Section 15. The city council of a city or a town may adopt a system of sewerage for a part or the whole of its territory, and may provide that assessments under section fourteen shall be made upon owners of land within such territory by a fixed uniform rate or a rate based upon a uniform unit method.

A fixed uniform rate shall be based upon the estimated average cost of all the sewers therein, according to the frontage of such land on any way in which a sewer is constructed, or according to the area of such land within a fixed depth from such way, or according to both such frontage and area; but no assessment in respect to any such land, which by reason of its grade or level or any other cause cannot be drained into such sewer, shall be made until such incapacity is removed. If the assessment is according to the area within such fixed depth, the lien therefor shall attach to the parcel assessed.

A uniform unit method shall be based upon sewerage construction costs divided among the total number of existing and potential sewer units to be served, after having proportioned the cost of special and general benefit facilities. Each sewer unit shall be equal to a single family residence. Potential sewer units shall be calculated on the basis of zoning then in effect. Existing and potential multifamily, commercial, industrial and semipublic uses shall be converted into sewer units on the basis of residential equivalents.

A city by ordinance or a town by by-law may separate the costs of general benefit facilities, including but not limited to pumping stations, trunk and force mains, from that of special benefit facilities, including but not limited to the sewer mains, serving adjacent properties. A portion of costs of the general benefit facilities may be apportioned by the uniform unit method on all areas to receive benefits within the pumping district or combination of districts. The cost of the general benefit facilities, attributable to undeveloped land not abutting a sewered street, may not be assessed until properties are serviced by public sewerage. The proportional cost of the special benefit and general benefit facilities may be assessed against all properties abutting a sewered street.