Administrative Code

Virginia Administrative Code

Article 6. Biological Treatment

9VAC25-790-670. Attached growth processes.

Article 6
Biological Treatment

A. The contactor, or media filled reactor, utilized for attached growth biological processes shall be preceded by primary clarification equipped with scum and grease collecting devices. Other pretreatment facilities equivalent to primary clarification may be proposed for evaluation by the department. The media shall provide sufficient surface area to support the attached biological growth necessary to achieve the desired performance standard. Recirculation of treated wastewater back to the contactor influent should be provided to maintain design loadings.

B. Trickling filters. Biological contactors called trickling filters shall be designed so as to provide either the reduction in biochemical oxygen demand required by the issued certificate or permit, or the treatment necessary to properly condition the sewage for subsequent treatment. This section provides performance criteria to achieve final effluent limits to meet federal secondary equivalency requirements for trickling filters. Such biological contactors may be designed to achieve higher degrees of treatment or used in conjunction with other unit operations. Where the design intent is to achieve other than secondary equivalency levels, the proposed design parameters shall be thoroughly reviewed during the preliminary engineering conference.

1. The hydraulic loading used for design of standard rate trickling filters shall be between two and four million gallons per acre per day with an organic loading between 400 and 800 pounds of BOD5 per acre foot per day.

2. The hydraulic loading used for design of high-rate filters shall be between 10 and 30 million gallons per acre per day with an organic loading between 1,200 and 3,300 pounds BOD5 per acre foot per day.

3. Other design loadings that are based on pilot studies and related to design and performance parameters through rational design equations or models will be evaluated by the department.

4. The performance of biological contactors can be detrimentally affected by diurnal loading conditions. The volume of media as determined from either pilot plant studies or from acceptable design equations shall be based upon the design peak hourly organic loading rate rather than the average rate. An alternative for reducing the design peak flow would involve provision of adequate flow equalization prior to the contactor.

5. Consideration should be given to the use of two-stage biological contactors in series operation where single stage reactors may not accomplish the required removals. Expected treatment efficiencies shall be calculated and documented.

C. Features. All hydraulic factors involving proper distribution of sewage on the contactor media shall be carefully calculated. For reaction type distributors, a minimum head of 24 inches between the low water level in siphon chamber and the horizontal elevation of the center of distribution arms shall be required. Surge relief to prevent damage to distributor seals shall be provided where sewage is pumped directly to the distributors. A minimum clearance of six inches between the media surface and the bottom of distributor arms shall be provided.

1. The sewage may be distributed over the contact reactor media surface by rotary distributors or other suitable devices that will permit reasonably uniform distribution to the surface area. At design average flow, the deviation from a calculated uniformly distributed volume per square foot of the filter surface shall not exceed plus or minus 10% at any point.

2. Sewage may be applied to the contactor media by siphons, pumps or by gravity discharge from preceding treatment units when suitable flow characteristics have been developed. Application of sewage should be continuous. In the case of intermittent dosing, the dosing cycles shall normally vary between five to 15 minutes with distribution taking place approximately 50% of the time. The maximum rest should not exceed five minutes based on the design average flow. Consideration shall be given to a piping system that will permit recirculation.

3. Underdrains with semi-circular inverts or equivalent shall be provided and the underdrainage system shall cover the entire floor of the filter. Inlet openings into the underdrains shall have an unsubmerged gross combined area equal to at least 15% of surface area of the filter. The underdrains shall have a minimum slope of 1.0%. Effluent channels shall be designed to produce a minimum velocity of two feet per second at the average daily rate of application to the filter. Provision shall be made for flushing the underdrains. The use of a peripheral head channel with vertical vents is acceptable for flushing purposes. Inspection facilities shall be provided.

4. The underdrainage system, effluent channels and effluent pipe shall be designed to permit free passage of air. The size of drains, channels, and pipe shall be such that not more than 50% of their cross-sectional area will be submerged under the design hydraulic loading. Provision shall be made in the design of the effluent channels to allow the possibility of increased hydraulic loading. Consideration should be given to the use of forced ventilation, particularly for covered trickling filters and deep (10 feet or more) contactors filled with a manufactured media.

5. The design should provide for variable rates of recirculation for various purposes; for example, to prevent drying of a standard rate filter between dosing. Devices shall be provided to permit measurement of flow to the filter process, including recirculated flows. The design should include provisions to flood filter structures where applicable.

6. All distribution devices, underdrains, channels and pipes shall be installed so that they may be properly maintained, flushed or drained. Mercury seals shall not be permitted. Ease of seal replacement shall be considered in the design to ensure continuity of operation.

7. A freeboard of four feet or more should be provided for all deep bed contactors with manufactured media that also utilize fine spray distributors, so as to maximize the containment of windblown spray.

8. Protection such as covers or windbreaks shall be provided to maintain operation and treatment efficiencies when climatic conditions are expected to result in problems due to cold temperatures.

D. Reactor media. Contact reactor media may be crushed rock, stone or specially manufactured material. The media shall be durable, resistant to spalling or flaking and relatively insoluble in sewage. The top 18 inches of rock or stone media shall have a loss by the 20-cycle, sodium sulfate soundness test of not more than 10% (as prescribed by ASCE Manual of Engineering Practice, "Filtering Materials for Sewage Treatment Plants," Manual of Engineering Practice No. 13, ASCE, New York, New York), the balance to pass a 10-cycle test using the same criteria. Stone media shall be free from iron. Manufactured media shall be chemically and biologically inert. The media shall be structurally stable to allow for distributor maintenance or a suitable access walkway shall be provided.

1. Rock or stone filter media shall have a minimum depth of five feet above the underdrains. Manufactured contactor media should have a minimum depth of 10 feet to provide adequate contact time with the wastewater. Rock and stone filter media depth should not exceed 10 feet and manufactured filter media should not exceed 30 feet except where special construction is justified through performance data or pilot plant studies.

2. Rock, stone, and similar media shall not contain more than five percent by weight of pieces whose longest dimension is three times the least dimension. They shall be free from thin elongated and flat pieces, dust, clay, sand or fine material and shall conform to the following size and grading when mechanically graded over vibrating screens with square openings:

a. Passing 4-1/2 inch screen—100% by weight

b. Retained on three-inch screen—95-100% by weight

c. Passing two inch screen—0-2% by weight

d. Passing one inch screen—0-1% by weight

e. Maximum dimensions of stone--five inches

f. Minimum dimensions of stone--three inches

3. Applications of manufactured media such as wood, plastic, etc., will be evaluated on a case-by-case basis. The handling and placement of the media should be specified.

E. Roughing reactors. Roughing contact reactors are used to reduce the organic load applied to subsequent oxidation processes. They are particularly applicable preceding an activated sludge process or a second stage filter in a treatment works receiving high strength wastewater (excessive organic loadings). Roughing filter designs differ from other contactors principally on the basis of the deeper depths and media design utilized for given loadings in comparison to high rate trickling filters. Since it is used to reduce the downstream organic loading rather than to provide a stabilized effluent, it is designed to receive organic loadings exceeding those applied to conventional biological contactors.

F. Granular media filters. Intermittently dosed biological sand filters utilized to process septic tank effluent to meet secondary treatment standards should be limited to schools, day camps and other installations that have part-time usage. These reactors should also be limited to those installations generating a sewage flow of 20,000 gallons per day or less and provide lengthy rest periods for filter operation. Biological sand filters may serve year-round residential dwellings if the design capacity is restricted to 1,000 gallons per day or less.

1. Biological sand filters shall not be used to treat raw wastewater and shall be preceded by a minimum of pretreatment designed to produce a settled sewage with adequate grease management. The use of biological sand filters designed to enhance effluent from other sewage treatment reactors shall be evaluated on a case by case basis.

