OAR 340-071-0290: Conventional Sand Filter Systems

(1) Criteria for approval. Construction of conventional sand filter systems may be approved for single-family dwellings or commercial facilities. (2) Sites approved for sand filter systems. Sand filters may be permitted on any site meeting requirements for standard onsite systems in OAR 340-071-0220 or for pressurized distribution systems in OAR 340-071-0275 if site conditions in this section can be met. (a) Separation from the temporary groundwater table must satisfy the requirements in this subsection. (A) The high level attained by a temporary groundwater table is: (i) Twelve inches or more below ground surface where: (I) The ground slope does not exceed 12 percent; (II) Equal distribution methods are achieved by gravity or using either a hydrosplitter or pressurized distribution method; and (III) A capping fill is placed under OAR 340-071-0265(2) and 340-071-0265(3)(a) through (c). (ii) Eighteen inches or more below ground surface where equal distribution methods are achieved by gravity or through the use of a hydrosplitter or pressurized distribution. (iii) Twenty-four inches or more below ground surface where serial distribution methods are used. (B) Methods used in OAR 340-071-0315 for tile dewatering systems may be used to achieve separation distances from temporary groundwater. (C) Absorption trenches may not be installed deeper than the highest level of the temporary water table. The minimum backfill depth within the absorption trenches is 6 inches for trenches using equal distribution methods and 12 inches for trenches using serial distribution. (b) Separation from the permanent groundwater table must satisfy the requirements in this subsection. (A) The highest level attained by a permanent water table does not exceed the minimum separation distance from the bottom of the absorption area as follows: (i) For gravel and Soil Group A: sand, loamy sand, sandy loam — 24 inches; (ii) For Soil Group B: loam, silt loam, sandy clay loam, clay loam — 18 inches; (iii) For Soil Group C: silty clay loam, silty clay, clay, sandy clay — 12 inches. (B) Shallow absorption trenches placed not less than 12 inches into the original soil profile may be used with a capping fill to achieve separation distances from permanent groundwater. The fill must be placed under OAR 340-071-0265(2) and 340-071-0265(3)(a) through (c). (C) Methods used in OAR 340-071-0315 for tile dewatering systems may be used to achieve separation distances from permanent groundwater. (c) Sand filter systems installed in soils with rapid or very rapid permeability as defined in OAR 340-071-0100(150)(a) and (b) in areas with permanent water tables may not discharge more than 450 gallons of effluent per 1/2 acre per day except where: (A) Groundwater is degraded and designated as a nondevelopable resource by the Oregon Water Resources Department; or (B) A detailed hydrogeological study determines loading rates exceeding 450 gallons per 1/2 acre per day would not increase nitrate-nitrogen concentration in the groundwater beneath the site or any downgradient location to above 5 mg/L. (d) Sand filter systems may be installed in soils, fractured bedrock, or saprolite diggable with a backhoe if, in the judgment of the agent, the soils, fractured bedrock, or saprolite is permeable to the extent that effluent will absorb adequately and not hinder the performance of the filter or absorption field. The agent may require that an absorption test be conducted to determine the permeability of the bedrock or saprolite. Test methods must be acceptable to DEQ. (A) Where ground slope does not exceed 12 percent, a capping fill, 12-inch deep trench may be installed under OAR 340-071-0265, except that when installed in fractured bedrock or saprolite, the cap material must be Soil Group B. (B) Where ground slope exceeds 12 percent but is not greater than 30 percent, a standard 24-inch deep trench may be installed. (e) A sand filter absorption facility may be installed on slopes of 30 percent or less if other conditions in this section are satisfied. (f) An absorption facility following a sand filter may be installed on slopes above 30 percent and up to 45 percent where: (A) Design flow does not exceed 450 gallons and the installation is sized under sand filter absorption area criteria; (B) The soil is diggable with a backhoe to a depth of at least 36 inches and 12 inches below the bottom of the trench; and (C) The temporary water table is at least 30 inches below the ground surface and 6 inches below the bottom of the trench. (g) Setbacks in Table 1 can be met, except the minimum separation distance between the sewage absorption area and surface waters must be at least 50 feet. (3) Absorption trenches. Absorption trenches for sand filter absorption facilities must satisfy the requirements in this section. (a) The minimum length of a standard absorption trench per 150 gallons of sewage flow is: (A) For gravel and Soil Group A: sand, loamy sand, sandy loam -- 35 linear feet; (B) For Soil Group B: loam, silt loam, sandy clay loam, clay loam -- 45 linear feet; (C) For Soil Group C: silty clay loam, silty clay, sandy clay, clay -- 50 linear feet; (D) For permeable saprolite or fractured bedrock -- 50 linear feet; (E) For high shrink-swell clays (Vertisols) -- 75 linear feet. (b) On lots created before January 1, 1974, which do not have sufficient, suitable area for an absorption facility sized under this section, the agent may allow seepage trenches if: (A) The design criteria and limitations in OAR 340-071-0280(2) are met; (B) The soil is not a high shrink-swell clay; (C) The temporary water table is at least 30 inches below the ground surface; and (D) All other requirements of this rule are met. (c) Trench designs in Vertisols. (A) Absorption trenches in Vertisols must contain 24 inches of drain media and 24 inches of soil backfill in areas with an