You lift the lid of the flush tank, or you look into the bowl after a flush, and there they are. Thin red threads, maybe 5 to 20 mm long, writhing in a figure-of-eight motion. Sometimes on the walls of the cistern. Sometimes settled at the bottom.
Almost everything you will find online about this is written for an American house. It will tell you the worms are breeding in your drain, that your toilet is dirty, and that you should scrub the bowl and pour something down the trap.
In an Indian apartment complex, that advice about red worms in flush water is usually wrong, because your flush water does not come from where an American house’s flush water comes from.
The reason Indian apartments are different: dual plumbing
Most modern Indian apartment complexes are built with a dual plumbing system. There are two separate water lines running through your building:
A fresh water line, to your kitchen, your washbasin, your shower. A flush water line, to your WC cistern, and usually to landscape irrigation and car washing.
The second line is not fresh water. It is treated sewage, water that has gone through your complex’s Sewage Treatment Plant, been treated, disinfected, stored in a holding tank, and then pumped back up to your flat to flush your toilet.
This is by design. It is good design, and it is why your complex can survive a dry summer.
But it changes the diagnosis completely. If something is living in your flush water, the most likely explanation is that it arrived in the supply, travelling down from the treated-water tank, not that it crawled up from your drain.
The five-minute test that settles it
Before anyone calls a plumber, a pest control company, or the STP contractor, do this. It costs nothing and it is the single most useful thing a resident can do when they find red worms in flush water.
Lift the cistern lid and look at the water inside the tank, the water that has not yet been flushed.
| What you see | What it means | Who owns the problem |
|---|---|---|
| Larvae in the cistern water, before it reaches the bowl | The organisms arrived in the supply. The source is the treated-water holding tank, the overhead flush tank, or the STP upstream of both. | Common area. This is an association matter, not a flat-owner matter. |
| Larvae only in the bowl, typically after the toilet has stood unused for days | Likely drain-side, drain fly or moth fly larvae living in the biofilm inside the bowl and trap. | The flat. Clean the bowl and trap. The STP is innocent. |
Do this in four to six flats, on different floors and different risers, and photograph what you find.
The distribution pattern is itself a diagnostic. One flat with larvae and eleven without points to a local branch or a dead leg in that riser. Larvae across multiple risers points to the common storage tank.
Getting this backwards is the expensive mistake. It sends a Managing Committee into an STP investigation when the actual fault was a drain problem, or, the other way round, into months of bowl-scrubbing while the real defect sits untouched in a water tank on the roof.
Five things that would make this test misleading
This is a good test, not a perfect one. Before you conclude anything:
A concealed or wall-hung cistern can’t be opened. Instead, run the flush-line angle valve into a clean glass jar and let it stand. A just-flushed cistern has been swept and refilled. Check a WC that has stood for several hours. A recent tank clean or shock chlorination will suppress visible larvae for days without fixing anything. Ask the facility manager what was done and when before you draw a conclusion. Larvae come in pulses, released by backwash events and tank level changes. One clear check does not clear the system. Check which line actually feeds your cistern. Some complexes with an STP still feed cisterns from potable or borewell water, the reuse was never commissioned, or the dual piping was never completed. If that is your building, larvae in the cistern implicate the potable storage instead. That is a more serious finding, not a lesser one.
What the worms actually are
The red worms in flush water are almost always chironomid larvae, the larvae of non-biting midges. They are commonly called bloodworms, and they are red because they carry haemoglobin, which lets them survive in water with very little oxygen.
Three things about them matter for diagnosis:
They get in as eggs, laid by a flying adult. An adult midge lands on an open water surface and lays a gelatinous egg mass at the waterline. It does not need a big opening. A tank vent without an insect mesh, a cracked or ill-fitting lid, an open inspection manhole, or an unscreened overflow pipe is a doorway.
They need something to eat. Organic sediment on the floor of a tank is their food. A tank that has not been drained and physically scrubbed in a long time is a nursery.
And this is the part that surprises most committees: chlorine does not kill them, and the gap is not close.
The only published protocol that reliably clears an established chironomid population uses a shock dose of around 70 mg/L of chloramine, held for one to two hours in a partially drained tank isolated from the system, repeated a week later.
