When a resident raises a flush water smell at a committee meeting, the reply is often some version of “it’s treated sewage, what did you expect?”
That reply is wrong, and it is worth knowing why.
Treated water reused for flushing in India has a prescribed quality. Two documents set it, and they carry very different weight. Start with the heavier one.
The national code puts it in mandatory language. Water supplied separately for a purpose other than drinking, which is exactly what a flushing line is, carries a quality requirement of its own:
NBCS 2026 (SP 7:2026), Part E ‘Plumbing Services’, Section 1, clause 4.2.3, printed page 20:
“For purposes other than drinking, water if supplied separately, shall be absolutely safe from bacteriological contamination so as to ensure that there is no danger to the health of the users due to such contaminants.”
The same sentence, punctuated very slightly differently, appears in NBC 2016, Part 9, Section 1, clause 4.2.3, printed page 13. Note the verb. It is shall, not should. The first sentence of the clause sets no numbers (it is a qualitative requirement, and you cannot take a reading against it), but it is not a suggestion, and it is not confined to drinking water. A second paragraph adds advisory temperatures to control legionella (cold water not above 20 °C, hot water at least 50 °C), but these are “advisable”, not “shall”. Whether a national code binds your building is a separate question, and we deal with it honestly in the note at the end of this article.
The measurable parameters come from a softer document, and odour is one of them. CPHEEO’s recommended norms of treated sewage quality for specified activities at point of use, published by the Central Pollution Control Board as Annexure I of its Guidelines for Reuse of Treated Sewage in Reference to Item of Circular Economy (February 2024), printed page 19, cover treated water reused for flushing. The odour entry (which applies across every reuse activity in the table, flushing included) reads: “Aseptic which means not specific and no foul odour.”
So a resident reporting a flush water smell is not being precious. They are reporting a parameter that has failed.
The useful question is which failure. There are six realistic causes, they cost wildly different amounts to fix, and two free tests will get you most of the way to the answer before you spend anything.
Test one: does the water smell, or does the room smell?
This matters more than anything else in this article, because many “STP smell” complaints turn out not to be STP problems at all.
Draw a bucket of water from the flush supply: the cistern inlet, or the flush tap if you have one. Carry it outside into open air. Let it stand ten minutes. Then smell it beside a bucket of fresh water from the tap.
- The water smells → it is a treatment or storage problem. Go to the causes below.
- The water doesn’t smell, but the bathroom does → it is a plumbing problem. The smell is sewer gas entering the room, and your STP is innocent.
To confirm the second: pour two litres of water into every floor trap and every unused WC in the affected flats, and check again after 24 hours. If the smell clears, you had dry trap seals: the water in the trap had simply evaporated, opening a direct path from the drain into the room.
This is extremely common in flats that stand empty for weeks, in servant toilets, and in second bathrooms nobody uses. It costs nothing to fix and it is misdiagnosed as an STP failure constantly.
It is worth knowing how little water we are talking about. There is a specified depth of water that is supposed to be standing in that trap, and BIS states it:
IS 5329:1983, Code of Practice for Sanitary Pipe Work Above Ground for Buildings, Table 2, printed page 14: Depth of water seal: water closets 50 mm, floor traps 50 mm, for all three of the plumbing systems the standard covers.
Fifty millimetres. That is the entire barrier between the drain and the bathroom, and in an unused bathroom in a Bengaluru summer it does not take long to go. Two litres of water puts it back.
A note on which clause to quote, because it matters if you are quoting it at a contractor. The same 50 mm also appears at clause 6.9.2, item (c), but clause 6.9 is headed “Design Data for Single Stack Plumbing”, so on its own terms it governs single-stack systems only. Table 2 is the one that gives the figure across all three systems, so Table 2 is what to cite unless you know your building is single-stack. The values in that clause are set in a two-column table on the page and do not read as a continuous sentence; anyone reproducing them should copy the table, not a run-on line.
Test two: measure the residual chlorine at the cistern
If the water itself smells, there is one number that will usually explain everything downstream of it.
The CPHEEO norms recommend a minimum residual chlorine of 1 mg/L in water reused for flushing, measured, again, at the point of use.
