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PDFWAC 246-272B-06750

Intermittent sand filters.

(1) Intermittent sand filters may be used to meet Treatment Level B.
(2) Pressure distribution to the filter media is required.
(3) The minimum dosing frequency is:
(a) Twelve equally spaced timed doses per day for coarse sand; or
(b) Six equally spaced timed doses per day for ASTM C-33 sand.
(4) The maximum hydraulic loading rate for the sand filter is one gallon per day per square foot.
(5) The minimum depth of sand media is twenty-four inches.
(6) The influent wastewater quality distributed to the filter media must meet or exceed effluent characteristics from a properly sized septic tank treating residential strength sewage.
(7) The minimum horizontal setback from the sand filter must meet the sewage tank requirements in Table 3 in WAC 246-272B-06050.
(8) The maximum depth of cover material over the distribution technology is twelve inches.
(9) The design engineer shall meet the sand specification requirements in WAC 246-272B-06700(4).
(10) The filter bed must be contained in:
(a) A flexible membrane-lined pit where the membrane has a minimum thickness of thirty mm and there is a three-inch layer of sand beneath the membrane; or
(b) A concrete vessel that is water tight, durable, and structurally sound.
(11) The underdrain must be designed with sufficient void storage volume for a single dose to the filter and maintain unsaturated filter material above the underdrain component.
(12) Filtrate may be collected and discharged from the bottom of the filter by either gravity flow or a pumpwell system.
(13) When filtrate is discharged by gravity, a boot and exit pipe must exit the side of the liner and be installed:
(a) By the manufacturer or the manufacturer's representative;
(b) So the boot outlet is bedded in sand;
(c) With the boot properly secured to the outlet pipe, such as with stainless steel bands, screws, and sealant strips, or as recommended by the manufacturer; and
(d) Watertight. If the boot will be submerged in a seasonal high water table, the installer shall test and verify it is watertight.
(14) Monitoring ports for intermittent sand filters must meet the requirements in WAC 246-272B-06400 (3)(b).
(15) Two monitoring ports must be installed every one thousand square feet in the sand filter and distributed uniformly throughout the filter area.
(a) One monitoring port must be installed to the top of the filter media; and
(b) One monitoring port must be installed to the bottom of the underdrain.
[Statutory Authority: RCW 70.118B.020. WSR 11-12-035, ยง 246-272B-06750, filed 5/25/11, effective 7/1/11.]
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