9VAC25-790-820. Postaeration.

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 = (C_s-C_a)/(C_s-C_b)
	where: r =	Deficit ratio
	C_s =	Dissolved oxygen saturation (mg/l)
	C_a =	Dissolved oxygen concentration above the weir, assumed to be 0.0 mg/l.
	C_b =	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.

Historical Notes

Former 12VAC5-581-880 derived from Virginia Register Volume 18, Issue 10, eff. February 27, 2002; amended and adopted as 9VAC25-790-820, Virginia Register Volume 20, Issue 9, eff. February 12, 2004.

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