OAR 340-071-0220: Standard Subsurface Systems

(1) Criteria For standard subsurface systems. Each site must meet all of the conditions in this section to be approved for a standard subsurface system. (a) Effective soil depth must extend 30 inches or more below the ground surface as shown in Table 3. A minimum 6-inch separation must be maintained between the layer that limits effective soil depth and the bottom of the absorption facility. (b) Water table levels must be predicted using standards in OAR 340-071- 0130(22). (A) The permanent water table must be at least 4 feet below the bottom of the absorption facility, except in defined geographic areas where DEQ has determined through a groundwater study that less separation will not degrade groundwater or threaten public health. In these exception areas, the permanent water table must be at least 24 inches below the ground surface. (B) A temporary water table must be 24 inches or more below the ground surface. An absorption facility may not be installed deeper than the top of the temporary water table. (C) A groundwater interceptor may be used to intercept or drain water from an absorption area on sites with adequate slope to permit proper drainage. An agent may require a demonstration that the site can be de-watered before issuing a site evaluation report approving the site. Where required, groundwater interceptors are an integral part of the system but do not need to meet setback requirements to property lines, wells, streams, lakes, ponds, or other surface water bodies that are required for the wastewater absorption area. (c) Except as paragraph (d) of this section provides, soil with rapid or very rapid permeability must be 36 inches or more below the ground surface. A minimum 18-inch separation must be maintained between soil with rapid or very rapid permeability and the bottom of absorption trenches. (d) Sites may be approved with no separation between the bottom of absorption trenches and soil with rapid or very rapid permeability as defined in OAR 340-071-0100(150)(a) and (b) and absorption trenches may be placed into such soil if any of the following conditions occur. (A) A confining layer occurs between the bottom of absorption trenches and the groundwater table and a minimum 6-inch separation is maintained between the bottom of absorption trenches and the top of the confining layer. (B) A layer of nongravelly (less than 15 percent gravel) soil with sandy loam or finer texture at least 18 inches thick occurs between the bottom of the absorption trenches and the groundwater table. (C) The design flow does not exceed a loading rate of 450 gallons per acre per day. (e) Slopes do not exceed 30 percent or the slope/effective soil depth relationship described in Table 3. (f) The site has not been filled or the soil has not been modified in a way that would, in the agent’s opinion, adversely affect the system’s functioning. (g) The site is not on an unstable land form that might adversely affect operation of the system. (h) The site of the initial and replacement absorption facility is not covered by asphalt or concrete or subject to vehicular traffic, livestock, or other activity that would adversely affect the soil. (i) The site of the initial and replacement absorption facility will not be subjected to excessive saturation from artificial drainage of ground surfaces, driveways, roads, roof drains, or other circumstances. (j) Setbacks in Table 1 except as modified by this subsection can be met. (A) Surface waters setbacks. Setback from streams or other surface waters must be measured from bank drop-off or mean yearly high water mark, whichever provides the greatest separation distance. (B) Lots created before May 1, 1973. For lots or parcels legally created before May 1, 1973, the agent may approve installing a standard or alternative system with a setback from surface waters of less than 100 feet but not less than 50 feet if all other applicable provisions of this rule can be met. (C) Water lines and sewer lines. Effluent sewer and water line piping constructed of materials that are approved for use within a building in the Oregon State Plumbing Specialty Code may be run in the same trench or may cross. Where the effluent sewer pipe material is not approved for use in a building, it may not be run or laid in the same trench as water pipe unless: (i) The bottom of the water pipe at all points is set at least 12 inches above the top of the sewer pipe; and (ii) The water pipe is placed on a solid shelf excavated at one side of the common trench with a minimum, clear, horizontal distance of at least 12 inches from the sewer pipe. (D) Septic tank setbacks. The agent must encourage placing septic tanks and other treatment units as close as feasible to the minimum separation from the building foundation to minimize clogging the building sewer. (E) Pressure transport pipe setback to well. Notwithstanding the setback distance in Table 1, the agent may allow the separation distance between a pressure transport pipe and a well to be less than 50 feet but no less than 25 feet when: [Table not included. See ED. NOTE.] (i) The pressure transport pipe is Schedule 40 or heavier pressure-rated piping; (ii) The pressure transport pipe is placed within a larger diameter Schedule 40 or heavier encasement pipe, with the pipe ends located at least 50 feet away from the well; and (iii) All pipe joints in the pressure transport pipe and encasement pipe are solvent-welded or heat welded in accordance with the manufacturer’s specifications. (k) The agent makes a determination based on the best available science that the nutrient load from the system would not significantly degrade or pollute public waters, or create a public health hazard. (2) Criteria for sizing absorption fields. Absorption fields must be designed and sized based on the criteria in this section. (a) Table 2, specifying quantities of sewage flows, or other information the