The sewer service charge schedule is net. On all accounts where payment is not received in full in the office of the Clarksburg Water Board, which is the collecting agent for the Sanitary Board, by 4:30 p.m., of the due date, which is twenty days after the date of the bill, a penalty of ten percent will be added to the amount shown on the bill. This delayed payment penalty is not interest and is collected only once for each bill where it is appropriate.