Not all of the phosphorus discharged into the Spokane River contributes to oxygen-robbing algae blooms in the reservoir behind Long Lake Dam, according to a new study from the University of Washington.
Some of the phosphorus is in complex molecular forms and the algae don’t have the enzymes to break it down, said the study, which could have implications for future river cleanup plans.
The study sampled effluent from six local dischargers that are testing new phosphorus removal methods. Researchers added algae to the effluent and tracked growth rates.
The treatments not only lowered the amount of total phosphorus flowing into the river, but they also appeared to target the “bioavailable” forms of phosphorus most likely to spur algae growth, said Michael Brett, a UW engineering professor who worked on the study with graduate student Bo Li.
“It’s getting out the most important phosphorus,” he said.
Brett said the study’s results challenge conventional thinking. Most treated wastewater is assumed to be high in phosphates, which act like Miracle-Gro for algae.
The study cited the pilot program at the city of Spokane’s wastewater treatment plant. The pilot treatment is reducing total phosphorus levels from 500 parts per billion to 50 parts per billion. At the same time, the amount of bioavailable phosphorus is dropping from 400 parts per billion to 10 parts per billion.
That’s a significant reduction, Brett said. The city of Coeur d’Alene’s pilot treatment had similar results. The study also found that treated wastewater from Inland Empire Paper Co.’s newsprint plant had low phosphorus bioavailability, because phosphorus sticks to other compounds found in the plant’s wastewater.
Inland Empire Paper is a subsidiary of Cowles Co., which also owns The Spokesman-Review.
Inland Empire Paper was active in lobbying for the UW study, after a smaller in-house study at the plant raised questions about the bioavailability of the phosphorus in its wastewater.
“What the study has shown is that only 9 percent of the phosphorus we release is bioavailable,” said Doug Krapas, environmental manager for the newsprint plant. “We’re asking for recognition of this.”
Krapas said the plant has spent $10 million to upgrade its treatment system, but still releases total phosphorus levels of about 70 parts per billion. The Washington Department of Ecology has proposed a new seasonal discharge limit of 36 parts per billion for the plant.
The $104,000 study – jointly paid for by the Ecology Department and the dischargers – comes as the department is finalizing new permits for entities that discharge into the Spokane River. The permits have strict limits on phosphorus releases, but the limits are based on total phosphorus instead of bioavailable phosphorus.
It’s too soon to speculate on whether the study will lead to greater leeway in total phosphorus discharges, said Dave Moore, the Ecology Department’s Spokane River water quality manager. The study is based on a relatively small sample size, and the issue may need research, he said.
“I think it’s going to take a couple of years to sort this all out,” said Lars Hendron, the engineer for Spokane’s wastewater treatment plant.
But Hendron said that study is encouraging for Spokane, because it affirms the success of the city’s pilot treatment program. A $150 million upgrade is planned for the wastewater treatment plant, which should be finished in early 2018.
That would be in time to meet the new phosphorus limits in the permit, which has a 10-year phase-in.
Brett predicted that the study’s result will spur lots of future discussion, and perhaps some follow-up research.
Regulators often take a conservative approach to phosphorous discharges, particularly when lakes and streams have oxygen deficiencies, he said.
“They don’t worry too much about how much phosphorus can be used,” Brett said. “The phosphorus that can support algal growth is a fraction of the total.”