agent determines is reliable with the following exception. [Table not included. See ED. NOTE.] A system must be sized on the basis of 300 gallons sewage flow per day plus 75 gallons per day for the third bedroom when the system: (A) Is proposed to serve a single - family dwelling on a lot of record created before March 1, 1978, that is too small to accommodate a system sized for a daily sewage flow of 450 gallons; or (B) Serves specifically planned developments with living units of three or fewer bedrooms and deed restrictions prohibit an increase in the number of bedrooms. (b) Table 4, specifying the minimum length of absorption trenches based on soil texture and effective soil depth. [Table not included. See ED. NOTE.] (c) Table 5, specifying the minimum length of absorption trenches based on soil texture and depth to temporary water. [Table not included. See ED. NOTE.] (d) Strength of the wastewater. If the strength of the wastewater exceeds the maximum limits for residential strength wastewater or the contents of the wastewater are atypical of residential strength wastewater or pose a threat to groundwater, public health, or the environment, the wastewater must be pretreated to acceptable levels before being discharged into a standard or alternative system. (3) Septic tank. (a) Liquid capacity. (A) The quantity of projected daily sewage flow and design flow for a facility must be estimated from Table 2. For establishments not listed in Table 2, the agent must determine the projected daily sewage flow and design flow. [Table not included. See ED. NOTE.] (B) A septic tank that serves a commercial facility must have a liquid capacity of at least two times the design flow or projected daily sewage flow, whichever is greater, unless the agent authorizes otherwise. In all cases the capacity must be at least 1,000 gallons. (C) The capacity of a septic tank that serves a single-family dwelling must be based on the number of bedrooms in the dwelling. For a dwelling with 4 or fewer bedrooms, the tank capacity must be at least 1,000 gallons. Septic tank capacity must be at least 1,500 gallons for dwellings with more than 4 bedrooms. (D) The agent may require a larger capacity than this subsection specifies as needed for special or unique waste characteristics, such as flow patterns, volumes, waste strength, or facility operation. (b) Installation requirements. (A) Septic tanks must be installed on a level, stable base that will not settle. (B) Septic tanks located in high groundwater area must be weighted or provided with an antibuoyancy device to prevent flotation under the manufacturer's instructions. (C) Tanks must be installed with at least one watertight riser extending to the ground surface or above. The riser must have a minimum diameter of 20 inches when the soil cover above the tank does not exceed 36 inches. The riser must have a minimum diameter of 30 inches when the soil cover above the tank exceeds 36 inches or when the tank capacity exceeds 3,000 gallons. A gasketed cover must be provided and securely fastened or weighted to prevent unauthorized access. (D) Tanks must be installed in a location that provides access for maintenance. (E) Where practicable, the sewage flow from an establishment must be consolidated into one septic tank. (F) The agent may allow a removable plug to be placed in the top of a septic tank inlet sanitary tee if the septic tank discharges directly into a gravity-fed absorption facility. (G) After installation all tanks must prove watertightness under OAR 340-073- 0025. (H) Unless the agent allows otherwise, an effluent filter meeting the requirements of OAR 340-073-0056 must be installed at the septic tank outlet if a tank serves a commercial facility. A service access riser and cover meeting the requirements of 340-071-0220(3)(b)(C) must be placed above the effluent filter. (c) Construction. Tank construction must comply with minimum standards in OAR chapter 340, division 073, unless otherwise DEQ authorizes otherwise in writing. (d) Multi-compartment tank requirement. (A) With the exception in paragraph (B) of this subsection, if a sewage ejector pump precedes a septic tank, the tank must have been manufactured as a multi- compartment tank under requirements in this division and OAR chapter 340, division 073. An effluent filter must be installed unless the agent allows other methods with equal or better performance in preventing suspended solids from passing to the drainfield. (B) If the sewage ejector pump preceding the septic tank at a single-family residence receives wastewater from only a clothes washing machine and a sink, a single-compartment septic tank may be used in lieu of a multi-compartment septic tank. The tank must meet the minimum capacity requirement in paragraph (a) of this section, and an effluent filter must be installed in the tank's outlet tee fitting. Alternatively, the agent may allow the filter to be placed in a separate vault and riser located just outside the septic tank or may authorize other alternatives as appropriate. (4) Distribution techniques. Absorption trenches must be constructed according to one of the methods in this section. (a) Gravity-fed equal distribution (including loop). (A) Equal distribution must be used on generally level ground. All trenches and piping must be level within a tolerance of plus or minus 1 inch. All lateral piping must be at the same elevation. (B) A pressure-operated hydrosplitter may be used to achieve equal distribution. (C) To determine the total useable area of a looped soil absorption facility, the agent must add the sum of the lengths of the parallel absorption trenches and the lengths of up to two absorption trenches intersecting the parallel trenches. (b) Serial distribution. Serial distribution is generally used on sloping ground. Each trench must be level within a tolerance of plus or minus 1 inch. Serial distribution may be a combination of equal distribution and