Section 880. Filtration

A. Rapid rate gravity filters acceptable for the treatment of water from surface water sources or groundwater sources under the direct influence of surface water.

1. Pretreatment is required where rapid rate gravity filters are utilized. Pretreatment shall include but not be limited to disinfection, coagulation, flocculation, and sedimentation.

2. At least two filtering units shall be provided at plants having a rated capacity of more than 100 gpm and less than 2 MGD. The total number of filters necessary at plants having a rated capacity equal to or greater than 2 MGD may be estimated utilizing the following formula:

N = 2.7 (Q)^0.5

(Formula as per Morrell and Wallace from Hardenbergh and Rodie's "WATER SUPPLY AND WASTE DISPOSAL 1960").

Where N equals number of filter units and Q equals plant capacity in million gallons per day.

3. The design rate of filtration shall be two gallons per minute per square foot of filter area.

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

a. The walls within the filter shall be vertical;

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

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

d. There shall be head room to permit normal inspection and operation;

e. The filter shall have a minimum depth of 8½ feet as measured from the normal operating water surface to the bottom of the underdrainage system;

f. A minimum water depth of three feet as measured from the normal operating water surface to the surface of the filter sand;

g. There shall be a water seal on the effluent line to prevent backflow of air to the filters;

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

i. A hand rail shall enclose each filter or filter bank;

j. 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 parameter, the linear velocity in pipes and conduits from settling basins to filters shall not exceed 1.0 foot per second;

k. Influent pipes or conduits where solids loading is heavy, or following lime soda softening, shall be straight and equipped with cleanouts.

l. Washwater drain capacity shall be sufficient to carry the maximum flow;

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

n. The normal operating water surface on a filter shall be at the same hydraulic grade level as the sedimentation basin.

5. Washwater troughs shall be so designed as to provide:

a. Bottom elevation of the trough above the maximum level of expanded media during washing;

b. A top elevation of the trough above the filter surface, not to exceed 30 inches;

c. A two inch freeboard at the maximum rate of wash;

d. A level top or edge;

e. Spacing so that each trough serves the same number of square feet of filter area; and

f. Maximum horizontal travel of suspended particles to reach trough not to exceed three feet.

6. Filter material.

a. Sand--A sieve analysis shall be provided by the design engineer. The media shall be clean silica sand having:

(1) A depth of not less than 27 inches and generally not more than 30 inches after cleaning and scraping; and

(2) An effective size of from 0.35mm to 0.5mm, depending upon the quality of the raw water and a uniformity coefficient not greater than 1.6.

b. Supporting media for the filter sand-A sieve analysis shall be provided by the design engineer. A three-inch layer of torpedo sand shall be used as a supporting media for the filter sand; such torpedo sand shall have:

(1) An effective size of 0.8mm to 2.0mm; and

(2) A uniformity coefficient not greater than 1.7.

c. Anthracite-A sieve analysis shall be provided by the design engineer. Clean crushed anthracite or a combination of sand and anthracite may be considered on the basis of data specific to the project; this media shall have:

(1) An effective size from 0.45mm to 0.8mm; and

(2) A uniformity coefficient not greater than 1.7.

d. Gravel, when used as the supporting media, shall consist of hard, rounded particles and shall not include flat or elongated particles. The coarsest gravel shall be 2 ½ inches in size when the gravel rests directly on the strainer system, and must 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 following size and depth distribution:

	SIZE	DEPTH
	2-1/2 to 1-1/2 inches	5 to 8 inches
	1-1/2 to 3/4 inches	3 to 5 inches
	3/4 to 1/2 inches	3 to 5 inches
	1/2 to 3/16 inches	2 to 3 inches
	3/16 to 3/32 inches	2 to 3 inches

Reduction of gravel depths may be considered upon application to the division where proprietary filter bottoms are proposed.

e. Granular activated carbon - See 12VAC5-590-960 B 6.

7. Porous plate bottoms shall not be used where iron or manganese may clog them or with waters softened by lime. The design of manifold type collection systems shall be such as to:

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

b. Assure even distribution of washwater and an even rate of filtration over the entire area of the filter;

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

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

e. Provide a manifold which has a cross sectional area which is 1 ½ to two times the total area of the laterals.

8. Surface wash facilities are required. Revolving type surface washers shall be provided; however, other types may be considered. All rotary surface wash devices shall be designed with:

a. Provisions for water pressures of 45 to 100 psi;

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

c. Provisions for adequate surface wash water to provide 0.5 to one gallon per minute per square foot of filter area; and

d. Air washing may be considered.