That is a completely different intervention from the 1 mg/L residual the CPHEEO norms ask you to maintain in a live flush line. Different chemical, different concentration, different method.
The residual is there to control bacteria, and it does that job. It was never a larval barrier and it was never designed to be one.
So when a complex shock-chlorinates the tank and the worms return three weeks later, that is not a mystery and it is not evidence of something exotic. It is the expected result. The control for larvae is physical, exclusion and substrate removal, not disinfection.
That single fact is why “just add more chlorine” is the most commonly repeated and least effective advice in this whole space.
The part nobody tells committees: your filters are not a barrier either
Many apartment tertiary trains run a pressure sand filter and an activated carbon filter. It is natural to assume anything living gets caught there.
One important qualification first. Where treated sewage is reused for flushing, an ultrafiltration membrane is expected in the train, and a UF membrane genuinely is an absolute barrier to a larva. So if your complex has UF, the question changes: is the membrane intact, is it bypassed at peak demand, and are the larvae entering downstream of it, in the treated-water holding tank or the overhead flush tanks? That is where this article’s own diagnostic already points.
Where the train is sand plus carbon only, which is common, neither stage is a barrier:
The published evidence says otherwise, and it is quite specific.
A granular activated carbon column was “ineffective in retaining Chironomidae larvae, leading to their escape within a short period.” A sand filter column built to the specification in common current use, effective size 0.7 mm, uniformity coefficient 1.7, “failed to act as a barrier for larvae.” Measured by larval stage: more than 80% of first-instar larvae and 3.3% of second-instar larvae passed straight through granular activated carbon. Only the later, larger instars were retained.
So the young larvae, the ones small enough to be invisible in a water sample, go through.
And there is a second implication, which is inference rather than a finding from those papers. A filter bed is dark, warm, permanently wet and organically loaded, conditions that suit larval development. Larvae hatched upstream pass through it as first instars. Larvae that hatch inside it grow there, undisturbed between backwashes, and get released in a pulse when the filter is backwashed.
So a sand-and-carbon train can be simultaneously a poor barrier and a plausible habitat. That is why backwash discipline and media replacement belong in the remedy list, and why “we have filters” is not on its own an answer to a worms complaint.
Both studies above are from drinking-water treatment in Korea, and their “widely used” sand specification is that context’s, not an Indian apartment pressure sand filter. The mechanism transfers; the exact specification may not.
Not all the worms are the same
Chironomid larvae (bloodworms), red, thread-like, 5–20 mm, cling to walls and sediment, writhe in a figure-of-eight. Tubifex and similar sludge worms, red but thinner and slower, living head-down in sediment. Their presence tells you specifically that there is an accumulated organic sludge layer in the tank. Mosquito larvae, these hang from the water surface to breathe and dart downward when disturbed, which distinguishes them instantly from chironomids that stay at the bottom and walls. This is the one finding that is a genuine public health matter. Aedes breeding in stored domestic water is well established in India, and an unscreened tank is a container like any other. Drain fly / moth fly larvae, small, pale, flattened with dark bands, living in drain slime. These are a drainage problem, not a water supply problem.
Identifying which one you have is not academic. It changes who you call and what you spend.
Is it dangerous?
Here is the honest answer, which is more useful than an alarming one.
Chironomid larvae in flush water are not a documented health hazard by that route. You are not drinking flush water.
One thing we will not skip past, because someone will raise it: chironomids are documented in the research literature as an environmental reservoir for Aeromonas and for non-epidemic Vibrio cholerae strains. That is a reservoir finding in drinking-water bodies, not a demonstration of transmission through a WC cistern, and we are not going to present it as one. What it does mean is that the organism is not inert, which is one more reason not to file a worms complaint under cosmetics. The known human health effect of chironomids is allergic, asthma and rhinitis from inhaling larval protein, and that is documented in aquarium keepers and fish-food factory workers, not in people whose cistern has larvae. We are not going to stretch that evidence to frighten you.
But the worms matter enormously as an indicator, and that is the real point.
