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Virginia Administrative Code
Title 9. Environment
Agency 25. State Water Control Board
Chapter 790. Sewage Collection and Treatment Regulations
8/5/2020

9VAC25-790-310. Design Factors.

Part III
Manual of Practice for Sewerage Systems and Treatment Works

Article 1
Collection and Conveyance Sewers

A. Sewage collection systems shall be designed and constructed to achieve total containment of the predicted sewage flows contributed from the established service area and population. New combined sewers receiving direct storm water drainage shall not be approved. Interceptor sewers for existing combined sewers shall be designed and constructed to prevent the discharge of inadequately treated wastes. Overflows from intercepting sewers shall be managed in accordance with the issued certificate or permit.

B. Basis. In general, sewer systems should be designed for the estimated ultimate tributary population with an upper limit consisting of the 50-year population growth projection, except when considering parts of the systems that can be readily increased in capacity. Consideration shall be given to land use plans and to other planning documents and to the maximum anticipated capacity of institutions, industrial parks, apartment developments, etc.

C. Factors. In determining the required capacities of sanitary sewers, the following factors shall be considered:

a. Maximum hourly sewage flow.

b. Additional maximum sewage or wastewater flows from industrial sources.

c. Ground water infiltration.

d. Topography of area.

e. Location of sewage treatment works.

f. Depth of excavation.

g. Pumping requirements.

h. Occupancy rates.

D. Capacity. New sewer system capacity shall be designed on the basis of an average daily per capita flow of sewage of not less than that set forth in Table 3 (9VAC25-790-460) of this chapter. These figures are assumed to include infiltration but do not address inflow. When deviations from the foregoing per capita rates and established peak flow factors are proposed, a description of the procedure used to establish those design flows shall be included with the submission for the purpose of this chapter, the following list defines the various collection system components that are to be designed to transmit peak flow rates:

1. "Lateral" means a sewer that has no other common sewers discharging into it.

2. "Submain" means a sewer that receives flow from one or more lateral sewers.

3. "Main or trunk" means a sewer that receives sewage flow from one or more submain sewers.

4. "Interceptor" means a sewer that receives sewage flow from a number of gravity mains, trunk sewers, sewage force mains, etc.

The minimum peak design capacity for lateral and submain sewers should be 400% of the average design flow.

Minimum peak design capacity of main, and trunk, sewers should be 250% of the average design flow.

Minimum peak design for interceptor sewers shall be 200% of the average design flow.

Statutory Authority

§ 62.1-44.19 of the Code of Virginia.

Historical Notes

Former 12VAC5-581-370 derived from Volume 18, Issue 10, eff. February 27, 2002; amended and adopted as 9VAC25-790-310, Virginia Register Volume 20, Issue 09, eff. February 12, 2004.

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