Virginia Law

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

4. The following operating equipment shall be provided:

a. A complete set of necessary tools and accessories.

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

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

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

a. Adjustable.

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

6. Sludge removal design shall provide that:

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

b. Entrance to sludge withdrawal piping will prevent clogging;

c. Valves are located outside the tank for accessibility;

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

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

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