Section 860. Filtration

A. Conventional design standards have been established for effluent filtration following unit operations for equalization, coagulation and chemical clarification. For conventional design, an equivalent level of pretreatment shall be provided. Filtration for other wastewater reuse alternatives and the design for nutrient removal will be evaluated by the department based on an evaluation of performance data. The owner shall accompany a proposal for nonconventional filtration design with appropriate pilot plant data or full scale unit operations data demonstrating acceptable treatment of similar wastewater. The average BOD₅ and suspended solids concentrations applied to the filter should not exceed twice the required values of filtrate BOD₅ and suspended solids concentrations in accordance with the issued discharge permit or certificate limitations.

B. General design. Conventional effluent filtration shall be accomplished at a uniform rate of one to five gallons per minute per square foot of surface area through filter media consisting of a specified depth of the following materials, either as a single media, or as an approved combination of multiple layers: (i) sand; (ii) anthracite; (iii) mineral aggregate; and (iv) other filter media considered on a case-by-case basis.

1. Equipment for the application of chemicals to the filter influent shall be provided if necessary, to enhance suspended solids removal and minimize biological growth within the media.

a. Multiple unit operations for filtration shall be provided to allow for continuous operation and operational variability for a system with an average design of 0.04 mgd or greater.

b. The operating head loss shall not exceed 90% of the filter media depth.

c. Each filter shall have a means of individually controlling the filtration rate.

2. The effluent filter walls shall not protrude into the filter media and the incoming flow shall be uniformly applied to flooded media, in such a manner as to prevent media displacement. The height of the filter walls must provide for adequate freeboard above the media surface to prevent overflows.

3. The filter shall be covered by a superstructure if determined necessary under local climatic conditions. There shall be head room or adequate access to permit visual inspection of the operation as necessary for maintenance.

C. Backwashing. The source of backwash water upflow to cleanse the filter media shall be disinfected and may be derived from filtered wastewater effluent, for all treatment works with an average design flow equal to or greater than 0.1 mgd.

A design uniform backwash upflow minimum rate of 20 gallons per square foot per minute, consistent with wastewater temperatures and the specific gravity of the filter media, shall be provided by the underdrain or backwash distribution piping. The backwash rate may be reduced in accordance with the demonstrated capability of other methods, such as air scour, provided for cleaning of filter media.

1. The design backwash flow shall be provided at the required rate by wash water pumps or by gravity backwash supply storage. Two or more backwash pumps shall be provided so that the required backwash flow rate is maintained with any single pump out of service. Duplicate backwash waste pumps, each with a capacity exceeding the design backwash rate by 20%, shall be provided as necessary to return backwash to the upstream unit operations.

2. Sufficient backwash flow shall be provided so that the time of backwash is not less than 15 minutes for treatment works with design flows of 0.1 mgd or more, at the design rate of wash. A reduced capacity can be provided if it can be demonstrated that a backwash period of less than 15 minutes can result in a similar clean media bed headloss and a similar filter operating period or run time.

3. The backwash control, or valves, as provided on the main backwash water line, shall be sized so that the design rate of filter backwash is obtained with the control or valve settings for the individual filters approximately in a full open position. A means for air release shall be provided between the backwash pump and the wash water valve.

4. Air scouring, if provided, should maintain three to five cubic feet per minute per square foot of filter area for two to three minutes preceding backwash at the design rate.

5. The bottom elevation of the channel or top of the weir shall be located above the maximum level of expanded media during back washing. In addition:

a. A backwash withdrawal arrangement for optimizing removal of suspended solids shall be provided.

b. A two-inch filter wall freeboard is to be provided at the maximum depth of backwash flow above the filter media.

c. A level top or edge is required to provide a uniform loading in gpm per foot of channel or weir length.

d. An arrangement of collection channels or weirs to provide uniform withdrawal of the backwash water from across the filter surface shall be provided.

D. Deep bed filters. The deep bed filter structure shall provide a minimum depth of 8-1/2 feet as measured from the normal operating wastewater surface to the bottom of the underdrain system. The structure should provide for a minimum applied wastewater depth of three feet as measured from the normal operating wastewater surface to the surface of the filter media.

