The strength of any industrial waste, the discharge of which is to be subject to surcharge, shall be determined monthly, or more frequently, from samples taken at the manhole described in Section 929.14 or at any other sampling point mutually agreed to by the Sanitary Board and the producer of the waste. The frequency and duration of the sampling period shall be such as, in the opinion of the Sanitary Board, will permit a reasonably reliable determination of the average composition of the wastes, exclusive of storm water runoff. Samples shall be collected, by a representative of the Sanitary Board, in proportion to the flow of waste, exclusive of storm water runoff, and composited for analysis in accordance with the current edition of Standard Methods for the Examination of Water and Sewage, as published by the American Public Health Association. Except as provided in this article, the strength of the waste so found by analysis shall be used for establishing the surcharge provided for in this article. However the Sanitary Board may, if it so elects, accept the results of routine sampling and analyses by the producer of the wastes in lieu of making its own samplings and analyses. (1969 Code §22-36)