Santa Rosa residents could see their monthly water and sewer bills increase by an average of $11 starting in July as the city looks to invest in improvements to its aging utility system. Rates are expected to go up by a combined 6.5% under the first year of a proposed five-year rate schedule, followed by increases of 5.4% to 5.8% over the next four years. The Santa Rosa City Council will consider the proposed increases on Tuesday. Santa Rosa Water is responsible for about $5 billion in infrastructure, including about 1,200 miles of water and sewer lines and the regional Laguna Wastewater Treatment Plant. It serves about 54,000 water customers and 52,000 wastewater customers.