Two things to be clear about. These are recommended norms, not notified law; CPCB’s own guideline says of them that “User-based reuse standards are provided in CPHEEO manual 2013, which needs to be revisited.” And on provenance: we cite CPCB’s published reproduction, which we hold and have opened at page 19. The norms originate in CPHEEO’s Manual on Sewerage and Sewage Treatment (2013), which we have not seen at source. With those caveats, this remains the only Indian reference we have found that sets flush-water quality where the water is actually used.
A DPD pocket colorimeter costs very little and gives a reading in under a minute. Take it at three places: the STP outlet, the treated-water holding tank, and a cistern on an upper floor.
- Zero at all three → the dosing system has failed. Check the pump, the hypochlorite stock, and the purchase records.
- A value at the outlet, zero at the cistern → the chlorine is decaying in storage. Your water is being held too long. This is the diagnosis almost nobody reaches, because almost nobody samples at the cistern.
That second pattern is worth dwelling on, because it explains a situation many committees find baffling: the operator’s test results are fine every month, and residents keep complaining. Both are true. The STP outlet is where the water is at its best. The cistern is where it is used.
The six causes, ranked
A note on the ranking: no Indian dataset ranks these by frequency. This ordering is engineering judgement, not a published statistic.
1. Under-aeration and septicity in the STP
The most common cause in Indian apartments, and it has a very specific and very human origin.
An activated sludge STP needs its blowers running to keep oxygen in the aeration tank. When the blowers stop, the biology goes anaerobic, sulphate-reducing bacteria take over, and hydrogen sulphide is generated. That is the rotten-egg smell.
And the plant gets switched off for two predictable reasons. Citizen Matters’ survey of Bengaluru apartment STPs found residents themselves switching plants off at night (sometimes to stop the blower noise, sometimes to save electricity) and reported that this is what makes them stink.
Which means a complex can generate its own odour complaint by acting on a noise complaint, and then treat the two as unrelated problems handled by different contractors. They are the same problem.
How to check it: read the blower run-hour meter, or the STP’s energy meter, for the last thirty days. Not the logbook, the meter. This is where “the blowers run 24×7” is confirmed or disproved.
2. Sludge carryover into the treated-water tank
Where the clarifier geometry is wrong, or sludge is not wasted often enough, sludge is carried over into the final treated-water tank, and then decays anaerobically in your flush water storage.
This is the one defect that produces odour, discoloration, and the organic sediment that feeds larvae, all at once. If your complex has both a flush water smell and a worms complaint, look here first.
3. No effective disinfection
The dosing pump has failed, or there is no chlorine contact tank, or the contact tank short-circuits so that its nominal detention time is fictional: water enters and leaves in a fraction of the time the design assumed.
Check whether hypochlorite has actually been purchased in the last three months. Procurement records are harder to fudge than a logbook.
4. The water is stored too long
Flush demand is roughly constant year-round. Landscape demand collapses in the monsoon. If your treated water is sized to serve both, then during the rains the holding tank turns over slowly, the residual chlorine decays, and the water goes septic in the tank rather than in the STP.
This is why some complexes have a flush water smell that appears every monsoon and vanishes in summer, and it is why the outlet sample keeps coming back clean.
We have not found an Indian standard setting a maximum storage time for treated water before reuse. That appears to be a genuine gap, and it means this cause has to be reasoned about rather than looked up.
One qualification, because we would rather state it than be caught by it. BIS published a water-reuse series in 2021 (IS 17662 and IS 17663), and we hold neither. IS 17662 is titled Guidelines for Water Quality Grade Classification for Water Reuse. We cannot tell you what is in a standard we have not read, so take the paragraph above as what we have been able to find, not as a survey of everything BIS has published.
The nearest provision in the national code runs the other way: it sets a minimum volume, not a maximum age:
NBCS 2026 (SP 7:2026), Part E ‘Plumbing Services’, Section 1, clause 4.4.10(3), printed page 23:
“in case of sewage treatment plant, for treated water storage in respect of flushing, one-day flushing demand shall be stored in UGT or treated effluent storage tank, which is part of the plant, and half a day demand shall be stored in OHT.”
