STP Capacity
STP capacity is the daily sewage volume a treatment plant is designed to process, expressed in kilolitres per day (KLD) — computed from occupancy and water-demand norms, and written into your SPCB consent conditions.
Why it matters on an MEPF project
The capacity number carries three lives: hydraulic (the tanks and blowers must actually process the flow), statutory (CTE/CTO consents state it, and discharge/reuse conditions hang off it), and spatial (the footprint is cast in concrete early). Undersizing shows up as odorous bypasses and consent violations; oversizing at small occupancy starves the biology that treatment depends on.
Sizing starts from water demand per occupancy class (the NBC Part 9 basis) with sewage typically estimated as a high fraction of supplied water, then peaks and equalisation shape the tankage. Reuse mandates increasingly close the loop — treated water to flushing, HVAC make-up and landscaping — which changes both the treatment train and the plumbing above it (dual piping).
How it's specified in practice
| Parameter | Typical / working position | Governing reference |
|---|---|---|
| Water demand basis | Litres/person/day by occupancy class | NBC 2016 Part 9 / IS 1172 basis |
| Sewage generation | High fraction of water supplied | design practice |
| Peak & equalisation | Peak-factor handling via equalisation tank | process design |
| Consent linkage | Capacity + discharge/reuse norms in CTE/CTO | SPCB consent conditions |
Common mistakes
- Sizing to plot potential instead of realistic phased occupancy — starved biology for years.
- No equalisation, so morning peaks slug the plant daily.
- Reuse plumbing omitted, making the mandate unmeetable later.
- Consent capacity and installed capacity drifting apart over expansions.
Related on this site
One of 68 working definitions in the MEPF & solar engineering glossary. The buying side of the same decisions is in the buyer's guides; the numbers are in the calculators.
Frequently asked
How is STP capacity calculated for a factory or society?
Occupancy × per-capita water demand (by use class) → sewage fraction → KLD, with peak/equalisation shaping tanks. Our STP calculator runs this chain; the consent application formalises it.
What happens if my STP underperforms its consent?
Discharge-quality violations invite SPCB action — notices, and in stressed areas closure directions. Underperformance is usually operational (aeration, sludge wasting) before it is capacity; an O&M review comes first.
Standards referenced: NBC 2016 Part 9 · IS 1172 (demand basis) · SPCB consent regimes