FLAGSTAFF, Ariz. (AP) â¿¿ The U.S. Environmental Protection Agency is facing a deadline to issue a final decision for controlling emissions from three Arizona power plants that it contends have impaired visibility at places like the Grand Canyon. The agency had proposed approving Arizona's air quality plan to reduce sulfur dioxide and soot at the Cholla, Coronado and Apache generating stations. But the EPA indicated earlier this year that the state's plan to lower nitrogen oxide emissions didn't go far enough. Instead of low nitrogen-oxide burners, the EPA had proposed that some of the older units at the plants be equipped with selective catalytic reduction technology to keep 17,000 tons nitrogen oxide from being released into the air and causing regional haze and visibility issues at 18 national parks and wilderness areas. Environmentalists had praised the proposal. Thursday marks the deadline for the agency to announce its decision. The state and the plants' operators have said that the EPA's proposal issued earlier this year would cost hundreds of millions of dollars with negligible improvement to air quality. "We're in favor of making sure our plant has state-of-the-art equipment," said Damon Gross, a spokesman for Arizona Public Service, which operates the 1,027-megawatt Cholla Power Plant near St. Johns. "The question becomes, what is the benefit you're getting versus the cost of what customers are going to assume because of that." EPA's proposal would mean a $436 million investment among Cholla's owners, which Gross said would be passed on to ratepayers. The bill for residential users would go up nearly $19 a year, while small business customers would pay almost $32 more annually, he said. The Salt River Project, which owns and operates the Coronado Generating Station near Holbrook, said it would be on the hook for another $119 million under the EPA's proposal. Kelly Barr, the senior director of environmental management policy and compliance for SRP, said that would be on top of a $500 million investment that partly came out of a previous consent decree with the EPA for upgrades.