Administrative Code

Virginia Administrative Code

Article 8. Advanced Treatment

9VAC25-790-830. Flow equalization.

Article 8
Advanced Treatment

A. Flow equalization is a unit process whereby the variability of wastewater flows, in terms of volume and strength, is lessened. Where flow equalization is utilized within a sewerage system or treatment works to reduce the peak flow conveyed to, or processed by, the treatment works, the performance of the treatment process should be improved in relation to the estimated conventional effluent values. The ability of a treatment works that is provided with flow equalization to meet permit or certificate effluent limitations shall be evaluated on a case-by-case basis.

B. Usage. Flow equalization shall be provided in the flow scheme ahead of advanced chemical-physical processes, unless engineering analysis shows that absence of flow equalization is more cost effective while maintaining the same degree of reliability and operational control.

1. Flow equalization should be provided upstream of biological treatment works designed to process a mean daily flow of 0.1 mgd or less, and receiving hourly peak flows in excess of twice the design flow, if such peak flows will occur daily in excess of 50 times annually.

2. Flow equalization shall be provided upstream of biological treatment works designed to process a mean daily flow of 0.1 mgd or less that are permitted with effluent limitations less than 20 mg/l of BOD5 or TSS, or a TKN of less than 5 mg/l, or a total phosphorus of less than 2 mg/l, unless approved downstream unit operations are also provided.

C. Design. The design of an equalization basin shall incorporate the evaluation and selections of a number of features as follows:

a. On-line versus off-line basins.

b. Basin volume providing for a total storage detention of one-third or more of the daily design flow.

c. Degree of compartmentalization relative to dry weather and wet weather peak flows.

d. Type of construction: earthen, concrete or steel.

e. Aeration and mixing equipment.

f. Pumping and control in order to uniformly introduce flow into the treatment process at approximately the daily design flow rate during peak flow events.

g. Location in treatment system to provide uniform loadings on downstream unit operations.

The design decisions shall be based on the nature and extent of the treatment processes used, the benefits desired and local site conditions and constraints.

1. The minimum mixing requirements for equalization basins receiving raw or untreated domestic wastewaters or sewage containing an average suspended solids concentration exceeding 45 mg/l, shall equal or exceed 0.02 hp/1,000 gallons at a depth providing at least one-third of the maximum storage volume. Oxygen shall be supplied at a rate of 15 pounds per hour per gallon. Multiple mixing and aeration units shall be provided for continuous operability.

2. Flow equalization basins receiving treated wastewater or sewage with an average suspended solids concentration of 45 mg/l or less shall be provided with a means of sludge removal or mixing equipment that shall have a minimum power input of 0.01 hp/1,000 gallons of maximum storage volume. Aerobic conditions shall be maintained. Multiple mixing and aeration units shall be provided for continuous operability.

3. Sufficient storage shall be provided to allow subsequent downstream unit operations that follow equalization to operate at or less than their ted design capacity.

a. Storage capacity shall be determined from flow data when available. Basin volume for equalization shall at a minimum be determined from an inflow mass hydrograph of the hourly fluctuations for a typical daily wastewater flow, where typical daily wastewater flow is defined as the desired flow rate out of the equalization basin. Additional equalization basin volume shall be provided to accommodate:

(1) Continuous operation of aeration and mixing equipment.

(2) Anticipated concentrated treatment works recycle flows.

(3) Unforeseen changes in diurnal flow.

b. An evaluation of infiltration/inflow shall be conducted where influent flow data are not available. The minimum detention time shall be eight hours of the estimated daily maximum flow as determined by the study.

4. Flow equalization basins with a storage capacity exceeding 20,000 gallons should be constructed as compartmentalized or as multiple basins. Single basin installation with a bypass to downstream treatment units may be used for treatment works with capacities less than 200,000 gpd that are not located in critical water areas. The storage basins shall be provided with the means to be dewatered.

5. Basins designed for a combination of storage of wet weather flows and equalization shall be compartmentalized to allow for utilization of a portion of the basins for dry weather flow equalization. Floating surface aerators shall have provisions to protect the units from damage when the tank is dewatered.

6. Multiple pumping units shall be provided that are capable of delivering flow to an overflow device so that the desired flow rate can be maintained from the equalization basin with the largest pumping unit out of service, unless a suitable gravity flow system is provided. Gravity discharge from equalization shall be regulated by an automatically controlled flow-regulating device. If a flow-measuring device is provided downstream of the basin to monitor and control the equalization discharge, then a raw sewage influent flow meter will not be required in accordance with this chapter.

