The governing body of any city may provide for the collection and disposal of garbage or trash or both of them and in doing so may provide for the collection and disposal of either one or both by contract or either one or both as a municipal function and any contract may be for not more than ten (10) years and such contract shall not be considered as being in conflict with the cash basis and budget laws: Provided, That any such contract may be for not more than twenty (20) years by any city of the first class having the commission form of government with three commissioners elected for terms of four years and a population in excess of one hundred twenty thousand (120,000), and any change in form of government or number or terms of commissioners or population shall not affect the power of any city qualifying at the time this act takes effect in the exercise of this power hereafter. If the city collects and disposes of garbage or trash or both as a municipal function it shall have power to purchase all necessary equipment, acquire all necessary land, build any necessary buildings, incinerators or other structures, to lease land or otherwise acquire the right to use land for the disposal of garbage or trash or both, and to do all other things necessary to a proper, effective, and sanitary disposal of garbage and trash: Provided, That garbage which may be disposed of through a sewage disposal plant may be disposed of through the sewage disposal plant rather than by burning, burying, or feeding or other disposal.