Each on-site subsurface sewage disposal system approved pursuant to 310 CMR 15.000 shall consist of a septic tank which discharges liquid effluent through a gravity distribution, dosing or pressure distribution network to a soil absorption system as hereinafter described. No modifications or alterations to the design criteria shall be allowed except pursuant to the alternative system and shared systems provisions of 310 CMR 15.280 through 15.293, or the local upgrade approval or variance procedures of 310 CMR 15.400.