This chapter is not to be construed as revoking, altering, or amending any provisions of law for sewer assessment now lawfully assessed by any city or town, but it is to be construed as authorizing a charge for the use of those sewers or sewer systems for disposal purposes, and in addition to all other sewer assessments now lawfully imposed by the city or town, under the provisions of any general, public or special law, charter, act, or resolve, and any ordinance now in effect.