Chicago Mayor Rahm Emanuel is proposing a tax increase on water and sewer usage to help pay down the city’s pension obligation. Under the proposal the average household would pay about $50 more a year for water and sewer starting in 2017. By 2020, it would increase to more than $200 more a year. The tax is part of an agreement Emanuel announced yesterday with the city’s unions to stabilize the Municipal Employees’ Annuity and Benefit Fund. The water and sewer tax increase still needs council approval.