(1) Septic tanks shall have the following capacities:

1. (a) For a single family dwelling unit with a design flow of less than 1,000 gallons per day, a minimum effective liquid capacity of 200% of the design flow or a minimum hydraulic detention flow of 48 hours, whichever is greater, shall be required. In no case shall the effective liquid capacity of the tank as measured below the outlet invert elevation be less than 1,500 gallons.
2. (b) When designed to serve facilities other than a single family dwelling unit or whenever the calculated design flow is 1,000 gallons per day or greater, a two compartment tank or two tanks in series are required. The design of the tanks shall be in accordance with 310 CMR 15.224 for multiple compartment tanks and 310 CMR 15.225 for tanks in series. At a minimum, the total, combined effective liquid capacity of both tanks in series or of the multiple compartment tank shall not be less than 1,500 gallons.
3. (c) When a domestic garbage grinder is proposed or installed, the minimum liquid capacity of the septic tank shall be 200% of the design flow with a minimum tank size of 1,500 gallons and a two compartment tank or two tanks in series shall be required which meet the design criteria specified in 310 CMR 15.223(1)(b). Domestic garbage grinders are prohibited in facilities which include an elevated septic tank constructed in accordance with 310 CMR 15.213 (construction in V-zones).