Virginia Law

A. On-line effluent polishing ponds (OLEPP) can be provided to receive discharges at locations where use of the receiving water requires a degree of performance reliability exceeding that provided by the design, operation and maintenance of the sewage collection system and treatment works. The design and construction of OLEPP's may be similar to that of stabilization ponds.

B. Useage. All sewage treatment works designed to produce a secondary effluent quality of 24 mg/l or more of BOD or suspended solids that discharge to shellfish waters such that shellfish harvesting restrictions may be imposed, shall be provided with an OLEPP, or sufficient off-line emergency storage, unless an exemption is granted by the director subsequent to a public hearing held to discuss the impacts of the discharge. An OLEPP should be required for all sewage treatment works (i) for which the design either does not achieve Class I reliability requirements, or is considered nonconventional in accordance with this chapter; (ii) that discharge to critical waters; or (iii) that are located where water quality conditions dictate the need for maximum protection of public health and welfare.

1. These effluent polishing ponds may be required for any Class I reliability discharge from treatment works that are not daily attended by operational personnel for a minimum period of 16 hours.

2. Those sewage treatment works for which sufficient information is provided to the department verifying that adequate performance reliability will exist in the form of continuously available operational staff and supplemental systems and resources, so that water quality and resources will not be damaged in a manner that produces socio-economic losses, may be granted an exception to the requirements for an OLEPP or emergency storage.

3. An OLEPP can be utilized in instances where an additional removal of BOD₅and suspended solids up to a maximum of 3.0% is desired from the effluent of a properly operated and properly loaded secondary treatment facility.

4. An OLEPP can be utilized to control residual chlorine through natural processes such as oxidation and UV light irradiation. The chlorine dosage applied to the pond influent shall be monitored and controlled.

5. A closure plan shall be provided in accordance with this chapter and standards contained in this chapter, prior to issuance of an operating permit.

6. Effluent from an effluent polishing pond shall be disinfected in accordance with this chapter, unless adequate disinfection can be provided for the pond influent, so that effluent disinfection is not deemed necessary.

7. Adequate disinfection of a three-day capacity effluent-polishing pond influent may require special consideration such as:

a. A minimum flow path length-to-width ratio within contact tanks of 40:1.

b. Expansion of detention volume to 60 minutes residence time.

c. Use of mixing devices for chlorine dosing to replace or supplement standard diffusers.

C. Design. The actual liquid depth of facultative polishing ponds shall not be less than five feet or more than 10 feet. The detention time shall not be less than one day nor more than three days, based on average daily flow.

1. In most cases, it should be necessary to provide postaeration facilities following facultative polishing ponds to meet effluent dissolved oxygen requirements, due to the depletion of oxygen in facultative ponds. If postaeration facilities are not provided, calculations shall be submitted to show that the required effluent dissolved oxygen concentrations can be maintained on a continuous basis. Postaeration shall occur during or following disinfection.

2. The influent line shall discharge below the liquid level of the pond near the edge of the pond embankment. The influent line shall enter the pond at a point opposite the effluent structure to prevent short-circuiting and to provide maximum detention time.

3. The effluent structure can be a single draw-off type with a draw-off point 12 to 18 inches below the normal liquid level or a multiple draw-off structure.

D. Aeration. The selection of aeration equipment shall be consistent with the depth of the lagoon.

1. The aeration equipment shall be sized to provide uniform dissolved oxygen concentration throughout the pond. Surface aerators should provide a minimum horsepower capacity of 0.01 hp per 1,000 gallons or provide equipment for which existing performance data has shown it to be sufficient to maintain solids in suspension and capable of dispersing the required level of oxygen uniformly. Diffused aeration systems must be adequately located and sized to provide uniform oxygen dispersion and maintain solids in suspension.

2. The number of surface aerators required shall be determined by the circle of influence of the aerator. The circle of influence shall encompass the entire pond and is defined as the area in which the return velocity is greater than 0.15 feet per second as certified by performance data. Without supporting data, the following may be used as a guide.

E. Features. For aerated OLEPP's the influent sewer shall discharge near one of the mechanical surface aerators. The outlet should be arranged to withdraw effluent from a point at or near the surface. In-pond baffling may be considered to improve hydraulics.

1. A sedimentation zone that has at least 1-1/2 hours of design detention or settling period and a surface loading not to exceed 700 gallons per square foot per day shall be provided. Provisions for sludge removal from the OLEPP, as necessary, shall be addressed in the final design.

2. Either concrete bottom, walls, or embankment walls, or soils-cement stabilization of bottom, walls and embankments should be evaluated in the final design. Earthen embankment walls one foot above and one foot below the normal water level shall be riprapped or stabilized with other suitable material to prevent erosion from wave action.