9. The following shall be provided for every filter:

a. A sampling tap shall be placed between the filter and the effluent rate of flow controller and shall be equipped with an auxiliary spigot at the point of connection to the effluent line;

b. Indicating and recording loss of head gauges shall be required on all filters having a capacity of greater than 100 gallons per minute. An indicating loss of head gauge shall be required on all filters having a capacity of 100 gallons per minute or less;

c. Indicating and recording rate of flow gauges shall be required on all filters having a capacity of greater than 100 gallons per minute. An indicating and totalizing water meter may be used in lieu of an indicating and recording gauge on filters having a capacity of 100 gallons per minute or less;

d. Effluent rate of flow controllers of the direct acting, indirect acting, constant rate, or declining rate types shall be required on each filter. All control devices used must 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, at the time of failure;

e. Provisions for draining the filter to waste (rewash) with appropriate measure for backflow prevention are required;

f. Hose bibb, hose, and suitable rack for storage of hoses are required; and

g. Indicating and recording turbidimeters on filter effluent with automatic high turbidity alarm are required at all plants having a capacity of 10 MGD or more.

10. Provisions shall be made for washing filters (backwashing) as follows:

a. A minimum rate of 15 gallons per square foot per minute, consistent with water temperatures and specific gravity of the filter media; a rate of 20 gallons per square foot per minute or more is recommended to provide for adequate expansion of the filter media;

b. Filtered water shall be provided at the required rate by washwater tanks, a washwater pump, from the high service main, or a combination of these;

c. Washwater pumps shall be in duplicate unless an alternate means of obtaining washwater is available;

d. The volume of washwater shall provide for not less than 15 minutes wash of one filter at the design rate of wash;

e. A washwater controller or valve shall be provided on the main washwater line to obtain the desired rate of filter wash with the washwater valves on the individual filters open wide;

f. The rate of flow indicator on the main washwater line shall be located so that it can be easily read by the operator during the washing process; and

g. Where backwash pumps are provided, a means for air release must be provided between the backwash pump and the washwater valve.

11. Miscellaneous:

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

b. Provisions must be made for continuous operation of all other filtering units while one filtering unit is out of operation; and

c. Automatic startup of filtering units is prohibited.

B. High rate gravity filters are acceptable for the treatment of water from surface water sources or groundwater sources under the direct influence of surface water. See 12VAC5-590-890 for design requirements.

C. Slow sand gravity filters are acceptable for the treatment of water from certain surface water sources or certain groundwater sources under the direct influence of surface water.

1. Source restrictions. Raw water quality for application to a slow sand filter without pretreatment shall meet the following requirements:

a. Not exceed a turbidity level of 5 NTU monthly average or 30 NTU peak day over a one year period;

b. Not exceed 800 total coliforms in 80% of a minimum of 50 samples taken over a minimum of a 52 week period;

c. Not exceed an apparent color level of 15 CU monthly average over a one year period; and

d. Groundwater sources under the direct influence of surface water shall pilot test to determine if the water contains sufficient nutrients for slow sand filtration to be a viable option.

2. Pretreatment. Raw waters that cannot meet the criteria listed in 12VAC5-590-880 C 1 a through c shall be treated to that quality prior to application to a slow sand gravity filter.

a. Presedimentation may be an appropriate pretreatment depending on the size and specific gravity of the turbidity particles.

b. Coarse media filtration of either a horizontal or vertical flow configuration may be appropriate for reducing levels of smaller size particles. Normally such roughing filters would be designed to accommodate periodic media removal, cleaning, and replacement.

c. Chemical flocculation and coagulation is normally not appropriate pretreatment for slow sand gravity filters.

d. Preoxidation is normally not appropriate pretreatment for slow sand gravity filters.

3. Number of filters. At least two filters shall be provided. In all cases the filters shall be capable of meeting the design maximum daily water demand with one filter out of service.

4. Filter media. Sand shall be clean silica sand that meets the following criteria:

a. The effective size shall be between 0.15 mm and 0.35 mm.

b. The uniformity coefficient shall not exceed 2.5.

c. The sand depth shall not exceed 55 inches. A minimum depth of 30 inches is required for normal operation.

5. Supporting media. Gravel shall meet the requirements of 12VAC5-590-880 A 6 d.

6. Structural details.

a. Sufficient head room shall be provided for normal movement on the filter by operating personnel for periodic sand removal operations.

b. Adequate manholes and access ports shall be provided for moving sand off and onto the filter.

c. There shall be no common wall between finished water and any lesser quality water.

d. Consideration should be given to providing facilities for dirty sand storage and washing, as well as for clean sand storage.

e. All slow sand filters should be covered.

