A. On relatively flat terrain, where the elevation difference of the ground surface does not exceed 6 inches in any direction within the absorption field, the septic tank effluent may be directed to the absorption field through a system of interconnecting distribution pipes;

B. On slightly sloping terrain, where the elevation difference of the ground surface does not exceed 28 inches in any direction within the absorption field, the septic tank effluent may be distributed by a distribution box, provided the final ground surface elevation of the lowest trench is at least 1 foot higher than the invert elevation of the outlets of the distribution box. The inverts of all outlets shall be at the same elevation as measured from a liquid surface, which is at least 4 inches above the distribution box floor. The inlet pipe invert shall be at least 1 inch higher than outlet inverts. Each absorption line shall be connected separately to the distribution box and shall not be subdivided. When the septic tank effluent is pumped to the distribution box, either a baffle wall shall be installed in the distribution box, or the pump discharge shall be directed against a wall of the box on which there is no outlet. The baffle shall be secured to the box and shall extend at least 1 inch above the crown of the inlet pipe. The distribution box shall be watertight with a removable cover, constructed of durable materials resistant to corrosion or decay, and shall have sufficient capacity to handle the maximum daily flow rate; and

C. On sloping terrain, where the elevation difference of the ground surface exceeds 28 inches in any direction within the absorption field, a serial distribution system shall be installed. The serial distribution system shall be connected with drop boxes or closed pipe relief lines, in such a manner that each trench is completely filled with septic tank effluent to the full depth of the gravel before effluent flows to succeeding trenches. The drop boxes or relief lines shall be placed on an undisturbed section of ground. The first drop box or relief pipe arrangement encountered shall not have the crown of the outlet pipe at its highest point above the invert of the septic tank outlet. At each drop box or relief arrangement, the invert of the inlet pipe shall be between 1 and 2 inches higher than the invert of the outlet pipe to the succeeding trench. The slope of the trench between the invert of the outlet and the invert of the inlet of successive drop box or relief pipe arrangements shall be 1 inch per 100 feet. When septic effluent is delivered to the drop box by a pump, the pump discharge shall be directed against a baffle wall or against a wall of the box on which there is no outlet. The drop box shall be watertight with a removable cover, constructed of durable materials resistant to corrosion or decay, and shall have sufficient capacity to handle the maximum daily flow rate.