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Virginia Administrative Code
Title 9. Environment
Agency 25. State Water Control Board
Chapter 790. Sewage Collection and Treatment Regulations
11/21/2024

9VAC25-790-480. Treatment works outfalls.

A. The effluent discharge line or outfall shall be designed with sufficient capacity to transmit the maximum expected flow in a manner so as to prevent flooding of the treatment process while providing optimum dispersion of the treated effluent into the receiving waters.

B. Velocity. The velocity in the outfall at design average flow shall be a minimum of 1-1/2 feet per second to avoid any settling of solids. Velocities should not exceed the recommendations of the pipe manufacturer with respect to maximum velocities to avoid pipe erosion or scour and should be less than eight feet per second to avoid excessive head loss or disruption of the receiving water channel.

C. Structures. The outlet structure should be submerged during design low flow conditions in the receiving water channel, unless adequate dispersion can be achieved without such submergence. The outlet structure should be designed to provide effective dispersion of effluent into the receiving body of water as established by the certificate or permit issued. Additional provisions for dispersion of effluent may be required, based on public health and welfare protection and water quality considerations in accordance with downstream uses.

1. The outfall, where practicable, shall be of gravity sewer design and extend beyond the design low water level of the receiving body of water and account for coastal erosion if necessary. The effluent discharge shall remain submerged if required to maintain water quality standards or protect public health and welfare.

2. Headwalls may be used where adequate dispersion will be obtained without requiring that the outfall be submerged. The design shall include measures to prevent erosion and foaming problems as a result of the discharge.

3. When a diffuser mechanism is provided, the design shall take into account the range of current velocities and directions in the vicinity of the diffuser.

Diffuser ports shall be spaced and located so as to avoid interference between adjacent jet plumes and sized so as to minimize head loss and maximize initial dispersion of the effluent into the receiving water.

D. Protection. The outfall shall be designed and constructed to protect against the effects of erosion, flood waters, tides, ice, boating and shipping, and other hazards, and to insure structural stability and freedom from stoppage.

Statutory Authority

§ 62.1-44.19 of the Code of Virginia.

Historical Notes

Former 12VAC5-581-540 derived from Virginia Register Volume 18, Issue 10, eff. February 27, 2002; amended and adopted as 9VAC25-790-480, Virginia Register Volume 20, Issue 9, eff. February 12, 2004.

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