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Virginia Administrative Code
Title 9. Environment
Agency 25. State Water Control Board
Chapter 790. Sewage Collection and Treatment Regulations

9VAC25-790-980. Factors for oxygen transfer.

A. Design. The oxygen supply requirements for diffused aeration systems serving suspended growth biological reactors should be established from the Alpha factor and the Beta factor (9VAC25-790-690 E 6).

B. Alpha factor. The alpha factor was once considered to be related only to wastewater characteristics, primarily surfactants. Additional investigations have shown that the alpha factor varies with other process conditions including mixing intensity, suspended solids concentration, and other factors, particularly the method of aeration. The alpha factor is unique for a particular wastewater treatment facility but is difficult to accurately determine; however, standards of practice for measuring the alpha factor have been proposed.

Reported observations on the variations of the alpha factor with diffuser type are as follows:

Bubble Size

Range of Alpha


0.4 - 0.55


0.7 - 0.8


0.8 - 0.9

Other studies have similarly indicated lower alpha factors for fine bubble diffusers in comparison to coarse bubble diffusers. Summaries of reported values for alpha factors for mechanical aerators indicate a general range of 0.8 to greater than 1.0 with some values as low as 0.6. Increasing mixing intensity tends to increase the alpha factor.

C. Beta factor. The beta factor has been observed to vary over a moderate range, although the variations are generally less than observed variations of the alpha factor. One method proposed for estimating the beta factor uses the TDS concentration of the wastewater and the Standard Methods chart for saturation dissolved oxygen concentrations at various chloride levels in which

Beta Factor = CSTC/CST,


CSTC-saturation dissolved oxygen concentration at temperature, T and chloride concentration, C

(Substitute chloride conc. for TDS conc. when using chart)


CST = saturation dissolved oxygen concentration at temperature, T and chloride concentration of 0.

Values reported for the beta factor for domestic wastewater are generally about 0.95 but considerable deviations from this value have been observed for industrial wastewater.

Statutory Authority

§ 62.1-44.19 of the Code of Virginia.

Historical Notes

Former 12VAC5-581-1040 derived from Virginia Register Volume 18, Issue 10, eff. February 27, 2002; amended and adopted as 9VAC25-790-980, Virginia Register Volume 20, Issue 9, eff. February 12, 2004.

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