1. (a) Multiple pass filters — Recirculating sand filters (RSF).

   1. (1) Recirculating sand filters pass effluent treated by a septic tank or ATU through a sand and/or gravel bed, built on site, prior to subsurface absorption or surface discharge.

   2. (2) Recirculating sand filters shall meet the following criteria:

      1. (A) A septic tank or ATU shall be required as the primary treatment of effluent prior to treatment by an RSF;
      2. (B) Effluent shall be dosed to an RSF by a pump housed in a filtered pump vault authorized by the Department of Health;
      3. (C) Effluent shall be distributed to an RSF by means of low-pressure distribution as outlined in 14 CAR § 21-709;
      4. (D) The collection line in the bottom of an RSF shall not be less than four inches (4”) perforated or slotted Schedule 40 PVC pipe;
      5. (E) The dosing rate for RSFs shall not be more than one-quarter (1/4) gallons per orifice per dose (gal/orifice/dose);
      6. (F)

2. (i) The initial recirculation rate for RSFs shall be 4:1.

   1. (ii) Depending upon water usage, the recirculation rate may be adjusted after system evaluation;

      1. (G) The loading rate for RSFs shall not be more than five (5) gallons per square foot per day (gal/ft²/day);

      2. (H)

         1. (i) RSFs may be installed either in or above ground.
         2. (ii) However, the top of the RSF bed shall not be installed below the finished grade of the ground surface;
      3. (I) A PVC liner not less than thirty (30) mils in thickness, with inlet and outlet boots shall be required in all RSFs in order to minimize groundwater infiltration into the system;

      4. (J)

         1. (i) Not less than two inches (2”) of gravel cover shall be applied atop the collection pipe in the bottom of an RSF.
         2. (ii) The gravel used shall be crushed rock or rounded rock, one-half inch (1/2”) to one and one-half inches (1 1/2”) in diameter;
      5. (K) Not less than four inches (4”) of three-eighths inch (3/8”) pea gravel shall be applied atop the crushed or rounded rock in the bottom of an RSF in order to prevent migration of filter sand into the collection piping and out of the filter;

      6. (L)

         1. (i) Not less than twenty-four inches (24”) of filter sand shall be applied atop the pea gravel in the bottom of an RSF.
         2. (ii) Filter media used shall be one and one-half millimeters (1 1/2 mm) to two millimeters (2 mm) in diameter, with a uniformity coefficient of 2.0 or less;

      7. (M)

         1. (i) Not less than three inches (3”) of three-eighths inch (3/8”) pea gravel shall be applied atop the filter sand in an RSF.
         2. (ii) The low pressure distribution system servicing an RSF shall be installed atop this layer of pea gravel;
      8. (N) Not less than three inches (3”) of gravel shall be installed as cover over the low pressure distribution system servicing an RSF; and
      9. (O) The top of the RSF filter bed shall be left uncovered.
3. (b) Other filters. Proprietary media filters such as peat filters, gravel filters, and geotextile filters shall be designed and installed in accordance with the manufacturer’s specifications as authorized by the department.

Codification Notes: “ATU” means aerobic treatment unit. "PVC" means polyvinyl chloride.