1. Porous plate and strainer bottoms are not recommended. The design of manifold type filtrate collection or underdrain systems shall:

a. Minimize loss of head in the manifold and baffles.

b. Assure even distribution of wash water and a uniform rate of filtration over the entire area of the filter.

c. Provide the ratio of the area of the underdrain orifices to the entire surface area of the filter media at about 0.003.

d. Provide the total cross-sectional area of the laterals at about twice the area of the final openings.

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

2. Surface wash means shall be provided unless other means of media agitation are available during backwash. Disinfected, filtered water or wastewater effluent shall be used as surface wash waters. Revolving type surface washers or an equivalent system shall be provided. All rotary surface wash devices shall be designed with:

a. Provisions for minimum wash water pressures of 40 psi.

b. Provisions for adequate surface wash water to provide 0.5 to 1.0 gallon per minute per square foot of filter area.

3. Deep bed filters shall be supplied with:

a. A loss of head gauge.

b. A rate of flow gauge.

c. A rate of flow controller of either the direct acting, indirect acting, constant rate, or declining rate types.

d. If necessary, continuous effluent turbidity monitoring.

e. A rate of flow indicator on the main backwash water line, located so that it can be easily read by the operator during the backwashing process.

E. Rapid rate filters. The conventional design rapid rate of filtration shall not exceed five gallons per minute per square foot of filter surface area. The selected filtration rate shall be based upon the degree of treatment required and filter effluent quality requirements.

1. A filtration media sieve analysis shall be provided by the design consultant. The media shall be clean silica sand having (i) a depth of not less than 27 inches and generally not more than 30 inches after cleaning and scraping and (ii) an effective size of 0.35 millimeters to 0.5 millimeters, depending upon the quality of the applied wastewater, and (iii) a uniformity coefficient not greater than 1.6.

2. A sieve analysis for supporting media shall be provided for the design. A three-inch layer of torpedo sand shall be used as the supporting media for the filter sand. Such torpedo sand shall have (i) an effective size of 0.8 millimeters to 2.0 millimeters and (ii) a uniformity coefficient not greater than 1.7.

3. A sieve analysis of anthracite media shall be provided for the design, if used. Clean crushed anthracite or a combination of sand and anthracite may be considered on the basis of experimental or operational data specific to the project design. Such media shall have (i) an effective size from 0.45 millimeters to 0.8 millimeters and (ii) a uniformity coefficient not greater than 1.7.

4. Gravel used as a supporting media 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 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 inch	3 to 5 inches
	1/2 to 3/16 inch	2 to 3 inches
	3/16 to 3/32 inch	2 to 3 inches

Reduction of gravel depth may be considered upon application to the department and where proprietary filter bottoms are proposed.

F. High rate gravity filters. The highest average filtration rate shall not exceed six gallons per minute per square foot unless the department can verify that a higher rate meets treatment needs based on evaluation of pilot plant studies or operational data. The selected filter rate shall be based upon the filter effluent quality requirements.

The media provided for high rate filtration shall consist of anthracite, silica sand or other suitable sand. Since certain manufacturers are presently utilizing multiple media and homogeneous media that are proprietary in nature, minimum standards are not established for filter media depth, effective size and uniformity coefficient of filter media, or the specific gravity of that media.

G. Shallow bed filters. The shallow bed filtration rate should not exceed 1-1/4 gallons per minute per square foot and shall not exceed two gallons per minute per square foot of filter area at average design flow.

1. Chlorination prior to shallow bed filtration shall be sufficient to maintain a chlorine residual of one mg/l through the filter for a system with average design flow of 0.1 mgd or greater.

2. Multiple unit operations shall be provided to allow for continuous operability and operational variability.

3. The filter media shall consist of a series of up to eight inch filter increments having a minimum total media depth of 11 inches. The sand media shall have an effective size in the range of 0.40 mm to 0.65 mm and a uniformity coefficient of 1.5 or less.

4. Filter inlets shall consist of ports located throughout the length of the filter.

5. The filter underdrainage system shall be provided along the entire length of the filter so that filter effluent is uniformly withdrawn without clogging of the outlet openings provided for collection and backwash.

