Section 250-RICR-150-10-6.37 - Advanced Pressure Drainfields (APDs)A. General 1. An advanced pressure drainfield must not be used without an advanced treatment unit(s) that has been approved by the Department to meet either Category 1 or Category 2 technology designations. An APD shall not be used with septic tank effluent. APDs include bottomless sand filters (BSFs) and pressurized shallow narrow drainfields (PSNDs). 2. Protecting the APD from Damage: The landscape over and immediately adjacent to any APD system shall be protected in accordance with § 6.33(T)(2) of this Part.B. APD Common Design Parameters 1. APD hydraulic loading rates: Sizing of the APD is based on soil texture, structure and consistence of the most restrictive horizon within three feet (3') below the proposed base of the APD (see § 6.37(B)(4) of this Part). A soil evaluation by a Class IV Soil Evaluator is required.2. Flow Differential Between the First and Last Orifice in APD Laterals: The maximum head differential between the first and last orifice on each APD lateral shall be no greater than ten percent (10%).3. Maximum Volume per Orifice per Dose: All APDs covered by this document shall be dosed up to a maximum of one quarter (0.25) gallon per orifice per dose.4. Hydraulic loading rates for Advanced Pressure Drainfields Soil Category | Soil Texture | Soil Structure | Soil Consistence In-Hand Using Soil Clods | Excavation Difficulty | Category 1 Technologies Loading Rate (gal/ft2/day) | Category 2 Technologies Loading Rate (gal/ft2/day) |
1 | cos, s, lcos, ls, cosl, fs | structureless-single grain or subangular blocky | loose friable | none | 2.3 | 1.5 |
2 | vfs, lvfs | structureless-single grain | loose | none | 2.7 | 1.9 |
3 | ls, sl, l | granular, subangular blocky | very friable to friable | low | 3.5 | 2.3 |
4 | lfs, lvfs, fsl, vfs | granular, subangular blocky | very friable to friable | low | 3.1 | 2.0 |
5 | sil, si, vfsl | subangular blocky | very friable to friable | low | 2.7 | 1.9 |
6 | lcos, cosl, lfs, ls, sl, l, | structureless massive | very friable to friable | low | 2.3 | 1.5 |
7 | fsl, vfsl, sil, si, vfs | structureless-massive | very friable to friable | low to moderate | 2.1 | 1.5 |
8 | all textures | structureless-massive | firm to very firm | moderate | 1.9 | 1.3 |
9 | all textures | platy or structureless- | firm to very firm | high | 1.5 | 1.0 |
10 | all textures | massive platy or structureless-massive | extremely firm | very high to extremely high | Not Allowed | Not Allowed |
Notes: (1) Loading rates shall be based on one hundred fifteen (115) gallons/bedroom and upon texture, structure, and consistence of the most restrictive horizon within three feet (3') below the proposed base of the APD. Please see §§ 6.76, 6.77, 6.79, 6.82, and 6.83 of this Part, Figures 18, 19, 21, 24, and 25. (2) Pressurized shallow narrow drainfields placed in cos, vcos, gravelly or very gravelly soils shall be installed over a leveled-off six inch (6") layer of sand meeting ASTM C-33 incorporated above at § 6.7(G) of this Part. |
C. Bottomless Sand Filters (BSFs) 1. BSF design guidelines: All BSFs shall conform to § 6.36(A) of this part "General design parameters for all pressurized drainfields." a. Vertical separation distances: All vertical separation distances defined in the Part 6 of this Subchapter must be met when using a BSF. The vertical separation distance shall be measured from the cover stone-sand interface below the PVC distribution manifold, to the seasonable high water table (SHWT) or to the impervious layer as defined by the Department. (See §§ 6.76 and 6.77 of this Part, Figures 18 and 19). The required separation distance to the SHWT shall be measured from twenty-four inches (24") above the base of the BSF (the minimum depth of sand media). b. Hydraulic loading rates: Hydraulic loading rates for BSFs will be based on the native receiving soil characteristics and the quality of wastewater being discharged by the preceding secondary treatment unit. These rates are provided in the table in § 6.37(B)(4) of this Part.c. Finished grade: Finished grade around any BSF shall be a minimum of six inches (6") and a maximum of twenty-four inches (24") below the top of the enclosure to prevent surface water from flowing onto the filter. One (1) layer of secured pressure treated timbers with minimum nominal dimensions of six inches by six inches (6" x 6"), (or other suitable structural support) shall be placed around the top perimeter (See §§ 6.76 and 6.77 of this Part, Figures 18 and 19).d. Fill perimeter: The land surface elevation two feet (2') below the cover stone-sand media interface shall be maintained for a distance of at least five feet (5') from