Any user of the services of the sewer system shall pay a sewer impact fee as established in section 78-121. The schedule of fees is subject to revision annually as may be necessary to keep the cost commensurate with changes in the cost of living as reflected by the Consumer Price Index or because of any regulatory or environmental factors which increase the costs of sewage collection, transmission and treatment. All revisions shall be done by resolution and shall become effective as of May 1 of each year. The basis of such rate increase shall be the Consumer Price Index U.S. city average, "all items" (82-84-100), published by the Bureau of Labor Statistics of the United States Department of Labor. The index number for the month of February 1991 shall be the base index number and the corresponding index number for the month of February in each succeeding year shall be the current index number. On May 1 of each year, the sewer impact fee shall be determined by dividing the current index number (CIN) by the base index number (BIN), in accordance with the following formula, dropping all digits after 1/100: