In Alaska's largest city, federal regulators are considering whether the town's water treatment plant will need upgrades, in part to comply with the endangered listing of the Cook Inlet beluga whale population.

The Clean Water Act of 1972 requires sewage-treatment plants to remove more than 90 percent of dissolved solids and most of the oxygen-depleting organics. Compounds such as nitrogen and phosphorous are also removed. But Anchorage's treatment plant doesn't have to meet that standard.The city has been operating with a waiver that has been receiving a series of temporary administrative extensions. Upgrading or replacing the main Anchorage plant to bring it up to the level of most U.S. cities could cost as much as $800 million.

As it is, Anchorage is one of only a few dozen sewage treatment plants in the U.S. to get such a waiver, allowing it to dump effluent with only primary treatment into local waters. But almost five years ago the U.S. Department of Fish and Wildlife listed Cook Inlet beluga whales as endangered. Since then, Anchorage officials have wrestled with how the listing might impact development, including its wastewater treatment.

Read more: Will feds grant Anchorage another sewage-treatment waiver?