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RULE §217.96Small Diameter Effluent Sewers

(a) Interceptor tank design. Septic tanks used as interceptor tanks must be designed and constructed according to §285.32(b)(1) of this title (relating to Criteria for Sewage Treatment Systems).

  (1) An outlet of an interceptor tank must have a commercially available effluent filter designed to remove particles larger than 1/16 inch.

  (2) The volume of an equivalent dwelling unit (EDU) interceptor tank must be based on the criteria in Chapter 285 of this title (relating to On-Site Sewage Facilities).

  (3) The volume of a multiple equivalent dwelling unit (MEDU) interceptor tank must be calculated using the following equations:

Attached Graphic

(b) Pretreatment units.

  (1) A non-residential contributor to an alternative collection system must provide a method for trapping and removing fats, oils, and grease (FOG) from the wastewater before the wastewater enters an interceptor tank.

  (2) A pretreatment unit must have at least two compartments.

  (3) The primary compartment volume must be at least 60% of the total tank volume.

  (4) A grease trap must meet the same requirements as an interceptor tank with regard to water tightness, materials of construction, and access to contents.

  (5) FOG retention capacity, in pounds, must be at least twice the pretreatment unit's flow capacity, in gallons per minute. The FOG retention capacity of a grease trap is the amount of FOG that it can hold before its efficiency drops below 90%.

  (6) Plumbing for a pretreatment unit must be designed to prevent wastes, other than FOG, from entering the pretreatment unit.

  (7) A pretreatment unit must be designed to allow monitoring of the sludge and FOG levels.

(c) Service pipe design.

  (1) Pipe materials used for service pipe must meet or exceed the performance characteristics of American Society for Testing and Materials (ASTM) D 2241 Class 200 polyvinyl chloride (PVC) pipe.

  (2) An interceptor tank must include a pumping unit if the interceptor tank's outlet elevation is below the main pipe elevation or if the hydraulic grade line is within a depressed section of a main pipe.

  (3) A service pipe for an EDU or MEDU must be sized to transport the highest flow expected from the building, but must not be less than 2.0 inches in diameter.

  (4) The diameter of a service pipe must be no greater than the diameter of the collection pipe it is connected to.

  (5) A service pipe of an interceptor tank that is subject to periodic backflow must include a check valve that:

    (A) is located immediately adjacent to the collection pipe;

    (B) is made from a corrosion-resistant material;

    (C) provides an unobstructed flow way; and

    (D) is a swing type valve with an external position indicator to show the open and closed positions.

(d) Collection system design.

  (1) Hydraulic design.

    (A) A small diameter effluent sewer (SDES) system with open channel flow must use a design depth of flow of 100% of the pipe diameter.

    (B) The minimum flow velocity in a collection pipe must be no less than 1.0 foot per second (fps).

    (C) The maximum flow velocity in any portion of an SDES system is 8.0 fps without thrust restraint and 13.0 fps with thrust restraint.

    (D) The engineering report must include velocity calculations for each pipe segment.

    (E) The elevation of the hydraulic grade-line at peak flow must be lower than an outlet invert of any upstream interceptor tank, unless the interceptor tank has on-site conveyance equipment.

    (F) The engineering report must include an analysis for each pipe showing the hydraulic grade line, energy grade line, and ground elevation in relationship to the outlet elevation of each interceptor tank being served by a collection pipe.

    (G) The engineering report must include an engineer's analysis of each segment of a variable grade effluent sewer.

    (H) Open pipe flow design must use a Manning's "n" value of 0.013.

    (I) Pressure flow design must use a Hazen-Williams "C" value of 120.

    (J) No pipe in an SDES may be smaller than 2.0 inches in diameter.

  (2) Vertical Alignment.

    (A) The vertical alignment of an SDES may be variable; however, the overall downhill gradient must allow the pipe to transport the peak flow.

    (B) Venting must be provided upstream and downstream of pipe segments that are below the hydraulic grade line.

    (C) The pipes must have a uniform profile (i.e., no abrupt or sharp changes in vertical direction).

    (D) A collection pipe must have a cleanout that extends to ground level and terminates in a watertight valve box at:

      (i) an upstream terminus;

      (ii) a minor junction;

      (iii) a change in pipe diameter; and

      (iv) intervals of no more than 1,000 feet.

    (E) Venting at a collection pipe summit must use a wastewater service air release valve or a combination air release and vacuum valve. The valve must be constructed of corrosion-resistant material and located in a vault.

    (F) Pipe material used in a collection system must meet the performance requirements of ASTM D 3034 SDR 26 PVC pipe, except for any segment under pressure flow conditions. Under pressure flow conditions, pipe material must meet the performance requirements of ASTM D 2241 Class 200 PVC pipe.

  (3) Odor Control. A collection pipe must have permanent odor control devices attached to the line and must prevent nuisance odors. Odor control devices must be accessible for maintenance.

Source Note: The provisions of this §217.96 adopted to be effective August 28, 2008, 33 TexReg 6843; amended to be effective December 4, 2015, 40 TexReg 8254

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