All present powers, duties and functions conferred upon municipalities within the basin in connection with the planning, construction, financing and operation of sewage or waste treatment facilities (excepting common sewers and other collector facilities considered ineligible for state grants under the provisions of RSA 486), or both, as are contained in RSA 485, 485-A, 149-I and applicable statutes, are transferred to the department. Personnel of municipalities engaged in the operation of sewage or waste treatment facilities, or both, as referred to in this subdivision, shall be given an opportunity to become employees of the department (with all benefits previously accrued) upon the effective date of the transfer of the municipal sewage or waste treatment facilities, or both, to the department. In no case shall personnel accepting state employment, as provided under this subdivision, be paid less than the salary paid such individuals as of January 1, 1973, nor shall they suffer a loss or reduction in benefits associated with tenure of service. It shall be the responsibility of the municipality previously employing the individual to supplement such state of New Hampshire benefits if they are less than the employee might have received if his employment had continued uninterrupted with the municipality.