12 Va. Admin. Code § 5-590-881 - Slow sand filtration

A. Slow sand filters shall be approved only after a pilot study demonstrates that the water supply contains sufficient nutrients for use of this treatment technology.

B. At least two filters shall be provided. In all cases, the filters shall be capable of meeting the design maximum daily water demand with one filter out of service.

C. Sand shall be clean silica sand that meets the following criteria:

1. The effective size shall be between 0.15 mm and 0.35 mm;

2. The uniformity coefficient shall not exceed 2.5; and

3. The sand depth shall not exceed 55 inches. A minimum depth of 30 inches is required for normal operation.

D. Supporting media gravel shall meet the requirements of 12VAC5-590-874 F.

E. Structural details.

1. All slow sand filters shall be covered.

2. Sufficient head room shall be provided for normal movement on the filter by operating personnel for periodic sand removal operations.

3. Adequate manholes and access ports shall be provided for moving sand off and onto the filter.

4. There shall be no common wall between the finished water and any water of lesser quality.

5. All filters shall be protected from freezing.

F. General design requirements.

1. Filter to waste shall be provided for all slow sand filters.

2. Water entering the filter shall be distributed in a manner so that the surface of the filter shall not be disturbed in any way.

3. The nominal rate of filtration range shall be from 45 to 150 gpd/ft² (0.031 to 0.10 gpm/ft²) of sand area.

4. The minimum depth of water over the filters shall be three feet. The maximum depth of water over the filters shall not exceed five feet. An overflow capable of handling the maximum flow to the filter shall be provided at the maximum filter water level.

5. Underdrains shall be provided to assure an even rate of filtration across the filter surface. The maximum velocity of water in the lateral underdrains shall be 0.75 ft/sec. The underdrain spacing shall not exceed three feet.

6. Each filter shall be capable of being filled with water from the bottom up.

7. Each filter shall be equipped with a loss-of-head gauge; a rate-of-flow control device such as an orifice, weir, or butterfly valve; a weir or effluent pipe designed to assure that the water level over the filter never drops below the sand surface; and filtered water sample taps.

8. Monitoring, indicating, and recording turbidimeters meeting the requirements of 12VAC5-590-770 B shall be provided for:

a. The source water;

b. The filter effluent from each filter unit; and

c. The CFE.

9. The filters shall be designed to operate to waste after scraping or replacement of the sand, until the ripening process is complete and the turbidity meets the requirements of 12VAC5-590-395 A 2 b (3).

Notes

12 Va. Admin. Code § 5-590-881

Derived from Virginia Register Volume 37, Issue 20, eff. 6/23/2021.

Statutory Authority: §§ 32.1-12 and 32.1-170 of the Code of Virginia.