Virginia Law

Article 4. Preliminary Processes

9VAC25-790-500. Screening.

Article 4
Preliminary Processes

A. Conventional preliminary treatment shall include adequate screening to remove solids and debris that could interfere with the performance of downstream unit operations. The process design shall address the means of proper waste management for screenings.

B. Requirements. Protection for pumps and other equipment shall be provided by installing large openings, one inch or more, coarse screens, or bar racks, and smaller openings screens. All screen equipment and facilities shall be readily accessible for maintenance. Small openings, fine screening, or comminution should follow grit removal, which should be preceded by coarse screening. Screen locations are to be provided as follows:

1. Manually cleaned screens shall be provided at all sewage treatment works unless adequate redundant mechanical screens are provided and manually cleaned screens shall be located in open areas with easy access. Mechanical screens that can be manually cleaned upon mechanical failure may be used to meet this requirement.

2. Manually cleaned screens located in deep pits shall be provided with stairway access, adequate lighting and ventilation, and convenient and adequate means for removing screenings.

3. Screening devices installed in a building where other equipment or offices are located shall be separated from the rest of the building, provided with separate outside entrances, and provided with adequate means of ventilation.

C. Design. Clear openings between the bars of coarse screens should be from one to 1-3/4 inches. Other size openings will be considered on a case-by-case basis. Coarse screen design shall provide for installation such that the screening equipment can be conveniently and safely accessed for maintenance and management of screenings.

1. Where a single mechanically cleaned screen that cannot be manually cleaned upon mechanical failure is used, an auxiliary manually cleaned screen shall be provided.

2. Where two or more mechanically cleaned screens are used, the design shall provide for taking any unit out of service without sacrificing the capability to handle the peak design flow.

3. Manually cleaned screens, except those for emergency use, shall be placed on a slope of 30 to 60 degrees with the horizontal.

4. All mechanical units that are operated by timing devices shall be provided with auxiliary controls which will set the cleaning mechanism in operation at predetermined high water levels.

5. The design or electrical fixtures and controls in enclosed places where gas may accumulate will be evaluated in accordance with the National Electrical Code specifications for hazardous conditions.

6. The use of fine mesh static or mechanical screens as an adjunct to or in lieu of sedimentation will be considered on a case-by-case basis.

D. Flow control. At the normal operating flow conditions, approach velocities should be no less than 1.25 feet per second, to prevent settling, and no greater than 3.0 feet per second, to prevent forcing materials through the openings.

1. The approach flow velocity shall be calculated from a vertical projection of the screen openings on the cross-sectional area between the invert of the channel and the flow line.

2. The screen channel invert shall be three to six inches below the invert of the incoming sewers. To prevent jetting action, the length and construction of the screen channel shall be adequate to reestablish hydraulic flow pattern following the drop in elevation.

3. Multiple channels, where provided, shall be equipped with the necessary gates to isolate flow from any one screening unit operation. Provisions shall also be made to facilitate dewatering each unit. The channel preceding and following the screen shall be shaped to eliminate settling and accumulation of solids. Fillets may be necessary.

E. Screening management. Properly sized facilities shall be provided for removal, storage, and disposal of screenings as required by the approved operation and maintenance manual or sludge management plan. Manually cleaned screening facilities shall include an accessible platform, in accordance with VOSH requirements, from which the operator may remove screenings easily and safely. Suitable drainage facilities shall be provided both for the platform and for storage areas, with all drain water returned to the raw or primary influent flow.

F. Comminution. Comminution should be provided in treatment works that do not provide primary sedimentation, unless other means of protecting downstream processes and equipment are provided. The term "comminutors" shall be understood to also include barminutors, or other shredding/grinding equipment.

1. Comminutors should be located downstream of any grit removal equipment. Areas containing comminution devices shall be provided with stairway access, adequate lighting and ventilation in accordance with VOSH requirements and convenient and adequate means for maintenance and device removal. Comminutors installed in a building where other equipment or offices are located should be accessible only through a separate outside entrance. Comminutor capacity shall be adequate to handle expected peak flows.

2. A bypass channel with appropriate screening shall be provided. Gates to isolate flow from the comminutor channel shall be installed. Each comminutor not preceded by grit removal shall be protected by a screen, trap, or other means to remove potentially harmful gravel.

3. Electrical equipment in comminutor chambers is to be designed in accordance with the applicable requirements of the National Electrical Code. Protection against accumulation of hazardous gases and accidental submergence shall be provided as required by state and federal regulations.