7. Equalization shall be preceded with screening and should be preceded by grit removal. Facilities shall be provided to flush solids and grease accumulations from the basin walls. A high-water-level takeoff shall be provided for withdrawing floating material and foam.

8. An overflow shall be provided for equalization basins so that such basins are not flooded, and these overflows are transmitted to downstream treatment units prior to the disinfection unit operation.

Statutory Authority

§ 62.1-44.19 of the Code of Virginia.

Historical Notes

Former 12VAC5-581-890 derived from Virginia Register Volume 18, Issue 10, eff. February 27, 2002; amended and adopted as 9VAC25-790-830, Virginia Register Volume 20, Issue 9, eff. February 12, 2004.

9VAC25-790-840. Chemical treatment.

A. Usage. Chemicals shall be compatible with the treatment works unit operation and have no detrimental effect upon receiving waters. Pilot plant studies or data from unit operations treating design flows of sewage or domestic wastewaters of similar characteristics (organic levels, metal concentrations, etc., within 25% of proposed design) shall be required to determine appropriate chemicals and feed ranges.

1. Space shall be provided where at least 30 days of chemical supply can be stored in dry storage conditions at a location that is convenient for efficient handling, unless local suppliers and conditions indicate that such storage can be reduced without limiting the supply.

2. Liquid chemical storage tanks must:

a. Have a liquid level indicator.

b. Have an overflow and a receiving basin or drain capable of receiving accidental spills or overflows.

3. Powdered activated carbon shall be stored in an isolated fireproof area, and explosion proof electrical outlets, lights and motors shall be used in all storage and handling areas in accordance with local, state and federal requirements.

4. Chemicals shall be stored in covered or unopened shipping containers, unless the chemical is transferred into an approved covered storage unit.

5. Solution storage or day tanks feeding directly should have sufficient capacity for 24-hour operation at design flow.

6. Acid storage tanks shall be vented to the outside atmosphere, but not through vents in common with day tanks.

B. Features. Provisions shall be made for measuring quantities of chemicals used to prepare feed solutions. Storage tanks, pipelines, and equipment for liquid chemicals shall be specific to the chemicals and not for alternates.

1. Chemicals that are incompatible (i.e., strong oxidants and reductants) shall not be fed, stored or handled in such a manner that intermixing of such compounds could occur during routine treatment operations.

2. Provisions shall be made for the proper transfer of dry chemicals from shipping containers to storage bins or hoppers in such a way as to minimize the quantity of dust that may enter the room in which the equipment is installed. Control shall be provided by use of:

a. Vacuum pneumatic equipment or closed conveyor systems;

b. Facilities for emptying shipping containers in special enclosures; or

c. Exhaust fans and dust filters that put the hoppers or bins under negative pressure in accordance with federal and state requirements.

3. Concentrated acid solutions or dry powder shall be kept in closed, acid-resistant shipping containers or storage units. Concentrated liquid acids shall not be handled in open vessels, but should be pumped in undiluted form from original containers to the point of treatment or to a covered day or storage tank.

4. For the handling of toxic chemicals, suitable carts, lifting devices, and other appropriate means shall be provided in accordance with the material safety data sheets and applicable state and federal requirements.

a. Provisions shall be made for disposing of empty containers by an approved procedure that will minimize exposure to the chemical.

b. The transfer of toxic materials shall be controlled by positive actuating devices.

5. Structures, rooms, and areas accommodating chemical feed equipment shall provide convenient access for servicing, repair, and observation of operation.

a. Floor surfaces shall be smooth but slip resistant, impervious, and well drained with a slope of 1/8-inch per foot minimum.

b. Open basins, tanks and conduits shall be protected from chemical spills or accidental drainage.

6. A minimum of two chemical feeders shall be provided for continuous operability. A standby unit or combination of units of sufficient capacity shall be available to replace the largest unit during shutdowns. The entire feeder system shall be protected against freezing and shall be readily accessible for cleaning.

