a. The administrative head or body, where applicable, may contract (1) for the purchase, from any legal entity having water available, of any quantity of water for resale within the water district on either a wholesale or retail basis, subject to the approval of the contract by the county legislative body, or (2) for the collection, conveyance, treatment or disposal of sewage, wastewater disposal, water quality treatment residuals, drainage and refuse with any public corporation or any county or town on behalf of any improvement district and also with any other sewer district, wastewater disposal district, water quality treatment district or private corporation. If the county drainage district has capacity in excess of its own needs, the county legislative body may contract for the conveyance of storm water and other waters, both surface and sub-surface, from municipalities, districts and persons, corporate or otherwise, outside the drainage district. If a Suffolk county sewer district has capacity in excess of its own needs, the administrative head may contract with municipalities, districts, public or private corporations, or individuals within or without the district for the collection, conveyance, treatment or disposal of sewage including scavenger waste.