Sewage is treated in an oxidation ditch at the water treatment plant in Ashland, Wis., on April 30, 2019. The oxidation ditch removes organic matter and pollutants and can hold about 900,000 gallons. Ashland’s City Wastewater Utility uses fine bubble aeration, which means air is being introduced into the wastewater to enhance treatment and help break down biomass. Danielle Kaeding/WPR
