Wyoming · Wyoming Water Quality Rules Chapter 25
Wyoming DEQ administers the program, with several counties operating delegated programs under agreement.
Minimum tank
One to three bedroom dwelling.
Design flow
Held two days for settling.
Cost index
Regional labour and permits.
Sets the design flow and the legal minimum.
Code sizes on bedrooms, not people.
Used to size the matching drainfield. Unsure? Sandy loam is the common default.
Semi-arid soils with wide variation — coarse alluvium in the river valleys, clay in the basins, and shallow bedrock across much of the state. Groundwater is generally deep, which eases separation.
Conventional trenches and seepage pits where soil depth allows. Large lot sizes make lagoons viable on some rural properties. Advanced treatment is required where groundwater protection warrants it.
| Dwelling | Design flow | Minimum tank | With disposal |
|---|---|---|---|
| 1 bedroom | 300 gpd | 1,000 gal | 1,250 gal |
| 2 bedrooms | 300 gpd | 1,000 gal | 1,250 gal |
| 3 bedrooms | 450 gpd | 1,000 gal | 1,250 gal |
| 4 bedrooms | 600 gpd | 1,250 gal | 1,500 gal |
| 5 bedrooms | 750 gpd | 1,500 gal | 1,750 gal |
Wyoming DEQ issues small wastewater permits, and several counties administer delegated programs. Lot sizes are large enough that setbacks are rarely the binding constraint — soil depth and rock usually are.
The state sets a floor. These Wyoming counties are where the largest septic populations sit, and where local requirements most often exceed it.
| County | What makes it different |
|---|---|
| Laramie | Cheyenne area; growth on the plains |
| Natrona | Casper; basin soils |
| Campbell | Gillette, energy sector development |
| Teton | Jackson; strict environmental requirements, high water table near the river, very high costs |
| Sheridan | Foothill development with variable soils |
A three-bedroom home at 150 gallons per bedroom produces 450 gallons per day of design flow. On sandy loam that needs roughly 750 square feet of absorption area. On clay loam it rises to about 2,250 square feet, and on clay a conventional field is usually not permitted at all. Run your own soil class through the drainfield calculator.
Advanced systems carry maintenance obligations under permit conditions. Use the pump schedule calculator to work out the interval for your tank size and household.
Regional labour and permit costs in Wyoming run about 98% of the national baseline. Here is what typical installations land at for a three-bedroom home:
| System type | Typical installed cost in Wyoming | When it applies |
|---|---|---|
| Conventional gravity | $8,800 – $15,700 | Good soil, adequate depth to water |
| Chamber system | $9,700 – $18,100 | Alternative to gravel trenches |
| Mound system | $16,700 – $33,000 | Shallow water table or thin soil |
| Aerobic treatment | $14,100 – $28,300 | Clay soil or small lots |
| Tank pump-out (service) | $310 – $640 | Every 3–5 years |
Replacing an existing system costs more than these figures — old-system removal and re-permitting add to every line. The replacement cost calculator covers that case, and the cost estimator adjusts all of the above for your bedroom count and soil.
Verify before you build. The figures above are state minimums compiled for planning, drawn from Wyoming Water Quality Rules Chapter 25, Small Wastewater Systems. County and health district rules routinely exceed them, code text changes, and only Wyoming DEQ — Water Quality Division, Small Wastewater or your local health department can confirm what applies to your parcel.
Send your sizing numbers to installers who work under Wyoming rules every day.