2. Sand filter media beds shall consist of level areas of sand beneath which there are graded layers of gravel surrounding the underdrains. Each filter bed shall have an impervious bottom. Sewage is discharged onto the beds through rotary distributors or pipes onto splash plates or, in the case of subsurface filters, through lines of drain tile laid with open joints. Open sand beds shall be surrounded by a concrete, brick or cinder block wall extending above the sand and at least one foot above ground level. For subsurface sand filters, the surrounding wall is not necessary except to prevent caving of the earth walls while the sand and gravel are being placed. The underdrainage system shall consist of open joint or perforated pipe tied together into a manifold and vented to the atmosphere. The minimum size for the underdrain shall be four inches in diameter. The underdrain pipes should be placed on a slope of not less than 1.0%.

3. Rock, gravel and sand media components shall be clean and free of organic matter, clay or loam soils and fine limestone material.

a. The media depth shall not be less than 30 inches. Sand media for intermittently dosed and recirculated effluent, shall have an effective size of 0.30 mm to 1.0 mm and 0.8 mm to 1.5 mm, respectively. The uniformity coefficient should not exceed 4.0. No more than 2.0% shall be finer than 0.177 mm (80 mesh sieve) and not more than 1.0% shall be finer than 0.149 mm. No more than 2.0% shall be larger than 4.76 mm (4 mesh sieve). Larger granular media up to 5 mm in effective size may be considered on a case by case basis.

b. The gravel base for sand media shall conform to the Virginia Department of Transportation's Road and Bridge Specifications (1974). The base gravel shall consist of No. 3 sized gravel with at least a three-inch depth above the sloped underdrains. The middle layer shall consist of at least three inches of No. 68 gravel, and the top layer shall consist of at least three inches of No. 8 gravel.

4. Dosing tanks with either siphons or pumps for sand filters shall have the capacities to effect the dosage volumes required. The siphons and the rotary distributor should be supplied by the same manufacturer. The influent line to the rotary distributor shall be equipped with a valved drain.

5. Sand filters designed for intermittent flooding should be divided into at least two beds for small filters and three beds for the larger filters. Distribution boxes must be provided for diverting the sewage onto the filter bed or beds desired, as it is often necessary to take one filter bed out of operation during scheduled rest periods. Providing such rest periods will prevent surface clogging that results in sewage ponding above filter media. When three filters are employed, only two beds are normally used at any one time, the other bed being held out of operation for rest periods or maintenance, if required.

6. In the design of intermittently flooded sand filters the area of the filter beds is normally based upon a rate of application of 2.3 gallons per square foot per day. Also, a sufficient amount of settled sewage should be discharged onto the sand bed surface to cover the sand to a depth of two inches.

7. A rotary distributor will accomplish uniform application of settled sewage over the sand filter surface. A uniform application will maintain the design treatment efficiency of the filter so that a relatively higher dosage rate may be utilized or, for equal sewage flows, the area of sand bed required may be less than other designs. The design of the area of the filter beds equipped with rotary distributors should be based on an application rate of 3.5 gallons per square foot per day. The amount of sewage applied to the sand filter at each discharge of the dosing siphon should be equal to a depth exceeding one-half inch over the entire sand bed area being dosed.

8. The rate of dosage onto a buried sand filter shall not exceed 1.15 gallons per square foot per day of settled sewage. Settled sewage shall be applied to the filter through lines of drain tile laid with open joints, with the tile placed in a 12-inch layer of No. 3 stone. The top of the filter may be finished with a 12-inch layer of stone. Where it is not feasible or desirable to finish the top of the subsurface filter with stone, a 3-inch layer of straw covered with a four to eight inch layer of top soil may be used. Open joint underdrain tiles shall be sloped one inch per 10 feet and shall be installed in the base gravel and connected to the effluent pipe. The ends of the distribution lines should be tied together into a manifold and should be vented to the atmosphere. All open joints shall be covered with collars of asphalt paper or other suitable material.

Distribution boxes must be provided for diverting sewage onto the filter beds through headers, with each header connecting to not more than four distribution lines, where multiple units are used. Each application must completely fill the tile lines in use.

9. Consideration should be given to providing recirculation for granular media filters to improve treatment performance. Recirculating sand filters should be designed using a hydraulic loading rate of 3-1/2 gallons per day per square feet, based on average daily flow, with an organic loading rate not to exceed 0.005 pounds of BOD5 per day per square foot of surface area. A recirculation ratio greater than 3:1 shall be provided. The use of granular media filters for nutrient removal will be evaluated on a case by case basis based on evaluation of performance data. Granular media filters shall be timer dosed and adjustable from one to 10 minutes of dosing per 30 minutes on time.

Statutory Authority

§ 62.1-44.19 of the Code of Virginia.

Historical Notes

Former 12VAC5-581-730 derived from Virginia Register Volume 18, Issue 10, eff. February 27, 2002; amended and adopted as 9VAC25-790-670, Virginia Register Volume 20, Issue 9, eff. February 12, 2004.

9VAC25-790-680. Rotating biological contactors.

A. The rotating biological contractor (RBC) treatment process may be used to accomplish carbonaceous and nitrogenous oxygen demand reductions. Expected performance of RBC equipment shall be based upon experience at similar full scale treatment works or through documented pilot scale testing with the particular wastewater.

B. Design. A minimum of two independent RBC units shall be provided for treatment works greater than 100,000 GPD. Provisions for positive and measurable flow control to individual contactors shall be provided. Piping shall permit each reactor to be operated in the parallel or series flow mode. The design of the RBC shaft and media support structures shall assure protection from structural failure for the design life of the treatment works.

1. In determining design loading rates, the following parameters shall be considered: design flow rate and influent waste strength; percentage of BOD5 to be removed; media arrangement, including number of stages and unit area in each stage; rotational velocity of the media; wastewater temperature; and percentage of influent BOD5 that is soluble. The maximum first stage loading shall not exceed three pounds soluble BOD5 per day per 1,000 square feet of media surface area.

2. The contactor basin should be designed to allow a submergence of 30% to 40% based on total media surface area.

a. The clearance between the tank floor and the bottom of the rotating media shall be four to nine inches to maintain sufficient bottom velocities and prevent solids deposition in the tank.

b. Suitable means shall be provided to dewater each basin.

3. Rotating biological contactors shall be covered to protect the biomass from cold temperatures and the media from direct sunlight.

4. Enclosures shall be constructed of corrosion resistant material. Adequate clearance shall be provided for normal maintenance and reasonable access to the rotating shafts and for observation of the biomass. Windows or simple louvered mechanisms shall be provided for adequate equipment ventilation. To minimize condensation the enclosure should be insulated or heated.

C. Features. Provisions shall be made to allow access to the shaft bearings for routine maintenance and removal. In addition, hydraulic load cells (i.e., bearing lift or electronic strain gauges) should be provided to allow a determination of total shaft weight, which in turn can be used to estimate the depth of attached growth or the biofilm thickness. The drives used for shaft rotation may be provided through either mechanical gear reducers or special media features that utilize aeration as a turning force. A stand-by drive assembly shall be provided to ensure continuous operability.

1. Rotational velocity directly affects the level of wastewater treatment by providing contact, aeration, and mixing between the biomass and wastewater. The optimum rotational velocity will vary with the specific installations and is generally in the range of one to two revolutions per minute (RPM).

2. RBC mechanical drive assemblies should have the capability to vary shaft rotational speed for dissolved oxygen and biofilm thickness control. Drive systems and motors shall be provided with protective coatings suitable for high humidity environments.

3. Supplemental aeration shall be provided for the first stage of all mechanically driven RBC units with first stage soluble organic (SBOD5) loadings greater than two pounds/1000 square foot of media surface. The air flow shall be supplied by air headers and diffusers located beneath the rotating media at a rate of not less than 1.25 cfm/1000 square foot of media surface area. The total design air flow rate may be provided by a single blower; however, two blowers, each providing 50% of the total air flow rate, are recommended. The design shall provide the capability to vary the volume of air delivered to handle fluctuations in the treatment works loading.

