After decades of squabbling between Detroit and its suburbs over water rates, a court-appointed lawyer has been looking into a possible solution that is sure to stir controversy: the prospect of the city selling off the third-largest water system in the country. F. Thomas Lewand was brought in to work with a group of businessmen, academics and former politicians who have come together to try to solve the long battle between the city and the suburbs. He recently submitted more than $155,000 in legal bills to the court overseer of the Detroit Water and Sewerage Department, noting that he was researching a sale of the system’s assets. No formal recommendations are expected until spring. But any talk of selling off the system, privatizing it or putting it under the control of a regional authority is certain to generate heated debate.