The income of a drainage utility system must be segregated and completely identifiable in municipal accounts. If drainage charges are solely for the cost of service, the municipality may transfer the charges in whole or in part to the municipal general fund, except for any part collected outside municipal boundaries and except for any part pledged to retire any outstanding indebtedness or obligation incurred, or as a reserve for future construction, repair, or maintenance of the drainage system. If the governing body has levied, in the drainage charge, an amount in contribution to the funding of future system improvements, including replacement, new construction, or extension, that amount is not transferable to the general fund.