Inputs
Example Data Table
| Scenario | Occupants | Per-capita (L/p/d) | Detention (days) | Safety | Infiltration (%) | Working Volume (L) | Total Volume (L) |
|---|---|---|---|---|---|---|---|
| Residential example | 5 | 150 | 2.0 | 1.20 | 10 | 2,880 | 3,312 |
Formula Used
Where N is occupants, q is per-capita flow, SF is safety factor, and I is infiltration percentage.
Where T is detention time in days.
Where rs and rc are the sludge and scum rates (L/person/year), and Y is pumping interval in years.
Vtotal = Vworking × (1 + FB/100)
Where FB is freeboard allowance as a percentage.
How to Use This Calculator
- Enter occupants based on typical use, not maximum parties.
- Set per-capita flow using local guidance or fixture intensity.
- Choose detention time (often 1.5–2.5 days for residential).
- Adjust safety and infiltration to reflect uncertainty and site risk.
- Set pumping interval and solids rates to match maintenance plans.
- Click Calculate, then download CSV or PDF for reporting.
Daily wastewater load assumptions
Select occupants and a per‑capita wastewater rate suited to the project. Residential planning often uses 120–200 L/person/day; high‑use staff areas can exceed 250. The calculator applies Qdesign = N×q×SF×(1+I/100) so you can include peak use and extra inflow. For newer, tight plumbing, I may be 0–10%; older or wet sites can justify 15–30%.
Detention time and treatment performance
Detention time sets how long wastewater stays in the tank for settling and scum separation. Many homes are sized at 1.5–2.5 days. Short detention can increase solids carryover; long detention increases excavation and cost. If usage is spiky (shift work, events), a modest increase can smooth peaks. Confirm any local limits before finalizing.
Sludge and scum storage planning
Beyond liquid detention, the tank must store sludge and scum between pump‑outs. Typical planning ranges are 30–60 L/person/year for sludge and 10–30 L/person/year for scum, but grease‑heavy use and disposals can raise both. Pumping every 2–5 years is common; longer intervals require more storage volume. The tool lets you tune rates to match maintenance policy or local guidance.
Freeboard and safety allowance
Freeboard adds reserve space above the working liquid level for surges and inlet turbulence. A 10–20% allowance is common for concept sizing. Safety factors often fall near 1.1–1.4 for stable residential occupancy and 1.5+ for uncertain loads or phased expansion. After calculating, compare total capacity to any jurisdictional minimum tank volume stated in liters or gallons.
Turning volume into dimensions
Using the working volume and selected liquid depth, the calculator proposes dimensions. Rectangular tanks typically perform well with L:W ratios of about 2.5–4.0; two compartments are commonly split near 2/3 and 1/3 of length. Cylindrical sizing solves diameter from volume and depth. The geometry check helps verify that dimensions match the computed working volume.
FAQs
1) What inputs matter most for sizing?
Occupants, per‑capita flow, and detention time drive the liquid volume. Pumping interval plus sludge/scum rates drive solids storage. Safety factor and infiltration increase the design flow when use is uncertain or inflow is likely.
2) How do I estimate occupants for a new building?
Use expected regular users, not maximum occasional visitors. For residences, many designers start with bedrooms × 2, then adjust for household size. For worker facilities, use peak shift headcount or realistic daily usage.
3) What per‑capita flow should I choose?
Start with local guidance if available. As a planning range, 120–200 L/person/day suits many homes, while higher‑use facilities may require 200–300+. Increase the value if multiple bathrooms, laundry, or heavy cleaning is expected.
4) Why are sludge and scum rates adjustable?
Rates vary with diet, grease, disposals, and maintenance. Adjustable rates let you match policy: longer pump intervals require more storage. If you lack data, use conservative mid‑range values and confirm with local practice.
5) Rectangular or cylindrical: which is better?
Both can work. Rectangular tanks often provide a longer flow path and easier two‑compartment splits. Cylindrical tanks can be efficient for precast options. Choose based on site constraints, availability, and the dimensions that fit your excavation.
6) Does freeboard replace code minimum capacity?
No. Freeboard is a reserve above the working liquid level, not extra treatment volume. Always compare the total volume against any code‑mandated minimum tank size and required compartment configuration, then adjust inputs or select the next larger standard tank.