For a larva to be in your flush tank, two things have to be true at once:
The physical containment has failed, a vent, lid, overflow or manhole is open to the outside, because that is the only way an adult midge gets in to lay. There is enough organic sediment to feed them, the tank has not been drained and scrubbed.
The first of those is not merely a housekeeping opinion. It is a code requirement, and it appears in more than one document.
NBCS 2026 (SP 7:2026), Part E ‘Plumbing Services’, Section 1 ‘Water Supply’, clause 4.4.4(f), printed page 22: “All overflow and vent pipes shall be provided with a mosquito proof brass grating to prevent ingress of mosquito, vermin and other insects”
NBCS 2026 (SP 7:2026), Part E, Section 1, clause 4.4.4(a), printed page 22: “Manholes shall be provided with locking arrangement to avoid misuse and tampering”
The older BIS code of practice for water supply inside buildings says the same about the tank itself, and adds the warning pipe:
IS 2065:1983, Code of Practice for Water Supply in Buildings, clause 11.3, printed page 26: “Every storage tank shall be of the prescribed kind and shall at all times be made and at all times be maintained watertight and shall be properly covered with a closed fitting dust, light and mosquito-proof lid fitted with a lock and key”
IS 2065:1983, clause 11.4, printed page 26: “Every tank shall be provided with an efficient mosquito-proof warning pipe.”
SP 7:2026 is a free download from the BIS portal, check the clause yourself.
Read those together and the diagnosis sharpens. An adult midge cannot lay eggs on water it cannot reach. For a larva to be in the tank, an adult got to the water surface, which means a grating, a lid, a cover or a warning pipe screen was missing, damaged or not seated. The larva is physical evidence that one of those requirements was not met on at least one opening. It is not proof of which opening, and finding that is the walk described further down.
A tank that is open to the outside and carries an unremoved sludge layer is not being protected against the things you can see. It is not being protected against the things you can’t, either, and faecal coliform is the one that actually matters.
The worm is the symptom you can see. The coliform is the one you can’t.
And there is one risk that is not indicative at all
The entire dual-plumbing arrangement rests on one thing: the flush line and the fresh water line never meeting. The national code permits the arrangement, and permits it conditionally.
NBCS 2026 (SP 7:2026), Part E ‘Plumbing Services’, Section 1 ‘Water Supply’, 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.”
Read the two halves separately, because they do not carry the same weight. The permission is conditional and mandatory in form, the system “shall be allowed in residences only if” proper provision is made against cross-connection. The dual piping is the means, and it is “recommended”, not required.
But do not stop at NBCS, because a second BIS standard is much less equivocal, and it is the one written for buildings like yours.
IS 12183 (Part 1) : 1987, Code of Basic Requirements for Plumbing in Multi-Storied 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 in “a .building”, and the grammar of “systems… is used”, are in the printed standard. We reproduce quotations exactly, including their errors.)
That is a “shall”, it is a standard rather than a handbook, it is reaffirmed and current, and it goes further than NBCS does: not merely the pipes but the entire system, storage, pumping, rising main and distribution, separate and independent. A single shared sump, a single shared pump, or a single shared riser does not satisfy it, even if the final branch pipes are separate.
If your complex reuses STP water for flushing, this is the sentence to put in front of your consultant.
The withdrawn 2016 edition, which is the edition Karnataka’s model building bye-laws still name, said nearly the same thing, with four extra words at the end:
NBC 2016 (SP 7:2016), Part 9 ‘Plumbing Services’, Section 1, clause 4.2.4, printed page 13, withdrawn by BIS on 30 April 2026; see the note at the foot of this article: “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. During use of treated waste water, it is recommended to have dual piping system to avoid cross-contamination.”
Those closing words, “to avoid cross-contamination”, were deleted from clause 4.2.4 in the 2026 edition. The reason did not disappear. It moved into a clause of its own, and it got stronger there:
NBCS 2026 (SP 7:2026), Part E, 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”
That is a “shall”. Physical segregation of the two systems is not advice in the current code; it is a duty. SP 7:2026 is a free download from the BIS portal, check the clause yourself.
That is not a hypothetical failure mode in India.