4. The design of an air drive system shall provide the capability to vary the volume of air delivered to handle fluctuations in treatment works loading or to control shaft rotational speed and biofilm thickness.

a. Air delivered shall not be less than 2.5 cfm/1000 square foot of media surface area to meet treatment objectives. For operational flexibility and biofilm thickness control, blowers shall be provided in multiple units, so arranged and in such capacities to allow delivery of 150% of the treatment air requirement with the single largest blower unit out of service.

b. Provisions shall be made for independent air flow metering and control to each RBC shaft.

5. At least two stages of RBC media for each flow stream shall be provided for secondary treatment applications. Additional stages shall be provided for nitrification or enhanced BOD5 removals.

6. Design consideration should be given to providing: (i) recirculation of secondary clarifier effluent; (ii) positively controlled alternate flow distribution, such as step feed; and (iii) combination air/mechanical drive systems.

Statutory Authority

§ 62.1-44.19 of the Code of Virginia.

Historical Notes

Former 12VAC5-581-740 derived from Virginia Register Volume 18, Issue 10, eff. February 27, 2002; amended and adopted as 9VAC25-790-680, Virginia Register Volume 20, Issue 9, eff. February 12, 2004.

9VAC25-790-690. Suspended growth (activated sludge) process.

A. A number of variations of suspended growth treatment systems can be designed, featuring combinations of reactors utilizing aeration to support suspended biomass, and secondary clarifiers to separate suspended solids from the secondary effluent, that are known as activated sludge processes. Design standards, operating data, and experience for some of these variations are not well established and may not be considered as conventional design.

B. Design. The possibility of nonconventional technology approval should be considered in selection of a process modification. The conventional process and its various modifications may be expected to consistently produce an effluent containing no more than 30 milligrams per liter of either Biochemical Oxygen Demand (BOD5), or total suspended solids (TSS), within the boundaries of the design parameters described in this chapter and with effective operation.

1. Designs to meet effluent limits more stringent than conventional secondary levels will be considered on a case-by-case basis when additional provisions such as flow equalization, increased clarifier capacity, or other process enhancement are proposed.

2. When the design includes multiple suspended growth reactors or aeration basins, provisions for combining the influent and return sludge and proportionally distributing the combined flows to each reactor shall be included to the extent practical. When the design includes multiple clarifiers, provisions for combining the effluent flows from all aeration basins and proportionally distributing the basin effluent with a uniform biomass concentration (mixed liquor suspended solids (MLSS)) to each secondary clarifier shall be included to the extent practical.

3. Effective removal of grit, debris and excessive oil or grease and grinding or fine screening of solids shall be accomplished prior to the activated sludge process. Aerated grit chambers alone will not provide adequate solids reduction.

C. Nitrification. The following requirements apply to activated sludge treatment works designed to provide nitrification.

1. The extended aeration modification shall be provided for single-stage activated sludge systems with a design flow of 0.5 mgd or less. Other modifications may be utilized for activated sludge systems with design flows greater than 0.5 mgd or two stage activated sludge systems; however, the design shall ensure that an adequate nitrifying bacteria population can be maintained during the required time period (i.e., seasonal or year-round) without excessive reactor biomass (MLSS). This requires (i) a longer detention time; (ii) a longer mean cell residence time (MCRT) with a relatively high ratio of the amount of biomass in the process compared to the rate of loss or wastage of biomass; and (iii) a lower organic loading rate than that required for carbonaceous organic removal alone.

2. The design for processes other than the extended aeration modification shall be based on satisfactory process performance obtained at full scale or pilot scale facilities. Performance data and information from such facilities shall be included with the design data submittal and shall particularly address temperature and pH dependence of the nitrification process.

3. Flow equalization or other proven methods to eliminate the likelihood of loss of biomass or activated sludge washout shall be provided for sewage treatment works subject to infiltration/inflow rates which could be expected to result in periodic biomass or activated sludge nitrifier washout.

4. Feed equipment for the addition of chemicals to maintain a minimum alkalinity of 50 mg/L in the aeration basin contents (mixed liquor) shall be provided when necessary, based on the characteristics of the influent wastewater. Approximately 7.2 pounds of alkalinity will be destroyed per pound of ammonia nitrogen oxidized. The design of the feed equipment shall meet the requirements of this chapter.

D. Reactor requirements. Multiple aerated suspended growth reactors (aeration basins) capable of independent operation shall be provided for all treatment works rated at greater than 40,000 gallons per day, with this exception: single units may be allowed for Reliability Class II and Class III treatment works having a capacity up to 100,000 gpd when the appropriate reliability and continuous operability requirements are satisfied, and provided that all aeration equipment is removable for inspection, maintenance and replacement without dewatering the reactor or clarifiers.

1. The size of the aeration basin for any particular adaptation of the process shall be based on such factors as (i) the design flow; (ii) degree of treatment desired; (iii) sludge age, (MCRT); (iv) mixed liquor suspended solids concentration (MLSS); (v) BOD5 loading; and (vi) food to microorganism ratio (F/M). Calculations shall be submitted to justify the basis of design of the aeration basin capacity and process efficiency.

2. Aeration basin detention times, recirculation ratios, and permissible loadings for the several adaptations of the process are shown in Table 5. Operational parameters (sludge age, F/M, and MLSS) for the various process modifications are also included in this table as a guide.

3. The dimensions of each independent aeration basin or any off-line reaeration basins shall be such as to maintain effective mixing and utilization of air. Liquid depths should not be less than 10 feet except in special design cases.

For very small basins (volume less than 40,000 gpd) or basins with special configuration, the shape of the basin or the installation of aeration equipment should provide for elimination of short-circuiting through the basin. Aeration basins should have a freeboard of at least 18 inches.

4. Inlets and outlets for each aeration basin shall be suitably equipped with valves, gates, stop plates, weirs or other devices to permit control of the flow and to maintain reasonably constant liquid level. The hydraulic properties of the system shall allow the anticipated maximum instantaneous hydraulic load or peak flow to be carried downstream with any single aeration basin out of service.

5. Channels and pipes carrying liquids with solids in suspension shall be designed to maintain self-cleaning velocities or the flow shall be mixed to keep such solids in suspension at all rates of flow within the design limits. The means for adequate flow measurement shall be provided in accordance with Table 6 of this section.

6. Foam control devices shall be provided for aeration basins. Suitable spray systems or other appropriate means will be acceptable. If potable water is used, approved backflow prevention shall be provided on the water lines. The spray lines shall have provisions for draining to prevent damage by freezing.


Process Detention Modification Time (Hr.)

Recirculation Flow Regime Ratio

MCRT (Days)

Food to micro-organism Ratio (F/M)

Reactor Loading #BOD5 per 1,000 cu. ft.

(MLSS) Suspended Solids (mg/L)

Conventional 4–8

PF 0.25–1.0





Complete Mix 4–8

CM 0.25–1.0





Step Aeration 4–8

PF 0.25–1.0





Contact Stabilization 0.5–1.5(1)


PF 0.25–1.5






Extended Aeration(3) 24

PF 0.25–1.5





High Purity Oxygen(4) Systems 1–5

CM 0.25–0.5






F indicates the amount of available organic substance in the influent to the reactor. M indicates the amount of viable biomass in the reactor measured as the volatile portion of the total suspended solids level (MLSS) in the reactor. PF indicates a plug flow hydraulic characteristic in which the measured residence time is 80% or more of the theoretical detention time. CM indicates a completely mixed basin whose contents have essentially the same characteristics as the average levels within the basin effluent. See 9 VAC 25-790-460 E (Table 4) for estimated values of secondary effluent from activated sludge reactors followed by secondary clarifiers.

