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Virginia Administrative Code
Title 9. Environment
Agency 25. State Water Control Board
Chapter 790. Sewage Collection and Treatment Regulations

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.

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