In October 2021, at a large apartment complex in the Yemalur area of Bengaluru, heavy rain flooded a basement. Around 340 children and adults became severely unwell, the figure rising over about a fortnight from some 140 cases reported by 23 October. Water testing was reported to have returned a coliform count of over 1,800, against IS 10500:2012, which requires coliform bacteria to be “Shall not be detectable in any 100 ml sample” (Table 6, clause 4.1.1, printed page 4).
Attribute the cause carefully, because the reports do not all say the same thing. The account that the sewage sump had been built directly beside the freshwater sump, and that sewage entered the freshwater system when the basement flooded, is reported as a resident’s account. The developer’s head of water management is reported as attributing the incident to 80 mm of rain falling on 23 October, and one report states plainly that the root cause was not established. We have not seen a laboratory report or a regulator’s finding, and we are not asserting one version over another.
Note what actually failed there. Not a pipe. A layout, two sumps put next to each other in a space that floods.
The code speaks to exactly that layout, though with a mix of strengths a committee should read carefully:
NBCS 2026 (SP 7:2026), Part E, Section 1, clause 4.4.7, printed page 23: “The underground tanks should not be located in low lying areas or near any public or private sewer, septic tank, leaching pool or soakage pit to prevent any contamination.”
“Care shall be taken to prevent backflow of local surface water into the tank in case of local flooding.”
“There should be no common wall between the tanks storing safe water and tanks storing water from unsafe sources.”
Where a sump sits, and whether it shares a wall with a sump holding unsafe water, are “should”, recommendations. Preventing backflow of surface water into the tank when the area floods is “shall”. So on the reported facts of that 2021 case, the part of the code with the strongest wording is the part about flooding, not the part about adjacency.
And note what the code does not give you: there is no minimum separation distance anywhere in NBCS Part E between a water tank and a sewer or a sewage sump. If someone quotes you a figure in metres, ask them where it comes from before you rely on it.
That is why a worms complaint deserves more than a tank clean:
A complex that cannot keep midges out of its treated-water tank has not demonstrated that it can keep treated sewage out of its drinking-water line. Those are the same discipline, physical separation, verified rather than assumed. The worm is free evidence that the discipline is absent.
What the code forbids, and what it will not accept as a fix
The older BIS code of practice puts the prohibition in the plainest language of any of these documents, and then closes the door a committee is most often shown as an escape:
IS 2065:1983, Code of Practice for Water Supply in Buildings, clause 7.2, printed page 14: “There shall, therefore, be no cross-connection whatsoever between a pipe or fitting for conveying or containing wholesome water and a pipe or fitting for containing impure water or water liable to contamination or of uncertain quality or water which has been used for any purpose. The provision of reflux or non-return valves or closed and sealed stop valves shall not be construed as a permissible substitute for complete absence of cross-connection.”
Treated STP effluent is, on any ordinary reading, “water which has been used for any purpose”. And the second sentence matters more than the first, because it forecloses the answer a committee is usually given when it finds a crossover between the two systems: there is a non-return valve on it. Under IS 2065, that is not a substitute for the connection not existing.
The current code is worded differently, and the difference is worth noticing rather than glossing over:
NBCS 2026 (SP 7:2026), Part E, 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”
NBCS 2026 (SP 7:2026), Part E, Section 1, clause 4.5.1, printed page 24: “The water supply system shall be designed, installed and maintained in such a manner so as to prevent contamination from non-potable liquids, solids or gases being introduced into the potable water supply system through cross-connections or any other connection to the system.”
NBCS allows a narrow exception where an approved backflow prevention assembly is installed. That is a specific engineered device with a specification and a test regime behind it, it is not the same object as the reflux or non-return valve IS 2065 rejects. We would not treat a valve found on site as satisfying either clause without establishing what it actually is, what it was approved against, and when it was last tested.
Then there is identification. The current code requires the non-potable pipework to be marked:
NBCS 2026 (SP 7:2026), Part E, Section 1, clause 4.5.4 ‘Identification of Non-potable Water Systems’, printed page 24: “Where non-potable water systems are installed, the piping conveying the non-potable water shall be identified either by colour marking, metal tags or tapes in accordance with the relevant standards and good engineering practices.”