6. Duplicate backwash pumps, each capable of providing the required backwash flow, shall be provided.

7. Facilities shall be provided for addition of filter aid to strengthen floc prior to filtration.

8. A skimmer shall be provided for each filter.

H. Pressure filtration. Pressure filter rates shall be consistent with those set forth in gravity filtration. Pressure filter media shall be consistent with that set forth in gravity filtration.

1. For pressure filter operation. The design should provide for:

a. Pressure gauges on the inlet and outlet pipes of each filter to determine loss of head.

b. A conveniently located meter or flow indicator with appropriate information to monitor each filter.

c. The means for filtration and backwashing of each filter individually, using a minimally complex arrangement of piping.

d. Flow indicators and controls convenient and accessible for operating the control valves while reading the flow indicators.

e. An air release valve on the highest point of each filter.

2. The top of the wastewater collection channel or weir shall be established at least 18 inches above the surface of the media.

3. An underdrain system to uniformly and efficiently collect filtered wastewater and that distributes the backwash water at a uniform rate, not less than 15 gallons per minute per square foot of filter area, shall be provided. A means to observe the wash water during backwashing should be established.

4. Minimum sidewall heights of five feet shall be provided for each filter. A corresponding reduction in sidewall height is acceptable where proprietary bottoms permit reduction of the gravel depth.

5. An accessible manhole should be provided as required to facilitate inspections and repairs.

I. Traveling bridge. This type of filter is normally equipped with a shallow bed divided into cells with a continuously operated reciprocating cell-by-cell traveling backwash system. This filter system shall comply with applicable design criteria set forth for shallow bed filters. Use of these filters will be evaluated by the department on a case-by-case basis.

J. Microstraining. Microstraining involves the passing of treated effluent through a horizontally mounted, rotating drum with a filtering fabric fixed to its periphery by a porous screen. Microstrainer equipment is typically used to improve treatment of biologically treated wastewater which has received secondary clarification. Thus, biological attached growth can accumulate on the filter fabric. Means to control such biological growth shall be addressed in the design.

1. The most common screen opening (aperture) sizes are 23, 35 and 60 microns, but other sizes may be available. Normally, the larger sizes are used in cases when only the coarser solids are desired to be removed. The type of mesh weave, when considered in conjunction with aperture size, greatly affects the hydraulic capacity of a microstrainer. Screen size selection must be based on the particle type and size to be removed.

2. Screens are made from a variety of woven metals and nonmetals, with stainless steel being the most commonly used material. Nonmetallic filter cloths are especially suitable for those applications where the presence of corrosive chemicals would be harmful to metallic cloths. Chlorination immediately ahead of microstraining units employing metallic cloths should be avoided.

3. The area of the submerged portion of the screening fabric helps to govern the hydraulic capacity. Normal submergence is 2/3 to 3/4 of the drum diameter. The speed of rotation of the drum should be based on particle type size to be removed. Decreasing the speed of rotation causes increased removal efficiencies but has the effect of increasing the head loss through the filter fabric and decreasing the hydraulic capacity of the unit. The design rotational speed should be about seven rpm.

4. The backwash system should be designed to serve the dual function of applying energy in the form of pressurized washwater spray to the screen to dislodge retained particles and to collect and transport the solids-laden washwater away from the microstrainer. The backwash system shall be designed to minimize splash-over (solids-laden backwash spray water that falls short or long of the washwater collector rather than into the collector as intended). The microstrainer design shall provide for solids retained on the screen which fall back into the drum pool. Backwashing shall be continuous. Backwash water requirements should be based on particle type and size to be removed. The volume of wash water required shall be determined on an individual basis. The normal source of backwash water is the microstrainer effluent collector. Normally only one-half of the backwash water volume actually penetrates the screen; the rest, called a splashback, flows into the effluent section. The backup system should minimize splashback. Increasing the backwash flow and pressure has the tendency to decrease the headloss through the screen. Up to 25% of the total throughput volume may be required for backwash purposes, but averages of 1.0% to 5.0% are typical. Adequate backwash waste storage and treatment facilities should be provided to dispose of the removed materials within the design limitations of other system components.

