Dakota Gas restores old stormwater pond

Dakota Gas recently restored a stormwater pond that hadn't been used for at least two decades. The newly restored pond - Pond D - makes it the fourth stormwater pond now available for use at Dakota Gas.

The need for another pond became apparent over the past couple of years, said Jake Miller, Basin Electric environmental engineer.

"Recently, we've had too much water and not enough capacity," he said, noting the last time the pond was used was about 20 years ago.

Typically, at Dakota Gas all the rainwater that falls within the plant outside of process areas is collected in three onsite ponds. "This water is normally discharged to the creek leaving the plant site after the water is tested and meets environmental specifications for discharge," said Dakota Gas Shift Superintendent Troy Johnson. "This winter we had some trouble meeting discharge parameters. The ammonia levels in the ponds prevented us from discharging the water off plant site."

In the newest stormwater discharge permit, the ammonia requirements have tightened, making it more challenging to meet the conditions required for discharge. The plant produces hundreds of thousands of tons of ammonia-based fertilizers every year, and there is tremendous effort to keep everything contained, but it only takes a few pounds of spillage to cause the storm water to deviate from the specification.

Last winter, the three ponds reached full capacity - approximately 20 million gallons - and so the team rented two 3.2-million-gallon temporary tanks. One of the tanks was used to store process cooling water and the other was used to treat stormwater for discharge. Through what is called a break-point chlorination process they were able to remove the ammonia from the stormwater in the temporary tank.

"This allowed us to meet the environmental discharge parameters and discharge the water," Johnson said, noting the rebuilt pond will give Dakota Gas a place to treat water for discharge when needed. "We used this tank to batch-treat 3 million gallons at a time. In the ponds, we also started aerating the water to promote algae growth. This, along with some plant cleanup, allowed us to start meeting discharge parameters in the main ponds where we could discharge the water without the need to treat."

Johnson said the nice thing about the new pond is that water needs to be pumped in or out. There is no runoff that directly enters the pond. This allows for a more consistent treatment of the water.

Restoring the pond involved digging out debris and excess soils, and then reconditioning the soil to form a compacted clay layer. This was done by using heavy machinery, compacting the clay to become waterproof and forming a seal in the pond. The compacted clay is 2 feet thick.

Miller said talk about revamping the pond started in June and work got underway in September. The project was completed in mid-October.

The project had a few challenges along the way, Miller said, but the team met them with bravado. He gave a nod to Bob Kohler, Basin Electric senior land surveyor, and Peter Moch Basin Electric senior civil engineer, for their help with the project.

Miller said the new pond will be a boon for several reasons, including those Johnson mentioned. It will allow for more stormwater storage and ensure that discharge water meets quality standards, something for which the team takes great pride.

"This earthen embankment will give us a little more capacity to hold water, especially in winter," he said. "We have been in situations in the past few years where we've had to rent tanks. Having this means we won't have to rent those in the future."

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