9.4 Dispersal Components & the DSPS Component Manuals
Key Takeaways
A design that follows a DSPS-recognized component manual needs no performance data at plan review, but any deviation makes it an Individual Site Design reviewed by state staff (SPS 383.60, 383.61).
The mound, at-grade and pressure distribution manuals cap design wastewater flow at 5,000 gpd, and the at-grade manual limits gravity designs to 600 gpd from one- and 2-family dwellings.
Each pressure dose must be at least 5 times the lateral void volume and, per the manuals, no more than 20% of design wastewater flow.
Mound distribution cells are no wider than 10 feet, at least 3 feet apart, on original slopes of 25% or less, with 12 inches of cover at the center and 6 inches at the outer edge.
Pressure networks need distal head of at least 5 feet for 1/8-inch orifices and 2.5 feet for 3/16- and 1/4-inch orifices, with 2 to 10 ft/s in the force main and manifold.
Why the manuals are on the reference list
The DSPS exam page lists five component manuals among the permitted references: At-Grade Pressure Distribution, Holding Tank, In-Ground Soil Absorption, Mound and Pressure Distribution. It also says candidates need "a working knowledge of the component manuals." Each manual is a product approval under SPS 384.10. The current DSPS versions (mound 2.1, at-grade 3.0, pressure distribution 2.1, holding tank 2.1) are dated May 2022 and approved through the end of May 2027.
How recognized methods work (SPS 383.60 and 383.61)
- A method recognized under SPS 384.10 (3) may be used on any site within its specifications without submitting performance data at plan review.
- Deviating from a recognized method's installation or maintenance specifications is a code violation. A design that doesn't fully comply becomes an Individual Site Design (ISD), which needs exclusive plan review by state staff.
- Approval letters impose conditions on every manual-based system: a copy of the approval letter goes with the plans; approved tanks; installation by properly licensed persons; cleanouts per SPS 382.35; grease interceptors for food service; a sanitary permit; and a soil report under SPS 385.40.
Pressure dosing rules in the code (SPS 383.44 (5))
Effluent goes by pressure distribution wherever Tables 383.44-2 or 383.44-3 require it, for example where the 24-inch reduced separation is used for pretreated effluent. Each dose must be at least 5 times the void volume of the distribution laterals. Pressure distribution is not required when rehabilitating a persistently ponded non-pressurized component that has at least 24 inches of unsaturated soil beneath it.
Pressure Distribution Component Manual (version 2.1)
| Item | Requirement |
|---|---|
| Design wastewater flow | 5,000 gpd or less |
| Particle size | 1/8 inch or less |
| Dose volume | At least 5 x lateral void volume and no more than 20% of design flow |
| Distal head | At least 2.5 ft for 1/4- and 3/16-inch orifices, 3.5 ft for 5/32-inch, 5 ft for 1/8-inch |
| Network pressure | Distal head plus 30% for fittings |
| Velocity in force main and manifold | 2 to 10 ft/s |
| Force main diameter | 0.75 to 6 inches |
| Manifold diameter | 1.25 to 3 inches |
| Lateral diameter | 0.75 to 3 inches |
| Orifice diameter | 0.156 to 0.25 in. for effluent over 30 mg/L BOD5 or TSS; 0.125 to 0.25 in. for 30 mg/L or less |
| Lateral spacing | 12 to 48 inches; half that spacing to the cell edge |
| Last orifice to cell end | 6 to 24 inches |
| Laterals | Level or sloped no more than 1 inch back to the manifold |
| Dose tank, simplex pump | Dose volume + reserve (at least one day's flow) + drain-back volume + 6 inches of tank volume |
Orifices smaller than 1/8 inch, including drip, are a restricted technology under Table 383.04-1, and the installer needs approved training.
Mound Component Manual (version 2.1)
| Item | Requirement |
|---|---|
| Design wastewater flow | 5,000 gpd or less |
| Influent | FOG 30, BOD5 220, TSS 150 mg/L monthly averages or less |
| Fill loading rate | 1.0 gal/ft2/day if BOD5 or TSS is over 30 mg/L; 2.0 if both are 30 or less |
| Dose | At least 5 x lateral void volume, no more than 20% of design flow |
| Linear loading | No more than 4.5 gal/ft/day where soil within 12 inches of fill has an application rate of 0.3 or less |
| Distribution cell width | 10 feet or less; long dimension parallel to contours |
| Spacing between cells | At least 3 feet |
| Fill depth at upslope edge | At least 6 in. (soils needing 36 in. or less treatment depth) or 12 to 36 in. (soils needing more) |
| Cell depth | At least 8 inches plus the pipe size |
| Cover | At least 12 in. at the top center and 6 in. at the top outer edge |
| Original slope | 25% or less under the basal area |
| In situ soil to groundwater or bedrock under basal area | At least 6 inches |
| Fill material | ASTM C33 fine aggregate |
| Distribution | Pressure network per the Pressure Distribution manual |
At-Grade Component Manual (version 3.0)
- Pressure designs: design flow up to 5,000 gpd; at least one orifice per 2 linear feet of cell; dose at least 5 x void volume and no more than 20% of design flow; linear loading no more than 4.5 gal/ft where soil within 12 inches of the surface is rated 0.3 or less.
- Gravity designs: only for one- and 2-family dwellings with design flow up to 600 gpd (public buildings require an ISD). Use a level distribution box or header at the high point, insulated, accessible within 12 inches of grade and checked every year after the frost leaves. Lateral segments no longer than 30 feet.
- Geometry: effective width no more than 10 ft; a 5-foot plowed perimeter on each side and end; absorption area = design flow divided by the soil application rate within the top 12 inches.
- The at-grade cell sits on plowed in situ soil with stone aggregate and fertile cover (less than 15% gravel).
In-Ground Soil Absorption Component Manual
The in-ground manual covers trenches and beds excavated into the in situ soil, fed by gravity or pressure. The same code limits apply: influent quality, separation to saturation (Table 383.44-3), application rates (Table 383.44-2), setbacks (Table 383.43-1) and pressure dosing where required. Two or fewer one- or 2-family dwellings using gravity distribution to an in-ground cell may be plan-reviewed by the county (Table 383.22-3). Distribution cell stone is washed aggregate 1/2 to 2-1/2 inches with Mohs hardness of at least 3. Geotextile over the cell must meet Table 384.30-12. Leaching chambers must meet ASTM F2418.
Choosing a component
| Site condition | Typical component |
|---|---|
| Deep, permeable soil (36 in. or more of suitable unsaturated soil below the trench) | In-ground trench or bed |
| Saturation or bedrock too shallow for in-ground, but at least 6 in. of in situ soil | Mound (fill provides the treatment depth) |
| Moderately shallow limits, relatively undisturbed sloping site | At-grade |
| No suitable soil, or a local option allows it | Holding tank (5 x flow or 2,000 gal minimum) |
Under the Pressure Distribution Component Manual, what minimum distal head is required at the end of a lateral with 1/8-inch orifices?
2.5 feet
3.5 feet
5 feet
10 feet
A one-family dwelling's at-grade system will use gravity distribution. What maximum design wastewater flow does the At-Grade Component Manual allow for gravity designs?
450 gpd
5,000 gpd
1,000 gpd
600 gpd
What happens when a POWTS design deviates from a DSPS-recognized component manual?
It becomes an Individual Site Design requiring state plan review
Nothing changes, as long as the county sanitarian approves the change
It must be converted to a holding tank designed for at least 2,000 gallons
The sanitary permit is automatically extended for 2 more years
Sections you finish are checked off in the contents.