Statutory Authority

§ 62.1-44.19 of the Code of Virginia.

Historical Notes

Former 12VAC5-581-560 derived from Virginia Register Volume 18, Issue 10, eff. February 27, 2002; amended and adopted as 9VAC25-790-500, Virginia Register Volume 20, Issue 9, eff. February 12, 2004.

9VAC25-790-510. Grit removal facilities.

A. Grit removal unit operations facilities shall be provided for all sewage treatment works with a design capacity of 0.15 million gallons per day or greater and are required for treatment works receiving sewage from combined sewers or from sewer systems receiving substantial amounts of grit. Grit removal facilities should be provided at all sewage treatment works.

1. Grit removal facilities should be located ahead of pumps and comminuting devices. Coarse bar racks and other suitable screens should be placed ahead of mechanically cleaned grit removal facilities.

2. Treatment works treating wastes from combined sewers shall have at least two mechanically cleaned grit removal units, with provisions for unit bypassing.

3. A single manually cleaned or mechanically cleaned grit removal unit, with a unit bypass, is acceptable for those sewage treatment works with a design capacity of less than 0.15 million gallons per day and serving separate sanitary sewer systems.

4. Minimum facilities for larger treatment works serving separate sanitary sewers shall be at least one mechanically cleaned unit with a unit bypass.

B. Design. The design of grit removal facilities shall be based on the requirements of the downstream treatment units. Local conditions and wastewater characteristics shall be evaluated in selecting the design size of particle to be removed. Flow turbulence into and through grit removal basins shall be minimized.

1. Horizontal flow basins shall be designed so as to provide controlled velocities as close as possible to one foot per second during average design flow conditions. The detention period shall be based on the size of particle to be removed. The design should take into consideration hydraulic inefficiencies and positioning of inlets and outlets.

2. Aerated chambers shall be designed to provide a minimum detention time of three minutes at average flow. An air flow of three to five cubic feet per minute per foot of chamber length should be maintained. Aerated chambers shall have adequate and flexible controls for agitation and air supply devices.

3. Other types of degritters will be approved on a case-by-case basis upon evaluation of satisfactory performance data.

4. Wherever possible, grit removal facilities should be located in open areas with easy access. Grit removal facilities located in deep pits shall be provided with mechanical equipment for pumping or hoisting grit to ground level. Such pits shall have a stairway, elevator, or manlift, adequate ventilation, and adequate lighting in accordance with VOSH requirements.

5. Provisions shall be made for dewatering each unit. Drain lines shall discharge to points within the system such that maximum treatment of the contents of the drained unit is provided.

6. The provision of grit washing facilities shall be a function of the ultimate disposal and transportation methods provided for the grit. Impervious surfaces with drains shall be provided for grit handling areas. If grit is to be transported, conveying equipment shall be designed to avoid loss of material.

Statutory Authority

§ 62.1-44.19 of the Code of Virginia.

Historical Notes

Former 12VAC5-581-570 derived from Virginia Register Volume 18, Issue 10, eff. February 27, 2002; amended and adopted as 9VAC25-790-510, Virginia Register Volume 20, Issue 9, eff. February 12, 2004.

9VAC25-790-520. Pre-aeration.

Aeration of pretreated sewage should be provided whenever the low dissolved oxygen, or anaerobic condition, of the sewage can interfere with downstream unit operation reliability. Pre-aeration may be used to prevent solids deposition problems in on-line or off-line equalization or storage basins.

1. Pre-aeration unit operations shall be designed so that removal from service will not interfere with normal downstream operation of the remainder of the treatment process.

2. Inlet and outlet devices shall be designed to ensure proper distribution and help prevent solids deposition, while minimizing any hydraulic short circuiting effects.

3. The aeration equipment shall be capable of obtaining both adequate mixing and self-cleaning velocities within the basin. Any of the types of equipment used for aeration of biological reactors may be utilized.

4. A satisfactory means of grit removal shall be provided for operation of pre-aeration basins.

Statutory Authority

§ 62.1-44.19 of the Code of Virginia.

Historical Notes

Former 12VAC5-581-580 derived from Virginia Register Volume 18, Issue 10, eff. February 27, 2002; amended and adopted as 9VAC25-790-520, Virginia Register Volume 20, Issue 9, eff. February 12, 2004.

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:

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:

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.