The size, slope, alignment, materials of construction of a building sewer and the methods to be used in excavating, placing of the pipe, jointing, testing and backfilling the trench shall all conform to the requirements of the appropriate specifications of the ASTM or WPCF Manual of Practice No. 9. All installation is to be subject to the express written approval of the city and/or the Western Carolina Regional Sewer Authority and in no case shall the size of the pipe installed be less than four inches in nominal diameter. (Prior code § 4-1-26)