7. Chemical feeders shall be of such design and capacity to meet the following requirements:

a. Feeders shall be able to supply, at all times, the necessary amounts of chemicals at an accurate rate throughout the range of feed.

b. Proportioning of chemical feed to the rate of flow shall be provided where the flow rate is not constant.

c. Diaphragm or piston type positive displacement type solution feed pumps should not be used to feed chemical slurries.

d. The treatment works service potable water supply shall be protected from contamination by chemical solutions or sewage by providing either an air gap between the portable water supply line and solution tank, or a suitable reduced pressure zone, backflow prevention device.

e. Chemical-contact materials and surfaces must be resistant to the aggressiveness of the chemical solutions.

8. Dry chemical feeder systems shall:

a. Measure the chemical volumetrically or gravimetrically.

b. Provide effective mixing and solution of the chemical in the solution pot.

c. Preferably provide gravity feed from solution pots.

d. Completely enclose chemicals and prevent emission of dust to the operation room.

9. Chemical feeders should be reasonably adjacent to points of application to minimize length of feed lines. Chemical feeders shall be readily accessible for servicing, repair and observation. Chemical feeding equipment should be provided with containment barriers or protective curbing so that chemicals from equipment failure, spillage or accidental drainage will be contained. Chemical feed control systems shall provide for both automatic and manual operation including:

a. Feeders that are automatically controlled should provide for reverting to manual control as necessary.

b. The feeders shall be capable or being manually started.

c. Automatic chemical dose or residual analyzers should be considered and, where provided, should include alarms for critical values and recording charts.

10. Solution tank dosing shall provide for uniform strength of solution, consistent with the nature of the chemical solution. Continuous agitation shall be provided to maintain slurries in suspension. Two solution tanks shall be required for a chemical to assure continuity of chemical application during servicing. Tank capacity should provide storage for 24 hours of operation and:

a. Each tank shall be provided with a drain.

b. Means shall be provided to indicate the solution level in the tank.

c. Make-up potable water shall enter the tank through an air gap.

d. Chemical solutions shall be kept covered, with access openings curbed and fitted with tight covers.

11. Subsurface locations for solution tanks shall:

a. Be free from sources of possible contamination.

b. Assure positive drainage for groundwater, accumulated water, chemical spills, and overflows.

c. Be protected from aggressiveness.

12. Solution tank overflow pipes shall:

a. Be turned downward.

b. Have free discharge.

c. Be located where noticeable.

d. Be directed so as not to contaminate the wastewater or receiving stream or be a hazard to operating personnel, in accordance with VOSH requirements.

13. Service water used in the feeder system shall be:

a. From sources acceptable to the department.

b. Protected from contamination by appropriate means.

c. Ample in supply and adequate in pressure.

d. Provided with means for measurement when preparing specific solution concentrations. Where a booster pump is required, duplicate equipment shall be provided.

14. Scales shall be provided as follows:

a. For volumetric dry chemical feeders.

b. Accurate to measure increments of 0.5% of load.

c. For weighing of carboys that are not calibrated volumetrically.

d. For large treatment works, indicating and recording type scales are desirable.

15. Chemical application equipment should:

a. Assure maximum efficiency of treatment.

b. Provide maximum protection of the receiving waters.

c. Provide maximum safety to operators.

d. Assure satisfactory mixing of the chemicals with the wastewater.

e. Provide maximum flexibility of operation through various points of application, when appropriate.

f. Prevent backflow or back-siphonage between multiple points of feed through common manifolds.

g. Provide for the application of pH affecting chemicals to the wastewater prior to the addition of coagulants.

C. Safety. Gases from feeders, storage, and equipment exhaust shall be conveyed to the outside atmosphere, above grade and remote from air intakes in accordance with applicable state and federal requirements.

1. Special provisions should be made as necessary for ventilation of feed and storage rooms in accordance with VOSH and applicable fire code requirements.

2. For each operator who will handle dry chemicals, protective equipment should be provided, including personal protective equipment for eyes, face, head, and extremities, and protective shields and barriers, in accordance with VOSH requirements.

3. Facilities should be provided for eye washing and showering, in accordance with VOSH requirements. Protective equipment and neutralizers shall be stored in the operating area.

Statutory Authority

§ 62.1-44.19 of the Code of Virginia.

Historical Notes

Former 12VAC5-581-900 derived from Virginia Register Volume 18, Issue 10, eff. February 27, 2002; amended and adopted as 9VAC25-790-840, Virginia Register Volume 20, Issue 9, eff. February 12, 2004.

9VAC25-790-850. Chemical clarification.