Here we have to report a gap in the standards themselves, and it is not any building’s fault. The BIS code of practice that would tell an installer which colour treats colour-coding as conditional in the first place, clause 8.1.13, printed page 17, applies it “where it is desirable to have some means of identifying the use of the various pipes”, and its identification table, at Appendix D, D-1.1, printed page 39, carries a single water entry: “Water: Sea green”. There is no separate colour in that table for recycled, treated or non-potable water.
Followed literally, then, a flushing riser and a drinking riser in the same shaft would both be sea green, which is the opposite of what identification exists to do. So “the pipes are colour-coded” is not, by itself, an answer to this question. The useful questions are what scheme the colours were taken from, whether it distinguishes the two systems at all, and whether a plumber standing at a shaft at 11 p.m. could tell them apart.
The related checks, tracing both lines from source to fixture, dye-testing from the treated-water tank at potable outlets, looking at sump adjacency and basement flood levels, verifying that the non-potable line is actually labelled and colour-coded, are exactly the sort of thing an O&M contractor is not positioned to audit on themselves.
What your flush water is actually supposed to be
Most residents, and a surprising number of committees, do not know that treated water reused for flushing has a prescribed quality at the point of use. It does.
The reference is 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. That is a free download from cpcb.nic.in, and the full toilet-flushing column reads:
| Parameter | Norm for toilet flushing |
|---|---|
| Turbidity | < 2 NTU |
| pH | 6.5 to 8.5 |
| BOD | ≤ 6 mg/L |
| Oil and grease | 10 mg/L |
| Nitrate nitrogen as TN | 10 mg/L |
| Total phosphorous as TP | 1 mg/L |
| Minimum residual chlorine | 1 mg/L |
| Faecal coliform | Nil per 100 mL |
| Colour | Colourless |
| Odour | “Aseptic which means not specific and no foul odour” |
Two things follow, and both are worth a committee’s attention.
First, “at the point of use” means at your cistern, not at the STP outlet. Almost nobody in India samples there. Water can leave the STP perfectly and go septic in a holding tank over the following days. Sampling only at the outlet is how a complex passes every test and still has worms.
Second, the faecal coliform norm for reuse is stricter than the discharge standard. The National Green Tribunal’s order of 30 April 2019 sets 100 MPN/100 mL as desirable and 230 as permissible for discharge. (Worth knowing, because it is the first thing anyone will challenge: the Expert Committee’s table put those figures on Mega and Metropolitan cities only, and gave weaker numbers to everywhere else. The Tribunal struck that down. Paragraph 14: “we accept the report of the Expert Committee with the modification that the standards recommended for Mega and Metropolitan Cities will also apply to rest of the country”, and it applied them to existing STPs too, disapproving the seven-year grace period.) The reuse norm for flushing is nil. A complex can therefore be fully compliant with its discharge consent and still be well outside the norm for the water it is sending to your bathroom.
Three honesty notes, because a committee should know exactly how much weight this table carries. First, it is recommendatory, not law, these are norms in an engineering manual, and CPCB’s own guideline says of them, in its own words, that “User-based reuse standards are provided in CPHEEO manual 2013, which needs to be revisited.” Second, on provenance: the values above are transcribed from the CPCB guideline named above, which we hold and have opened at page 19. The norms originate in CPHEEO’s Manual on Sewerage and Sewage Treatment (2013); we have not seen that manual at source, so we cite CPCB’s published reproduction of it rather than a table number we cannot check. Third, we have not found a BIS standard putting a number on flush water quality, and IS 10500, which people often cite, is the drinking water standard and does not govern flush water.
And one open item we are not going to hide. BIS published a water-reuse series in 2021, IS 17662 and IS 17663, and IS 17662 is titled Guidelines for Water Quality Grade Classification for Water Reuse. We do not hold either standard and have not read them. A title like that may well bear on this question. So read the sentence above as “we have not found one”, which is what we can support, and not as “there is none”, which we cannot. If your consultant has the series, ask them.