Read that carefully, because it is easy to misread as filling the gap. It tells you how much treated water you must be able to hold. It says nothing about how long water may sit before it is used, nothing about residual chlorine at the cistern, and nothing about what to do when landscape demand collapses in the monsoon and the tank stops turning over. A complex can comply with clause 4.4.10(3) exactly and still send septic water to a flat. The gap stands.
5. Plumbing: trap seals and venting
Covered in test one above, but worth listing properly among the causes because it is so often the answer.
What a trap is actually for, in the words of the BIS handbook on the subject:
SP 35 (S&T) : 1987, Handbook on Water Supply and Drainage (with Special Emphasis on Plumbing), clause 6.4.6, printed page 108:
“A trap is a device to prevent the passage of air, odours and vermin through it from sewers escaping through a plumbing fixture into the building.”
SP 35 is a handbook (guidance), not a mandatory standard. We cite it because it explains the mechanism better than anything else we hold, not because it binds anybody. One point of precision: IS 5329, which is a standard, describes the water seal as a barrier to the passage of air. The “vermin” in that sentence is SP 35’s word, not IS 5329’s, and if you are quoting it at a contractor, quote it from SP 35.
The same clause sets out the four ways a seal is lost, and the remedy for two of them:
“Traps are useless unless they retain their seals at all times. The seals may be broken in the following ways: a) siphonage, b) air compression, c) momentum, and d) evaporation. The remedy for (a) and (b) is adequate means of trap ventilation.”
That list maps onto what you will actually find in a complex:
- Dry trap seals: that is (d), evaporation, in unused fixtures. The 50 mm of water specified in IS 5329 clause 6.9.2, item (c) has simply gone.
- Induced siphonage: that is (a). A discharge elsewhere on the stack pulls the seal out of a neighbouring trap. IS 5329 defines it exactly: “Induced Siphonage – The extraction of water from a trap by siphonage set up by reduction of pressure at the outlet of the trap.” (IS 5329:1983, clause 2.14, printed page 5.) The signature is a gurgle, followed later by a smell.
- Blocked, undersized or missing vent stack, a broken gully trap, or a cistern overflow discharging into a waste line. This is the one worth reading SP 35 twice on: the remedy it gives for siphonage and air compression is trap ventilation. So a venting defect and a siphonage complaint are not two problems. If the smell comes back within a day of pouring water into the traps, stop looking at the traps and look at the vent stack.
None of these have anything to do with the STP, and all of them are routinely blamed on it.
6. Cross-connection between a sewage line and the flush line
Rare, and the most serious. The signature is different from the others: visible solids, a sewage smell rather than a sulphide smell, high ammonia, and high faecal coliform.
Three clauses of the current national code bear on this, and it pays to be precise about which one says what.
First, the permission itself is conditional. Using treated wastewater for flushing in a residence is allowed only if provision is made against cross-connection:
NBCS 2026 (SP 7:2026), Part E ‘Plumbing Services’, Section 1, clause 4.2.4 ‘Waste Water Reclamation’, printed page 21:
“This supply system shall be allowed in residences only if proper provision is made to avoid any cross-connection of this treated waste water with domestic water supply system for which it is recommended to use dual piping system.”
Two different strengths in one sentence. The provision against cross-connection is a condition of being allowed to run the system at all. Dual piping is recommended as the way to achieve it: recommended, not required.
A second BIS standard is far less equivocal, and it is the one written for multi-storeyed buildings:
IS 12183 (Part 1) : 1987, Code of Practice for Plumbing in Multi-Storeyed Buildings, Part 1 Water Supply, clause 8.3 ‘Dual Supply’, item (c), printed page 15:
“Wherever systems using recycled treated water is used in a .building for flushing or air-conditioning purposes, the entire system of storage, pumping, rising main and distribution system shall be separate and independent of the domestic supply system.”
(The stray full stop and the grammar are in the printed standard; we quote exactly.) That is a “shall” in a current standard, and it reaches further than NBCS: storage, pumping, rising main and distribution, all separate and independent. A shared sump or a shared pump does not satisfy it.