the edge of the BSF. Land surface regrading adjoining this five foot (5') perimeter must maintain a minimum of 3:1 (run:rise) slope down gradient (See § 6.76 of this Part, Figure 18).e. BSF protection: The proposed BSF location shall be staked out and protected in accordance with § 6.33(T)(2) of this Part prior to any site preparation activities.f. Setbacks to trees and shrubs: A minimum setback of ten feet (10') shall be maintained between BSFs and neighboring trees and shrubs. Where the ten foot (10') setback cannot be maintained, a root barrier fabric shall be placed between the trees and shrubs and the filter.g. No structures, permanent features, or large, heavy or numerous decorations shall be placed on top of the BSF that would obstruct, prevent or hinder operation and maintenance or access to the BSF.2. Additional components of the BSF: Bottomless sand filters shall also conform to the components in § 6.36(B) of this Part: "Common components for all pressurized drainfields." a. BSF Enclosures (See § 6.79, Figure 21) (1) The walls of BSFs must be lined with a thirty (30) mil flexible PVC liner with all boots, patches, repairs, and seams having the same physical properties as the liner material.(2) Any penetration through the PVC liner wall shall be done with a PVC boot attachment glued to the liner with the appropriate resilient sealer.(3) Support walls are needed to prevent caving of the filter walls during construction. These walls shall be rigid and made of sacrificial plywood or particle board (or equivalent; plywood is intended to decompose over time) and supported by at least one (1) row of six inch by six inch (6" x 6") nominal dimension pressure treated timbers (or equivalent) above the finish grade.(4) A permanent top frame structure (such as pressure treated six inch by six inch (6" x 6") nominal dimension timbers, or other suitable structural support) must be provided on any portion of a BSF that is installed above grade. The top frame structure shall be a minimum of six inches (6") but no higher than twenty four inches (24") above grade. The design shall include cross-bracing to maintain structural integrity of the frame. Below grade use of timbers is prohibited.b. Bottomless Sand Filter Media Specifications: All media within the enclosure and below the cover stone shall have an effective size (D10) of 0.33 mm (+/-) and uniformity coefficient (D60/D10) of 2.0 to 4.0. The maximum allowable percentage of fines passing through a Number 200 sieve shall be one percent (1%). Other than the gradation and fine content specified above, the sand media shall meet the other ASTM C-33 sand specifications.c. BSF distribution laterals (1) General: Influent applied to a BSF shall be distributed over the sand surface using small diameter, pressure rated SCH 40 PVC pipe.(2) Orifices: A series of clean, uniform, one eighth inch (1/8") diameter holes (orifices) shall be drilled in the distribution laterals and spaced no less than fourteen inches (14") and no more than twenty-four inches (24") apart. Two (2) orifices in each lateral shall be drilled pointing up (12 o'clock position) and be located approximately one third (1/3) and two thirds (2/3), respectively, along the length of each lateral. All other orifices shall be drilled pointing down (6 o'clock position). Orifice shields shall be placed over each orifice (above or below the lateral, as required). Orifice shields placed below any orifice shall contain slots or holes to provide free draining (usually referred to as cold weather orifice shields, see § 6.80 of this Part, Figure 22).(3) Laterals: Laterals shall be spaced between fourteen inches (14") and twenty-four inches (24") on center and shall be no longer than fifty feet (50').(4) Lateral ends: The distal end of each BSF lateral shall be fitted with a forty-five degree (45°) elbow and closed off with either a ball valve or a threaded end and cap (see § 6.81 of this Part, Figure 23).(5) Orifice square grid and space to liner: Lateral spacing and orifice spacing shall be as close to square as practicable. The space from the ends of the laterals to the liner shall be close to half the orifice spacing and be able to accommodate the fittings (i.e.: forty-five degree (45°) elbow, threaded end adapter and cap) and have sufficient space for maintenance activities. See § 6.78 of this Part, Figure 20.d. BSF inspection well: One (1) inspection well shall be installed in the approximate center of the filter and extend down to the sand and native soil interface (See §§ 6.76, 6.77, and 6.78 of this Part, Figures 18, 19 and 20). Larger zoned BSFs shall have at least one (1) inspection well per zone. e. BSF cover stone: A three eighths to