annual rainfall of 25 inches or less, minimum slopes of 5 percent, and a temporary water table at least 48 inches below the ground surface. (B) Seepage trenches in Vertisols containing less than 24 inches of drain media may be used if designed under the criteria and limitations in OAR 340-071-0280 in areas with an annual rainfall of 25 inches or less, minimum slopes of 5 percent, and a temporary water table at least 48 inches below the ground surface. (d) Seepage bed construction. (A) Seepage beds may be used instead of absorption trenches in soil as defined in OAR 340-071-0100(150)(b). (B) The effective seepage area must be based on the bottom area of the seepage bed. The area must be at least 70 square feet per 150 gallons per day waste flow. (C) Materials and construction requirements from 340-071-0275(4)(d) apply, except as otherwise provided for in this section. (4) Bottomless sand filter. Sites may use a bottomless sand filter if the site meets the criteria in this section and section (3) of this rule. (a) Saprolite; fractured bedrock; gravel; or soil textures of sand, loamy sand, or sandy loam occur in a continuous section at least 2 feet thick in contact with and below the bottom of the sand filter. (b) The agent determines the saprolite, fractured bedrock, gravel, or soil is permeable over the basal area to the extent that effluent will absorb adequately and not hinder the performance of the filter. The agent may require that an absorption test be conducted to determine the permeability of the basal area. Test methods must be acceptable to DEQ. (c) The application rate is based on the design flow in OAR 340-071-0220(2)(a) and the basal area of the sand. (d) The water table is at least 24 inches below the ground surface throughout the year, and a minimum 24-inch separation is maintained between a water table and the bottom of the sand filter. (5) Materials and construction. (a) All materials used in sand filter system construction must be structurally sound, durable, and capable of withstanding normal installation and operation stresses. Component parts subject to malfunction or excessive wear must be readily accessible for repair and replacement. (b) All filter containers must be placed over a stable, level base. (c) In a gravity-operated distribution system, the invert elevation of the outlet end of the underdrain pipe must be at or above the final settled ground elevation of the highest absorption trench. (d) Piping and fittings for the sand filter distribution system must comply with the requirements for pressure distribution systems in OAR 340-071-0275. (e) Septic tanks, dosing tanks, and other components must comply with the requirements in OAR 340-071-0220 unless this rule specifies different requirements. (f) The design and construction requirements in OAR 340-071-0295 must be met. A bottomless sand filter unit does not require a watertight floor, but does require watertight walls unless otherwise authorized by the agent. (g) A bottomless sand filter unit does not require a minimum 10-foot separation between the original and replacement unit. (6) Gravelless absorption method. (a) Absorption trenches following a sand filter may be constructed without using drain media if they meet the criteria in this section. (A) Absorption trenches must be 12 inches wide by 10 inches deep and incorporate pressurized distribution and a chamber constructed of half sections of 12-inch diameter plastic irrigation pipes or an equivalent pipe material as determined by the agent. DEQ may consider deviations to the depth requirement in this rule for alternative drainfield products. (B) Trenches must be level end to end and across their width. (C) The agent may allow trenches on minimum 3-foot centers maintaining at least 2 feet of undisturbed earth between parallel trench sidewalls. (D) Pressurized distribution piping must meet the requirements of OAR 340-071- 0275(4)(b), except that orifice shields are not required. (E) Distribution piping must be perforated with 1/8 inch diameter orifices on maximum 2-foot centers at the 12 o'clock position. The hydraulic design must provide at least a 2-foot residual head at the distal orifice. (F) The chambers must have an adequate footing to support the soil cover and all normal activity and at a minimum must be constructed of 12-inch plastic irrigation pipe rated at 43 pounds per square inch and meeting the appendix standards of ASTM D-2241.The agent may allow other pipe materials that demonstrate equivalent durability and strength to 12-inch plastic irrigation pipe. Each line must be equipped with a minimum 6-inch diameter inspection port. (b) Except as noted in paragraph (a) of this section, all construction and siting criteria for conventional sand filter systems in this division must be met. This includes but is not limited to the absorption field sizing for sand filter systems in OAR 340-071-0290(3) and area sizing for an initial and replacement absorption facility meeting standard trench separations in OAR 340-071-0220(7)(a)(E). Plans must verify that a system can be installed on the parcel that will meet the requirements in OAR 340-071-0290(3) and 340- 071-0220(7)(a)(E) and all other applicable rules before a gravelless absorption method is approved. (c) A gravelless absorption method may be used wherever this division allows a standard or alternative-type absorption trench for sand filter systems, except in Vertisols. (d) A method to prevent burrowing animals from entering the chamber must be provided in areas where this is likely to occur. (7) Operation and maintenance. The operation and maintenance of conventional and other sand filter systems, including service contracts for commercial sand filters permitted on or after March 1, 2005, and residential sand filters permitted after January 1, 2014, must meet the requirements under OAR 340-071-0132. [ED. NOTE: All tables are found in OAR 340-071-0800.]