A note on the older edition, because the wording changed. NBC 2016’s clause 4.2.4 carried an extra sentence: “During use of treated waste water, it is recommended to have dual piping system to avoid cross-contamination.” (NBC 2016, Part 9, Section 1, clause 4.2.4, printed page 13.) NBCS 2026 deleted the words “to avoid cross-contamination” from clause 4.2.4, and BIS withdrew NBC 2016 on 30 April 2026. So do not attach the cross-contamination rationale to 4.2.4 in the current code. It did not vanish; it moved, and it got stronger.
Second, physical segregation is mandatory.
NBCS 2026 (SP 7:2026), Part E ‘Plumbing Services’, Section 1, clause 4.4.11, printed page 24:
“Adequate measures and precautions shall be taken for physical segregation for sanitary purposes and to avoid cross-contamination in the distribution system and to prevent back contamination of water supply sources”
Shall. This is where the cross-contamination duty now lives, and it is a stronger verb than the “recommended” in 4.2.4.
Third, the cross-connection itself is prohibited outright.
NBCS 2026 (SP 7:2026), Part E ‘Plumbing Services’, Section 1, clause 4.5.3 ‘Cross-Connection Control’, printed page 24:
“Cross-connections shall be prohibited, except where approved back flow prevention assemblies/devices are installed to protect the potable water supply”
So a flush line found joined to the domestic line is not untidy plumbing to be tidied up at the next AMC renewal. It is the thing the code prohibits.
If the other five causes have been excluded, this needs a dye or tracer test, not another water sample.
The safety point that actually matters
Hydrogen sulphide has an odour threshold around 0.01 ppm. People smell it far, far below any concentration that is hazardous. So the honest position on a smell in a bathroom is that it is a nuisance and a failed standard, not an acute health emergency, and we are not going to tell you otherwise.
The serious H₂S hazard is confined-space entry: STP wet wells, sumps and manholes. That risk falls on your operators and cleaning crews, not on residents.
And on this point the position is stronger than “it would be good practice to provide safety equipment.” Parliament has legislated to prohibit the cleaning of a sewer or a septic tank without that equipment.
The Prohibition of Employment as Manual Scavengers and their Rehabilitation Act, 2013 defines the term at section 2(1)(d):
“‘hazardous cleaning’ by an employee, in relation to a sewer or septic tank, means its manual cleaning by such employee without the employer fulfilling his obligations to provide protective gear and other cleaning devices and ensuring observance of safety precautions, as may be prescribed or provided in any other law, for the time being in force or rules made thereunder”
And section 7 prohibits it:
“No person, local authority or any agency shall, from such date as the State Government may notify, which shall not be later than one year from the date of commencement of this Act, engage or employ, either directly or indirectly, any person for hazardous cleaning of a sewer or a septic tank.”
The section’s own marginal heading is “Prohibition of persons from engagement or employment for hazardous cleaning of sewers and septic tanks.”
Read the two together and the shape of the duty is clear. The Act does not ban cleaning a sewer or a septic tank. It bans cleaning it unprotected, and section 7 reaches anyone who engages or employs a person for it “either directly or indirectly”.
Those three words are the ones a Managing Committee should sit with. They are plainly aimed at arrangements where the worker is on somebody else’s payroll. Whether they make your association an engager in your particular case is a question for your advocate, and we are not going to answer it for you. What we will say is that a committee which assumes the answer is “no” has assumed the more dangerous of the two possibilities, and has done so without asking anybody qualified to tell it.
Two caveats, and we are not going to paper over either of them.
First, section 7 takes effect from such date as the State Government may notify. We have not established whether Karnataka, Telangana or Tamil Nadu has issued that notification. We are not going to tell you the section is in force in your State when we have not confirmed that it is. What we will say, as engineers rather than as lawyers, is this: Parliament has written down what safe entry into a sewer or a septic tank looks like (gear, devices, safety precautions actually observed), and no committee should want to explain, after a man is hurt, why it settled for less than that while it argued about commencement dates.
Second, we are engineers, not lawyers, and nothing here is legal advice. Where we quote a code, a standard or a statute, we give the clause and the printed page so you can check it against your own copy. Where we do not know something, we say so rather than round it up. Questions about what your association is liable for belong to your advocate, and questions about what your corporation will actually enforce belong to your local authority.
