(a) All domestic sewage treatment permit applicants, all permittees who construct treatment facility expansions, and all permittees who apply for increases in their permitted effluent flows who propose to dispose of treated sewage effluent by discharge directly into Segment 1428 of the Colorado River shall, at a minimum, achieve the following level of effluent treatment:(1) 10 milligrams per liter of biochemical oxygen demand, based on a 30-day average;(2) 15 milligrams per liter of total suspended solids, based on a 30-day average;(3) 2 milligrams per liter of ammonia nitrogen, based on a 30-day average; and(4) 5 milligrams per liter, at least, of dissolved oxygen content, based on a 30-day average.(b) Existing plants which discharge treated domestic sewage effluent directly into Segment 1428 of the Colorado River must meet the level of treatment set forth in subsection (a) of this section, by June 1, 1990. The commission may modify the date for compliance with the treatment levels based on consideration of economic and technical feasibility, actual or potential environmental impacts, or other appropriate factors.(c) The treatment level in subsection (a) of this section may be modified if the results of water quality studies show that this is necessary.