(1) The degree of waste treatment required to restore and maintain the quality of surface waters shall be based on the surface water quality standards and the following:
(a) the state's policy of nondegradation of existing high water quality as described in 75-5-303, MCA;
(b) present and anticipated beneficial uses of the receiving water;
(c) the quality and nature of the flow of the receiving water;
(d) the quantity and quality of the sewage, industrial waste or other waste to be treated; and
(e) the presence or absence of other sources of pollution on the same watershed.
(2) For design of disposal systems, stream flow dilution requirements must be based on the minimum consecutive seven-day average flow which may be expected to occur on the average of once in ten years. When dilution flows are less than the above design flow at a point discharge, the discharge is to be governed by the permit conditions developed for the discharge through the waste discharge permit program. If the flow records on an affected surface water are insufficient to calculate a ten-year seven-day low flow, the department shall determine an acceptable stream flow for disposal system design. For total nitrogen and total phosphorus, the stream flow dilution requirements must be based on the seasonal 14Q5, which is the lowest average 14 consecutive day low flow, occurring from July through October, with an average recurrence frequency of once in five years.
(3) Where the department has determined that the disposal of sewage may adversely affect the quality of a lake or other state waters, the department may require additional information and data concerning such possible effects. Upon review of such information the department may impose specific requirements for sewage treatment and disposal as are necessary and appropriate to assure compliance with the Water Quality Act, Title 75, chapter 5, MCA.