(a) General Requirement. The type and size of an OSSF shall be determined on the basis of the soil and site information developed according to §285.30 of this title (relating to Site Evaluation).
(b) Suitability. A standard subsurface absorption system may be used if all the soil and site criteria are determined to be suitable under §285.91(5) of this title (relating to Tables). If one or more of the soil and site criteria categories are determined to be unsuitable, a standard subsurface absorption system cannot be used except as noted in §285.91(5) of this title. If it is determined that a standard subsurface absorption system cannot be used, either a proprietary or a non-standard system may be used, provided all soil and site criteria for that system can be met as required in §285.91(13) of this title.
(c) Surface drainage criteria.
  (1) Topography. Uniform slopes under 30% are suitable for standard subsurface absorption systems. If the slope is less than 2%, steps shall be taken to ensure there is adequate surface drainage over any subsurface disposal field. The excavation for a standard subsurface absorption system shall be parallel to the contour of the ground.
  (2) Flood hazard. Any potential OSSF site within a 100-year floodplain is subject to special planning requirements. The OSSF shall be located so that a flood will not damage the OSSF during a flood event, resulting in contamination of the environment. Planning materials shall indicate how tank flotation is eliminated. Additionally, if the site is within the regulated floodway, a professional engineer shall demonstrate that:
    (A) the system shall not increase the height of the flood;
    (B) all components, with the exception of risers, chlorinators, cleanouts, sprinklers, and inspection ports, shall be completely buried without adding fill; and
    (C) non-buried components (e.g. alarms, junction boxes, and compressors) shall be elevated above the 100-year flood elevation.
(d) Separation requirements. OSSFs shall be separated from features, in the area where the OSSF is to be installed, that could be contaminated by the OSSF or could prevent the proper operation of the system. The separation requirements are in §285.91(10) of this title.

Source Note: The provisions of this §285.31 adopted to be effective June 13, 2001, 26 TexReg 4115