A. General design. Design unit operation detention time shall be estimated as the ratio of the design basin volume to the design flow rate (into that basin) unless adequate test data is made available verifying that a different value of detention time can be utilized. Multiple unit operations for mixing, flocculation and clarification, including duplicate basins and equipment used for chemical feeding, controlled mixing and for final clarification, shall be provided as follows:

1. Advanced treatment works having a rated capacity greater than 40,000 gallons per day.

2. Treatment works consisting of physical-chemical unit operations.

3. Unit operations for controlled mixing shall be in series or parallel.

4. Provisions for unit operations to be taken out of service without disrupting operation shall be included.

5. Multiple stage unit operations shall be provided when a conventional operation cannot be achieved otherwise.

B. Mixing. All treatment works shall provide appropriate mixing unit operations upstream from required chemical clarification and filtration unit operations.

Rapid or high intensity mixing may be accomplished either within basins or in-line within closed channels. Basins should be equipped with mechanical mixing devices; other arrangements, such as baffling, are acceptable only under special conditions. Where mechanical mixing devices are utilized, duplicate mechanical mixing units or spare mixing equipment shall be provided.

The rapid or high intensity detention period (T) should not be less than 10 seconds.

1. The design of the rapid mixing unit operations should be based upon the mean temporal velocity gradient (G) (expressed in inverse units of seconds). Typical values for G and T are:

T (Seconds)

G (Seconds-1)











For optimization, the design should establish the proper values of (G) and (T) from appropriate test or performance data.

2. Multiple points of application shall be provided to enable the provision of maximum mixing intensity.

3. The physical configurations of the mixing basin shall be designed to eliminate vortexing.

4. The speed variation of rapid mix equipment should be approximately 50% of the average speed requirement range.

C. Flocculation. Flocculation basins shall be designed to optimize the effects of coagulation through increased opportunity for solids contact, and thus inlet and outlet design shall prevent short-circuiting and destruction of the developed suspended particles or floc.

Flocculation and sedimentation basins shall be as close together as physically possible. The velocity gradient of the flocculated water through pipes or conduits to settling basins shall not be greater than the velocity gradient utilized in flocculating the water. Where velocity gradient is not used as a design parameter, the linear velocity in pipes and conduits from the flocculators to the settling basin shall not exceed 1/2 foot per second. Allowances shall be made to minimize turbulence at bends and changes in direction.

1. A drain and overflow shall be provided for each basin.

2. Multiple unit operations shall be provided for continuous operability for design flows greater than 40,000 gallons per day.

3. Baffling may be used to provide for flocculation in small scale unit operations (less than 2,000 gallons in volume).

4. Flocculation basins shall be provided separately from other unit operations except where a reactor clarifier or clarifiers are provided.

D. Low intensity mixing. The minimum detention time for the low intensity mixed volume shall be 20 minutes, unless acceptable operational or test data establishes that adequate flocculation can be accomplished within a reduced detention time.

1. The design of the low intensity or contact type flocculation units shall be based upon the value of the product of the mean temporal velocity gradient times the detention time (GT), which is ordinarily in the range of 20,000 to 200,000.

2. The design should also establish the optimum value of GT for flocculation from appropriate test data. Variable speed drive units shall be designed to allow speed variation throughout the design range.

3. Successive mixed or contact compartments should be provided. Special attention shall be given to providing properly sized ports effectively located between compartments to minimize short-circuiting.

Tapered flocculation should be provided. Wing walls or stators shall be provided to prevent vortexing in basins utilizing vertical shaft flocculators.

E. Conventional clarifiers. Circular clarifiers of the center feed, peripheral feed and spiral flow type will be considered on an individual basis for gravity settling of coagulated and flocculated sewage effluent (chemical clarification).

1. Multiple basins shall be provided as required for continuous operability of treatment works with design flow capacity of more than 40,000 gallons per day or for treatment works utilizing chemical-physical unit operations.

2. The design surface loading (overflow rate) shall be established on a case-by-case basis as a function of the types of coagulants or use of enhanced settling devices or configurations, such as modular tube-type sections utilized within shallow depth clarifiers. Surface loading rates shall not exceed 600 gpd/square foot for alum sludges, 800 gpd/square foot for iron sludges and 1,000 gpd/square foot for lime sludges, in processes utilizing flocculation, unless adequate pilot plant data is presented verifying that higher loading rates are acceptable.

