STP Hydraulic Load Calculator
Before an STP is specified, pin down the flow it has to take. Use measured sewage flow where you have it; otherwise estimate it from domestic water demand and the share you state returns to the sewer. Add growth — and, separately, the hours sewage arrives and a peak factor for the hydraulic peak.
What this tool estimates
The present and future mean daily sewage flow in KLD — from a measured flow, or from domestic water demand × the return fraction you state — and, shown separately, the average and peak hourly flow over the hours sewage arrives at the peak factor you set.
What it cannot decide
- The treatment capacity or process: BOD, COD and TSS, and how they vary, need their own data.
- Anything about industrial process effluent. A domestic return fraction is never applied to it; it needs its own characterisation and treatment.
- Which return fraction or peak factor fits your site — you enter both, and measured or logged flows should back them.
- The equalisation volume, the plant footprint, or the discharge and reuse standards your consent sets.
Result
Estimated from domestic water demand. Leave process water out of that demand: industrial effluent is characterised and treated on its own, and a domestic return fraction says nothing about it.
| Step | Value |
|---|
| Input | Growth | Future mean |
|---|
1 KLD = 1 m³/day = 1,000 litres a day. Where the treated water could be reused, test the case with the treated-water reuse savings calculator.
Method version 6 October 2026. A screening estimate, not a design: see the limits below.
How this calculator works — and where it stops
What it calculates
The hydraulic load on a sewage treatment plant: the present and future mean daily sewage flow in KLD — from a measured flow, or from domestic water demand × the return fraction you state — and, shown separately, the average and peak hourly flow over the hours sewage arrives. It does not declare a treatment capacity.
Inputs
- Basis: a measured mean sewage flow, or an estimate from domestic water demand. Use the measurement whenever there is one.
- Measured basis: the mean daily sewage flow over a representative period, domestic sewage only, in KLD, m³/day or L/day.
- Estimated basis: the domestic water demand (process water left out) and the share of it that returns to the sewer, as a percentage — your stated basis.
- Expected growth in the mean flow, as a percentage (0 for none).
- Optional, for the peak: the hours a day sewage arrives, and a peak factor — peak hourly flow ÷ average hourly flow over those hours, at least 1, from logged flows or the design basis.
Method
- Present mean sewage = the measured mean flow; or, when estimating, domestic water demand × the return fraction.
- Future mean sewage = present mean × (1 + growth).
- 24-hour average flow (m³/h) = future mean (m³/day) ÷ 24.
- Average flow while sewage arrives (m³/h) = future mean ÷ the hours a day it arrives.
- Peak hourly flow (m³/h) = that average × your peak factor; also shown in L/s and as a multiple of the 24-hour average. Without the hours or the factor, no peak is shown — none is assumed.
- Sensitivity: the future mean is recomputed with the return fraction ±10 points (or a measured flow ±10%) and growth ±10 points.
Assumptions
- The water demand entered is domestic only. A domestic return fraction says nothing about process effluent, which is characterised and treated on its own.
- The return fraction and the peak factor are yours and should rest on measurement — water-meter readings, an inlet flow meter, logged hourly flows. The tool suggests neither.
- Growth is applied to the mean flow; whether the peak grows by the same proportion depends on how the new load is scheduled.
Limitations
- Hydraulic load only. The organic load — BOD, COD and TSS, and how they vary over the day and week — needs influent test data, and a process engineer sizes the treatment from both. No plant size or technology is recommended.
- Kitchens, canteens, laundries and hospital wards change the strength of the sewage and sometimes need a different design basis; process effluent may need an ETP.
- Equalisation volume, plant footprint, civil tank sizes and equipment selection are outside the tool.
- The discharge or reuse standards, and so the treatment train, are set by the consent from the State Pollution Control Board (or Committee) — not by this tool.
Worked example
Produced by running this calculator with the inputs below.