5. The most suitable pressure differential through the screen shall be determined on an individual basis. Usual pressure differential under normal operating conditions is 12 to 18 inches. The pressure applied to the screen affects the flow rate through the screen. The low pressure requirement is one of the microstrainer's advantages. The secondary effluent should not be pumped, but allowed to flow by gravity to the microstrainer unit to minimize the shear force imparted to the fragile biological floc.

6. Hydraulic capacity of the microstrainer is affected by the rate of clogging of the screening fabric. The accumulation or build-up of attached bio-mass on the screen over time must be prevented. The use of ultraviolet light may reduce the rate of such accumulation. Microstrainers shall not be utilized to treat wastewaters containing high grease and oil concentrations, due to their clogging effects. Iron and manganese buildups also tend to clog the screen. Periodically, the screen must be taken out of service and cleaned. Microstraining units shall be provided in sufficient numbers and capacities to maintain 100% operability of the microstraining process. Automatic control of drum speed and backwash pressure based on head loss through the screen shall be utilized to help overcome this sensitivity problem.

7. Pilot plant studies can be conducted to determine the applicability and design of the microstraining unit to the specific wastewater to be treated. The hydraulic capacity of a microstrainer is determined by the following: head applied, concentration of solids, size of solids, nature of solids, rate of clogging, drum rotational speed, drum submergence, mesh weave and aperture size. These factors are interrelated such that a change in any one of them will cause a change in some or all of the remaining factors.

K. Nonfixed beds and upflow. Continuously backwashed and other nonfixed bed filters are considered as nonconventional technology. Conventional design standards may be established through evaluation of performance data as provided for in this chapter.

L. Membrane, ultra and micro. Filtration of treated effluent through membranes and other media involving molecular sized removal is considered nonconventional technology. Application of this technology will be considered based on evaluation of performance data as provided for in this chapter.

M. Carbon adsorption. Carbon adsorption involves the interphase accumulation or concentration of dissolved substances at a surface or solid-liquid interface by an adsorption process. Activated carbon, which is generally a wood or coal char developed from extreme heat, can be used in powdered form (PAC) or granular form (GAC). Generally, carbon adsorption is used as the polishing process to remove dissolved organic material remaining in a wastewater treated to a secondary or advanced level. Activated carbon adsorption can also be used for dechlorination.

1. Parameters with general application to design of carbon adsorption units are carbon properties, contact time, hydraulic loading, carbon particle size, pH, temperature and wastewater composition, including concentrations of suspended solids and other pollutants.

2. The adsorption characteristics of the type of carbon to be used shall be established. Such characteristics may be established using jar test analyses of various activated carbons in reaction with the waste to be treated. Adsorption isotherms for each form of carbon proposed for use shall be determined. The source and availability of replacement carbon, as designed, shall be addressed.

3. Pilot plant studies shall be performed upon the selected carbon using the wastewater to be adsorbed, where industrial and domestic wastes are present to determine: breakpoint, exhaustion rate, contact time to achieve effluent standards; and if applicable, the backwash frequency, pressure drop through the fixed bed columns, and the carbon regeneration capacity required. Where strictly domestic waste is to be treated, data from similar full scale unit operations or pilot plant data will be acceptable.

4. Where carbon regeneration is provided, carbon loss due to transportation between the columns and regeneration furnace in the range of five to 10 percent total carbon usage shall be considered normal for design. The rate at which carbon will lose adsorption capacity with each regeneration should be established.

5. If fixed-bed GAC carbon columns must be backwashed to remove solids entrapped in the carbon material, then backwash facilities shall provide for expansion of the bed by at least 30%.

6. Carbon adsorption unit operations may be provided in parallel or series. Sufficient capacity shall be provided to allow for continuous operability of the carbon adsorption process.

7. Nonfixed bed carbon adsorption unit operations may be operated in the upflow or downflow mode. Duplicate pumping units shall be provided for such unit operations.

8. Carbon adsorption unit operations should provide for purging with chlorine or other oxidants as necessary for odor and bio-mass control.

Historical Notes

Former 12VAC5-581-920 derived from Volume 18, Issue 10, eff. February 27, 2002; amended and adopted as 9VAC25-790-860, Virginia Register Volume 20, Issue 09, eff. February 12, 2004.

Statutory Authority

§ 62.1-44.19 of the Code of Virginia.