3. Conventional chemical clarification shall provide a minimum of four hours effective settling time unless adequate operational data is submitted to verify that adequate treatment can be achieved at a reduced value of detention time. Effective settling time will be calculated using the settling zone volume of the basins extending from the inlet entrance to the basins to the submerged effluent orifices or weirs.

4. Rectangular sedimentation basins shall be designed with a length to width ratio of at least four to one.

5. Inlets shall be designed to distribute the wastewater equally and at uniform velocities. Open ports, submerged ports, stilling walls or similar entrance arrangements are required. Where stilling walls are not provided, a baffle shall be constructed across the basin in a manner to redirect flow from the inlet and shall project several feet below the water surface to dissipate inlet velocities and provide uniform flows across the basin settling zone.

6. Outlet devices shall be designed to maintain velocities suitable for settling in the basin and to minimize short-circuiting. The use of submerged orifices or submerged weirs shall be provided where flocculation precedes filtration. The maximum velocity gradient in pipes and conduits from the settling basins to the filters shall not exceed that used in the flocculation. Where velocity gradient is not used as a parameter in the design of outlet devices, the linear velocity in pipes and conduits from settling basins shall not exceed one foot per second.

7. The velocity through settling basins shall not exceed one foot per minute. The basins shall be designed to minimize short circuiting.

8. An overflow weir (or pipe) shall be installed to be compatible with the maximum water level desired above the filter media where filters follow sedimentation. The overflow shall discharge with a free fall at a location where the discharge may be observed.

9. Settling basins used for chemical clarification shall be provided with a means for dewatering. Basin bottoms shall slope toward the drain not less than one foot of fall in 12 feet of length.

10. Automatic continuous sludge removal equipment shall be provided for chemical clarification. Provision shall be made for the operator to observe or sample sludge being withdrawn from the clarifier.

11. Consideration shall be given to the provision of control of climatic factors, such as wind and temperature through use of enclosures or superstructures.

F. Reactor clarifiers. Reactor type flocculation and chemical clarification basins may be considered where wastewater characteristics are evaluated by the department and deemed to be uniform.

Reactor clarifiers shall be designed for the maximum uniform flow rate and shall be adjustable to changes in flow which are less than the design rate.

1. Multiple reactor clarifiers are required to maintain continuous operability.

2. For reactor clarifiers a minimum of 30 minutes shall be provided for flocculation and mixing. The clarification detention time shall be established on the basis of the raw wastewater or sewage characteristics and other local conditions that affect the operation of the unit. Based on design flow rates, the minimum detention time shall be two hours for reactor clarifiers.

3. Reactor clarifiers shall be equipped with orifices if they precede filtration. Orifices shall produce uniform rising or overflow rates over the entire area of the tank and shall provide an exit velocity not to exceed one foot per second. Upflow rates shall not exceed one gallon per minute per square foot of area of the horizontal zone of sludge separation (blanket), for the design mode of operation of the clarifier.

4. The following operating equipment shall be provided:

a. A complete set of necessary tools and accessories.

b. Adequate piping with suitable sampling taps so located as to permit the collection of samples of wastewater from critical portions of the units.

c. Conventional equipment to maintain feeding, mixing, and flocculation operation.

5. Weirs should be designed so that surface water does not travel over 10 feet horizontally to the overflow point or tops of weirs (launders). Weir loading shall not exceed 20 gallons per minute per foot of weir length. Where weirs are used they shall be:

a. Adjustable.

b. At least equivalent in length to the perimeter of the tank.

6. Sludge removal design shall provide that:

a. Sludge pipes shall be not less than three inches in diameter and so arranged as to facilitate cleaning;

b. Entrance to sludge withdrawal piping will prevent clogging;

c. Valves are located outside the tank for accessibility;

d. The operator may observe or sample sludge being withdrawn from the unit;

e. Automatic continuous sludge control shall be provided; gravity control should be utilized.

7. Superstructures. Consideration shall be given to providing a superstructure to enclose the reactor clarifier and associated sampling valves and piping.

Statutory Authority

§ 62.1-44.19 of the Code of Virginia.

Historical Notes

Former 12VAC5-581-910 derived from Virginia Register Volume 18, Issue 10, eff. February 27, 2002; amended and adopted as 9VAC25-790-850, Virginia Register Volume 20, Issue 9, eff. February 12, 2004.

9VAC25-790-860. 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:



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.

Statutory Authority

§ 62.1-44.19 of the Code of Virginia.

Historical Notes

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

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