(1)Contact Unit

(2)Solids Stabilization Unit

(3)Includes Oxidation Ditch Systems

(4)Reactors in Series


Flow Stream

Treatment Works Design Capacity, Q, MGD

Q 0.04

0.04 < Q 1.0

Q > 1.0

Influent Sewage to each aeration basin(1)



Indicating & Totalizing(2)

Air to each aeration basin




Return Activated Sludge to each aeration basin(1)



Indicating & Totalizing(2)

Waste Activated Sludge

Indicating & Totalizing

Indicating & Totalizing

Indicating, Recording & Totalizing


(1)Where it can be verified by calculations or pilot studies that proportional flow distribution to each aeration basin can be maintained, then flow measurement devices for the influent and return activated sludge to each basin may not be required. However, as a minimum, the total influent and return activated sludge flows shall be provided with flow measuring devices to measure each flow separately.

(2)Recording and totalizing may not be required where adequate flow control is provided and totalizing refers to the total flow not individual basin flow.

E. Aeration. Oxygen requirements generally depend on BOD5 loading, degree of treatment and level of biomass or suspended solids concentration to be maintained in the aeration basin (MLSS). Aeration equipment shall be designed to meet the oxygen demands of the activated sludge process and provide adequate mixing to rapidly mix the influent with the reactor contents and maintain the reactor biomass (MLSS) in uniform and complete suspension.

1. When the applied wastewater contains a substantial portion of industrial wastes which have characteristics significantly different from domestic wastes, then experimentally derived data shall be submitted to support the proposed oxygen requirements for the process. Calculations shall be submitted to justify the oxygen requirements and the equipment capacity.

2. The oxygen requirements for domestic waste shall be a minimum of 1.2 pounds of oxygen per pound of applied BOD5 for the extended aeration process and a minimum of 1.1 pounds of oxygen per pound of applied BOD5 for other processes listed in Table 5 of this section. In addition, oxygen requirements for nitrification of ammonium nitrogen shall be a minimum of 4.6 pounds of oxygen per pound of applied ammonium nitrogen for the extended aeration process, and for other processes, unless the proposed operation procedures will preclude nitrification by employing a low sludge age (MCRT).

3. The oxygen shall be supplied at a rate that can maintain a minimum aeration basin dissolved oxygen concentration under critical environmental conditions (i.e., temperature, pressure) of: 2.0 mg/l at average design organic loading, or 1.0 mg/l at peak design organic loading, whichever is greater.

4. The peak organic loading rate shall be the maximum organic loading applied to the aeration basin during a six-hour period. When influent data is not available or for new treatment works, the peak organic loading rate shall be two times the design average daily organic loading rate.

5. Certified test data shall be obtained for regulatory evaluation prior to installation that demonstrates the standard clean water oxygen transfer capabilities of the proposed diffused aeration equipment for treatment works with a design flow greater than 100,000 gpd and for proposed mechanical aeration equipment for all treatment works. The test data shall be developed using similar reactor and aerator configuration, basin depth, aerator depth as applicable, and air or energy input rates as proposed in the design. The procedures for conducting the clean water oxygen transfer tests shall be in accordance with the latest ASCE Standard for Measurement of Oxygen Transfer in Clean Water (see Part IV (9VAC25-790-940 et seq.) of this chapter).

6. The field oxygen transfer rate shall be calculated from the standard clean water oxygen transfer rate using the following equation:

Equation 1:

OTRf =

(Alpha)(SOTR)(Theta(T 20))(Tau*Beta*Omega*C*20-C)/C*20


OTRf =

Field oxygen transfer rate estimated for the system operating under process conditions at a D.O. concentration, C-mg/l, and temperature, T-°C.

Alpha =

Oxygen transfer correction factor for wastewater = (average wastewater KLA)/(average clean water KLA)


Standard Oxygen Transfer Rate for clean water at standard conditions.

Theta =

Empirical temperature correction factor; usually taken as 1.024.

T =

Temperature in mixed liquor at design operating conditions, °C

Tau =


C*st =

Tabular dissolved oxygen surface saturation value for clean water at standard barometric pressure of 1.00 atm, 100% relative humidity, and critical design operating temperature, mg/L.

C*s20 =

Tabular dissolved oxygen surface saturation valve for clean water at standard barometric pressure of 1.00 atm, 100% relative humidity, and standard temperature of 20°C, mg/L.

Beta =

Dissolved oxygen saturation correction factor for wastewater = (dissolved oxygen saturation value for wastewater at standard conditions)/(dissolved oxygen saturation value for clean water at standard conditions).

Omega =

Pressure correction factor



Pb =

Critical design operating barometric pressure, atm.

Ps =

Standard barometric pressure of 1.00 atm.

C*20 =

Dissolved oxygen saturation valve for a given aeration device at standard barometric pressure of 1.00 atm and standard temperature of 20°C.

7. A discussion of the Alpha and Beta factors is provided in Part IV (9VAC25-790-940 et seq.) of this chapter. Further description and discussion of terms are provided in the ASCE Standard and Annexes for the Measurement of Oxygen Transfer in Clean Water and other related publications.

8. When conventional diffused air equipment performance data is not submitted, then minimum air supply to meet the oxygen requirements in terms of cubic feet of air per minute per pound of applied BOD5 to the aeration basin shall be 1,500 CF /lb. per day BOD5 for the conventional, complete mix, step aeration, and contact stabilization processes and 2100 CF /lb. BOD5 for the extended aeration process.

9. Air supply for mixing requirements shall be 20 to 30 cubic feet per minute of air per 1,000 cubic feet of aeration basin volume. Air supply volume requirements shall be increased for aerated channels, pumpwells, or other air-use demands.

10. The air supply blowers shall be provided in multiple units, so arranged and in such capacities as to meet the maximum air demand with the single largest unit out of service. The design shall also provide for varying the volume of air delivered in proportion to the load demand of the treatment works. Time clocks or variable speed drives are acceptable. In addition, positive displacement blowers shall be equipped with either multispeed pulleys with sufficient horsepower or other means to change the speed from the motor drive up to the highest speed and capacity. The specified capacity of blowers or air compressors, particularly centrifugal blowers, shall take into account that the air intake temperature may reach 40°C (104°F) or higher and the pressure may be less than normal. Air supply intake filters shall be provided in numbers, arrangement and capacities to furnish at all times an air supply sufficiently free from dust to prevent clogging of the diffuser system used.

11. The spacing of diffusers in basins or channels shall be in accordance with the oxygenation requirements through the length of the basin or channel and should be designed to facilitate adjustments of their spacing without major revision to airheader piping. The arrangement of diffusers should also permit their removal for inspection, maintenance and replacement without shutting off the air supply to other diffusers in the basin or otherwise adversely affecting treatment performance.

12. Individual assembly units of diffusers shall be equipped with control valves, preferably with indicator markings for throttling or for complete shutoff. Diffusers in any single assembly shall have substantially uniform pressure loss.

13. The mechanism and drive unit for mechanical aerators shall be designed for the expected conditions in the aeration basin in terms of the proven performance of the equipment. The aeration equipment shall be designed to provide the total projected oxygen requirements. Minimum power input shall be 0.5 to 1.3 horsepower per 1,000 cubic feet of aeration basin volume for mixing. The design basis for determining mechanical mixing requirements shall be submitted. Due to the heat loss incurred by surface mixing, consideration shall be given to protecting treatment unit operations from ice and freezing effects.

14. Multiple mechanical aeration unit installations shall be so designed as to meet the maximum air demand with the largest aeration unit out of service. The design shall also provide for varying the amount of oxygen transferred in proportion to the organic loading. Time clocks, variable speed drives or variable aeration basin level controls are acceptable. A spare aeration mechanism shall be furnished for single unit installations.

