Whenever the county government shall determine it necessary to acquire additional lands or interests in lands or to acquire or to construct additional trunk, interceptor and outfall sewers, pumping stations, treatment and disposal works and appurtenances, lateral sewers or other facilities, or whenever the county government shall determine it necessary for the purpose of the operation and maintenance of such facilities to increase, improve and reconstruct the facilities thereof, including the acquisition of additional lands or interests in lands, it may cause the same to be accomplished without further authorization; provided, however, that if it is proposed to finance any part of the cost thereof by the issuance and sale of county obligations, such improvement shall not be undertaken unless authorized by proceedings taken in the same manner as the proceedings taken for the original establishment or extension of the district except that the sole determination which the county government shall make is whether such improvement is in the public interest, and, if zones of assessment have been established, said county government shall further determine the allocation of the cost thereof as between such zones.