DES MOINES — The water utility serving a half a million customers in central Iowa including the city of Des Moines says it has switched on its nitrate removal system. Des Moines Water Works CEO Bill Stowe says nitrate concentration in untreated water from river sources is approaching 10 milligrams per liter. That’s a level the Environmental Protection Agency says isn’t safe for babies to drink. Stowe says processed water remains safe but it costs extra to run the denitrification equipment.