“No number” is not the same as “no requirement”
That last point gets misread, including by us in an earlier draft of this article, so it is worth stating carefully. We have found no BIS standard setting a numeric limit for flush water, subject to the two standards named above that we have not read. What is certainly not true is that the national code is silent on the quality of water piped to your flat for non-drinking use. It is not silent, and what it says is mandatory:
NBCS 2026 (SP 7:2026), Part E ‘Plumbing Services’, Section 1 ‘Water Supply’, 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 withdrawn 2016 edition carries the same sentence, with the comma one word earlier, reproduced here as printed:
NBC 2016 (SP 7:2016), Part 9 ‘Plumbing Services’, Section 1, clause 4.2.3, printed page 13: “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.”
That is a “shall”, and it has been there through both editions. It changes the standing of a bad flush-water result. Faecal coliform in the water arriving at your cistern is not merely outside a recommendatory engineering norm that CPCB itself says needs revisiting. It is contrary to a requirement of the national code, and, on the point below about which edition your bye-law names, this requirement reads the same in both.
What the code does not do is tell you the number at which water stops being “absolutely safe from bacteriological contamination”. For that you are back to CPHEEO’s recommended nil per 100 mL, with every caveat above still attached. So the honest position is this: the requirement is mandatory and qualitative; the only number available is recommendatory. A committee arguing this with its operator should lead with the requirement, and use the number to show the requirement is not being met.
What a Managing Committee can do this week, at no cost
Lift cistern lids in four to six flats across different risers and floors. Photograph. Establish cistern-versus-bowl.
Walk to the treated-water tank and every overhead flush tank. Look at the vents and the overflow pipes. Is there insect mesh? Is it intact? Is the lid seated? Is a manhole open? Cover any open opening the same day. A missing or corroded grating is not untidiness, it is a departure from NBCS 2026 clause 4.4.4(f), quoted above.
Run the wire test on a cistern. The BIS handbook on plumbing gives a pass/fail test that needs no instrument:
SP 35 (S&T) : 1987, Handbook on Water Supply and Drainage (with Special Emphasis on Plumbing), clause 6.4.9, printed page 110: “The cistern is provided with a removable cover. The cistern shall be mosquito proof, that is, there should be no clearance to permit a 1.6 mm wire to pass through the cistern.”
Take a piece of 1.6 mm wire and try it at every gap on the cistern, the lid seam, the inlet, the lever spindle, the overflow. If the wire goes through anywhere, the cistern is not mosquito proof by the handbook’s own measure. Two honest caveats: SP 35 is a BIS handbook, explanatory guidance, not a mandatory standard, so nobody is in breach of anything by failing it; and this test is written for the cistern, so do not carry the 1.6 mm figure across to an overhead tank vent. That opening is governed by NBCS clause 4.4.4(f) above, which requires a mosquito proof brass grating and gives no dimension.
Check the chlorine dosing system, is there hypochlorite in the day tank, is the pump actually stroking? Then check the purchase records. If no hypochlorite has been bought in three months, no disinfection has been happening, whatever the logbook says.
Read the blower run-hour meter or the STP energy meter for the last 30 days, and compare it against what the logbook claims.
Pull the last lab report and compare it against the flushing column above. Most committees have never made that comparison.
Every one of those is free, and together they will usually tell you whether you have a maintenance failure, a containment failure, or a treatment failure.
What needs a contractor, and what needs someone independent
A contractor can: fit and seal corrosion-resistant insect mesh on every vent and overflow, and replace lid gaskets, do this first, it is cheap and it is the actual defect; drain and mechanically scrub the treated-water and overhead tanks to remove the sediment that feeds them; backwash the sand and carbon filters to a written schedule and replace exhausted media; repair the dosing pump; desludge the STP and clean the diffusers; flush the riser and cut out dead legs serving vacant flats.
Order matters here. Cleaning a tank that still has an open vent buys you a few weeks. Exclusion first, habitat removal second, chemistry last, and if larvae persist after the first two, a one-off oxidant knockdown is reasonable. As a standing answer, it is just paying to reset a clock.
How often should the tank be cleaned?
