340-071-0295 Conventional Sand Filter Design and Construction (1) Criteria for sizing. Systems must be sized based on quantities of sewage flow under OAR 340-071-0220(2)(a). (2) Minimum filter area: (a) A sand filter proposed to serve a single-family dwelling must have an effective medium sand surface area of at least 360 square feet. If the design flow exceeds 450 gallons per day, the medium sand surface area must be determined with the following equation: Area = design flow divided by 1.25 gallons per square foot. (b) A bottomless sand filter following an ATT system must have an effective medium sand surface area of at least 250 square feet. If the design flow exceeds 450 gallons per day, the medium sand surface area must be determined with the following equation: Area = design flow divided by 1.80 gallons per square foot. (c) Sand filter influent may not exceed concentrations of 300 mg/L BOD5, 150 mg/L TSS, or 25 mg/L oil and grease. (3) Design criteria. (a) The interior base of the filter container must be level or constructed at a grade of 1 percent or less to the underdrain piping elevation. (b) Except for sand filters without a bottom, underdrain piping must meet the requirements in OAR 340-073-0060(2) and must be installed in the interior of the filter container at the lowest elevation. The piping must be level or on a grade of 1 percent or less to the point of passage through the filter container. The pipe perforations or slots must be oriented in the upright or sideways position. (c) The base of the filter container with the underdrain piping in place must be covered with a minimum of 6 inches of underdrain media. Unless the agent waives it, the underdrain media proposed for a sand filter must be sieved to determine conformance with the criteria in OAR 340-071-0100(173) and a report of the analysis must be provided to the agent. The underdrain piping must be enveloped in an amount and depth of drain media to prevent migration of the underdrain media to the pipe perforations. (d) A minimum of 24 inches of approved sand filter media must be installed over the underdrain media. The sand filter media must be damp at the time of installation. The top surface of the media must be level. Unless waived by the agent, the sand filter media proposed for each sand filter must be sieve-tested to determine conformance with the criteria in OAR 340-071-0100(126), and a report of the analysis must be provided to the agent. (e) A minimum of 3 inches of clean drain or underdrain media is required below the distribution laterals, and sufficient media is required above the laterals to meet or cover the orifice shields to provide a smooth, even cover. (f) A pressurized distribution system meeting the requirements of OAR 340-071-0275(4) and (5) must be constructed as described in paragraph (e) of this section. (A) Distribution laterals must be spaced a maximum of 30 inches center to center. Orifices must be spaced no more than 30 inches apart. (B) The ends of the distribution laterals must be designed and constructed to allow flushing of the piping, collectively or individually, using a corrosion- resistant and accessible valve or threaded endcap. The flushed effluent may be discharged to the septic tank or into the sand filter. (C) The diameters of the distribution manifold and laterals must be at least 1/2 inch in diameter. (D) A sand filter must be dosed at a rate not to exceed 10 percent of the projected daily sewage flow. (g) The top of the media in which the pressure distribution system is installed must be covered with filter fabric. (h) The top of the sand filter area must be backfilled with a soil cover free of rock, vegetation, wood waste, and other materials that may harm the filter. The soil cover must have a textural class no finer than loam unless otherwise authorized by the agent. The soil cover must be at least 6 inches and no more than 12 inches deep. (i) All piping passing through the sand filter container must be watertight. (4) Container design and construction. (a) A reinforced concrete container with watertight walls and floors must be used where watertightness is necessary to prevent groundwater from infiltrating into the filter or to prevent the effluent from exfiltrating from the filter except as otherwise allowed in this division or OAR chapter 340, division 073. The container structure may require a building permit for construction. (b) The container may be constructed of materials other than concrete where equivalent function, workmanship, watertightness, and at least a 20-year service life can be documented. (A) Flexible membrane liner (FML) materials must have properties at least equivalent to 30 mil unreinforced polyvinyl chloride (PVC) described in OAR 340-073-0085. For FML materials to be approved for installation: (i) Field repair instructions and materials must be provided to the purchaser with the liner; and (ii) The final materials must have factory-fabricated boots suitable for field bonding onto the liner to facilitate the passage of piping through the liner in a waterproof manner. (B) Where accepted for use, flexible sheet membrane liners must be installed as OAR 340-073-0085 requires. (C) The backfill around the container must be no steeper than a 3:1 slope (3 feet for every vertical foot) unless otherwise authorized by the agent. (5) Internal pump option. Where a pump is used to discharge effluent from a sand filter to another treatment unit, a distribution unit, or an absorption facility, the design and construction of the filter may include an internal pump station if the following conditions are met. (a) The location, design, and construction of the pump station must not conflict with design, construction, and operation of the sand filter system. (b) The design and construction of the pump, discharge plumbing, controls, and alarm must meet the requirements in OAR 340-073-0055 except subsections (4)(d) and (4)(h). (c) The pump and related apparatus must be housed in a corrosion-resistant vault designed to withstand stresses and prevent the migration of drain media, sand, or underdrain media to its interior. The vault must have a durable, affixed floor. The vault and lid must provide watertight access to finished grade with a diameter equal to that of the vault and designed to receive treated effluent from the bottom of the sand filter. (d) The depth of underdrain media and the operating level of the pump cycle and alarm may not allow effluent to come within 2 inches of the bottom of the sand filter media. The pump off-level may be no lower than the invert of the perforations of the underdrain piping. (e) The internal sand filter pump must be electrically linked to the sand filter dosing apparatus to prevent effluent from entering the sand filter if the internal sand filter pump fails. (6) Required inspections. The agent must perform the following inspections for each installed sand filter system, unless waived by the agent: (a) For bottomless sand filter only, inspect the absorption area for scarification, soil texture, and moisture content (b) Inspect the box or container construction. (c) Inspect the media size, cleanliness, and installation. (d) Inspect the construction of the pressure distribution system. (e) Inspect the installation of filter fabric over the top of the media the distribution system is installed in. (f) Inspect the sand filter soil cover. (g) For conventional sand filter only, inspect the absorption facility before covering. (h) Inspect to confirm the pump, floats, and controls are powered on, set according to the approved plans, and functioning properly. The agent may require the certified maintenance provider under contract for inspecting, operating, and maintaining the system to conduct this inspection and submit a completed start-up report on a DEQ-approved form to the agent. Statutory/Other Authority: ORS 454.625 & 468.020 Statutes/Other Implemented: ORS 454.615, 454.775 & 454.607 History: DEQ 29-2025, amend filed 10/01/2025, effective 01/01/2026 DEQ 15-2017, amend filed 10/31/2017, effective 11/01/2017 DEQ 14-2013, f. 12-20-13, cert. ef. 1-2-14 DEQ 11-2004, f. 12-22-04, cert. ef. 3-1-05 DEQ 16-1999, f. & cert. ef. 12-29-99 DEQ 12-1997, f. & cert. ef. 6-19-97 DEQ 27-1994, f. 11-15-94, cert. ef. 4-1-95 DEQ 15-1986, f. & ef. 8-6-86 DEQ 5-1982, f. & ef. 3-9-82 DEQ 10-1981, f. & ef. 3-20-81