The Washington Supreme Court ruled Thursday that the Department of Ecology can impose new water-protection requirements on Puget Sound sewer plants without considering the costs or taking public comments.

Justices unanimously upheld Ecology’s new limits on nitrogen discharged into Puget Sound, which the department set without going through a formal rule-writing process.

Ecology set limits on 29 sewer plants. The department exempted small plants and only partially fulfilled a commitment it made to Northwest Environmental Advocates to place caps on 107 plants.

Writing for the court, Justice Helen Whitener said Ecology’s new requirement wasn’t a rule because it wasn’t applied to every sewer plant.

Because Ecology retained discretion and wasn’t bound by its commitment to the Portland-based environmental group, the new requirement didn’t have “general applicability,” Whitener wrote.

The ruling opens the way for Ecology to unilaterally and selectively impose new requirements on irrigation districts and farmers, Washington Water Resources Association executive director John Stuhlmiller said.

“It says you can go beyond the laws and rules as long as you don’t make it uniformly applicable,” he said. “To me that’s unconscionable. That’s not how you regulate.”

Nitrogen causes plant growth that depletes oxygen in the water. Most sewer plants aren’t equipped to screen out nutrients, and upgrades are expensive, according to court records.

Tacoma, Kitsap County and two Puget Sound sewer districts sued Ecology. The plaintiffs said they didn’t necessarily oppose limits, but wanted Ecology to consider the financial, environmental and social costs.

Tacoma warned it might have to cancel building permits if it couldn’t meet the new cap on nitrogen, worsening a housing shortage.

A Thurston County judge and an appeals court agreed Ecology had violated the Administrative Procedure Act by adopting a rule without a public process. Ecology appealed to the Supreme Court.

Ecology downplayed its commitment to Northwest Environmental Advocates as an “unenforceable statement” and claimed it had the power to set limits on nitrogen on a case-by-case basis all along.

Ecology’s authority to address the “nutrient crisis” in Puget Sound “should be determined by this court,” according to Ecology’s appeal.

The Washington Conservation Action Education Fund and Suquamish and Squaxin Island Tribes submitted a friend-of-the-court brief asking the Supreme Court to rule in Ecology’s favor.

The Building Industry Association of Washington submitted a brief arguing that Ecology adopted a rule without considering its impact on housing costs.

King County, which has four sewer plants discharging into Puget Sound, said it didn’t oppose stricter regulations, but argued they should be science-based and adopted through a transparent public process.

King County estimated it will have to spend $9 billion to $15 billion in 10 to 15 years on removing nitrogen, causing rates to increase 23% to 40%.

Treating for nutrients will generate more effluent sludge that will need to be disposed of, and removing nitrogen from wastewater converts it into nitrous oxide, a potent greenhouse gas, according to King County.

Stuhlmiller said the ruling encourages “seat-of-the-pants” governing.

“The bottom line is you can make a rule and if you have some discretion, you don’t have to go through rule-writing,” he said. “We like discretion in the hands of an agency that’s reasonable, but if an agency becomes unreasonable, how do you rein it in?”