NAC445A.533. Standards for operation of facility for filtration.

A facility for filtration must be operated in accordance with the following requirements:

     1. A plant for conventional and direct filtration must be operated at a rate of flow not to exceed 3 gallons per minute per square foot for single media filters and 6 gallons per minute per square foot for deep bed, dual or mixed media filters under conditions of gravity flow. For pressure filters, the rates of filtration must not exceed 2 gallons per minute per square foot for single media filters and 3 gallons per minute per square foot for dual, mixed media or deep bed filters.

     2. A slow sand filter must be operated at a rate of filtration not to exceed 0.1 gallons per minute per square foot. The filter bed must not be dewatered except for cleaning and maintenance.

     3. A diatomaceous earth filter must be operated at a rate not to exceed 1 gallon per minute per square foot.

     4. During normal operating conditions, any filter removed from service must be backwashed upon start-up.

     5. Any membrane filtration unit must be subjected to a direct integrity test upon start-up.

     6. Rates of filtration must be increased gradually when placing filters back into service after backwashing or any other interruption in the operation of the filter.

     7. In a plant using conventional and direct filtration, the turbidity of filtered water from any individual filter after backwashing or any other interruption must be less than 0.5 units of nephelometric turbidity after 4 hours of the initial operation of the filter. The level of turbidity must never exceed 1 unit of nephelometric turbidity before placing the filter back into operation.

     8. A pressure filter must be inspected physically and evaluated annually for occurrences such as media condition, formation of balls of mud and short circuiting. A written record of the inspection must be maintained at the treatment plant.

     9. Coagulation and flocculation unit processes must be in use at all times when a plant using conventional and direct filtration is in operation. The effectiveness and optimization of these processes must be demonstrated by jar testing, pilot filter column testing or other means acceptable to the Division or the appropriate district board of health.

     10. The level of turbidity of filtered water from each filter unit must be monitored with a continuous turbidity meter and recorded at a minimum of every 15 minutes by a sampling program approved by the Division or the appropriate district board of health. If this monitoring indicates that any filter unit is not performing as required by 40 C.F.R. § 141.175(b)(4) or 141.563(c), as adopted by reference in NAC 445A.4525, the filter must be inspected to determine the cause of its inadequate performance by implementing a composite correction program. The Division or the appropriate district board of health may require that the filter be taken out of service.

     11. To obtain approval for rates higher than those specified in subsections 1, 2 and 3, a supplier of water must demonstrate to the Division that his or her filters can ensure the same water quality at the increased rates of flow.

     (Added to NAC by Bd. of Health, eff. 11-29-90; A by Environmental Comm’n by R126-05, 10-31-2005; R194-08, 10-27-2009)