Each and every waterworks corporation holding consumer deposits shall allow to each depositor simple interest on the amount deposited. The interest rate to be applied to the amount deposited shall be a composite yield of intermediate term, A-rated corporate bonds, as reported in financial publications, less costs of administering deposits of 1.75 per centum per annum. The rate shall be updated effective January 1st each year, based upon yield information available on the first Friday of October of the preceding year. The commission will issue an advisory letter which publicizes the information to assure consistency. Each corporation shall credit such interest to each depositor whose deposit shall have been held for one or more years by paying such interest in cash or deducting it from the amount of a bill for water service rendered such depositor thereafter, but not later than the next bill rendered after the next succeeding first day of October and at each one-year interval thereafter.