The Act is published in the Gazette of India, Extraordinary, Part II Section 1, No. 35, dated 19 September 2013. Both quotations above are from the Act’s own text; read the sections in full rather than taking ours as complete.
So if a Managing Committee takes one safety action after reading this, make it that one: confirm that nobody enters a wet well, sump or manhole in your complex without gas testing, forced ventilation, a harness and a standby person, and that this holds for the contractor’s men exactly as it holds for your own staff. That is where the genuine acute risk sits, and it falls squarely within the association’s responsibility.
What a committee can do this week, free
- The bucket test: water-borne or room-borne.
- Pour two litres into every floor trap and unused WC in the complaining flats; recheck in 24 hours.
- Read the blower run-hour or energy meter for 30 days and compare it to the logbook.
- Check the hypochlorite day tank and the purchase records.
- Measure residual chlorine at a cistern, not just at the STP outlet.
- Increase turnover of the treated-water tank, especially during the monsoon when landscape demand drops.
- Pull the last six months of lab reports and compare them against the flushing column of the CPHEEO norms: turbidity under 2 NTU, residual chlorine at least 1 mg/L, faecal coliform nil, no foul odour.
That last one is the exercise most committees have never done, and it frequently ends the argument on its own.
When you need someone who isn’t the operator
Everything above is diagnosis a committee can drive itself. There is one question it cannot answer alone, and it is the one that decides the money:
Is this STP mis-designed, or has it simply been mis-operated?
The answer separates a repair from a replacement, and an order of magnitude in cost. Your O&M contractor cannot answer it, because they are the party being assessed. That is not a criticism of them; it is a structural conflict, and no amount of goodwill removes it.
An independent audit answers that question, verifies the logbook against instrument data rather than against itself, samples at the point of use rather than the outlet, and checks whether the lab named on the report was actually the lab that ran the test.
The Nemmadi RWA Health Check is a periodic engineering audit of your complex’s common assets: structural condition, the full MEP set including STP, WTP, DG sets, lifts and fire systems, functionality testing of the equipment that is supposed to work when you need it, and a review of your AMC documentation. Our engineers produce a prioritised plan your committee can act on.
We are never paid to fix what we find.
Related reading: STPs in Bangalore Residential Societies: Challenges, Solutions and Best Practices
A note on the National Building Code, and which version applies to you
The Bureau of Indian Standards withdrew the National Building Code of India 2016 on 30 April 2026 and published the National Building Construction Standards 2026 (NBCS 2026) in its place. The BIS portal record for SP 7:2016, read on 7 September 2026, states: “Withdrawn, On – 30 Apr, 2026, Superseded by IS: SP 7:2026”.
But BIS withdrawing a code does not, by itself, change what your building is held to. BIS has never had the power to make the National Building Code binding. NBCS 2026 says so about itself, in its own front matter:
“As the regulation of land development and buildings is a State subject and is dealt by the States and local bodies such as urban (and rural) local bodies within their jurisdiction through building regulatory documents like building regulations, development control rules building byelaws, and fire regulations, this document is voluntary in nature and is non-binding. The implementation depends on adoption or adaptation by concerned parties or stipulations in a contract or by appropriate adoption by the concerned authorities.”
NBCS 2026 (SP 7:2026), front matter, page vii
A national code becomes enforceable only when a State or local authority writes it into its building bye-laws. And in Bengaluru, what the bye-laws point at is genuinely unresolved, and older than you would expect.
- The Bangalore Mahanagara Palike Building Bye-Laws 2003 carry this note: “All references of clauses of the National Building Code in these bye-laws is referred to the publication of the National Building Code of India 1983. If these numbers are changed in subsequent additions, corresponding numbers shall be substituted.” (page 48; “is referred to” and “additions” are as printed). That points at the 1983 edition, with a clause that carries the reference forward as editions change.
- The five city corporation amendment bye-laws gazetted on 14 May 2026 (Bengaluru Central, East, North, South and West) each amend “the Bengaluru City Corporation Building Bye-Laws, 2003”, made under the Greater Bengaluru Governance Act 2024. They mention no national code at all.
