40 CFR § 62.14412 - What stack opacity and visible emissions requirements apply?
(a) Your HMIWI (regardless of size category) must not discharge into the atmosphere from the stack any gases that exhibit greater than 6 percent opacity (6-minute block average).
(b) Your HMIWI as defined in § 62.14400(a)(2)(ii) and utilizing a large HMIWI must not discharge into the atmosphere visible emissions of combustion ash from an ash conveying system (including conveyor transfer points) in excess of 5 percent of the observation period (i.e., 9 minutes per 3-hour period), as determined by EPA Reference Method 22 of 40 CFR part 60, appendix A-7, except as provided in paragraphs (b)(1) and (2) of this section.
(1) The emissions limit specified in paragraph (b) of this section does not cover visible emissions discharged inside buildings or enclosures of ash conveying systems; however, the emissions limit does cover visible emissions discharged to the atmosphere from buildings or enclosures of ash conveying systems.
(2) The provisions specified in paragraph (b) of this section do not apply during maintenance and repair of ash conveying systems. Maintenance and/or repair must not exceed 10 operating days per calendar quarter unless you obtain written approval from the state agency establishing a date when all necessary maintenance and repairs of ash conveying systems are to be completed.