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

9VAC25-790-530. Clarifiers.

A. Conventional solids settling basin design information shall apply to clarifiers not preceded by chemical flash mix and flocculation. Where clarifiers are preceded by chemical flash mix and flocculation, chemical clarification requirements shall apply.

B. Design. Conventional clarifiers shall be designed to dissipate the inlet velocity, to distribute the flow uniformly across the basin, and to prevent short-circuiting hydraulic currents.

1. Inlet channels should be designed to maintain a velocity of at least one foot per second at 1/2 design flow. Corner pockets and dead ends shall be eliminated, and corner fillets or channeling used where necessary.

2. Provisions shall be made for elimination or removal of floating materials in inlet structures having submerged ports.

3. The minimum length of flow from inlet to outlet of a clarifier should be 10 feet unless special provisions are made to prevent short circuiting.

4. The liquid depth of mechanically cleaned clarifiers shall be as shallow as practicable but not less than 10 feet for an overflow rate of 300 gpd per square foot. For each three-foot increase or decrease in depth, the overflow rate shall be increased or decreased by 200 gpd per square foot respectively. Final clarifiers receiving flow from biological reactors should not be less than 12 feet in depth.

5. A minimum of two hours of average design flow detention volume should be provided within the settling zone of conventional clarifiers, at the design loading.

6. Multiple clarifiers capable of independent operation shall be provided at treatment works having a capacity of more than 40,000 gallons per day; however, single clarifiers may be allowed at Reliability Class II and Class III treatment works having a capacity up to 100,000 gpd when appropriate reliability and continuous operability requirements are satisfied.

7. Where multiple clarifiers are utilized in suspended growth processes, provisions for combining the effluent from the reactors (aeration basins) and proportionally distributing the reactor effluent to each clarifier shall be included, for the purpose of evenly distributing the biomass to the clarifiers.

8. Overflow weir plates shall be adjustable. In cases in which clarifier designs have a potential for short circuiting hydraulic inefficiencies, weir loadings rates should not exceed 10,000 gallons per day per linear foot for treatment works designed for average flows of 1.0 mgd or less. Special consideration will be given to weir loading rates for treatment works designed for flows in excess of 1.0 mgd, but such loading rates should not exceed 15,000 gallons per day per linear foot if short circuiting problems may affect performance. If pumping is required, pump capacity shall be related to clarifier design to avoid excessive weir loading.

9. The tops of beams and similar construction features which are submerged shall have a minimum slope of 1.4 vertical to 1 horizontal. The underside of such features should have a slope of one to one to prevent the accumulation of scum and solids. Effective scum collection and removal facilities, including baffling, shall be provided ahead of the outlet weirs on all clarifiers. Provisions may be made for discharge of scum with the sludge; other provisions may be necessary to dispose of floating materials which may adversely affect sludge handling and management.

10. Clarifier design should include provisions for reasonable access for maintenance and protection of operators. Such features may include slip resistant stairways and walkways, protective handrails, etc., in accordance with VOSH requirements. If side walls are extended some distance above the liquid level to provide flood protection, or for other purposes, stairs and walkways with handrails should be provided to facilitate housekeeping and maintenance. Access for cleaning and maintenance of weirs should also provide proper safety features in accordance with VOSH requirements.

11. Where primary clarifiers are used, provisions for emergency bypassing, or discharging sewage which has received preliminary treatment directly to the biological treatment unit operation, may be desirable.

12. Shallow depth sedimentation will be considered on a case-by-case basis.

C. Loadings. Conventional clarifier design should provide for established surface settling rates (flow rate per unit surface area) for optimum performance reliability.

1. Surface settling rates for primary clarifier should not exceed 1,000 gpd per square foot at design average flows or 2,500 gpd per square foot at peak hourly flows. Clarifier sizing shall be calculated for both flow conditions, and the larger surface area shall be used.

2. Surface settling rates for secondary clarifiers following attached growth biological reactors shall not exceed 1,200 gpd per square foot, based on peak hourly flows, or 500 gpd per square foot, based on average daily design flow, whichever loading results in a larger clarifier volume.

3. The hydraulic design of clarifiers following the activated sludge process shall be based on the anticipated peak hourly overflow rate from the clarifier. The hydraulic loading, except as noted, shall not exceed the following peak hourly surface settling (overflow) rates:

Type of Process



1,200 gpd/sq. ft.

step aeration

1.200 gpd/sq. ft.

contact stabilization

1,200 gpd/sq. ft.

carbonaceous stage of separate stage nitrification

1,200 gpd/sq. ft.

extended aeration

1,000 gpd/sq. ft.

nitrification stage of separate stage nitrification

800 gpd/sq. ft

4. The peak hour surface settling (overflow) rates for sewage treatment works with an average design flow of 0.1 mgd or less shall not exceed 800 gpd/sq. ft.

5. The established surface settling rates may be reduced by up to 30% for treatment works employing flow equalization, prior to the clarifier, provided that such a reduction will not result in turbulence and density currents that may be associated with a smaller clarifier design surface area.

6. The solids loading shall be evaluated at both peak hourly and average daily flow conditions in the design of secondary clarifiers, for comparison to the hydraulic loading. The larger surface area established by design loadings shall be utilized to establish the required clarifier size. The following values for solids loading shall apply:

Type of Treatment

Solids loading (lb/sq. ft./hour)



attached growth process



extended aeration



other activated sludge



D. Sludge removal and handling. Sludge collection and sludge withdrawal facilities shall be designed to minimize density currents and to permit rapid and continuous sludge removal.

1. Final clarifiers in activated sludge treatment works greater than 0.25 mgd shall be provided with positive scraping devices.

2. If multiple sludge hoppers are provided for sludge collection, means for individually and variably controlling sludge withdrawal from each hopper shall be provided in order to overcome any variations in the quantities of settled sludge in the various hoppers.

3. Each sludge withdrawal line shall be individually valved. Pumped withdrawal lines shall be at least four inches in diameter, and gravity withdrawal lines shall be at least six inches in diameter. The size of sludge withdrawal lines for airlift sludge removal shall be determined by the sludge removal rate.

4. The depth or head available for gravity withdrawal of sludge shall be at least 30 inches of water. A sludge well or other appropriate equipment shall be provided for viewing and sampling the sludge.

5. The minimum slope of the hopper side walls for primary basins shall be 1.7 vertical to 1.0 horizontal. Hopper wall surfaces should be made smooth with rounded corners to aid in sludge removal. Hopper bottoms should have a maximum dimension of two feet. The use of sludge hoppers for sludge thickening purposes is not recommended.

Statutory Authority

§ 62.1-44.19 of the Code of Virginia.

Historical Notes

Former 12VAC5-581-590 derived from Virginia Register Volume 18, Issue 10, eff. February 27, 2002; amended and adopted as 9VAC25-790-530, Virginia Register Volume 20, Issue 9, eff. February 12, 2004.

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