Steam generation requires access to large volumes of water. Our plants fully comply with regulations governing the use of regional water resources as set forth in our water permits. We take great care to ensure we don’t negatively impact the rights of other users. That policy has served us well and has helped ensure a balance between environmental and economic interests. None of the areas from which our company withdraws water are subject to high degrees of water stress, and our plants do not cause significant impacts to water quality or availability for other uses.
We use several strategies to conserve water and preserve water quality at our plants. We optimize water use by recycling water in recirculating cooling systems at two locations and in other plant processes, resulting in approximately 8,000 megaliters being reused system-wide. In 2019 we converted to using dry ash-handling equipment at all generating facilities. We manage water quality to ensure that levels of certain parameters—such as total suspended solids, oil and grease, and metals—in the outflow stay within permit limits to protect environmental and human health. In 2019 we did not have any water use violations, fines, or penalties.
We estimate water use through various means, including calculations based on pump ratings and physical methods such as water meters.
The Minnesota Pollution Control Agency oversees water-quality programs and discharge regulations at Hoot Lake Plant. The Minnesota Department of Natural Resources (DNR) administers water appropriations.
Most of the water appropriated for the plant comes from nearby Wright Lake and is classified as either consumptive or nonconsumptive. Consumptive water is consumed within the plant, while nonconsumptive water is returned to the Otter Tail River under the plant’s water discharge permit. Nonconsumptive water generally is used in the plant’s once-through cooling system, where it’s thermally heated by condensers and doesn’t come into contact with any other industrial processes. The plant complies with strict thermal limitations in its discharge permit to protect the aquatic environment.
Additionally, the plant’s thermal mixing zone moderates water temperatures immediately after discharge. During 2019 nonconsumptive use totaled 34,330 megaliters of water, and consumptive use totaled 0.3 megaliters. A total of 34,329 megaliters was appropriated from Wright Lake, while 1.2 megaliters was appropriated from groundwater. Stormwater runoff onsite is managed through a Stormwater Pollution Prevention Plan.
Coyote Station appropriates water from the Missouri River under a permit issued by North Dakota's State Water Commission. The plant uses a closed-cycle cooling tower to meet its cooling needs. We discharge a small portion of the water appropriated for cooling tower operation to the Missouri River under a permit approved by the North Dakota Department of Environmental Quality. This thermally heated water travels through an underground pipeline for nearly 25 miles before returning to the river, giving it ample time to cool down. For this reason, our state discharge permit doesn’t have a thermal limitation.
In 2019 Coyote Station appropriated 4,875 megaliters of water from the Missouri River and discharged 262 megaliters back into the river. Over three-quarters of the water appropriated at Coyote Station evaporates after going through various plant processes. We use and recycle the remaining water within the plant. We contain most of the stormwater runoff onsite and reuse it in various plant processes. When runoff isn’t contained onsite, we treat it onsite using a settling pond before discharging it through a permitted outfall.
Big Stone Plant withdraws water from Big Stone Lake under a permit granted by the South Dakota Department of Environment and Natural Resources. The permit includes restrictions that limit water withdrawals when the lake is below prescribed levels. During 2019 we withdrew 3,481 megaliters of water from Big Stone Lake.
Virtually all water used at Big Stone Plant is treated and eventually recycled. Big Stone Plant’s unique cooling-water design eliminates the need to discharge heated water or process water into any natural body of surface water. As a zero-discharge facility, Big Stone Plant doesn’t require an industrial wastewater discharge permit from the state. All stormwater runoff is contained within the facility grounds.
Since 2002 Big Stone Plant has provided the adjacent POET Biorefining ethanol plant some of its process steam to use in the plant’s ethanol production. This is one of the region’s first and largest combined heat and power arrangements of its kind. The arrangement benefits both plants by lowering production costs at both facilities.