9.3 Design Flow, Septic & Holding Tanks and Tank Construction
Key Takeaways
Design wastewater flow is at least 150% of estimated daily flow; a dwelling's combined flow is estimated at 100 gallons per bedroom per day (60 gray and 40 black when segregated).
A holding tank must hold at least 5 times the estimated daily flow or 2,000 gallons, whichever is greater, with its service opening within 25 feet of an all-weather road.
Tank manhole openings are at least 23 inches; septic tank manholes rise to within 6 inches of grade, while those over pumps and holding tank service openings extend at least 4 inches above grade.
Holding tank suction and discharge lines are at least 3 inches with a quick-disconnect service port at least 2 feet above grade, and may not allow gravity or siphon drainage.
Septic tanks are pumped when sludge plus scum equals 1/3 of tank volume; holding tanks when the level is 1 foot below the inlet invert.
Estimating flow (SPS 383.43 (2) to (6))
| Flow type | Equation (B = bedrooms, 2 persons each) |
|---|---|
| Combined graywater, clear water and blackwater from a dwelling | 100 gallons x B |
| Segregated graywater and clear water | 60 gallons x B |
| Segregated blackwater | 40 gallons x B |
| Public buildings and places of employment | Measured flow or a detailed per-capita or per-use estimate |
Design wastewater flow must be at least 150 percent of the estimated daily flow to cover peaks.
Worked example. A 4-bedroom house: estimated flow = 100 x 4 = 400 gpd; design flow = 400 x 1.5 = 600 gpd. A restaurant on the same POWTS adds its own estimate from the SPS 383 Appendix or metered data. Its grease interceptor for a POWTS must be at least 1,000 gallons (SPS 382.34 (5)).
General design requirements (SPS 383.43 (8))
- Gravity piping between components: at least 1 ft/s computed velocity at half full.
- Pressurized piping upstream of in situ soil components: at least 2 ft/s.
- Gravity piping inside an in situ soil component: level, or pitched downstream no more than 4 inches per 100 feet.
- Alarms: mechanical treatment or distribution devices need an automatic visual or audible failure alarm on a circuit separate from the device. Pump alarms stay on until manually silenced, and duplex pumps must switch over automatically. A holding tank needs an alarm that warns when servicing is needed.
- Tanks in saturated soil must be anchored against flotation. Components inside buildings must be gastight.
- Every component that needs maintenance must be accessible.
Tank construction (SPS 384.25)
| Topic | Requirement |
|---|---|
| Watertightness, concrete with cover | Hold a vacuum of 2 in. Hg for 60 minutes, or hold water for 1 hour after a 24-hour soak and refill to the top of the cover |
| Watertightness, steel with cover | 5 psig air for 15 minutes, or 5 psig water for 60 minutes |
| Concrete durability | Protective coating above the lowest liquid level, or concrete with a water-cement ratio of 0.45 or less |
| Steel | Coated inside and out; buried tanks need cathodic protection (UL 1746 or STI-P3) |
| Venting | Every tank (except camping-unit transfer containers) vents gases; vent terminals per SPS 382.31 (16) |
| Manholes | At least 23 inches in least dimension |
| Septic (anaerobic) tanks below ground | Manhole over the inlet of the first compartment, in each compartment, and over all treatment apparatus and pumps; manholes within 6 inches below grade, but those over apparatus and pumps at least 4 inches above grade |
| Holding tank service openings | At least 4 inches above grade, at least 23 inches, over any pumps or siphons |
| Security | Covers at or above ground on openings larger than 8 inches need locking devices or other effective measures |
| Warning label | Covers on openings over 8 inches get a permanent warning label, at least 4 x 5 inches, lettering at least 1/2 inch |
| Tank labels | Manufacturer, capacity of each compartment or model number; NSF 40 aerobic units carry 2 label plates with rated daily flow |
Holding tanks (Holding Tank Component Manual, version 2.1, May 2022, valid through May 2027)
| Specification | Requirement |
|---|---|
| Capacity | 5 x estimated daily flow, or 2,000 gallons, whichever is greater |
| Service access | Farthest service opening or pump-out port within 25 feet of an all-weather road |
| Alarm | High-level warning per SPS 383.43 (8) (e) 2. and SPS 384.25 (10) |
| Vent | 2-inch inside diameter, Table 384.30-1 material, opening oriented down |
| Setbacks | Table 383.43-1 |
Service lines (SPS 383.43 (8) (j)):
- Suction and discharge lines at least 3 inches.
- A quick-disconnect service port with a removable plug, at least 2 feet above grade, labeled with a permanent sign and secured to a support.
- The line may not allow the tank to drain by gravity or siphon.
- A lift station discharge may be 2 inches, with a keyed switch at the port.
Worked example. A seasonal cabin with 2 bedrooms: estimated flow 200 gpd, and 5 x 200 = 1,000 gallons. The 2,000-gallon minimum is greater, so the tank must hold 2,000 gallons. A 6-bedroom lodge (600 gpd) needs 5 x 600 = 3,000 gallons. Above 3,000 gpd design, plan review moves from the county to the department or a designated agent (Table 383.22-2).
When tanks must be pumped (SPS 383.54 (3) and (4))
| Tank | Service trigger |
|---|---|
| Anaerobic treatment (septic) tank | When combined sludge and scum equal 1/3 of tank volume |
| Holding tank | When wastewater reaches 1 foot below the inlet invert |
| RV transfer tank | Serviced so nothing discharges to soil or ground surface |
These triggers apply to systems installed before July 1, 2000 as well. The management plan must specifically address servicing if a tank bottom is more than 15 feet below the servicing pad elevation or more than 150 feet horizontally away. Servicing is done by licensed septage operators under NR 113 and NR 114.
Grease and remodels
Kitchen and food wastes from non-dwelling occupancies on a POWTS pass through a grease interceptor of at least 1,000 gallons. In remodels where separating kitchen and toilet waste isn't practicable, the combined waste may go to a POWTS tank sized with the interceptor capacity added to the septic tank capacity (SPS 382.34 (5) (b) 2.).
What is the estimated daily wastewater flow for a 3-bedroom dwelling under SPS 383.43 (3) (a), and what is its design flow?
300 gpd estimated; 450 gpd design
150 gpd estimated; 225 gpd design
450 gpd estimated; 675 gpd design
300 gpd estimated; 300 gpd design
What minimum capacity does the Holding Tank Component Manual require for a dwelling with an estimated daily flow of 300 gallons?
1,000 gallons
1,500 gallons
2,000 gallons
3,000 gallons
When must an anaerobic treatment (septic) tank be serviced under SPS 383.54 (3) (b)?
Every 12 months regardless of contents
When the liquid reaches 1 foot below the inlet invert
When sludge alone fills half the tank
When combined sludge and scum equal 1/3 of the tank volume
Sections you finish are checked off in the contents.