RIVERDALE – Riverdale has been asking its water users to conserve this summer as it seeks to resolve an algae problem impacting its water treatment system.

Riverdale City Council member Jerry Orth said this week the coagulant in the city’s water treatment system is not capturing and removing the algae, which then passes through to and clogs the water treatment plant’s membrane filters.

Orth said the filters are being cleaned daily rather than the usual once every week or two. Employees are working seven days a week to accomplish this, he said.

The result has been a reduction in the daily water supply from what typically would be 600,000-800,000 gallons a day in the summer to fewer than 200,000 gallons a day, Orth said. Riverdale is producing enough water to continue giving users an adequate and safe drinking water supply, but other uses are restricted.

Forcing water through clogged filters to supply more water would cripple the membranes and create a situation in which the membranes would need to be replaced. Ordering and replacing the membranes would be a lengthy process, Orth said.

“We are working with our engineering team. They are going to be at the next meeting with several ideas,” Orth said. He said Riverdale already tried a suggestion from its engineers for pre-filtering but that filter clogged as well after about 30 minutes of use.

The algae issue started in May, although Riverdale’s plant operator was seeing signs of a problem developing in April, Orth said.

“The algae is not going away. If it does, it will come back,” he said, citing a similar issue experienced upstream on the Missouri River in Montana.

Greg Wavra, director of the Drinking Water Program in the North Dakota Department of Environmental Quality’s Division of Municipal Facilities, said the department is unaware of issues in other communities with Missouri River and Lake Sakakawea water supplies. Riverdale sells water to Sheridan-McLean Water District, Pick City and Underwood, which have a combined population of about 1,525 residents, he said.

Communities with their own systems use different types of Intakes and treatment systems. Riverdale gets its water through the stock pens in the Garrison Dam, where water is deep and cold, Wavra said. Algae usually grows in shallower, warmer water.

It’s uncertain whether the current low lake levels might be a factor in Riverdale’s experience, Wavra said. Organics also can cause membrane clogging when there is runoff into the lake during times when lake levels are low, he said.

The Northwest Area Water Supply project that is scheduled to begin bringing water from Lake Sakakawea to Minot and area communities later this year will use a different treatment system from the one used in Riverdale.

Jason Sorenson, Utilities director in the Minot Public Works Department, said particulate will be removed before the water goes through ultraviolet light and chlorine treatment at the biota plant in Max. The process for removing particulate is called dissolved air flotation (DAF), which is followed by filtration. DAV involves dissolving air into pressurized water and releasing it, which causes microscopic bubbles to attach to contaminants so they float to the surface.

“DAF isn’t really common in typical water treatment systems, but is seen more commonly in wastewater treatment. DAF is a process that is specifically used for algae, so the biota plant shouldn’t have any issues,” Sorenson said. “As long as it’s removed in Max, the Minot plant shouldn’t have any concerns.”

Brian Houle, an environmental scientist who runs the Harmful Algal Blom Monitoring Program in the North Dakota Department of Environmental Quality, said algae in the water, called filamentous algae, differs from algal blooms on the water’s surface that can prompt advisories or warnings when thresholds get high.

Algal blooms are actually bacteria, he said, and there hasn’t been a report on Lake Sakakawea since monitoring began in 2016. Typically larger lakes with deep water aren’t prone to harmful algal bloom, he said. There have been a few reports on Lake Audubon but cyanotoxins have been low.

Filamentous algae, such as the green strings that a canoe paddle might pull up, don’t carry the same danger as the algal blooms, Houle said. That type of algae isn’t monitored.