F. Biomass control. The design of an activated sludge process shall include methods for returning settled biomass (secondary sludge) back to the inlet section to the aeration basin. The minimum secondary sludge return rate of withdrawal from the secondary clarifier or clarifiers is a function of the concentration of suspended solids in the aeration basin (mixed liquor) that are contained in the secondary clarifier influent. In addition, the secondary sludge volume index (as determined by Standard Methods for the Examination of Water and Wastewater) and the length of time that a design depth of sludge (blanket) is to be retained in the settling basin should be considered when selecting a sludge return rate.

1. The rate of sludge return expressed as a ratio of the average design flow shall generally be variable between the limits set forth in Table 5. The rate of sludge return shall be varied by means of variable speed motors, drives, air assisted withdrawal, flow control methods, or timers for such operations.

2. If motor driven sludge return pumps are used, the maximum return sludge capacity shall be obtained with the largest pump out of service. If air lifts are used for returning sludge from each clarifier basin, no standby unit will be required, provided the design of the air lifts are such as to facilitate their rapid and easy cleaning and if other suitable standby measures are provided.

3. Suction and discharge piping shall be designed to maintain a velocity of not less than two feet per second when sludge return facilities are operating at normal return sludge rates. Suitable devices for observing, sampling and controlling secondary sludge return flow from each secondary clarifier shall be provided.

4. The design of activated sludge processes shall provide methods for controlling the rate at which secondary sludge (waste sludge) is transferred to further treatment. For those treatment works with a capacity of one mgd or higher, the daily capacity for waste sludge transferal to sludge handling and treatment facilities should equal or exceed 20% of the total aerated reactor volume. For treatment works with a design capacity of less than one mgd, such waste sludge facilities should provide a minimum return rate of 10 gallons per minute. Means for observing, sampling and controlling waste sludge flow shall be provided.

G. High purity oxygen. The following additional requirements apply to activated sludge systems which utilize high purity oxygen for aeration.

1. The design of activated sludge processes utilizing pure oxygen aeration shall provide for covered and compartmentalized reactors to provide a series of stages for biological growth. Sampling ports shall be provided for each compartment of the biological reactors. An enclosed air-oxygen exhaust system shall be provided to collect and vent the reactor off-gases.

2. Mixing equipment shall be sufficient to maintain solids in suspension. Normally, the power input should be 0.5 to 1.3 horsepower per 10 cubic feet of aerator volume. The design basis for determining mixing requirements shall be submitted. Provisions shall be included for rapid removal or cleaning of the mixers.

3. The high purity oxygen storage and generation facilities and piping shall be remotely located from areas where flammable or explosive substances may be present. Warning signs shall be posted in the area of the oxygen storage and generation facilities. The covered aeration basins should be equipped with explosive atmosphere monitors and alarms in accordance with applicable state and federal regulations. An influent hydrocarbon monitor shall be included at the headworks to initiate operation of purge air blowers to vent reactor oxygen when explosive mixtures could occur.

4. At least two sources of oxygen shall be provided. On-site storage of oxygen for emergencies and peak demands is required. Storage of oxygen shall be determined by engineering analysis of the availability and delivery of oxygen to the treatment works site.

H. Biomass support systems. Modifications to the activated sludge process in which attached growth supports are located within the aeration basins will be considered on a case-by-case basis evaluation of performance data and approved through the provisions of this chapter.

Statutory Authority

§ 62.1-44.19 of the Code of Virginia.

Historical Notes

Former 12VAC5-581-750 derived from Virginia Register Volume 18, Issue 10, eff. February 27, 2002; amended and adopted as 9VAC25-790-690, Virginia Register Volume 20, Issue 9, eff. February 12, 2004.

9VAC25-790-700. Oxidation ditches.

A. An oxidation ditch process typically employs an extended aeration type of activated sludge process with a single channel or multiple interconnected concentric channels used as an aeration basin with a detention volume of 18 hours or more at the design flow rate. However, they may utilize some batch type operational principles.

B. Design. The geometry of the channels can vary; however, the oval is the most common configuration. Design requirements involving the use of duplicate oxidation ditches within the flow range of 40,000 gpd to 200,000 gpd shall be determined by the reliability class of the treatment works (Class I, II or III), the nutrient removal requirements, and the use of conventional dual final clarifiers. For design flows up to 100,000 gpd, a single oxidation ditch should be sufficient for secondary treatment of discharges to Class I reliability waters, if provided with external duplex clarifiers. In Class II and Class III waters, a single oxidation ditch may be acceptable for secondary treatment of flows up to 200,000 gpd. However, for treatment works permitted with effluent limits less than secondary or nutrient removal requirements, duplicate reactors and clarifiers shall be provided. In other cases, the treatment works size and location may allow for an exception for specific designs.

1. The multiple concentric channel basin can have any number of interconnected channels. This channel design scheme provides some process flexibility, since with minor modifications it can be changed to other activated sludge process modes. Typically, the outer channel (if multiple channels are present) receives unsettled raw sewage with a loading of 15 pounds per 1,000 cubic feet of volume or less. Shallow channels are usually four to six feet deep with 45° sloping walls. Deep channels have vertical side walls and are normally 10 to 12 feet deep.

2. The channels are characteristically lined to prevent erosion and leakage. Ditch lining should be constructed of reinforced concrete, asphalt or plastic liners. Shallow channels with sloped sidewalls are often constructed of concrete poured against earth backing and reinforced with welded wire mesh. Deep vertical wall channels require reinforced concrete walls.

3. Oxidation ditches may also be operated in alternating modes through on/off operation of aeration/mixing devices with intermittent changes in flow rates or direction. Influent wastewater can be diverted through one or more multiple reactors in which different operational phases (anoxic, aerobic, etc.) may occur. Effluent clarification may be accomplished within the reactor or within a separate clarifier. Automatically controlled weirs regulate flow direction and alternating operation of aeration/mixing equipment controls the operating mode. As with either standard continuous flow, or batch-type processes, the design duration of each operating phase is critical to performance reliability.

C. Aeration. Since oxidation ditches are considered a variation of the extended aeration modification of the activated sludge process, the requirements set forth in this chapter are applicable except as follows:

1. The mixing system shall be capable of maintaining a minimum velocity throughout the oxidation ditch cross-section of 1.0 fps at maximum design depth and solids concentration. For designs utilizing in-channel suspended solids removal the mixing system shall provide for all necessary variations in flow velocity to achieve adequate separation of suspended solids. Calculations and certified performance data for the mixing system shall be submitted to substantiate the adequacy of the proposed design.

2. Designs based on anoxic operation shall provide mixing and aeration system capacity for aerobic operation with adequate turndown capability to operate in the anoxic mode. Flexibility to allow for operation in the anoxic mode should be considered for all designs.

3. Designs should provide for variation in the oxygen supply independent of the mixing function.

4. The outlet from the oxidation ditch shall be separated from the inlet in such a manner as to prevent discharging of partially treated effluent.

5. Intra-channel clarifiers may be utilized if conventional settling rates are maintained and sludge handling, treatment and management provisions are satisfactorily addressed.

Statutory Authority

§ 62.1-44.19 of the Code of Virginia.

Historical Notes

Former 12VAC5-581-760 derived from Virginia Register Volume 18, Issue 10, eff. February 27, 2002; amended and adopted as 9VAC25-790-700, Virginia Register Volume 20, Issue 9, eff. February 12, 2004.

9VAC25-790-710. Sequencing batch reactors (SBR).

A. Usage. In accordance with the requirements of this chapter and standards contained in this chapter, batch operation modifications of the activated sludge process will be considered as conventional secondary treatment processes. Adequate performance data and information from a full scale treatment works of similar design, including the levels of influent wastewater characteristics that produce hydraulic and organic loading rates within 25% of values used in the proposed design, shall be provided if an SBR design is to accomplish an effluent quality more advanced than a secondary level.

