Rohnert Park is raising its sewer rates by about seven percent. The rate increase, approved by the Rohnert Park City Council this week, will take effect in October. Monthly bills for single-family homes are expected to increase from $42.20 to $45.33. There will be incremental rate hikes during each of the next three years. The city says it needs to keep up with the rising cost of managing and treating its wastewater. Rohnert Park currently has the least expensive sewer rates in Sonoma County. Cotati will take that title in October.