one half inch (3/8" - 1/2") round or sub-rounded, screened or crushed, uniform in size so that no more than five percent (5%) of the sample is greater than one half inch (1/2") and no more than five percent (5%) shall pass a three eighths inch (3/8") sieve, non-shale or other soft stone, double washed, containing little or no fines shall be used for cover stone on the BSF. Total depth of the stone shall be eight to nine inches (8" - 9"), depending on the size of the lateral (see § 6.79 of this Part, Figure 21 and installation procedures).3. BSF Installation Specifications a. The proposed BSF location shall be staked out and protected prior to any site preparation activities.b. Installation of BSF media (1) BSF base: Sod, vegetation, or dead or decaying organic litter or any organic soil horizon shall be removed from the area planned for the BSF installation. Once the proper design elevation for the BSF base has been reached and the enclosure is in place, three inches (3") of the native soil material shall be scarified and thoroughly mixed with three inches (3") of the sand media (see § 6.79 of this Part, Figure 21). Perimeter stripping is prohibited. Excavation of soil beneath the established native soil and filter sand interface is prohibited unless a boulder, stone, fill, or other unexpected condition is encountered. Only approved gravel (§ 6.33(M) of this Part) or BSF sand media shall be placed for backfilling the base of the BSF before placement of the required sand media.(2) Placing sand media: All sand media placed within the BSF enclosure and below the cover stone must meet the requirements of § 6.37(C)(2)(b) of this Part and must be a minimum of twenty-four inches (24") deep. The excavator or backhoe bucket used to place media in the filter shall be washed thoroughly to remove any mud or fines before the loading process begins. The sand media shall be placed in level eight inch (8") lifts in the filter. Each lift of sand media shall be lightly compacted.(3) Placing stone cover: After the required amount of filter sand has been added to the filter, place three inches (3") of three eighths inch (3/8") double washed stone over the filter sand. After the distribution laterals and orifice shields have been assembled atop the cover stone, six more inches (6") of cover stone shall be added. The total depth of cover stone over the sand media will be eight inches (8") to nine inches (9"), depending on the size of lateral pipe employed (see § 6.79 of this Part, Figure 21).(4) Burial precautions: BSFs shall not be buried or covered by topsoil or any other material which will limit the gas or oxygen movement into and out of the filter. Designer shall also note on their plans that the area of the BSF shall be treated as a wastewater utility as tampering with a BSF may present a public health risk. The BSF shall be accessed and serviced by trained professionals only.D. Pressurized Shallow Narrow Drainfield 1. PSND Design Guidelines: PSNDs shall also conform to § 6.36(A) of this part, "Common design parameters for all pressurized drainfields." a. Vertical separation distances: All vertical separation distances defined in §§ 6.33(H), 6.33(I), 6.43(E), and 6.44(C) of this Part must be met when using a PSND. The vertical separation distance shall be measured from the base of the PSND (the natural receiving soil surface), to the SHWT or to impervious layer. The bottom of each lateral must be kept level. The required separation distance to the SHWT shall be twenty-four inches (24") statewide; the required separation distance to any impervious material shall be forty-eight inches (48") statewide unless otherwise specified by permit. b. Trench spacing: The minimum trench spacing shall be two and one half feet (2 1/2') on-center (one and one half feet (1 1/2') edge-to-edge).c. Shape and geometry: PSND laterals shall be placed lengthwise along the existing site contours. Laterals can be broken into zones of different shapes, so long as the total basal area of each shape (zone) is the same as the other zones. d. Finished grade: Finished grade shall prevent surface water ponding and prevent surface runoff over the PSND area.e. Fill perimeter: The land surface elevation of the infiltrative surface of each trench shall be maintained for a distance of five feet (5') from the edge of the PSND. Land surface re-grading adjoining this five foot (5') perimeter must maintain a minimum of 3:1 (run:rise) slope down gradient for at least twenty-five feet (25') before meeting existing grade.f. Soil between trenches: Preservation of the native soil between trenches and minimizing its disruption and compaction during construction is essential to maintaining soil