- The Karnataka Model Building Bye-Laws 2017 (Corporations notification) name NBC 2016 by name on 27 pages, for example, “All structural design/safety aspects as per latest BIS Codes & NBC 2016 shall be complied along with consideration of weight of Fire Engine & its manoeuvrings.”
Which of those instruments governs your building today is a question we cannot answer for you, and we are not going to pretend otherwise. The 2026 amendments are built on the 2003 bye-laws, which is a strong indication, but whether the 2017 Model Bye-Laws also apply to Bengaluru is an open question, and it is not ours to settle.
So we cite NBCS 2026 (it is the current national standard and the best available statement of good engineering practice), and we name NBC 2016 where the comparison matters. What we will not do is tell you that either one is what your corporation will enforce. Put that question to your local authority, and to your association’s advocate, with the bye-law in front of you.
A note on how we quote. Bold inside a quotation is our emphasis, not the original’s. Clause text, spelling and punctuation are reproduced exactly as printed, including the occasional printing error in the source, which we do not silently correct.
Sources
- Bureau of Indian Standards, portal record for SP 7:2016 (National Building Code of India 2016), read 7 September 2026: “Withdrawn, On – 30 Apr, 2026, Superseded by IS: SP 7:2026”.
- Central Pollution Control Board, Guidelines for Reuse of Treated Sewage in Reference to Item of Circular Economy (February 2024), Annexure I, printed page 19. Downloaded from cpcb.nic.in and opened. The norms originate in CPHEEO’s Manual on Sewerage and Sewage Treatment (2013), which we have not seen at source.
- IS 5329 : 1983, Code of Practice for Sanitary Pipe Work Above Ground for Buildings (Reaffirmed 2022). Bureau of Indian Standards. Clause 2.14, printed page 5; clause 6.9.2, item (c), printed page 19.
- IS 12183 (Part 1) : 1987, Code of Basic Requirements for Plumbing in Multi-Storied Buildings, Part 1 Water Supply, Bureau of Indian Standards, clause 8.3 ‘Dual Supply’, item (c), printed page 15. A standard, not a handbook, and current (reaffirmed 2019).
- Karnataka Model Building Bye-Laws 2017 (Corporations notification).
- Nath, S. & Singh, B.S. (10 June 2022). “How to fix problems with apartment STPs.” Citizen Matters. https://citizenmatters.in/how-to-fix-problems-with-apartment-stps/
- Nath, S. & Singh, B.S. (28 June 2022). “Six reasons why Bengaluru apartments are not reusing their wastewater.” Citizen Matters. https://citizenmatters.in/six-reasons-why-bengaluru-apartments-are-not-reusing-wastewater/
- National Building Code of India 2016 (SP 7:2016), Part 9 “Plumbing Services”, Section 1. Clause 4.2.3, printed page 13; clause 4.2.4, printed page 13. Withdrawn by BIS on 30 April 2026.
- National Building Construction Standards 2026 (SP 7:2026), Part E “Plumbing Services”, Section 1 “Water Supply”. Bureau of Indian Standards. Clause 4.2.3, printed page 20; clause 4.2.4, printed page 21; clause 4.4.10(3), printed page 23; clause 4.4.11, printed page 24; clause 4.5.3, printed page 24; front matter, page vii.
- National Green Tribunal, Order dated 30/04/2019 in O.A. No. 1069 of 2018.
- The Prohibition of Employment as Manual Scavengers and their Rehabilitation Act, 2013 (Act 25 of 2013), ss. 2(1)(d) and 7, quoted verbatim. Published in the Gazette of India, Extraordinary, Part II Section 1, No. 35, dated 19 September 2013.
- Schellenberg, T. et al. (2020). Frontiers in Environmental Science 8:30. https://doi.org/10.3389/fenvs.2020.00030
- SP 35 (S&T) : 1987, Handbook on Water Supply and Drainage (with Special Emphasis on Plumbing). Bureau of Indian Standards. Clause 6.4.6, printed page 108. A handbook: guidance, not a mandatory standard.