7. Hydraulic design.

a. Filter to waste shall be provided for all slow sand filters.

b. Water entering the filter shall be distributed in a manner such that the surface of the filter shall not be disturbed in any way.

c. The nominal rate of filtration may range from 45 to 150 gpd/ft² (0.031 to 0.10gpm/ft²) of sand area.

d. The minimum depth of water over the filters shall be three feet. The maximum depth of water over the filters shall not exceed five feet. An overflow capable of handling the maximum flow to the filter shall be provided at the maximum filter water level.

e. Underdrains shall be provided to assure an even rate of filtration across the filter surface. The maximum velocity of water in the lateral underdrains shall be 1.0 ft/sec. The underdrain spacing shall not exceed 12 feet.

f. Each filter shall be capable of being filled with water from the bottom up.

g. Each filter shall be equipped with a loss-of-head guage; a rate-of-flow control device such as an orifice, weir, or butterfly valve; a weir or effluent pipe designed to assure that the water level over the filter never drops below the sand surface; and filtered water sample taps.

8. Performance report. At the conclusion of at least 12 months but no more than 18 months operation of the full scale plant an engineering report shall be submitted to the division that summarizes operating conditions and establishes optimum filter curing time, optimum filter run times, raw and finished water bacteriological and turbidity data, and any other pertinent factors.

D. Diatomaceous earth filtration is essentially a straining process. The use of these filters is acceptable for application to surface waters or groundwaters under the direct influence of surface water with low turbidity and low bacterial contamination, and may be used for iron removal for groundwaters.

1. Source restrictions. Raw water quality for application to a diatomaceous earth filter without pretreatment shall meet the following requirements:

a. Bacteria shall not exceed 50 total coliforms in any sample.

b. Color shall not exceed 15 apparent CU units in any sample.

c. Turbidity shall not exceed 5 NTU in any sample.

2. Pretreatment. If the raw water can be treated to meet the above source restrictions diatomaceous earth filtration may be utilized.

3. Pilot plant study. Installation of a diatomaceous earth filtration system shall be preceded by a pilot plant study on the water to be treated.

a. Conditions of the the study, such as duration, filter rates, head loss accumulation, slurry feed rates, turbidity removal, bacteria removal, and other relative information shall be approved by the division prior to the study.

b. Satisfactory pilot plant results shall be obtained prior to submission of final construction plans and specifications.

c. The pilot plant study shall demonstrate the ability of the system to meet applicable drinking water standards at all times.

4. Types of filters. Pressure or vacuum diatomaceous earth filtration units will be considered for approval.

5. Treated water storage. Treated water storage capacity in excess of normal requirements shall be provided to:

a. Allow operation of the filters at a uniform rate during all conditions of system demand at or below the approved filtration rate, and

b. Guarantee continuity of service during adverse raw water conditions without bypassing the system.

6. Number of units. At least two filtering units shall be provided at plants having a rated capacity of more than 100 gpm.

7. Precoat.

a. Application. A uniform precoat shall be applied hydraulically to each septum by introducing a slurry to the tank influent line and employing a filter-to-waste or recirculation system.

b. Quantity. Diatomaceous earth in the amount of 0.2 lb/ft² of filter area or an amount sufficient to apply a minimum of ⅕ inch coating shall be used with recirculation.

8. Body feed. A body feed system to apply additional amounts of diatomaceous earth slurry during the filter run is required.

a. Quantity. Rate of body feed is dependent on raw water quality and characteristics and must be determined in the pilot plant study.

b. Adequate accessibility to the feed system and slurry lines is required.

c. Continuous mixing of the body feed slurry is required.

d. Consideration should be given to providing a coagulant coating (alum or suitable polymer) of the body feed.

9. Filtration.

a. Rate of filtration. The recommended nominal rate is 1.0 gpm/ft² of filter area and shall not exceed 1.5 gpm/ft². The filtration rate shall be controlled.

b. Head loss. The head loss shall not exceed 30 psi for pressure diatomaceous earth filters, or a vacuum of 15 inches of mercury for a vacuum system.

c. Recirculation. A recirculation or holding pump shall be employed to maintain differential pressure across the filter when the unit is not in operation in order to prevent the filter cake from dropping off the filter elements. A minimum recirculation rate of 0.1 gpm/ft² filter area shall be provided.

d. Septum or filter element. The filter elements shall be structurally capable of withstanding maximum pressure and velocity variations during filtration and backwash cycles, and shall be spaced such that no less than one inch is provided between elements or between any element and a wall. Means shall be provided to check the septum(s) for cleanliness or damage. Consideration should be given to providing septum assemblies where individual septums can be removed, cleaned, repaired, and replaced.

e. Inlet design. The filter influent shall be designed to prevent scour of the diatomaceous each from the filter element.

