INDUSTRIES / COMMERCIAL BUILDINGS AND AIRPORTS
STPs and water recycling for commercial buildings
so the chillers run on recycled water.
In an air-conditioned mall, office tower or IT park, the cooling towers usually drink more water than the people do. We design, build and operate STPs that feed treated water back to cooling towers and flushing, treat the cooling water itself, and measure every litre, with tests from our own NABL-accredited laboratory.
In short
What a commercial building needs from its water systems.
An STP for a commercial building treats sewage from toilets, food courts and pantries so it can be reused for toilet flushing, cooling tower make-up and landscaping through a separate, colour-coded pipe network (dual plumbing). In malls, offices and IT parks, HVAC cooling towers are usually the largest single water use, so recycling STP water into them gives the biggest saving. A typical scheme combines screening, an equalisation tank, a compact MBBR, SBR or MBR stage, filtration, disinfection and, for cooling towers, softening. Under the EIA Notification, 2006, large building projects need environmental clearance, which normally requires an on-site STP, reuse and rainwater harvesting.
- 45L/person/day
- National Building Code water demand for offices, used to size office STPs
- 20,000m²
- Built-up area from which building projects need environmental clearance under the EIA Notification, 2006
- 1.5kW
- Fan power of an RCD cooling tower for 200 TR, against about 7.5 kW for a conventional tower
- NABL
- Accredited laboratory for STP, cooling tower and drinking water tests
Where the water goes
Where does a commercial building use water?
A commercial building uses most of its water for things other than drinking and bathing.
- HVAC cooling towers: water evaporates to carry away heat from the chillers, and some is bled off (blowdown) to stop salts building up. In centrally air-conditioned buildings this is often the largest single use, especially in summer.
- Toilets and urinals: the largest sewage source; flushing alone is a big share of domestic use and can run entirely on recycled water.
- Food courts, restaurants and pantries: greasy wastewater that needs grease traps before the STP.
- Landscaping: lawns, podium gardens and green walls; large in IT parks and campuses.
- Housekeeping, car wash and parking: floor washing, basement cleaning and vehicle wash bays.
- Drinking water RO and softeners: RO reject and softener regeneration water, high in salts.
Sewage and greywater
Blackwater (from toilets) and greywater (from wash basins, pantries and showers) usually go to one STP in commercial buildings. Greywater alone is weaker and easier to treat, so some green buildings collect it separately for a smaller recycling plant, but a single well-run STP is simpler to operate for most owners.
How much sewage will the building produce?
The National Building Code figure for offices is about 45 litres per person per day, of which a large part is flushing. Malls, food courts, multiplexes and hotels inside a complex add much more. A 10,000-seat IT campus therefore produces roughly 350 to 450 KLD of sewage, but its cooling towers may need a similar volume again as make-up water. We size from metered use and footfall data where the building already runs.
| Stream / parameter | Typical range | Why it matters |
|---|---|---|
| Sewage BOD | 150 to 350 mg/L (higher with food courts) | Sets the size of the biological stage; office sewage is concentrated because little bathing water dilutes it |
| Sewage COD | 300 to 700 mg/L | Total organic load, including slowly biodegradable matter |
| Sewage TSS | 150 to 400 mg/L | Must be very low before reuse in flush valves and cooling towers |
| Oil and grease | 20 to 50 mg/L after traps; far higher from food courts | Grease blinds MBBR media and membranes and causes odour |
| Cooling tower blowdown TDS | Typically 3 to 5 times the make-up water TDS | Too high and the tower scales; too low and water is wasted |
| Raw water hardness | 200 to 600 mg/L as CaCO₃ in many borewells | Scale in condensers and cooling towers raises chiller power |
Rules and standards
What rules apply to commercial building STPs?
