NAC445A.9688. Intermittent sand filter system.

     1. In an intermittent sand filter system, the effluent must be pumped or siphoned into the absorption area and through a distribution network located in the upper part of a sand filter containment vessel. The upper part of the vessel must be made of coarse aggregate. The effluent must pass through the aggregate and then infiltrate the filter media below. Treatment of the effluent must occur as it passes through the filter media and into the unsaturated zone of the natural soil.

     2. The pressurized wastewater delivery system must provide even distribution in the sand filter through good engineering practice. The design engineer must:

     (a) Specify all necessary controls, pipes, valves, orifices, filter cover materials, gravel or other distribution media, including, without limitation, monitoring and servicing components in the design specifications submitted pursuant to NAC 445A.9614; and

     (b) Ensure that the topsoil cover is not less than 6 inches or more than 12 inches in depth and graded to drain off the top of the sand filter.

     3. Pressurized wastewater delivery must be applied from the septic tank or separate watertight treatment unit chamber with a pump or siphon that is sized and controlled to deliver the pretreated wastewater to the top of the intermittent sand filter. The dosing rate must not be less than 4 doses or more than 24 doses per day.

     4. The vessel containing the sand filter must be watertight, structurally sound, durable and capable of withstanding stress from installation and operational service. The intermittent sand filter system may be placed above grade, partially buried or fully buried depending on site and service specifications and must be justified by the design engineer in the design specifications submitted pursuant to NAC 445A.9614.

     5. The filter media used in the intermittent sand filter system must consist of washed, durable granular material with less than 1 percent organic matter by weight. The effective size of the filter media must not be less than 0.25 millimeter or more than 1.0 millimeter. The uniformity coefficient of the filter media must be less than 4.

     6. The depth of the filter media must be at least 24 inches, and the top and bottom surfaces of the filter media must be level.

     7. The filter media used in the intermittent sand filter system must meet the following criteria:

Filter Media Specifications

Sieve Size	Effective Particle Size	Percent by Weight Passing Sieve
3/8"	9.50 mm	100
4	4.75 mm	95-100
8	2.36 mm	80-100
16	1.18 mm	50-85
30	0.60 mm	25-60
50	0.30 mm	10-30
100	0.15 mm	2-10

Source: ASTM C-33, “Specifications for Fine Aggregate”

The filter media must not have more than 45 percent passing any one sieve and retained on the next consecutive sieve of those shown in the table above. The fineness modulus must not be less than 2.3 or more than 3.1. As used in this subsection, “fineness modulus” means the sum of the cumulative percentages retained in the sieve analysis, divided by 100, for the sieve sizes shown in the table above.

     8. The required size of the intermittent sand filter system depends on the projected daily sewage flow of the on-site sewage disposal system. The maximum wastewater loading rate is 1 gallon per day per square foot of inlet surface at the rated projected daily sewage flow. For example, a system with a flow of 4,000 gallons per day would require a sand filter of at least 4,000 square feet.

     9. The native soil portion of the absorption system must be designed to ensure that the linear loading rate does not exceed the disposal capability at the site.

     10. The following is a diagram of an intermittent sand filter system:

     (Added to NAC by Environmental Comm’n by R194-07, eff. 8-26-2008)