LARGO — Sewer rates are expected to jump at least 15 percent next year and could go even higher to pay for a $37 million plan to revamp the city's system.City officials don't have much choice: The state is making them upgrade the system so untreated sewage will not end up in Largo streets and waterways.Largo officials plan to give the go-ahead Monday for construction to begin. The project, which is expected to take up to 1 ½ years to complete, will involve 14 miles of sewer pipes and the reconstruction of seven pumping stations. When finished, the upgrades will not only bring Largo into compliance with state environmental rules and prevent the overflow of sewage into city streets, it will also expand the system to handle expected population growth through 2030."The big driver is (that) it's going to stop the overflows," city engineer Leland Dicus said.The sewer improvements are the result of problems that started about 15 years ago when the system overflowed during the heavy rains of that hurricane season, Dicus said. The overflow came out through manhole covers and dumped sewage in the street that eventually made its way to the bay. In all, Dicus said, about 30 million gallons of overflow came out of the system."There's not enough capacity" during those heavy rainfalls, he said. "The sewer system backs up and it will overflow."Largo studied the system and made some improvements that included lining and sealing old pipes and manhole covers to stop leaks. Some of those remedies continue today. But that wasn't enough to cope with the 2004 hurricane season when Hurricanes Charley, Frances, Ivan and Jeanne hit the state. Each time Largo's system overflowed, between 3 million and 4 million gallons of sewage poured into the streets and ended up in the bay. The state Department of Environmental Protection moved in.Largo and the DEP signed an agreement in 2006 that gave the city 10 years to bring its system up to par so that it could withstand a "10-year rain event." That's a heavy rainfall that's expected to happen only once every 10 years. They also agreed to a set a fine should Largo experience any overflows during the 10 years. It's unclear how much Largo has been fined. City officials did not have records readily available.Since then, Largo has spent about $4.3 million studying the system to decide the best and most economical way to comply with the DEP's rules and to make sure the system is big enough to absorb Largo's expected population growth. Garney Cos. won the contract with a $39.4 million bid. That has since been reduced to about $37.1 million during negotiations with Garney, and city officials expect that to drop to about $36.7 million by the time the project is done.Largo is borrowing money from the state at 1.07 percent interest to finance the construction. The money will be repaid from the city sewer fund, which gets its money from customer billing. Sewer rates are expected to go up 15 percent in the 2015-16 fiscal year to help pay off the debt, said Amy Davis, who oversees the city's Office of Management and Budget. Currently, she said, plans are for a second 15 percent increase three years later, but that is not set in stone."The second one is a moving target right now," Davis said.Although Garney will receive the go-ahead next week, Dicus said construction is unlikely to begin until March. Construction will be done in phases and the city will place door hangers in affected areas before crews move into the area. Sewer service is not expected to be interrupted, but traffic might be. For information on the construction, see largo.com. Residents will also be able to sign up for email notification of the project's progress.Contact Anne Lindberg at [email protected] or (727) 893-8450. Follow @alindbergtimes.