Four sets of rules shape a commercial building's water systems: environmental clearance for large projects, the SPCB consent, groundwater rules and local building bye-laws. The outlet standard is set by the consent, usually based on the MoEF&CC STP notification of 2017 or the stricter values recommended by the NGT in 2019.
| Parameter | 2017 notification: metro cities and most state capitals | 2017 notification: other areas | NGT-recommended values (2019) |
|---|---|---|---|
| pH | 6.5 to 9.0 | 6.5 to 9.0 | 5.5 to 9.0 |
| BOD | 20 | 30 | 10 |
| Total suspended solids | Below 50 | Below 100 | 20 |
| COD | Not specified | Not specified | 50 |
| Total nitrogen / total phosphorus | Not specified | Not specified | 10 / 1.0 |
| Faecal coliform (MPN/100 mL) | Below 1,000 | Below 1,000 | Below 230 (desirable below 100) |
Treatment
How a commercial building STP and reuse system works.
The aim is not just to meet the outlet limit, but to make water clean enough that tenants accept it in toilets and the chiller plant accepts it in the towers.
| Stage | What happens |
|---|---|
| 1. Grease traps at food courts and kitchens | Sized on covers served, emptied on a fixed schedule written into tenant agreements. |
| 2. Screening and equalisation | Bar screens catch wipes, cups and plastics. The equalisation tank holds the weekday daytime peak so the STP runs evenly through the night and weekend. |
| 3. Biological treatment | MBBR for most buildings, SBR for campuses, MBR for tight basements and high reuse targets. An anoxic zone removes nitrogen. |
| 4. Clarification | Tube settler or clarifier; not needed with SBR or MBR. |
| 5. Tertiary treatment | Pressure sand and activated carbon filters, sometimes ultrafiltration, for clear, odour-free water. |
| 6. Disinfection | UV and a small chlorine residual to keep the flushing network free of slime. |
| 7. Softening for cooling towers | Treated water is softened, or partly passed through RO, so the towers can run at higher cycles of concentration without scale. |
| 8. Recycled water storage and distribution | A separate tank and pumps feed the flushing risers, cooling tower make-up and irrigation lines, with flow meters on each. |
| Option | Best for | Footprint | Watch-outs |
|---|---|---|---|
| MBBR + filters + UV | Office towers and malls with basement plant rooms | Small | Carrier screens and grease control |
| SBR | IT parks and campuses with strong weekday/weekend swings | Medium | PLC and level sensors must be reliable |
| MBR | Premium buildings with indoor reuse and little space | Smallest | Higher power and membrane care |
| BIO-STP (nature-based) | Campuses and business parks with open land | Larger | Needs area; very low power, no blowers |
BIO-STP is Rispri's nature-based sewage treatment: water flows by gravity through planted beds instead of being aerated by blowers, so power use is a fraction of a conventional STP. It suits campuses and townships with landscaped area to spare.
Our NABL-accredited laboratory tests STP outlet, cooling tower water and drinking water: BOD, COD, TSS, ammonia, hardness, TDS, coliforms and Legionella. You get one report for the facility manager, the board and the green building assessor.
Reuse
How much water can a mall or IT park recycle?
In a well-designed commercial building, recycled water can meet most non-potable demand: flushing, cooling tower make-up, landscaping and car washing. Since these uses often add up to more than half of total demand, a building with a good STP and dual plumbing can cut its fresh water purchase substantially.
Cooling tower make-up: the biggest opportunity
A cooling tower loses water mainly by evaporation, roughly 1% of the circulating flow for every 5 to 6 °C of cooling. In practice, make-up for a chiller plant at full load is in the order of 7 to 9 litres per hour per TR (ton of refrigeration), including blowdown. A 1,000 TR plant running 12 hours a day at an average 70% load therefore needs about 60 to 75 KLD of make-up, all of which can come from softened STP water.
Cycles of concentration
The more times water circulates before it is bled off (the cycles of concentration, COC), the less make-up is needed. Raising COC from 3 to 5 typically cuts blowdown by more than half. Doing so safely needs soft make-up water and a sound scale, corrosion and biocide programme.
Lower cooling energy too
Rispri's RCD cooling towers use rotating disks rather than packed fill, so the fan works against very little resistance: about 1.5 kW of fan power for a 200 TR tower, against about 7.5 kW for a conventional tower, with no droplet drift. See low-energy cooling towers.
Airports
Airport water and wastewater management.
