(1) All components of sewage treatment systems must be designed and installed in accordance with Department Circular DEQ-4, Department Circular DEQ-2, or other applicable department circular and are subject to the following restrictions:

(a) systems designed in accordance with Department Circular DEQ-2 may not be used for individual, shared, or multiple-user systems, except as provided in Department Circular DEQ-4; and

(b) experimental systems are allowed only pursuant to a waiver granted in accordance with ARM 17.36.601.

(2) Multiple-user systems with design flows greater than or equal to 2500 gallons per day must be designed by a professional engineer and are subject to the requirements in ARM 17.36.121.

(3) Subsurface wastewater treatment systems must comply with ARM Title 17, chapter 36, subchapter 3, and Circular DEQ-4.

(a) The applicant must provide the following minimum information for wastewater submittals:

(i) site evaluation information, soil profile descriptions, percolation tests if required, and other pertinent soil information for each proposed drainfield, and seasonal high ground water information;

(ii) drainfield dimensions, including lateral length and lateral width based on application rate and design flow; drainfield location and type of system; and direction and percentage of slope across the treatment area or a contour map with a contour interval of at least two feet; 

(iii) setback information as required by ARM 17.36.323; and

(iv) criteria necessary to make a non-degradation significance determination.

(b) Additional design details showing compliance with DEQ-4 will be required by the reviewing authority if:

(i) the proposed wastewater system is multiple-user or public;

(ii) the discharge is not residential strength;

(iii) the local health department does not issue septic permits for parcels reviewed under Title 76, chapter 4, MCA; or

(iv) the site will require strict adherence to the lot layout at the time of installation, as determined by the reviewing authority.

(4) For subsurface systems, a minimum separation of at least four feet of natural soil must exist between the infiltrative surface or the liner of a lined system and a limiting layer. Exceptions to this rule are:

(a) at least six feet of natural soil must exist between the infiltrative surface or the liner of a lined system and a limiting layer on a slope of greater than 15 percent; and

(b) for elevated sand mounds constructed in accordance with Department Circular DEQ 4, the depth of the key may be included as part of the separation distance between the infiltrative surface and a limiting layer.

(5) The proposed subsurface sewage treatment area must include an area for 100 percent replacement of the system, except that the replacement area for elevated sand mounds may be allowed as provided in Department Circular DEQ-4. If a size reduction is approved for a system, the replacement area must have area sufficient for the system without the size reduction. Unless a waiver is approved by the department pursuant to ARM 17.36.601, the replacement area must meet the same requirements as the primary area. If the replacement area is not immediately adjacent to the primary area, or if there is evidence that site conditions for the replacement area may vary from those for the primary area, the applicant shall submit adequate evidence of the suitability of the replacement area.