10. Backwash. Provision shall be made for periodic backwashing of filter. A satisfactory method to thoroughly remove and dispose of spent filter cake shall be provided.

11. Appurtenances. The following shall be provided for every filter:

a. Sampling taps for raw and filtered water,

b. Loss of head or differential pressure gauge,

c. Rate-of-flow indicator, preferable with totalizer,

d. A throttling valve used to reduce rates below normal during adverse raw water conditions.

12. Monitoring. Turbidity monitoring is required for filter effluent. The monitoring may be done by recorder or daily periodic measurements.

E. Direct filtration.

1. General. Direct filtration refers to the filtration of high quality and seasonally consistent raw water without prior sedimentation. Design shall be preceded by a pilot study acceptable to the division. An in-plant demonstration study may be appropriate where a conventional treatment plant is to be converted to direct filtration.

2. Preliminary engineering report. A report shall be prepared and submitted to the division which included the following specific items, in addition to those listed in 12VAC5-590-200 C:

a. Historical summary of meteorological conditions.

b. Historical summary of raw water quality covering a period of at least one year with special reference to fluctuation in quality and possible sources of contamination. The following raw water parameters should be evaluated:

(1) Apparent color

(2) Turbidity

(3) Bacterial concentration

(4) Microscopic biological organisms

(5) Temperature

(6) Total solids

(7) General inorganic and organic chemical characteristics

(8) Additional parameters as required by the division.

c. Description of the pilot plant study methods and work to be done.

3. The pilot plant or in-plant demonstration study shall be conducted over a sufficient time to treat all expected raw water conditions throughout the year. The pilot plant filter shall be of a similar type and operated in the same manner as proposed for full-scale operation. The following items, as a minimum, shall be addressed:

a. Chemical mixing conditions including shear gradients and detention periods.

b. Chemical feed rates.

c. Use of various coagulant and filtration aids including polymers.

d. Flocculation conditions and contact time necessary for optimum filtration for each coagulant proposed.

e. Filtration rates.

f. Filter gradation, types of media, and depth of media.

g. Filter breakthrough conditions and backwash requirements.

4. Final engineering report. A final report including the engineer's design recommendation shall be prepared and submitted prior to the submission of plans and specifications.

5. Treatment facilities.

a. Flash mixing and flocculation. The design shall be based on the results of the pilot plant or in-plant demonstration study and the requirements in 12VAC5-590-870 C and D.

b. Filtration. Filters shall be dualmedia or multimedia gravity filters. The final design shall be based on the results of the pilot plant or in-plant demonstration study and the requirements in 12VAC5-590-890. Turbidity at the sand-coal interface of each filter shall be monitored by indicating and recording equipment.

6. Plant siting. The plant design should allow for the future installation of sedimentation basing.

F. Rapid rate pressure filters.

The use of these filters may be considered for iron and manganese and other clarification processes. Pressure filters shall not be used in the filtration of polluted water, water from surface water sources, groundwater under the direct influence of surface water, or following lime soda softening.

1. Minimum criteria relative to number, rate of filtration, structural details and hydraulics, filter media, etc. provided for rapid rate gravity filters also apply to pressure filters where appropriate.

2. The normal rate of filtration shall be 3 gpm/ft² of filter area.

3. The filters shall be designed to provide:

a. Loss of head gauges on the inlet and outlet pipes of each filter;

b. An easily readable meter or flow indicator on each battery of filters. A flow indicator is recommended for each filtering unit;

c. Filtration and backwashing of each filter individually with an arrangement of piping as simple as possible to accomplish these purposes;

d. Minimum sidewall shell height of five feet. A corresponding reduction in sidewall height is acceptable where proprietary bottoms permit reduction of the gravel depth;

e. The top of the washwater collection trough to be at least 18 inches above the surface of sand;

f. The underdrain system to collect efficiently the filtered water and to distribute the backwash water at a rate not less than 15 gpm/ft² of filter area;

g. Location of flow indicators and controls that is easily readable while operating the control valves;

h. Air release valve on the highest point of each filter;

i. Accessible manhole to facilitate inspections and repairs;

j. Means to observe the wastewater during backwashing; and

k. Construction to prevent cross connection.

Historical Notes

Derived from VR355-18-009.04 § 3.25, eff. August 1, 1991; amended, Volume 09, Issue 17, eff. June 23, 1993.

Statutory Authority

§§ 32.1-12 and 32.1-170 of the Code of Virginia.