B. Design. The design shall meet the applicable loading requirements. The operating cycle normally consists of FILL, REACT, SETTLE, DRAW, and IDLE sequences with alternating sequences of mixing and aeration on and off.

1. A minimum of two basins shall be provided for design flows in excess of 0.1 mgd. The minimum total basin volume shall be equal to the design daily influent flow volume and either upstream in-line or off-line storage is necessary to minimize influent flow during settling and decanting. Effective scum collection and removal equipment shall be provided.

2. The design basis for meeting oxygen requirements shall consider the variation in liquid level depth and the aeration sequence time.

3. The basin depth shall be sufficient to provide optimum separation of the settled biomass and the point of effluent withdrawal.

4. Adequate mixing shall be provided to resuspend settled solids at the start of the FILL sequence and maintain solids in suspension over the design liquid volume range.

5. A high liquid level overflow shall be provided between basins. The overflow shall be located as far as possible from the outlet device and in no case be closer than 10 feet from the outlet device.

6. Inlets to each basin shall be located as far as possible from the outlet and in no case be closer than 10 feet from the outlet.

7. Scum baffles or other suitable arrangements shall be provided to prevent scum from being withdrawn with the effluent.

8. Outlet facilities shall be designed to prevent resuspension of the settled solids in the basins. An adjustable flow rate control device shall be provided on each basin outlet.

9. Waste sludge control facilities shall have a rate per day equal to 50% of the total basin volume.

10. The FILL and DRAW sequences for an individual basin shall not overlap.

C. Features. Automatic control valves and switches shall be provided for controlling the operating sequences of each basin. Automatic control valves shall be capable of manual operation. Control sequences shall be adjustable to allow flexibility in operating time periods for each sequence. The control system shall provide automatic operation of the inlet valve and outlet valve to each basin, the air supply valve to each basin and the blowers or mechanical aerators.

1. The control system shall provide for automatic operation of downstream units or equipment as necessary. The control system should also provide automatic control of the waste sludge removal system. A spare automatic control unit shall be provided.

2. A monitoring system shall be provided that will indicate control system status and actual valve position of each automatically operated valve. Also, the control system status and the actual operational status of both the air supply system (blowers or mechanical aerators) and sludge removal system shall be monitored, when equipped for automatic operation. The monitoring system shall include an alarm to indicate a malfunction of the control system.

3. Dual hydraulic pumps and air compressors shall be provided when such facilities are utilized in conjunction with valve operators. Electrically operated valves shall be designed so as to fail only in the open position.

4. A high liquid level alarm for each basin shall be provided to signal an alarm condition prior to reaching the overflow pipe or port.

5. Influent flow and effluent flow measurement for the treatment facility shall be provided. The extent of the flow measurement equipment shall be in accordance with this chapter.

6. Disinfection and other downstream treatment units shall be sized based on the maximum design DRAW sequence flow rate.

Statutory Authority

§ 62.1-44.19 of the Code of Virginia.

Historical Notes

Former 12VAC5-581-770 derived from Virginia Register Volume 18, Issue 10, eff. February 27, 2002; amended and adopted as 9VAC25-790-710, Virginia Register Volume 20, Issue 9, eff. February 12, 2004.

9VAC25-790-720. Sewage stabilization ponds and aerated lagoons.

A. General design. Basins with surface areas many times larger than conventional biological reactors, that utilize relatively low (less than 500 mg/l) levels of biomass, are typically referred to as stabilization ponds (if unaerated) but are referred to as facultative lagoons if aerated. This section provides criteria for achieving final effluent levels of 45 mg/l BOD5 and 45 mg/l, or higher suspended solids, as permitted limits applicable to the geographic allowance for sections of Virginia. This level of treatment has been established in accordance with the federal requirements for secondary treatment equivalency as achievable through the use of stabilization ponds and facultative lagoons. The design information contained herein pertaining to features other than biological treatment performance criteria shall apply to the construction of earthen basins used in the treatment of sewage.

Stabilization ponds or facultative lagoons may be designed to achieve a higher degree of treatment or used as a biological treatment phase in conjunction with other unit processes. Proposed design parameters to achieve other than 45 mg/l BOD5 effluent limits shall be thoroughly reviewed with the area engineer during the preliminary engineering conference. Necessary features for protecting public health and welfare and preventing potential violations of water quality standards shall be addressed in the design report.

1. The engineering design report shall contain pertinent information on location, geology, soil conditions, area for expansion, and any other factors that may affect the feasibility and acceptability of waste stabilization ponds or aerated lagoons used for sewage treatment. Specifically, the report shall contain the following supplementary field survey data.

a. The location and direction of all residences, commercial development, recreation areas and potable water supplies within one-half mile of the proposed pond or lagoon site. If practicable, ponds and lagoons should be located so that local prevailing winds will be in the direction of uninhabited areas.

b. Borings or other necessary geophysical analyses required to determine surface and subsurface characteristics of the immediate area and their effect on the construction and operation of ponds or lagoons located on the site.

c. Data demonstrating anticipated permeability at the elevation of the proposed pond or lagoon bottom.

d. A description, including maps showing elevations and contours, of the site and adjacent areas suitable for expansion.

e. A closure plan shall be submitted to the department prior to issuance of an operating permit.

2. The proximity of ponds or lagoons to potable water supplies and other water resources subject to potential contamination and location in areas of porous soils and fissured rock formations within the depth directly affected by the ponds or lagoons shall be reported to avoid area contamination. Monitoring and more stringent construction requirements may be required after consideration of such factors as distance from water sources, water uses, installation size, liner design, and wastewater characteristics. Adequate provisions shall be made to divert storm water around the ponds or lagoons and otherwise protect pond embankments.

3. Access control for the immediate area surrounding the ponds or lagoons shall be addressed by sufficient means, such as a woven wire fence at least six feet high. Vehicle access control shall be provided. Any access gate(s) shall be provided with locks.

a. Appropriate signs shall be provided along the secured perimeter or fence around the ponds or lagoons to designate the nature of the facility and advise against trespassing. The size of the sign and lettering used shall be such that it can be easily read by a person with normal vision at a distance of 50 feet.

b. Access for maintenance equipment, transporting chlorine cylinders and inspection shall be provided by an all-weather entrance road.

B. Loading design. For stabilization pond design with relatively uniform organic and hydraulic loading, the maximum loading shall be 30 pounds of BOD5per day per total surface acreage, measured at the four-foot water depth level. For stabilization ponds that are not intended to meet federal secondary treatment equivalency limitations but will be used for pretreatment, higher loading rates may be acceptable.

1. In no case shall the detention time be less than 45 days, based on a four-foot operation level. For purposes of design, evaporation is to be considered equal to rainfall. At a minimum, a pond system shall consist of two physically separated ponds providing three separate treatment cells. For treatment works receiving an average design flow of less than 0.04 mgd, a minimum of one pond with two treatment cells may be acceptable. Organic loading to the first upstream or primary cells receiving sewage influent shall be a maximum of twice the total design loading for the system.

2. The shape of all cells shall be designed to provide even distribution of flow throughout the system. Round or square ponds are acceptable; however, rectangular ponds with high length to width ratios (up to 10:1) are considered most desirable. If round or square ponds are used, appropriate aeration arrangements and baffling shall be provided in order to minimize short-circuiting. Earth dikes shall be rounded at corners to minimize accumulations of floating materials.

3. Multiple sections of pond volume or cells designed so as to be capable of receiving design loadings under both series and parallel operation are required for all except small treatment works (one-half acre of pond surface or less). The minimum freeboard shall be two feet above the maximum operation depth, except for treatment works receiving less than 40,000 gpd. Operation depth requirements include:

a. The minimum operation depth shall be two feet, excluding any sludge storage section.

b. The maximum operating depth shall be five feet, excluding any sludge storage section.

