LYNN – Water and sewer ratepayers will pay an additional $20 on an average $779 annual bill under a proposed 2.5 percent increase scheduled for Water and Sewer Commission review on June 3.If approved by the commission following the 6 p.m. public hearing in Water and Sewer?s 400 Parkland Ave. offices, the hike will come on the heels of last year?s 3.1 percent increase. Water and Sewer Treasurer David Travers said the commission kept rates from rising in 2011, 2012 and 2013.Travers said the average residential ratepayer uses 8,000 cubic feet of water – an amount translating into an average annual bill of $779.20. The proposed rate hike would boost the average bill to $798.40, he said.?We will basically have had about a five percent increase in five years,” he said.By contrast, rates rose more than three percent annually every year between 2000 and 2005 with the steepest increases between 2006 and 2008.Neighboring communities use a variety of formulas to calculate water and sewer rates. Some, but not all, are comparable to Lynn?s formula. Nahant?s current combined water and sewer rate is $15.61. Saugus? combined rate is currently $11.03.Lynnfield residents pay water rates that differ depending on which of the two town water districts they live in. The minimum water rates residents pay range from $60 to $132,Commissioners are set to review next year?s Water and Sewer budget and capital spending plan at their May 11 meeting. Travers said the commission?s ability to keep costs down by refinancing Water and Sewer debt has helped keep rates in check, but he said the commission has long-range expenses, including a $5 million upgrade to the sewage treatment complex incinerator.The commission has budgeted $2.5 million to convert its Parkland Avenue water treatment system from chlorine gas to liquid chlorine and the costs associated with a large-scale plan to separate sewage from stormwater and end West Lynn flooding are still being calculated.?We have submitted a supplemental plan to regulatory authorities and we are waiting to hear back on what the next step is,” he said.Travers said the commission also has a $1 million meter change-out program to replace older meters that, he said, can “under register consumption.” He said Water and Sewer workers are about halfway through replacing 20,000 meters located across the city.