Treated-Water Reuse Savings Calculator
Check whether reusing treated water pays before dual piping is drawn. Enter the compliant treated water you have, the demand it can replace, what each cubic metre of fresh water costs you and what reuse adds — the tool gives the annual net saving and, when that is positive, a simple payback.
What this tool estimates
The treated water you can actually reuse — the smaller of compliant supply and matching demand — over your operating days, the annual net saving after incremental treatment, pumping and fixed O&M, and a simple payback on the capital cost when that saving is positive.
What it cannot decide
- Whether the treated water meets the quality each reuse needs, or whether the reuse is permitted — those are prerequisites, not outputs.
- What fresh water really costs you at the margin: enter only costs that actually stop.
- The design or cost of tertiary treatment, storage and separate (dual) piping.
- Tariff escalation, financing, tax or equipment life — the payback is simple and undiscounted.
Result
As entered, reuse costs more each year than it saves. That is the result, not an error: check the avoided cost (only costs that actually stop), the incremental cost and the volume before deciding.
| Step | Value |
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| Change | Annual net saving | Payback |
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Quality and permission come first: reuse only water treated to the standard its use needs, as your consent and the authority allow. The figures here assume that is settled. Find the sewage flow with the STP hydraulic load calculator and the site's demand with the water demand 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
Whether reusing treated water pays: the volume that can actually be reused — the smaller of the compliant treated water available and the demand it can replace — over the operating days, the annual net saving after incremental treatment, pumping and fixed O&M, and a simple payback on the capital cost when that saving is positive.
Inputs
- Compliant treated water available per day — measured output that already meets the quality the reuse needs, not the plant's rating.
- Matching reuse demand per day — the fresh-water use it can replace: flushing, landscaping, cooling-tower make-up, washdown.
- Operating days per year on which both exist.
- Avoided fresh-water cost in ₹/m³ — only costs that actually stop: the tariff per m³, tanker charges, or borewell pumping and treatment.
- Incremental treatment and pumping cost in ₹/m³ — what reuse adds, not the cost of running an STP the site needs anyway.
- Fixed O&M added by reuse in ₹ a year, and optionally the capital cost of the reuse system for a payback.
Method
- Water reused (m³/day) = the smaller of the treated water available and the matching demand. The surplus treated water and any demand still on fresh water are shown, so reused + surplus = available and reused + unmet = demand.
- Water reused per year = reused per day × operating days.
- Annual net saving = water reused per year × (avoided cost − incremental cost per m³) − fixed O&M. A negative result is shown as a loss.
- Simple payback = capital cost ÷ annual net saving, undiscounted — shown only when the saving is positive; otherwise there is no payback.
- Break-even avoided cost = incremental cost + fixed O&M ÷ water reused per year.
- Sensitivity: the net saving and payback are recomputed with the avoided cost, the incremental cost and the fixed O&M each ±20%, and with 20% less water reused.
Assumptions
- Quality and permission are settled before this calculation: the treated water meets the standard each reuse needs and the reuse is allowed under the consent. The tool cannot check either.
- Every cubic metre reused displaces one cubic metre of fresh water at the avoided cost entered. Fixed charges on a water bill do not fall with volume and belong outside that rate.
- No cost is counted twice. If the STP must run to meet its discharge consent anyway, its running cost is not a cost of reuse; only the polishing, disinfection, storage and pumping that reuse adds are.
- Costs are level from year to year — no tariff escalation, financing, tax or replacement is modelled.
Limitations
- Does not design or price the tertiary treatment, storage, pumps or the separate (dual) piping the reuse needs — enter their capital cost if you have it.
- Takes the treated water and demand as daily averages over the operating days; where they do not coincide in time, storage is needed, and seasonal demand (landscaping, cooling towers) changes the volume reused.
- Says nothing about the quality standard for any reuse; that comes from the consent and the authority, and the reuse point's own needs (cooling-tower make-up has to suit the equipment).
- A simple payback is not a project appraisal: with long lives, high capital cost or escalating tariffs, a discounted analysis should replace it.
