Current through Register Vol. 41, No. 3, September 23, 2024
A. Postaeration
design may involve mechanical aeration, diffused air injection, or cascade type
aeration. Other methods may be utilized and will be evaluated on a case-by-case
basis by the department.
B.
Mechanical aeration. Multiple aeration basins for continuous operability should
be provided at all treatment works with a design flow of 40,000 gallons per day
or more, unless other means of maintaining an adequate level of dissolved
oxygen (D.O.) in the effluent are available.
1. The aeration equipment transfer efficiency
shall be determined utilizing the manufacturer's certified rating for the
particular equipment being considered. The transfer efficiency shall be
adjusted to reflect anticipated field conditions of temperature, atmospheric
pressure, initial D.O., and composition of the wastewater being oxygenated.
When the detention time within the aeration basin exceeds 30 minutes,
consideration shall be given to oxygen requirements resulting from biological
activity in the postaeration basin. For aeration basins equipped with a single
mechanical aeration unit, a minimum of one mechanical aeration unit shall be
maintained in storage at the treatment works site for immediate
installation.
2. Aeration basins
shall be designed to minimize short circuiting of flow and the occurrence of
dead spaces. Vortexing shall be prevented.
C. Diffused aeration. Multiple aeration
basins shall be provided for continuous operability of treatment works having a
design flow capacity of 40,000 gallons per day or greater, except where
diffusers may be removed from the basin for maintenance.
1. Diffused aeration basins shall be designed
to eliminate short-circuiting and the occurrence of dead spaces. For maximum
efficiencies, sufficient detention time shall be provided to allow the air
bubbles to rise to the surface of the wastewater prior to discharge from the
basin.
2. When the detention time
in the aeration basin exceeds 30 minutes, consideration shall be given to the
oxygen requirements resulting from biological activity in the aeration
unit.
3. Diffused air aeration
systems shall be designed utilizing Fick's Law (the rate of molecular diffusion
of a dissolved gas in a liquid) in the determination of oxygen requirements.
Supporting experimental data shall be included with the submission of any
proposal for the use of diffusers which are considered nonconventional. Such
proposals will be evaluated on a case-by-case basis by the
department.
4. Blower design shall
be such that with any single unit out of operation, the oxygen requirements
will be provided for maintaining effluent D.O. A minimum of one standby blower
shall be stored at treatment works where single aeration basins are
utilized.
D. Cascade
type. Effluent aeration may be achieved through a turbulent liquid-air
interface established by passing the effluent downstream over either a series
of constructed steps, or a rough surface that produces a similar opportunity
for transfer of dissolved oxygen to the effluent.
1. The following equation shall be used in
the design of cascade type aerators:
| rn =
(Cs-Ca)/(Cs-Cb)
|
| where: r = |
Deficit ratio |
| Cs = |
Dissolved oxygen saturation (mg/l) |
| Ca = |
Dissolved oxygen concentration above the weir,
assumed to be 0.0 mg/l. |
| Cb = |
Dissolved oxygen concentration in the effluent from
the last or preceding step |
| n = |
The number of equal size steps |
| r = 1 + (0.11) (ab) (1 + 0.046 T) (h)
|
| where: T = |
Water temperature (°C) |
| h = |
Height of one step (ft) |
| a = |
1.0 for effluents (BOD of 15 mg/l or less) |
| = |
0.8 for effluents (BOD of 15 mg/l to 30 mg/l) |
| b = |
1.0 for free fall and 1.3 for step weirs |
2. The equation for determining the number of
steps is dependent upon equidistant steps; and, if unequal steps are used,
transfer efficiencies must be determined for each separate step.
3. The effluent discharge to a cascade type
aerator shall be over a sharp weir to provide for a thin sheet of wastewater.
Consideration shall be given to prevention of freezing.
4. The final step of the cascade type aerator
shall be above normal stream flow elevation and the cascade aerator shall be
protected from erosion damage due to storm water drainage or flood/wave
action.
5. When pumping is
necessary prior to discharge over the cascade aerator, multiple, variable speed
pumps must be provided except when preceded by flow equalization.
Statutory Authority
§ 62.1-44.19 of the Code of Virginia.
