Current through Register Vol. 41, No. 3, September 23, 2024
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.
| Nameplate Horsepower |
Radius in feet |
| 5 |
35 |
| 10-25 |
50 |
| 40-60 |
50-100 |
| 75 |
60-100 |
| 100 |
100 |
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.
Statutory Authority
§ 62.1-44.19 of the Code of Virginia.