Worked example
Produced by running this calculator with the inputs below.
Inputs
- Compliant treated water available 10 m³/day; matching reuse demand 8 m³/day
- 300 operating days a year
- Avoided cost ₹100/m³; incremental treatment and pumping ₹30/m³
- Fixed O&M added by reuse ₹18,000 a year; capital cost ₹6,00,000
Working
- Water reused = smaller of 10 and 8 = 8 m³/day — limited by the matching demand; 2 m³/day of treated water is left over.
- Water reused per year = 8 × 300 = 2,400 m³.
- Annual net saving = 2,400 × (₹100 − ₹30) − ₹18,000 = ₹1,68,000 − ₹18,000 = ₹1,50,000.
- Simple payback = ₹6,00,000 ÷ ₹1,50,000 = 4 years.
- Break-even avoided cost = ₹30 + ₹18,000 ÷ 2,400 = ₹37.5/m³ — below that, reuse costs more than it saves.
Result. ₹1,50,000 (₹1.5 lakh) a year net, paying back ₹6,00,000 in 4 years, undiscounted — provided the treated water is fit and permitted for the reuse.
Sensitivity — what moves the answer
- The avoided cost moves the answer most: 20% lower (₹80/m³) cuts the saving to ₹1,02,000 and stretches the payback to 5.9 years; 20% higher (₹120/m³) gives ₹1,98,000 and 3 years.
- Reusing 20% less water than planned — a wet season for landscaping, a cooling tower off — gives ₹1,16,400 and 5.2 years. Treated water beyond the matching demand adds nothing: at 50 m³/day available the saving is still ₹1,50,000.
- The incremental cost matters less here: 20% higher (₹36/m³) gives ₹1,35,600 and 4.4 years; fixed O&M 20% higher gives ₹1,46,400 and 4.1 years.
How engineers use the result
As the first test of a reuse idea, before dual piping is drawn: if the saving survives a lower avoided cost and less volume, it is worth a water balance and a treatment design; if it is marginal, the money may be better spent elsewhere — unless the consent requires reuse anyway, in which case the question is how, not whether.
When a professional design must replace it
Before committing capital: confirm the treated-water quality from test results against what each reuse needs, the permission under the consent, the volumes from metered data across the seasons, the avoided cost from actual bills or invoices, and the capital cost from a design — then appraise it with discounting if the life and cost are large.
Sources
- CPHEEO (Ministry of Urban Development), Manual on Sewerage and Sewage Treatment Systems, 2013 — Part A: Engineering, Appendices — Appendix A 7.3, case studies in recycling and reuse of sewage, discusses reuse for toilet flushing and the separate piping and treatment it needs; cited for context — no figure from it is used
- Annual net saving = volume × (avoided − incremental cost) − fixed O&M; simple payback = capital cost ÷ annual net saving — arithmetic; no published rate or benchmark is applied
Questions people ask before using it
What information do I need before using the reuse savings calculator?
The treated water you can reuse per day — the plant's measured output that meets the quality the reuse needs, not its rated capacity — and the demand it can replace per day: flushing, landscaping, cooling-tower make-up, washdown. Then the days a year both exist, what each m³ of fresh water costs you at the margin (the tariff, tankers, or borewell pumping and treatment), the extra treatment and pumping cost per m³, the fixed O&M that reuse adds, and the capital cost if you want a payback.
Which site conditions could change the reuse saving?
Seasons — landscaping needs little in the monsoon and cooling-tower make-up peaks in summer; the storage needed when treated water and demand do not coincide in time; quality excursions that send treated water to drain; whether the fresh-water bill actually falls (fixed charges do not); tariff changes; and consent conditions that may already require reuse, in which case running the STP is not a cost of reuse.
What should I send an engineer for a reuse review?
The site water balance with meter readings, the STP's treated-water test results and its consent conditions, the reuse points with the quality each needs, recent fresh-water bills or tanker invoices, the existing plumbing — whether flushing is already on a separate line — and where treatment, storage and pumps could sit.
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