5 Colo. Code Regs. § 1003-5-3.12
All sand filters shall comply in all respects with the standards of the NSF covering such filters. Sand filters shall be properly sized to achieve the proper turnover rate without exceeding the maximum flow rate for the filter design. Rapid sand filters shall be sized for a filter rate not to exceed 3-5 gal./min./sq.ft. High rate sand filters shall be sized to operate at a filter flow rate of 15-20 gpm/sq.ft. The underdrain system shall be of corrosion resistant and durable material so designed and of such material that the orifices or other openings will maintain an approximate constant area. It shall be designed to provide even collection or distribution of the flow during filtration and backwashing. The filter system shall be provided with a rate of flow indicator, a method of air release of the filter tank, and pressure gauges, all in good operating condition. A backwash sight glass shall be provided. The recommended sand specifications must be provided with each filter on a plate on the filter tank in an easily readable location. Backwash lines shall not be restricted in any way between the filter and the sanitary sewer.
5 CCR 1003-5-3.12