Safe drinking water has returned to four cities in southwest Iowa.

A membrane filtration system that failed at a Creston-based water treatment plant left 20,000 customers without safe tap water since June 1. The problem was finally fixed late last week and Creston residents, on Sunday, were the first to be told they no longer had to boil water before using it for food preparation, brushing teeth, or drinking.

Three more cities were given the all-clear Monday — Afton, Diagonal, and Lenox.