4. For Class I reliability, the treatment works should provide for operation under winter conditions. The design should include considerations for, but not limited to, winter storage and supplemental aeration, to prevent effluent deterioration during cold weather conditions.

5. Installations provided for intermittent operation at a higher than normal loading for a relatively short portion of the year will be individually considered, taking into account the ability of the volume of the pond system to absorb shock loads.

6. The pond design shall include provisions for sludge storage. The volume of sludge storage should be based on a 20-year design life. The sludge storage section should be located in the upstream portion of the primary cells of the pond system.

7. Piping should be provided around the first cell in order to allow for parallel operation of the first two upstream cells in a pond system.

C. Features. Embankments and dikes shall be constructed of relatively impervious materials and compacted sufficiently to form a stable structure. Vegetation should be removed from the area upon which the embankment is to be placed. Embankment material shall be free of vegetative material and large rocks (more than six inches in length). Topsoil relatively free of debris may be used as outer slope cover material. Construction details including methods of construction, compaction details, inspection and construction certification shall be included in the design specifications. Soils used in constructing the side slopes shall either be compacted within 3.0% of the optimum moisture content to at least 90% Standard Proctor Density, or compacted in accordance with the proper site specific geotechnical recommendations.

1. The minimum embankment top width should be eight feet to permit access of maintenance vehicles. Lesser top widths will be considered for lagoons designed to serve 200 persons or 0.040 mgd or less. The top width must be designed to allow adequate maintenance.

2. Outer slopes should not be less than three-horizontal-to-one-vertical and the inner slope should not be less than three-horizontal-to-one-vertical nor greater than four-horizontal-to-one-vertical.

3. Exposed embankments and excavated areas shall be protected against erosion by suitable seeding, sodding or other methods. Additional protection for embankments, such as riprap, may be necessary to protect against wave action and flood currents. A method shall be specified that will prevent vegetation growth one foot above and below the operating water levels.

4. The pond shall be as level as possible at all points. Finished elevations shall not be more than three inches from the average elevation on the bottom. The bottom shall be cleared of vegetation and debris. Organic material thus removed shall not be used in the dike core construction.

D. Liners. A liner shall be provided for all ponds in order to minimize seepage. Material shall be of acceptable standard to assure uniform placement and quality. Standard ASTM procedures or acceptable similar methods shall be used for all tests. Natural soil and enhanced soil (bentonite, cement, etc.) material used as liners should be capable of achieving a maximum coefficient of permeability of one tenth of one millionth of one centimeter each second (1X10-7cm/sec) or approximately three centimeters per year or less. Following the specified level of compaction, liner material used for the pond's side and bottom shall have a coefficient of permeability of one millionth (1X10-6) cm/sec or less. Bentonite, asphalt, and other sealant additive materials should be considered to enhance the impermeability of natural soil liners.

1. Synthetic liner material shall be selected considering the application and manufacturer's use recommendations. Minimum requirements for generally used materials are:

a. Plastic film (nonreinforced, covered)—thickness equal or greater than 0.020 inches.

b. Plastic film (nonreinforced, noncovered)—thickness equal or greater than 0.050 inch.

c. Asphalt panels (covered)—thickness equal to or greater than 0.25 inch.

d. Asphalt panels (noncovered)—thickness equal to or greater than 0.50 inch.

2. Construction should be planned and implemented to assure liner integrity throughout the coverage area for the design life of the liner. The design specifications shall include details of construction, inspection, and certification. Services of qualified soil scientists, manufacturer material certification and inspection, and other qualified means of assuring proper material installation should be used. The liner substrate should be free of organic material, graded, rolled and be level and smooth in nature. The preparation of a stable and adequately smooth substrate is important for liner installation.

3. Natural soil or enhanced soil liners shall be compacted at or up to 4.0% above optimum moisture content to at least 95% Standard Proctor Density (or 90% Modified Proctor Density) throughout the bottom and side coverage area. Soil liners shall not contain rock fragments greater than two inches in the longest dimension and shall have a compacted thickness of at least 12 inches. Soil layers shall be applied in multiple compacted lifts of six inches or less.

4. Soil enhancers (bentonite, cement, hot asphalt) used to improve soil impermeability can be used to reduce the required liner thickness. Although thickness may be reduced with improved impermeability, a minimum thickness of two inches shall be provided. The enhanced soil liner soil matrix should be screened and free of stones greater than 3/4-inches in the longest dimension. Reduced thickness enhanced soil liners should be covered with a six-inch compacted protective soil layer. All layers should be applied in lifts of six inches or less. Presence of smaller gravel will assist in erosion protection.

5. Synthetic liners shall be constructed in accordance with the manufacturer's applicable instructions for liner usage. Generally, these liners should be covered by a protective layer of soil to prevent surface damage and deterioration. The liner shall be top anchored with a minimum berm set back and anchor depth of 18 inches. Unless the manufacturer specifies otherwise, all seams should be perpendicular to the slope with the overlap in the down slope direction. The pond should be subsurface drained or the liner vented to protect against damage due to gas accumulation under the liner. Special care and design will be required to assure a tight seal around inlet and outlet structures. Pads will be required in areas of aerator action and other sources of high velocity flow.

a. If mechanical equipment may result in damage to liner, then a protective layer of soil or other material shall be provided.

b. The pond bottom liner shall be located at least two feet above the seasonal high water table.

E. Hydraulics. The influent line to the pond system shall conform to acceptable material requirements of this chapter. A manhole shall be installed at the terminus of the influent sewer line, preceding the pond system, and shall be located as close to the dike as topography permits. Its invert shall be at least six inches above the maximum operating level of the initial upstream pond to provide sufficient hydraulic head without surcharging the manhole. The influent line to the initial upstream pond shall slope uniformly to the inner toe of the sloping embankment. A bend may be used where the influent line changes direction at the inner toe of the dike embankment and pond bottom.

The sewer upstream from the manhole should not be surcharged unless the means to routinely flush the influent pipeline is provided. If sewage is discharged to the pond system through a force main or mains, an antisiphoning device shall be provided on the force main.

1. Influent and effluent piping shall be located as far apart as possible along the flow path to minimize short-circuiting within the pond.

a. The influent line to each pond should be located approximately at the center of the influent area provided to uniformly distribute influent flow. Influent lines or interconnecting piping to downstream or secondary cells of multiple cells in the pond system, that are operated in series, may consist of pipes through the separating dikes.

b. Influent mixing or dispersion shall be provided for ponds having two acres or more of water surface area. All gravity lines shall discharge horizontally above an erosion resistant surface. Force mains shall discharge vertically upward and shall be submerged at least two feet when operating at the three feet depth. Velocity in the force main at normal pumping rate must be sufficient to prevent deposit of grit in the force main.

c. A concrete-lined pad with a minimum size of four feet square or a surface with equivalent resistance should be provided to prevent erosion at the influent point of discharge to the pond.

2. The outlet structure shall be placed on the horizontal pond floor adjacent to the inner toe of dike embankment. A permanent type walkway from top of dike to top of outlet structure for access shall be provided. The outlet structure shall consist of a well or box equipped with multiple-valved pond draw-off lines. An adjustable draw-off device is also acceptable. The outlet structure shall be designed such that the liquid level of the pond can be varied from a three-foot depth to a five-foot depth in increments of one-half foot or less. Withdrawal points shall be spaced so that effluent can be withdrawn from depths of 0.75 feet to 2.0 feet below pond water surface, irrespective of the pond depth. The lowest draw-off lines shall be 12 inches off the bottom to control eroding velocities and avoid pick-up of bottom deposits. The overflow from the pond shall be taken near but below the water surface. The structure shall also have provisions for draining the pond. A locking device shall be provided to prevent unauthorized access to level control facilities. An unvalved overflow placed six inches above the maximum water level shall be provided.

