(1) For the purposes of this section, the rate at which sewage is discharged into, or at which it flows through, a pipe or sewer shall be measured in units which are herein referred to as “discharge units” (abbreviated reference d.u)

(2) The rate at which sewage flows through a pipe or sewage flows through a pipe or sewer at a particular point therein shall be the sum of the rates at which such sewage is discharged into such pipe or sewer in accordance with section 34 at all points above that particular point.