1. (a) Interceptor tank design. Septic tanks used as interceptor tanks must be designed and constructed in accordance with §285.32(b)(1) of this title (relating to Septic Tanks).

   1. (1) An outlet of an interceptor tank must have a commercially available effluent filter designed to remove particles larger than 1/16 inch.
   2. (2) The volume of an equivalent dwelling unit (EDU) interceptor tank must be based the criteria in Chapter 285 of this title (relating to On-site Sewage Facilities).
   3. (3) Multiple equivalent dwelling unit (MEDU) interceptor tanks size must be calculated using the following equations:
      Attached Graphic
      

2. (b) Pre-treatment units.

   1. (1) An MEDU must provide a method for trapping and removing fats, oils, and grease (FOG) from the wastewater before the wastewater enters an interceptor tank.
   2. (2) A pretreatment unit must have at least two compartments.
   3. (3) The primary compartment volume must equal at least 60% of the total tank volume.
   4. (4) Construction of 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. (5) FOG retention capacity in pounds must be equal to at least twice the pretreatment unit's flow capacity in gallons per minute. The FOG retention capacity of a trap is the amount of FOG that it can hold before its efficiency drops below 90%.
   6. (6) Plumbing for a pretreatment unit must be designed to prevent wastes other than FOG from entering the pretreatment unit.
   7. (7) A pretreatment unit must be capable of monitoring the sludge and FOG levels.

3. (c) Service pipe design.

   1. (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. (2) An interceptor tank must include a pumping unit if its outlet elevation is below the main pipe elevation or the hydraulic grade line in a depressed section of a main pipe.
   3. (3) A service pipe for an EDU or MEDU must be sized to meet the hydraulic requirements of the building, but must be at least 2.0 inches in diameter.
   4. (4) The diameter of a service pipe must be no greater than the collection pipe it is connected to.

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

      1. (A) is located immediately adjacent to the collection pipe;
      2. (B) is made from a corrosion resistant material; and
      3. (C) provides an unobstructed flow way.
   6. (6) A collection pipe must have in-line odor control devices that are accessible for maintenance.

4. (d) Collection system design.

   1. (1) Hydraulic design.

      1. (A) A small diameter effluent sewer (SDES) system with open channel flow must use a design depth of flow of 100% of pipe diameter.
      2. (B) The minimum low velocity in a collection pipe must be at least 1.0 foot per second (fps).
      3. (C) The maximum flow velocity in any portion of an SDES system is 8.0 fps without velocity protection and 13.0 fps with velocity protection.
      4. (D) The report must include velocity calculations for each pipe segment.
      5. (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.
      6. (F) The 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.
      7. (G) The report must include an engineer's analysis of each segment of a variable grade effluent sewer.
      8. (H) Open pipe flow design must use a Manning's "n" value of 0.013.
      9. (I) Pressure flow design must use a Hazen-Williams "C" value of 120.
      10. (J) No pipe in a SDES may be smaller than 2.0 inches in diameter.

   2. (2) Vertical Alignment.

      1. (A) The vertical alignment of an SDES may be variable; however, the overall downhill gradient must allow the pipe to transport the expected peak flow.
      2. (B) Venting must be provided upstream and downstream of pipe segments that are below the hydraulic grade line.
      3. (C) The pipes must have a uniform profile with no abrupt or sharp changes.

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

         1. (i) an upstream terminus;
         2. (ii) a minor junction;
         3. (iii) a change in pipe diameter; and
         4. (iv) intervals of no more than 1,000 feet.
      5. (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.
      6. (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.

Source Note:The provisions of this §217.96 adopted to be effective August 28, 2008, 33 TexReg 6843.