The owner of the premises, the occupant thereof and the user of the service shall be jointly and severally liable to pay for the water and sewer service on the premises, and the service is furnished to the premises by the city only upon condition that the owner, occupant and user are jointly and severally liable therefor to the city. All bills for service shall be rendered as of the first day of the period succeeding the period for which the service is billed and shall be payable not later than the close of business on the fifteenth day of the same month for monthly billings and on the thirtieth day of the same month for semiannual billings. A penalty of 10% shall be added to all bills not paid within the allowable time. When the date of billing is a Sunday or a legal holiday, then the bills shall be payable on the next succeeding secular day, without any additional penalty.