3. Interconnecting piping for multiple pond installations operated in series should be valved or provided with other arrangements to regulate flow between structures and permit flexible depth control. Interconnecting piping and outlets shall be of materials meeting the requirements of this chapter.

Statutory Authority

§ 62.1-44.19 of the Code of Virginia.

Historical Notes

Former 12VAC5-581-780 derived from Virginia Register Volume 18, Issue 10, eff. February 27, 2002; amended and adopted as 9VAC25-790-720, Virginia Register Volume 20, Issue 9, eff. February 12, 2004.

9VAC25-790-730. Aerated lagoons.

A. Low intensity aerated basins containing relatively low levels (less than 500 mg/l) of biomass are also known as aerated lagoons. The designed construction details of aerated lagoons are often similar to stabilization ponds. However, the aerated lagoon liquid depth shall be sufficient to provide for uniform distribution of dissolved oxygen in the design range of six feet to 15 feet.

B. Design. Not less than two physically separated basins providing a minimum of three treatment cells shall be used to provide the detention time and basin volume required by the lagoon system design. For treatment works less than 0.04 mgd, one basin with two treatment cells may be acceptable. The basins shall be designed to receive established loadings for both parallel and series operation. The air diffusion equipment shall be capable of maintaining sufficient mixing and oxygen concentration in the aerated volume under maximum seasonal demand conditions. Consideration should be given to fixed or floating-type in-pond baffle walls with carefully placed openings, to minimize short circuiting effects and to maximize flow path length. Deep ponds with depths exceeding 10 feet shall be provided with baffling to ensure adequate flow distribution and proper detention.

1. Detention time is dependent on many variables including type of waste, temperature, effective volume and nutrient balance. For a typical sewage influent strength of 300 mg/l or less of BOD5 or TSS, the lagoon system design shall require total detention times in the range of 20 days. In addition to adequate volume to achieve the desired detention time, the design for primary lagoons shall include a minimum of 10% additional volume for sludge storage.

2. The initial upstream, primary cell receiving influent flow shall contain a minimum of one third of the total system volume. For small treatment works (design flow of 0.04 mgd or less) the primary cell shall contain at least one half of the total design volume.

3. Design requirements, as with detention time, may be dependent on many variables. Generally, mixing energy to maintain adequate solids suspension will be the limiting factor. All aerated lagoon systems shall be designed to maintain a normal dissolved oxygen concentration of two mg/l throughout the system. Minimum aeration requirements shall be based on established mass transfer models considering the treatment variables involved. Aeration equipment shall be capable of transferring two pounds of oxygen per pound of BOD5 applied to the basin. Calculations shall be submitted to justify equipment and aeration patterns.

4. The influent to a lagoon shall discharge into a highly turbulent area, if applicable, to facilitate mixing effects. Baffles and pipe diffusers shall be considered for provision of uniform distribution of flow into basins with a surface area of 10 acres or more. All systems shall be designed with piping flexibilities to permit isolation of any cell without affecting the transfer and discharge capabilities of the total system. In addition, the ability to discharge the influent waste load to a minimum of two cells or all primary cells in the system shall be provided. Screening shall be provided on influent lines to prevent damage to mechanical surface aerators.

5. The outlet structure shall be located in a quiescent zone, at such a depth and at the most remote location possible with respect to the basin inlet, so as to minimize suspended solids carryover and maximize basin detention. The outlet structure shall provide for withdrawal at controlled rates for multiple depth levels, such that the liquid level in the basin can be drained and can be varied in an easily accessible manner. A minimum of three incremental withdrawal elevations should be provided, including the minimum and maximum operating depths.

6. Provisions shall be made to allow final solids settling prior to discharge. This provision should be made through the use of either a final settling basin or by providing an adequate quiescent zone toward the end of the final treatment cell. If a final settling basin is used, it shall provide a minimum of 1.5 hours settling time and conform to applicable requirements specified in this chapter.

7. It may be desirable to provide for concrete or soil cement stabilization of bottoms, walls and embankments. However, they will not be required initially unless experience dictates their necessity. Adequate concrete pads shall be provided under mechanical surface aerators to prevent bottom scour. For surface aeration, earthen embankment walls one foot above and one foot below the normal water level must be riprapped or stabilized with other suitable material to prevent erosion by wave action.

C. Mechanical aeration. Not less than two aeration units shall be used to provide the horsepower required. Aerators shall be located such that their circles of influence touch. The circle of influence is that area in which return velocity is greater than 0.15 feet per second as indicated by certified data. Without supporting data the following may be used as a guide:

Nameplate Horsepower

Radius in feet











1. The horsepower shall be sufficient to provide the oxygen required for BOD5satisfaction and mixing. In no case shall the horsepower be less than 10 horsepower per million gallons of basin volume.

A sufficient number of aerators shall be provided so that a design level of dissolved oxygen within a particular cell shall be maintained with the largest capacity aerator in that cell out of service. Installation of the backup aerator should not be required, provided that it can be placed into service prior to a detrimental decrease in dissolved oxygen levels.

2. Floating surface aerators should be anchored in at least three and preferably four directions. Interconnection of floating aerators is discouraged. Flexible cables are preferred over rigid ones.

3. Surface aerators should be designed to prevent icing. Consideration should be given to the installation of splash plates for control of misting. For platform mounted aerators, the platform legs should be spaced at a sufficient distance from the aerator to minimize the effect of ice build-up caused by splashing.

a. Aerator design should provide for periodic and major maintenance and repairs and shall provide for removal of the aerators for replacement if necessary.

b. Provisions shall be made for independent operation of each aerator by on/off switches, time clocks, etc.

D. Diffused aeration. The design for compressed air volume requirements shall include the basin aeration requirements together with air used in other channels, pumps, or other air-use demands. The air diffusion equipment shall be capable of maintaining sufficient mixing and oxygen concentration in the aerated volume under maximum seasonal demand conditions. Provisions shall be made for removal of deposits for unclogging of air diffuser openings. Consideration should be given to minimizing the points of access necessary for cleaning.

1. The specified capacity of blowers or air compressors, (particularly centrifugal blowers), shall take into account that the air intake temperature may reach 40°C (104°F) or higher and the pressure may be less than normal. Air filters shall be provided in numbers, arrangement, and capacities to furnish at all times an air supply sufficiently free from dust to protect equipment and prevent clogging of the diffuser system used.

2. The blowers shall be provided in multiple units, so arranged and in such capacities as to meet the maximum air demand with the single largest unit out of service. The design shall also provide for varying the volume of air delivered in proportion to the design load for individual cells of the lagoon system.

3. Calculations shall be provided to verify that blower pressure is sufficient to dewater the diffuser lines at saturation conditions under normal operating depths.

4. Diffusers shall be arranged in each basin to provide tapered aeration with maximum intensity near the inlet. The spacing of diffusers shall be in accordance with the oxygenation requirements of the total process, i.e., the organic loading in each cell. Diffuser spacing should be designed to facilitate adjustments without major revision to air header piping. The arrangement of diffusers should also permit their removal for inspection, maintenance, and replacement without completely dewatering the basin and without shutting off the air supply to other diffusers in the basin.

5. Individual assembly units of diffusers shall be equipped with control valves, preferably with indicator markings for throttling or for complete shut-off. Provisions must be made for subsequent air flow or pressure measurements and necessary air flow adjustments. Diffusers in any single assembly shall have substantially uniform pressure loss.

Statutory Authority

§ 62.1-44.19 of the Code of Virginia.

Historical Notes

Former 12VAC5-581-790 derived from Virginia Register Volume 18, Issue 10, eff. February 27, 2002; amended and adopted as 9VAC25-790-730, Virginia Register Volume 20, Issue 9, eff. February 12, 2004.

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