You will be quoted “every six months” by vendors. We have found no Indian statutory requirement for six months and we are not going to repeat one. There is, however, a published figure, and it is worth having in front of you when a quotation arrives:
IS 2065:1983, Code of Practice for Water Supply in Buildings, clause 13.1, printed page 31: “It is, however, desirable that the communication pipes and the storage cisterns are thoroughly cleaned at least once every year in order to remove any suspended impurities that may have settled in the pipes or the tanks.”
Note the modal verb, because it is the whole point: “desirable”. Annual cleaning is what a BIS code of practice recommends, not a duty it imposes. The wording that is mandatory carries no interval at all:
IS 2065:1983, clause 15.1, printed page 33: “Storage tanks shall be regularly inspected and shall be cleaned out if necessary.”
And the current national code requires only that cleaning be possible:
NBCS 2026 (SP 7:2026), Part E, Section 1, clause 4.4.5, printed pages 22–23: “Every storage tank shall be easily accessible and placed in such a position as to enable thorough inspection and cleaning to be carried out.”
Put together: inspection and cleaning when needed is mandatory, at least annually is desirable, and a tank that cannot be reached and emptied is itself a defect. If your treated-water tank has never been drained, or cannot be drained because of the way it was built, that is the finding worth writing down, not the number of months since somebody last went in.
What a contractor cannot do is tell you whether the problem is the design or the operation, because if it is the operation, they are the party being judged.
That is the question that actually decides your money. “Is the STP undersized and does it need replacing?” and “has this STP simply not been run properly?” have answers that differ by an order of magnitude in cost, and the operator cannot be the one to answer it.
When red worms in flush water persist, an independent audit is how a committee gets that answer, verifies the logbook against instrument data rather than against itself, and samples at the point of use rather than at the outlet.
The Nemmadi RWA Health Check is a periodic engineering audit of your complex’s common assets, structural condition, the full MEP set including the STP, WTP, DG, 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. It is carried out by our engineers, and it produces a prioritised plan your committee can act on.
We are never paid to fix what we find.
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.
A note on the two NBC 2016 quotations in this article. The text layer of our NBC 2016 copy is corrupted, search software returns nonsense from it, though the pages themselves are perfectly legible. Both quotations were therefore taken by rendering the page and reading it optically, and each was then cross-checked against a second, independent copy of the same volume. We mention it because we would rather tell you how a quotation was obtained than have you assume.
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
- Central Pollution Control Board, Guidelines for Reuse of Treated Sewage in Reference to Item of Circular Economy (February 2024), Annexure I, printed page 19 — “CPHEEO Recommended Norms of Treated Sewage Quality for Specified Activities at Point of Use”. Downloaded from cpcb.nic.in and opened; every value quoted in this article was read off that page. The norms originate in CPHEEO’s Manual on Sewerage and Sewage Treatment Systems, Part A: Engineering (2013), Ministry of Urban Development — a document we have not seen at source, which is why we cite CPCB’s published reproduction rather than a table number we cannot verify.
- Schellenberg, T., Subramanian, V., Ganeshan, G., Tompkins, D. & Pradeep, R. (2020). “Wastewater Discharge Standards in the Evolving Context of Urban Sustainability — The Case of India.” Frontiers in Environmental Science 8:30. https://doi.org/10.3389/fenvs.2020.00030
- National Green Tribunal, Order dated 30/04/2019 in O.A. No. 1069 of 2018 — effluent standards.
- IS 10500:2012, Drinking Water — Specification (Second Revision), Bureau of Indian Standards. https://law.resource.org/pub/in/bis/S06/is.10500.2012.pdf
- National Building Construction Standards 2026 (NBCS 2026 / SP 7:2026), Part E “Plumbing Services”, Section 1 “Water Supply”, Bureau of Indian Standards — clauses 4.2.3 (quality of separately supplied non-drinking water), 4.2.4 (waste water reclamation), 4.4.4 (storage tank openings), 4.4.5 (accessibility for cleaning), 4.4.7 (underground tanks), 4.4.11 (physical segregation), 4.5.1, 4.5.3 (cross-connection control) and 4.5.4 (identification of non-potable systems). Printed pages 20–24. SP 7:2026 is a free download from the BIS portal.
