Louisville’s Metropolitan Sewer District board on Monday approved a 5.5 percent rate increase, which will raise the average customer’s bill by $29 a year. The change is slightly smaller than the 5.8 percent increase ratepayers saw last year.MSD is about halfway through a 19-year consent decree with the federal government to overhaul the city’s sewer systems and decrease the frequency of sewer overflows.The new rate goes into effect on Friday.