structure and therefore water and gas movement in the soil around the trenches. For this reason, construction is to be trench-by-trench (relief from this requirement may be granted by the Department on a case-by-case basis when informed of unanticipated site conditions encountered during construction, see § 6.82 of this Part, Figure 24). Excavation equipment with minimal pressure impact (i.e.: mini excavators) shall be used when constructing PSNDs.2. Additional Components of the PSND: PSNDs shall also conform to the components in § 6.36(B) of this Part, "Common components for all pressurized drainfields." a. Orifices: A series of clean, uniform, one eighth inch (1/8") diameter holes (orifices) shall be made in the top of the distribution laterals (12 o'clock position) and spaced according to the dosing requirements of the system. Every fifth (5th) orifice along the lateral shall be drilled from the bottom of the pipe (6 o'clock positions and eliminating the up-facing orifice in that location) to allow drainage after a dose and to prevent lateral freezing in cold weather.b. Lateral clean outs: SCH 40 PVC or equivalent sweep elbows shall be attached to the distal end of each PSND lateral to facilitate maintenance and inspection (§ 6.83 of this Part, Figure 25). A standard ninety degree (90°) elbow shall not be used here because it will interfere with maintenance activities.c. Lateral access ports: The ends of the sweep elbows shall be readily accessible by means of a six (6") to eight inch (8") diameter access box or port brought to finish grade (see §§ 6.82 and 6.83 of this Part, Figures 24 and 25). Access ports shall be placed every twenty feet (20') maximum along each lateral but no more than two (2) access ports are required for each lateral (one (1) at the midpoint of the lateral and one (1) at the clean out).d. Drainfield Cover: The dome-like covering over the PSND should be made of twelve inch (12") diameter PVC plastic or an approved equivalent (see §§ 6.82 and 6.83 of this Part, Figures 24 and 25). Support bars (see § 6.37(D)(3)(c) of this Part, "PSND Installation Specifications") shall be used along the trench to provide a larger bearing surface that keeps the thin edge of the dome (PIP pipe only) from digging deep into the infiltrative soil surface. e. Trench Maximum Length: Maximum trench length shall not exceed fifty feet (50').3. PSND Installation Specifications a. The proposed PSND location shall be staked out and protected prior to any site preparation activities. b. Trenches: Each trench base must be level. Do not over-dig the width or depth of the individual trenches; minimal backfilling on bottom and sides provides a more stable enclosure. Scarify each trench bottom before installing components. PSNDs placed in cos, vcos, gravelly or very gravelly soils shall have a leveled-off six inch (6") ASTM C-33 sand layer. Do not remove the soil between the trenches. If the presence of boulders, heavy roots, or other obstacles make trench construction impractical, the basal area may be excavated as necessary, backfilled with a maximum of ten inches (10") of ASTM C-33 sand (concrete sand) to the design elevation of the bottom bed, the PSND constructed and backfilled with native soil material (see § 6.82 of this Part, Figure 24). Prior approval by the Department and notification of the designer of record is required in order to remove material between or under trenches.c. Support bars: One inch (1") diameter by fourteen inch (14") long Schedule 40 PVC support pipes shall be used to support the dome (and pressure pipe), to act as a spreader device and to provide a greater bearing surface for the dome. These support/spreader pipes shall be spaced approximately four feet (4') apart or whenever a drainfield cover joint occurs. Notches shall be cut one inch (1") from either end of the support pipes for the cover to fit into.d. Laterals: Laterals shall be as specified in § 6.37(D)(2) of this Part and shall be installed over the support bars.e. Drainfield cover: The dome-like cover should snap over the spreader bars and into the notches. A minimum of three inches (3") overlap shall exist between drainfield cover joints. Filter fabric shall cover the overlap joint.f. Inspection/access ports and ends of trenches shall be as specified in § 6.37(D)(2) of this Part and installed in access holes in the drainfield cover.g. Grass cover: PSNDs shall be kept free of debris and planted to a sod cover or seeded with grass and protected. A watering program shall be in place until a healthy and sustainable grass cover is obtained.250 R.I. Code R. 250-RICR-150-10-6.37
Amended effective 11/25/2018
Amended effective 12/28/2021
Amended effective 7/1/2022