- National Building Code of India 2016 (SP 7:2016), Part 9 “Plumbing Services”, Section 1, Bureau of Indian Standards — clauses 4.2.3 and 4.2.4, printed page 13. Withdrawn by BIS on 30 April 2026; see the note on the two codes above.
- IS 2065:1983, Code of Practice for Water Supply in Buildings, Bureau of Indian Standards — clause 7.2 (cross-connection, printed page 14), clause 8.1.13 (colour-coding, printed page 17), clauses 11.3 and 11.4 (tank lid and warning pipe, printed page 26), clause 13.1 (cleaning, printed page 31), clause 15.1 (inspection and cleaning, printed page 33), Appendix D, D-1.1 (identification colours, printed page 39). 7b. 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). Quoted exactly, including the printed text’s own errors. 7c. IS 10500 : 2012, Drinking Water — Specification, Bureau of Indian Standards (reaffirmed 2023, with Amendments 1 to 4) — Table 6 ‘Bacteriological Quality of Drinking Water’, clause 4.1.1, printed page 4: coliform bacteria “Shall not be detectable in any 100 ml sample”.
- SP 35 (S&T) : 1987, Handbook on Water Supply and Drainage (with Special Emphasis on Plumbing), Bureau of Indian Standards — clause 6.4.9, printed page 110 (mosquito-proof cistern; the 1.6 mm wire test). A handbook, not a mandatory standard.
- Container preference of Aedes aegypti for oviposition, Delhi (2023). Epidemiology and Health e2023073. https://www.e-epih.org/journal/view.php?doi=10.4178%2Fepih.e2023073
- Laviad, S. & Halpern, M. (2016). “Chironomids’ Relationship with Aeromonas Species.” Frontiers in Microbiology 7:736. https://doi.org/10.3389/fmicb.2016.00736
- Kuttuva, P., Lele, S. & Villalba Mendez, G. (2018). “Decentralized Wastewater Systems in Bengaluru, India: Success or Failure?” Water Economics and Policy 4(2). https://doi.org/10.1142/S2382624X16500430
- University of Florida IFAS Extension, EENY-716, “Drain Fly Psychoda spp.” https://ask.ifas.ufl.edu/publication/IN1226
- Kwon, H-J. et al. (2023). “Comprehensive Solutions to Prevent Larvae Breakout in Water Filtration Systems.” Sustainability 15(20):14881. https://doi.org/10.3390/su152014881 — GAC ineffective at retaining larvae; sand filter at ES 0.7 mm / UC 1.7 failed as a barrier.
- “Passability of Chironomid Larvae in Granular Activated Carbon” (2022). Int. J. Environ. Res. Public Health 19(2):1005. https://pubmed.ncbi.nlm.nih.gov/35055831/ — >80% of first-instar and 3.3% of second-instar larvae passed through GAC.
- Broza, M., Halpern, M., Teltsch, B., Porat, R. & Gasith, A. (1998). “Shock chloramination: potential treatment for Chironomidae (Diptera) larvae nuisance abatement in water supply systems.” Journal of Economic Entomology 91(4):834–840. https://doi.org/10.1093/jee/91.4.834 — ~70 mg/L chloramine for 1–2 hours; LC50 32 mg/L at 75 min for fourth instars.
- Reporting on the October–November 2021 water contamination at a Yemalur, Bengaluru apartment complex. The “around 340” figure and the coliform count of over 1,800 are from the Hans India report; the account of the two adjacent sumps is reported there as a resident’s account, and the developer’s head of water management is reported as attributing the incident to 80 mm of rain on 23 October, with the root cause not established. Deccan Herald, https://www.deccanherald.com/india/karnataka/bengaluru/over-300-infected-by-contaminated-water-at-bengaluru-apartment-building-1047474.html · The New Indian Express, https://www.newindianexpress.com/cities/bengaluru/2021/Nov/04/200-kids-ill-after-drinking-dirty-water-in-yamlur-2379437.html · The Hans India, https://www.thehansindia.com/news/cities/bengaluru/more-than-300-people-have-been-affected-by-polluted-water-in-bengaluru-714318
