(a) Except as provided in subsections (b) and (c) of this section, all discharges of treated sewage effluent into the waters of the state in the tributaries of Segment 1428 of the Colorado River or directly into Segment 1427, Onion Creek, of the Colorado River Basin and its tributaries shall, at a minimum, achieve the following level of effluent treatment:
  (1) five milligrams per liter of biochemical oxygen demand, based on a 30-day average;
  (2) five milligrams per liter of total suspended solids, based on a 30-day average;
  (3) two milligrams per liter of ammonia nitrogen, based on a 30-day average; and
  (4) one milligram per liter of phosphorus, based on a 30-day average.
(b) Subsection (a) of this section does not apply to any existing facilities which discharge treated domestic sewage effluent into tributaries of Segment 1428 of the Colorado River or Segment 1427, Onion Creek, and its tributaries, of the Colorado River Basin, so long as that facility remains at its permitted flow for the facility currently constructed. However, subsection (a) of this section shall apply to all permitted facilities and phased facility expansions not under substantial construction as of September 25, 1986, no later than June 1, 1990. This subsection shall not preclude the commission from imposing more stringent treatment levels to such facilities in the future if the results of water quality studies show that such is necessary.
(c) The City of Austin's Walnut Creek wastewater treatment facility, located at the south side of FM Road 969, approximately one mile east of the intersection of FM Road 969 and United States Highway 183 in Travis County, must by October 1, 1997, meet, at a minimum, the effluent treatment level in subsection (a) of this section unless the discharge is relocated to the main stem of the Colorado River.
(d) The treatment level in subsection (a) of this section may be modified if the results of water quality studies show that this is necessary.

Source Note: The provisions of this §311.43 adopted to be effective October 19, 1986, 11 TexReg 4167; amended to be effective April 29, 1994, 19 TexReg 2958.