340-071-0302 Recirculating Gravel Filter (RGF) (1) Siting and absorption area construction criteria. (a) RGFs approved for treatment standard 1 may be sited and sized as follows. (A) In areas with a temporary water table, as specifications for sand filters in areas with temporary groundwater in OAR 340-071-0290 require. (B) In areas with permanent groundwater, where 4 feet of separation can be maintained between the bottom of the trench and groundwater and the other criteria in OAR 340-071-0290 can be met. (C) On sites meeting criteria for standard onsite systems in OAR 340-071-0220 or for pressurized systems in OAR 340-071-0275. (b) RGFs used in conjunction with approved disinfection and approved nitrogen reduction processes and expected to meet treatment standard 2 may be sited and sized as follows. (A) On sites meeting the criteria for treatment standard 1 in paragraph (a) of this section. (B) In areas with a permanent water table, as specifications for sand filters in areas with a permanent water table in OAR 340-071-0290 require. (c) Any type of absorption area permitted for a sand filter system, including the gravel- less absorption method, may be permitted for an RGF system. (2) Design criteria. (a) Filter design and dosing. (A) The filter’s basal or bottom area must be sized based on a maximum organic load. For residential strength wastewater that has been pretreated through a septic tank, the maximum hydraulic load allowable is 5 gal/ft2/day. (B) For BOD5 waste strengths stronger than residential strength wastewater but not exceeding 400 mg/L, the filter size must be increased proportionately. (C) Higher strength wastewaters must be pretreated or will require special consideration. In no case may the concentration of greases and oil applied exceed 30 mg/L. (b) Filter media. (A) Where CBOD5 removal must be at least 85 percent based upon the raw sewage concentration applied to the septic tank and nitrification of wastewater is necessary, a filter media must consist of 3 feet of very fine washed gravel, 100 percent passing a 3/8-inch sieve with an effective size between 3 and 5 millimeters and a uniformity coefficient of 2 or less. Washed means that negligible fines (less than 1.0 percent) pass a No. 10 sieve. (B) Where additional removal of BOD5 and denitrification is intended or required, a treatment media may consist of 2 feet of very coarse washed sand, 100 percent passing a 3/8-inch sieve with an effective size between 1.5 and 2.5 millimeters and a uniformity coefficient of 2 or less. Washed means that negligible fines (less than 4.0 percent) pass the No. 100 sieve. (C) Sieves of 3/8 inch, 1/4 inch, and Nos. 4, 6, 8, 10, 50, and 100 must be used in gradation analysis. (D) The permittee must provide fresh samples of the intended media for each project before shipment to the project site. A laboratory gradation analysis must be performed and the gradation data plotted on semi-log paper as a gradation curve. The permittee must submit lab data, gradation curve, and a 5-pound sample of the media to the agent for approval. Only approved media may be used. (c) Filter media must be overlain by a 3-inch bed of 1/2-inch to 3/4-inch washed gravel. The media and gravel may only lightly cover the distribution piping. Unless otherwise authorized, each orifice must be covered by an orifice shield to prevent aerial spray drift. (d) Filter dosing must use a low pressure distribution piping system operating under adequate head to pressurize the system. The operating head must be a minimum of 5 feet at the remotest orifice and have no more than 10 percent flow variation between the nearest and remotest orifice in an individual unit. Each lateral pipe end must terminate with a screwed plug or cap accessible for removal and flushing. Wherever practical, a valved backflush system must be installed to flush groups of laterals back to a septic tank or elsewhere. (e) Pressure-distribution piping must be spaced 2 feet center to center in a parallel grid. Orifice spacing must be every 2 feet on laterals. Piping grid edges should be within 1 foot of the filter basal edge. (f) Filter media must be underlain by a 6-inch bed of a 1/2 to 3/4-inch washed gravel underdrain media. No filter fabric may cover the underdrain media. (g) Perforated collection pipes must meet requirements in OAR 340-073-0060(2) and be bedded in the underdrain media. Pipes must be at least 4 inches in diameter with no filter fabric wrap. At least 15 lineal feet of collection pipe is required for each 225 square feet of filter basal area. (h) The filter container must be watertight to suit the design conditions. Underflow must be contained. Groundwater must be excluded. A concrete container may be used. Other materials may be used if equivalent function, workmanship, watertightness, and at least a 20-year service life can be expected. (3) Recirculation/dilution tank. (a) A recirculation tank receives septic tank effluent and underflow from the filter. A pumping system at this tank delivers flow to the filter dose piping network according to a project design. The recirculation tank volume measured from tank floor to tank soffit must be at least equal to the projected daily sewage flow volume. (b) The recirculation ratio at design flow must be at least 4. Recirculation ratio is the daily volume of recycle divided by design daily volume of the wastewater. A fabricated "T" or "Splitter T" float valve located in the recirculation tank must be used whenever possible. Minimum recirculation tank liquid volume must be at least 80 percent of the gross tank volume when a float valve is used. Alternatively, where required and reasonable, a splitter basin using orifice or weir control may be used to divide underflow 20 percent to the absorption field and 80 percent to recycle on a daily basis. This alternative must use orifice control wherever possible. Minimum recirculation tank liquid volume must be at least 50 percent of the required tank volume when a splitter basin is used. (c) Evaluation of and design for overflow and surge control at the recirculation tank must be included in the design plans. (d) An audible or visual high water alarm must be included in the recirculation tank immediately below the overflow level. A latching electrical relay must retain the audible or visual alarm until a site attendant acknowledges it. (e) Parallel pump start/stop electric controls (usually floats) must be installed to correct any unforeseen high liquid level event and keep sewage contained. This pump start function precludes overflow and must operate in parallel with the start/stop function of a timer and must not interfere with or depend upon a timer position. (f) All areas of the filter must be wetted 48 times a day or every 30 minutes to achieve the recirculation ratio of at least 4 unless the agent authorizes otherwise. (g) Testing must demonstrate the recirculation tank is watertight. The designer must witness the testing. Test protocol must be included in the design plans. (h) A fence or other effective means must restrict access onto the filter. Design and construction must prevent surface water entry onto the filter. (i) Access openings to the recirculation tank must be provided at each end. Larger tanks must have additional openings. The smallest dimension of any access must be 18 inches. Larger openings must be provided if partially obstructed with piping or other objects. Provisions must be made to remove dregs (settleable solids). Pumps must be readily removable and replaceable without demolition of piping or other components. (4) Operation and Maintenance standards. The operation and maintenance of a RGF system must meet the requirements under OAR 340-071-0132 and this rule. (5) Operation and maintenance manual. The designer of an RGF system must ensure that comprehensive and detailed operation and maintenance instructions are provided to the onsite system owner and agent at the time of installation. The instructions must emphasize operating and maintaining the entire system within the parameter ranges for which it is designed. The information must be presented in a manner that can be easily understood by the owner and include at a minimum: (a) As-built plans with the name and contact number of the installer; (b) A description of how the process functions, including diagrams illustrating basic system design and flow path; (c) A maintenance schedule for all critical components; (d) Requirements and recommended procedures for periodic removal of residuals from the system; (e) A detailed procedure for visually evaluating the function of system components; (f) A description of olfactory and visual techniques for confirming correct process parameters and system performance; (g) A recommended method for collecting and transporting effluent samples; (h) Safety concerns that may need to be addressed; and (i) Emergency contact numbers for maintenance providers and pumpers. Statutory/Other Authority: ORS 454.625 & 468.020 Statutes/Other Implemented: ORS 454.615, 454.607, 468B.050 & 468B.055 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 27-1994, f. 11-15-94, cert. ef. 4-1-95