An airport is a commercial complex, a light industrial site and a large paved catchment in one. New airports and terminal expansions need environmental clearance under the EIA Notification, 2006, and each stream needs its own answer.
| Stream | What it contains | Usual treatment |
|---|---|---|
| Terminal sewage | Toilets, food and beverage outlets, lounges; strong daily and seasonal peaks | Large STP (SBR or MBBR) with tertiary treatment and reuse for cooling towers and landscaping |
| Aircraft toilet waste | Concentrated waste with toilet fluid chemicals, emptied at a disposal point | Screened and dosed gradually into the STP so it does not shock the biology |
| Flight kitchen effluent | High fat, oil and organic load | Grease traps, oil skimming, then the STP or a dedicated pre-treatment unit |
| Aircraft and vehicle washing | Detergents, oil, metals and grit | Oil-water separation, coagulation and filtration, with wash water recycled |
| Fuel farm and hangar oily water | Jet fuel, lubricants, hydraulic fluid | Oil interceptors and separators, then oily wastewater treatment |
| Apron and runway stormwater | Rubber, oil and solids in the first flush | Oil and grit interceptors, detention ponds, recharge or harvesting |
Cost
How much does an STP for a commercial building cost?
Cost depends on flow, the reuse quality needed, basement or open siting and how much is automated.
| Sewage flow | Typical building | Indicative STP cost with reuse treatment* |
|---|---|---|
| 25 to 50 KLD | Mid-size office building or small mall | ₹10 to 22 lakh |
| 100 to 200 KLD | Large mall, office tower, small IT park | ₹25 to 60 lakh |
| 300 to 500 KLD | IT campus, mixed-use complex | ₹60 lakh to ₹1.5 crore |
| 1 MLD and above | Business park, airport terminal | ₹1.2 crore and upwards |
*Indicative 2026 ranges for MBBR or SBR plants with filtration, disinfection and softening for cooling tower reuse: equipment, electricals and installation, excluding civil tanks, land, taxes and the dual plumbing network. MBR typically adds 30 to 60%. Running cost is usually ₹8 to 20 per kilolitre, which is often less than tanker or municipal water. We give a firm price after a site visit and, for running buildings, a water audit and sewage test.
What drives capital cost
- Flow and weekday peak
- MBBR, SBR, MBR or BIO-STP
- Basement siting, ventilation and odour control
- Softening or RO for cooling tower reuse
- Meters, PLC and online monitoring
We audit the water balance, fix the STP's tertiary stage, add softening and connect recycled water to cooling towers and flushing, then run it under an O&M contract with online monitoring.
Troubleshooting
Common problems in commercial building water systems.
- 01The problem
Tenants complain that flush water is yellow or smells, so recycling is switched off.
What we doRepair the settler, replace sand and carbon media, set UV and chlorine correctly and track turbidity daily.
- 02The problem
Cooling towers scale up when fed with STP water.
What we doAdd softening, control cycles of concentration on conductivity, and adjust the inhibitor programme to the recycled water's chemistry.
- 03The problem
The STP is starved at weekends and overloaded on weekdays.
What we doUse the equalisation tank to even the week, run blowers on dissolved oxygen control with variable speed drives, and recirculate at low flow.
- 04The problem
Food court grease blocks pumps and media.
What we doResize traps, put emptying schedules into tenant contracts and add oil skimming in equalisation.
- 05The problem
Nobody knows where the water goes, and the CGWA or board asks for records.
What we doInstall meters on sources, STP and reuse lines, connect them to a dashboard and telemetry, and produce monthly water balance reports.
What we offer
Water services for commercial buildings and airports.
STPs and recycling
MBBR, SBR, MBR and BIO-STP plants with tertiary treatment for flushing and cooling tower reuse.
STP solutions →Smart metering and OCEMS
Flow meters, CGWA telemetry, online analysers and dashboards for the water balance.
Smart water →Cooling tower treatment
Scale and corrosion inhibitors, biocides and RO chemicals, with dosing and monitoring.
Chemicals →Softeners
Softeners for cooling tower make-up, kitchens and hot water systems.
