N.Y. Comp. Codes R. & Regs. tit. 6, § 750-1.10 – Effluent limitations in issued SPDES permits.

(a) In the application of effluent limitations, water quality standards, and other applicable requirements, pursuant to this Part, the department may specify daily average, monthly average, seven-day average, annual average, 12-month rolling average, peak hourly, annual maximum, instantaneous maximum and daily maximum quantitative limitations for the level of pollutants in the authorized discharge in terms of weight or, as in the case of flow, pH, temperature, and for any other pollutants not appropriately expressed by weight, in other appropriate terms. The department may, in addition to or in lieu of the specification of daily quantitative limitations by weight or by other terms, specify other limitations, such as average or maximum concentration limits, on the pollutants in the authorized discharge.
(b) Any point source, the construction of which is commenced after October 18, 1972, and which is so constructed or physically modified, provided the cost of such modification exceeds 50 percent of the initial capital costs of the facility or new source which meets the applicable promulgated new source performance standards before the commencement of the discharge, to meet all applicable standards of performance set forth in the point source SPDES permit and fact sheet shall not be subject to a more stringent technology based standard of performance (under section 301[b][2] of the act) requiring the construction of additional treatment facilities during a 10-year period beginning on the date of completion of such construction or during the period of depreciation or amortization of such facility for the purposes of section 167 or 169 (or both) of the Internal Revenue Code of 1986, whichever period ends first; provided, however, that if the operation of such source causes or contributes to any contravention of any State water quality standard the department shall require that abatement action be taken by the permittee and modify the permit pursuant to section 750-1.18 of this Subpart.
(c) Notwithstanding any other provision of this Part, when effluent limitations are established they must be at least as stringent as the effluent limitations previously required unless the department determines that an exception is warranted because the circumstances on which the previous permit was based have materially and substantially changed since the time the permit was issued and would constitute cause for permit modification. A permit may be modified to contain a less stringent effluent limitation applicable to a pollutant, if:
1. (1) material and substantial alterations or additions to the permitted facility occurred after permit issuance, which justify the application of a less stringent effluent limitation;
  1. (i) information is available, which was not available at the time of permit issuance (other than revised regulations, guidance, or test methods), which would have justified the application of a less stringent effluent limitation at the time of permit issuance; or
  2. (ii) the department determines that technical mistakes or mistaken interpretations of law were made in issuing the permit under this Part;
    (3)
  3. (i) a less stringent effluent limitation is necessary because of events over which the permittee has no control and the permittee demonstrates that it has thoroughly studied or implemented all feasible alternative means to remedy the situation, and is still unable to meet the limitations; or
  4. (ii) the department reviewed the data supplied by the permittee and is in agreement that no feasible alternatives to remedy the situation exist; provided that:
  (2)
2. (4) the permittee has received a permit modification under section 301(c), (g), (h), (i), (k), (n), or 316(a) of the act (see section 750-1.25 of this Subpart);
3. (5) the permittee has installed the treatment facilities required to meet the effluent limitations in the previous permit and has properly operated and maintained the facilities but has nevertheless been unable to achieve the previous effluent limitations, in which case the limitations in the reviewed, reissued, or modified permit may reflect the level of pollutant control actually achieved (but shall not be less stringent than required by effluent guidelines in effect at the time of permit modification);
4. (6) the discharge is to non-attainment waters for the limited pollutant and the limit is based on a final or interim TMDL or other waste load allocation method and the TMDL is set to assure attainment of the water quality standard; or
5. (7) the limit is for attainment waters for the limited pollutant and based on a final or interim TMDL or other waste load allocation method and the TMDL is set to assure attainment of the water quality standard and the limit has been subjected to an anti degradation review in accordance with the department's anti degradation policy;
6. (8) the modifications allowed in paragraphs (6) and (7) of this subdivision shall not apply to any revised waste load allocations or any alternative where the cumulative effect of such revised allocations results in a decrease in the amount of pollutants discharged into the concerned waters, and such revised allocations are not the result of a discharger eliminating or substantially reducing its discharge of pollutants due to complying with the requirements of this Chapter or for reasons otherwise unrelated to water quality.
(d) In no event may a permit be modified in accordance with subdivision (c) of this section to contain an effluent limitation that is less stringent than required by effluent guidelines, including variances thereto, in effect at the time the permit is modified. In no event may such a permit to discharge into waters be modified in accordance with subdivision (c) of this section to contain a less stringent effluent limitation if the implementation of such limitation would result in a violation of a water quality standard or guidance value applicable to such waters as set forth in Parts 700-706, et seq. of this Title.