(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 17.36.314.

(3) 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, except that at least six feet of natural soil must exist on a slope of greater than 15 percent.

(4) 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.