Inputs
- Basis: estimated from domestic water demand
- Domestic water demand 20 KLD; return to sewer 80%
- Expected growth 25%
- Sewage arrives over 12 hours a day; peak factor 2 (example values)
Working
- Present mean = 20 KLD × 80% = 16 KLD.
- Future mean = 16 × (1 + 25%) = 20 KLD (20 m³/day).
- 24-hour average = 20 ÷ 24 = 0.83 m³/h.
- Average while sewage arrives = 20 ÷ 12 = 1.67 m³/h.
- Peak hourly flow = 20 ÷ 12 × 2 = 3.33 m³/h (0.93 L/s) — 4 × the 24-hour average.
Result. A future mean hydraulic load of 20 KLD, with a peak of 3.33 m³/h for the sewers, pumps, screens and equalisation. The treatment capacity still needs the organic load from influent tests.
Sensitivity — what moves the answer
- The return fraction passes straight through: at 70% the future mean is 17.5 KLD, at 90% it is 22.5 KLD.
- Growth does the same: 15% gives 18.4 KLD and 35% gives 21.6 KLD at an 80% return.
- The arrival window drives the peak more than the daily volume: the same 20 KLD at factor 2 peaks at 1.67 m³/h spread over 24 hours, 3.33 m³/h over 12 hours and 5 m³/h over 8 hours. A factor of 3 over 12 hours also gives 5 m³/h.
How engineers use the result
To agree the design flow before the process design starts — the future mean for the plant's daily throughput and the consent application, the peak for the sewers, lifting pumps, screens and equalisation — and to show early whether influent testing and flow logging are needed before anyone sizes the treatment.
When a professional design must replace it
Before the plant is specified or the consent applied for: a process engineer should fix the flow from measured data where it exists, establish the influent strength by testing, set the peak factor and equalisation from logged flows, keep process effluent out of the domestic basis, and design the treatment to the discharge or reuse standards in the consent.
Sources
- CPHEEO (Ministry of Urban Development), Manual on Sewerage and Sewage Treatment Systems, 2013 — Part A: Engineering, Appendices — the general approach only: Appendix A 3.7 works sewage flow from the per-capita water supply and applies a peak factor to it as an illustration. No figure from the manual is applied here — the return fraction and peak factor are yours
Questions people ask before using it
What information do I need before using the STP hydraulic load calculator?
Ideally a measured sewage flow — from a flow meter at the STP inlet, logged over a representative period. Without one, the domestic water demand in KLD from water-meter readings or a demand calculation (process water left out) and the return fraction you will use, with its basis. Then the growth you are planning for, and for the peak: the hours a day sewage arrives and a peak factor from logged flows or your engineer's design basis.
Which site conditions could change the sewage load?
Shift changes and canteen breaks that send a large part of the day's flow in a short window; plant holidays that leave the plant without feed; kitchens, laundries or process drains connected to the sewer, which change the strength as well as the flow; stormwater or groundwater getting into the drains; treated water reused for flushing, which comes back as sewage; and headcount growth.
What should I send an engineer for an STP review?
The water balance with meter readings, the headcount by shift and the facilities served (canteen, hostel, laundry), any logged hourly flow, influent test results — BOD, COD, TSS, oil and grease, pH — with their sampling times, details of the existing STP and its consent conditions, and where the treated water is meant to go.
Optional: ask an engineer to look at it
The result above is yours with no form. If you want an engineer to check it against your drawings, schedules or bills, send a request. Nothing is sent until you press the button.
Request saved
It reached our team with exactly what the preview showed. An engineer will use the number you gave if they need your drawings or data.
Prefer to talk? WhatsApp or the contact form.
Got your number — what next?
A calculator gives you a first figure. These take it further.
The work itself
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- STP Cost for a Factory: Capacity, Technology Choice and What Drives It
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- STP vs ETP for Industrial Wastewater: What an Expansion Needs
- Expanding an STP: Check Both Hydraulic and Organic Load
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