Section 15.232 - Distribution Boxes(1) For all gravity flow distribution systems, a water tight distribution box designed to provide equal distribution of septic tank effluent to the distribution lines of the soil absorption system shall be provided between the septic tank and soil absorption system.(2) Construction of the distribution box shall be in conformance with the provisions of 310 CMR 15.221 (general construction requirements) and 15.226 (septic tank construction), with the following exceptions: (a) The distribution box may be constructed of plastic or other materials approved by the Department if it is installed level and true to grade on a level stable base that has been mechanically compacted and onto which six inches of crushed stone has been placed to minimize uneven settling or on a concrete pad which is at least six inches in thickness and 1.5 times the bottom surface area of the distribution box.(b) The minimum inside dimension of the distribution box, regardless of material, shall be 12 inches.(c) The minimum wall thickness for reinforced concrete shall be two inches.(3) The distribution box shall conform to the following design specifications:(a) When the soil absorption system is to be dosed or the slope of the inlet pipe exceeds 0.08 feet per foot, an inlet tee, baffle or splash plate extending to one inch above the outlet invert elevation shall be provided to dissipate the velocity of the influent.(b) The invert elevation of all outlets shall be equal to each other and located at least two inches below the invert elevation of the inlet. The distribution lines leading from the distribution box shall all have the same invert elevation as determined by flooding the distribution box to the height of the distribution line invert after all lines have been sealed in place. If all inverts are not the same elevation, they shall be adjusted by filling with durable and non-deformable material permanently fastened to the line or reconstructing the lines until all inverts are at the same elevation.(c) Outlet distribution lines shall be level for a minimum of the first two feet of their length. There shall be at least one outlet for each effluent distribution line.(d) Every distribution box shall have a water tight cover or in the case of systems with a design flow greater than 2,000 gpd, water tight manhole with cover.(e) Every distribution box shall have a minimum sump of six inches as measured below the outlet invert elevation.(f) Distribution boxes buried greater than nine inches below grade shall be equipped with risers.