Water softeners →Water and Legionella testing
NABL-accredited tests for STP outlet, cooling towers and drinking water.
Microbiology →Waste management
Organic waste and sludge handling for malls, food courts and campuses.
Waste management →Buying guide
What to ask before you buy a commercial building STP.
Use this list with any supplier, including us.
- Is there a full water balance, including cooling towers, not just a sewage figure?
- Which outlet values are guaranteed in writing, and to which standard?
- Is the treated water fit for cooling towers, with softening or RO where needed?
- How does the plant cope with weekends and holidays?
- What is the power use per day, and are blowers on variable speed?
- Are flows metered at the inlet, outlet and each reuse line?
- How are odour and noise controlled in the basement or near parking?
- Who operates it and who tests it? Ask for trained operators and NABL-accredited reports.
Why Rispri
STP, cooling water and metering from one team.
Our own NABL lab
Tests on sewage, recycled water, cooling towers and drinking water, so design and compliance reports come from one place.
Engineering since 1999
1,498+ plants and turnkey projects and 499+ clients across industries, hotels, hospitals and cities.
Low-energy technologies
Field-tested BIO-STP and RCD cooling towers that cut blower and fan power. We work in collaboration with IIT Bombay, IIT Kharagpur and ICT Mumbai.
Operate and maintain
O&M contracts with trained operators, chemicals and monthly reports, from offices in Jaipur, Agra and Ahmedabad.
Questions
Commercial building STP and water recycling FAQs.
Is an STP mandatory for a commercial building?
Building projects of 20,000 m² built-up area and above need environmental clearance under the EIA Notification, 2006, which normally requires an on-site STP and reuse. Smaller buildings may need one under state bye-laws, development authority rules or the SPCB consent, especially without a sewer connection.
What size STP does an office building need?
Start from the National Building Code figure of about 45 litres per person per day for offices, take 80 to 90% as sewage, and add food courts and other uses. An office with 2,000 occupants produces roughly 72 to 81 KLD before food courts; metered data from a running building is better.
Can STP water be used in cooling towers?
Yes, and it is usually the best use of it. The water must be well filtered and disinfected, and normally softened so the towers can run at higher cycles of concentration without scale. A matching inhibitor and biocide programme is essential.
How much water does a cooling tower use?
Roughly 7 to 9 litres per hour per TR at full load, including blowdown. A 1,000 TR plant at average 70% load for 12 hours a day needs about 60 to 75 KLD of make-up.
What is dual plumbing?
Dual plumbing is a second, separate pipe network that carries recycled water to toilets, cooling towers and gardens. It is colour-coded and never connected to drinking water, and it is far cheaper to install during construction.
How much does an STP for a mall or IT park cost?
As an indication, a 100 to 200 KLD STP with reuse treatment costs about ₹25 to 60 lakh, and 300 to 500 KLD about ₹60 lakh to ₹1.5 crore, for equipment and installation excluding civil work. We quote after a site visit and water audit.
Do commercial buildings need a CGWA NOC for borewells?
Generally yes. Under the CGWA guidelines of 2020, commercial and infrastructure users of groundwater need a no-objection certificate, with flow meters and often telemetry, subject to state rules. Recycling sewage reduces the groundwater you need.
Is rainwater harvesting compulsory for commercial buildings?
In most states, yes, above a set plot size under the building bye-laws, which follow the MoHUA Model Building Bye-Laws, 2016. Environmental clearance conditions also require it for large projects.
What is a water audit and why do we need one?
A water audit measures where water comes from and where it goes in the building, then finds losses and reuse opportunities. It is the basis for cutting tanker bills, CGWA compliance and IGBC or GRIHA water credits.
How do airports treat wastewater?
Terminal sewage goes to a large STP with reuse, aircraft toilet waste is dosed into it gradually, flight kitchen water is de-greased, wash bays recycle their water, and fuel farm and apron runoff passes through oil interceptors before discharge or recharge.
Do you operate STPs in commercial buildings?
Yes. We run STPs under annual O&M contracts with trained operators, chemicals, scheduled maintenance and monthly NABL-accredited test reports, and take over plants built by others.
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