serial distribution. (c) Pressurized distribution systems. Pressurized distribution must satisfy the requirements in OAR 340-071-0275. (5) Distribution boxes and drop boxes. (a) Construction. Distribution box and drop box construction must comply with standards in OAR 340-073-0035 and 340-073-0040. (b) Foundation. All distribution boxes and drop boxes must be bedded on a stable, level base. (c) In all gravity distribution techniques, the connection of the effluent piping to the distribution piping must include at least one distribution or drop box or other device acceptable to the agent as a means for locating and monitoring the absorption field. (6) Dosing tanks and dosing septic tanks. (a) Tank construction must comply with the standards in OAR chapter 340, division 73 unless DEQ authorizes otherwise in writing. (b) The tank must be installed on a stable, level base at a location that provides access for maintenance. (c) The tank must be provided with at least one watertight service access riser extending to the ground surface or above. The riser must have a minimum diameter of 20 inches when the soil cover above the tank does not exceed 36 inches. The riser must have a minimum diameter of 30 inches when the soil cover above the tank exceeds 36 inches. A gasketed cover must be securely fastened or weighted to prevent unauthorized access. (d) A tank located in a high groundwater area must be weighted or provided with an antibuoyancy device to prevent flotation under the tank manufacturer's instructions. (7) Absorption trenches. (a) Absorption trenches must be constructed under the standards in this section unless otherwise authorized in this division. (A) Minimum bottom width of trench — 24 inches. (B) Minimum depth of trench: (i) Equal or looped distribution —18 inches. (ii) Serial distribution — 24 inches. (iii) Pressure distribution — 18 inches. (C) Maximum depth of trench — 36 inches. (D) Maximum length of an individual trench — 150 linear feet, unless the agent authorizes otherwise in writing. (E) Minimum distance of undisturbed earth between trenches — 8 feet. (b) The bottom of the trench must be level within a tolerance of plus or minus 1 inch end to end and level from side to side. (c) When the sidewall within a trench has been smeared or compacted, sidewalls must be raked to ensure permeability. (d) Trenches must be constructed to prevent septic tank effluent from flowing backwards from the distribution pipe to undermine the distribution box, the septic tank, or any portion of the distribution unit. (e) Drain media must extend the full width and length of the trench to a depth of at least 12 inches with at least 6 inches of drain media under the distribution pipe and at least 2 inches over the distribution pipe. (f) Before backfilling the trench, the drain media must be covered with filter fabric, untreated building paper, or other material the agent approves. (g) If trenches are installed in sandy loam or coarser soils, filter fabric or other nondegradable material the agent approves must be used to cover the drain media. (8) Trench backfill. (a) The installer must backfill the system. Backfill must be carefully placed to prevent damage to the system. (b) A minimum of 6 inches of backfill is required. 12 inches is required in serial systems. (c) Backfill must be free of large stones, frozen clumps of earth, masonry, stumps, waste construction materials, or other materials that could damage the system. (9) Header pipe. Header pipe must be watertight, have a minimum diameter of 3 inches, and be bedded on undisturbed earth. Where distribution boxes or drop boxes are used, the header pipe between the box and the distribution pipe must be at least 4 feet in length and be installed level. (10) Distribution pipe. (a) Distribution pipes must have a minimum diameter of 3 inches. (b) Each disposal trench must have distribution piping that is centered in the trench and laid level within a tolerance of plus or minus 1 inch. (c) Distribution pipe must comply with standards in OAR 340-073- 0060(4). (d) All perforated pipe must be installed with centerline markings up. (11) Effluent sewer. The effluent sewer must extend at least 5 feet beyond the septic tank before connecting to the distribution unit. It must be installed with a minimum fall of 4 inches per 100 feet and at least 2 inches of fall from one end of the pipe to the other. In addition, there must be a minimum difference of 8 inches between the invert of the septic tank outlet and either the invert of the header to the distribution pipe of the highest lateral in a serial distribution field or the invert of the header pipe to the distribution pipes of an equal distribution absorption field. A minimum 18-gauge, green-jacketed tracer wire or green color-coded metallic tape must be placed above the effluent sewer pipe. (12) Curtain drain construction. Unless the agent authorizes otherwise, curtain drains must comply with the following requirements. (a) Ground slope must be at least 3 percent, or other landform features such as an escarpment must allow for effective drainage. (b) The curtain drain must extend at least 6 inches into the layer that limits effective soil depth or to a depth adequate to effectively dewater the site. (c) Trench width must be a minimum of 12 inches. (d) Perforated pipe must have a minimum diameter of 4 inches and must meet the requirements in OAR 340-073-0060(4). (e) Perforated pipe must be installed at least 2 inches above the bottom and along the full length of the trench and must be covered by a minimum of 10 inches of drain media. (f) The curtain drain must be filled with drain media to within 12 inches of the ground surface. (g) Outlet pipe must be rigid, smooth-wall, solid Schedule 40 minimum pipe with a diameter of at least 4 inches. A flap gate or rodent guard must be installed. (h) Filter fabric must be placed over the drain media. [ED. NOTE: All tables are found in OAR 340-071-0800.]