provide a manifold that has a cross-sectional area which is 1-1/2 to two times the total area of the laterals.

H. Surface wash or air scouring of filters shall be provided.

1. All rotary surface wash devices shall be designed with:

a. Provisions for water pressures of at least 45 psig;

b. A vacuum breaker or other device or assembly to prevent backsiphonage; and

c. Adequate surface wash water to provide 0.5 - 1.0 gpm/ft² of filter area.

2. Air scouring shall provide for:

a. An air flow rate of three to five scfm/ft² of filter area when air is introduced in the underdrain. A lower air flow rate shall be used when the air scour distribution system is placed above the underdrain.

b. A method for avoiding loss of filter media during backwashing.

c. A fluidization backwash following air scour sufficient to restratify the filter media. The backwash water delivery system shall be in accordance with this section except the rate of flow should not exceed 8.0 gpm/ft² unless operating experience demonstrates that a higher rate is necessary to remove scoured particles from the filter media.

I. Turbidity monitoring.

1. Indicating and recording turbidimeters meeting the requirements of 12VAC5-590-770 B shall be provided for:

a. The source water;

b. The settled water from each sedimentation basin;

c. The filter effluent from each filter; and

d. The CFE.

2. Finished water indicating and recording turbidimeters shall be considered if chemical pH adjustment occurs following filtration.

3. The location of the turbidity sample tap shall allow turbidity to be monitored for both the filtered water and the filter-to-waste water.

4. The design may incorporate an operator selected filter effluent high turbidity alarm.

J. Appurtenances.

1. A sampling tap shall be placed between each filter and the effluent rate-of-flow controller to sample filtered water and filter-to-waste water. The location of sample taps shall allow turbidity to be monitored of both the filtered water and the filter-to-waste water.

2. Indicating and recording loss-of-head gauges shall be provided on all filters having a capacity of greater than 100 gpm. An indicating loss-of-head gauge shall be provided on all filters having a capacity of 100 gpm or less.

3. Indicating and recording rate-of-flow gauges shall be provided on all filters having a capacity of greater than 100 gpm. An indicating and totalizing water meter may be used instead of an indicating and recording gauge on filters having a capacity of 100 gpm or less.

4. Effluent rate-of-flow controllers of the direct acting, indirect acting, or constant rate types shall be provided on each filter.

a. All control devices used shall incorporate an auxiliary shutoff valve in the filter effluent line. Indirect and direct acting effluent rate-of-flow control devices shall start operation from the closed position. Failure of indirect acting controllers shall not result in any increase in the rate of flow.

b. Filter effluent rate-of-flow control that simply maintains a constant water level on the filter is prohibited.

c. Control devices shall be configured to prevent exceeding the design filter hydraulic loading rate when any filter is taken out of service.

5. Provisions for draining the filter-to-waste (rewash) with appropriate backflow prevention and rate control shall be provided on each filter. The filter-to-waste design flow rate shall be equal to the filtration rate.

6. A high pressure hose and hose rack shall be provided to allow washing down filter walls.

K. Backwash provisions.

1. Filtered or finished water shall be applied uniformly across the filter in an upflow direction to provide at least 50% media expansion during all operating conditions. This will normally require backwash flow rates of up to 20 gpm/ft² depending on media size, media specific gravity, uniformity coefficient, and water temperature.

2. The backwash water shall be provided at the required rate by backwash pumps, backwash water tanks, the high service main, or a combination of these methods. Consideration should be given to including provisions to obtain backwash water from the distribution system or other sources and to supply backwash water during plant start-up or during catastrophic events.

3. At least two backwash water pumps shall be installed unless an alternate means of obtaining backwash water is available.

4. The volume of backwash water provided shall be sufficient to backwash one filter at the design backwash flow rate and duration during the warmest water temperature. This backwash water volume shall be in addition to any other water storage requirements.

5. A backwash water controller or valve shall be provided on the main backwash water supply line to obtain the desired rate of filter wash with the backwash water valves on the individual filters open wide.

6. Consideration shall be given to provide for seasonal adjustments of the backwash flow rate to ensure proper backwashing while preventing media loss and to conserve water.

7. The rate-of-flow indicator on the main backwash water supply line shall be located so that it may easily be read by the operator during the backwashing process.

8. Where backwash water pumps are provided, a means for air release shall be installed between the backwash water pump and the backwash water valve.

L. Other design considerations.

1. Roof drains shall not discharge into the filter or basins and conduits preceding the filters.

2. Provision shall be made for continuous operation of all other filtering units while one filtering unit is out of operation.

3. High rate filtration shall be provided with precise coagulation control. A multiple six-gang stirring machine for performing jar tests shall be provided in addition to one or more of the following means of controlling the coagulation process:

a. Zeta potential, as measured by microelectrophoresis.

b. Pilot filters. Where dual pilot filters are used, two units shall be provided. Each pilot filter shall consist of a small filter (about six inches in diameter) containing the same type and depth of media as the plant filters. The pilot filter shall be equipped with recording turbidimeters on the effluent to measure the filterability of the water as reflected by turbidity monitoring.

c. Streaming current monitor, defined as a continuous sampling instrument that measures the electric current generated when water flows past suspended particles contained in the water.

4. High rate filtration shall be provided with indicating and recording pH monitoring equipment for:

a. The source water;

b. The rapid mix effluent; and

c. The finished water leaving the treatment plant.