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Compliance9 min read

Is an STP Mandatory for Your Apartment? What the Law Requires

When an apartment building legally needs a sewage treatment plant, who enforces it, what a society risks without one, and how to retrofit one that passes inspection.

Technically reviewed by Prashant Dwivedi, Co-Founder & Director ·

Short answer: there is no single national rule that says every apartment must have an STP. The requirement comes from your State Pollution Control Board's consent conditions and state building rules, and it depends mainly on the size of the project, whether it is new or existing, and whether the area is already served by an underground sewer. But enforcement is tightening — India treats only about 20,235 MLD of the estimated 72,368 MLD of sewage its urban centres generate, and boards under CPCB direction are pushing every large generator, including housing societies, to treat on site (CPCB).

Why are pollution boards looking at apartment buildings at all?

Because apartments are a big share of the untreated sewage problem. CPCB estimated urban sewage generation at 72,368 MLD against an installed treatment capacity of only 20,235 MLD, leaving roughly 52,133 MLD discharged untreated (CPCB). CPCB identified 351 polluted river stretches across the country, with domestic sewage the dominant polluter (MoEF&CC, Lok Sabha answer).

To force action, CPCB issued directions under Section 18 1(b) of the Water (Prevention and Control of Pollution) Act, 1974 in April 2015 to all State Pollution Control Boards for setting up STPs in their states, and under Section 5 of the Environment (Protection) Act, 1986 in October 2015 to the municipal authorities of 66 metropolitan cities and state capitals (MoEF&CC, Lok Sabha answer). Those directions cascade down to individual developments through consent conditions — which is where your society comes in.

Is an STP legally mandatory for my apartment building?

It is mandatory when your State Pollution Control Board's consent for the building says so, and most large projects get such a condition. The pattern across states:

  • New projects above a size threshold — consent to establish is granted with an STP as a condition, along with dual plumbing for reuse.
  • Projects in areas with no underground sewerage — on-site treatment is the only option, so it is required for even smaller projects.
  • Existing buildings — many older buildings pre-date these conditions, and boards are now chasing them; there is no automatic grandfathering.

Two worked examples make this concrete:

  • Karnataka: the government amended the rules in 2024 so that in areas served by underground drainage, an in-situ STP is mandatory only for apartment complexes with more than 120 units producing more than 80 KLD of sewage; in areas yet to be served by underground drainage, it is mandatory for all projects with more than 20 units (CSIS Engaging Indian States). Earlier, the threshold was 50 units.
  • Maharashtra: STPs must obtain Consent/Authorisation from the Maharashtra Pollution Control Board and be run according to the conditions laid down in that consent (MPCB FAQ). Any activity likely to discharge sewage or trade effluent into the environment needs consent under the Water Act, 1974 — and that includes sewage treatment and discharge from large buildings.

The practical test for a society committee: find your building's Consent to Establish/Consent to Operate. If an STP is a consent condition, it is a legal requirement for you, whatever your neighbour three lanes away does.

Who enforces the STP requirement — CPCB, the State PCB or the municipality?

Enforcement sits with the State Pollution Control Board, acting on CPCB directions and the conditions in its own consent orders. The National Green Tribunal has reinforced this: it directed in its order dated 30.04.2019 in O.A. No. 1069/2018 that all existing and upcoming STPs must meet the stricter sewage treatment standards it prescribed, and its subsequent monitoring orders require the boards to report compliance (NGT order, 23 Nov 2021).

The boards have real teeth. MPCB's own FAQ lists its powers: issuing notices with time limits to comply, closure of the defaulting unit in grave cases, and prosecution in serious violations (MPCB FAQ). The municipal body (water supply and sewerage utility) is a second lever where a sewer network exists — it can refuse or disconnect the sewerage connection for buildings that discharge untreated sewage.

What actually happens if your apartment has no STP?

In Bengaluru, this stopped being theoretical in 2020. KSPCB issued closure notices to 268 apartments and individual houses between January and March 2020 for discharging untreated sewage, demanding environmental compensation of ₹5 lakh where an STP existed but underperformed — and ₹3 crore from apartments that had no STP at all, along with threats to disconnect power and water (Citizen Matters, 1 Mar 2020). Thirty-four complexes went to the Karnataka High Court, which granted interim stays and directed that proper hearings be held before any coercive action (Citizen Matters).

Reported (press, not a board document), but it tells you the risk ladder a society faces:

  1. Inspection or sampling of your discharge by board officials.
  2. Show-cause notice with a short reply window.
  3. Environmental compensation demand — which, in the Bengaluru cases, ran from ₹5 lakh to ₹3 crore.
  4. Closure or disconnection notices, and possible prosecution under the Water Act, 1974.

Note the fairness caveats: the courts did push back on procedure, and several boards have since formalised environmental compensation formulas. But "we never got a notice before" is not a defence; it is a delay.

Can an existing building retrofit an STP — and how big should it be?

Yes, and retrofitting is routine now. The practical path:

  • Size it from people, not plot. A plant is rated in KLD (kilolitres per day). Working out resident count × water use gives the sewage flow; a 300-flat society with about 1,000 residents typically lands in the 100–150 KLD range, but the definitive number comes from your consent document and a consultant's water balance — never guess at sizing.
  • Pick a compact technology for tight basements. MBBR and MBR systems are the common choices where space is scarce; if treated water is meant for toilet flushing and gardening, MBR's polished output simplifies reuse compliance. We compare the trade-offs in MBR vs SBR for buildings.
  • Plan dual plumbing at the same time. CPCB's draft guideline on reuse of treated sewage sets out quality requirements for non-potable uses such as toilet flushing and landscaping (CPCB). An STP without a reuse route just creates a discharge problem somewhere else.
  • Budget for operation, not just construction. Consent is renewable, samples are taken without warning, and the treated water must meet the notified limits — BOD of 30 mg/L, or 20 mg/L in metro cities, as notified for STP discharge into water bodies.
  • Get the paperwork before the plant. Consent from the State PCB (Consent to Establish for the retrofit works, Consent to Operate before running), and the utility's nod where a sewer connection exists.

For buyers: an apartment in a building without an STP is a building with a pending compliance bill. Ask for the consent document and the latest treated-water lab report before you book.

Checklist for a society committee inheriting a building with no STP

  1. Pull the Consent to Establish/Operate from the builder or the board's regional office — check whether an STP is a stated condition.
  2. Confirm whether your locality is served by underground sewerage; this changes both the requirement and your options.
  3. Get a water balance done (residents × consumption → sewage flow) and size the plant with margin for peak flow.
  4. Approach the State PCB before construction — a retrofitted STP still needs consent, and boards respond better to societies that come to them first.
  5. Shortlist vendors who quote O&M and lab testing alongside capex, and who will train your operator. See our buyers guide for the questions that filter out the bad ones.
  6. Put treated-water reuse (flushing, gardening) into the plumbing scope now; it is the fastest way to shrink both your fresh-water bill and your discharge compliance load.

Frequently asked questions

Is a small building with 20 flats required to have an STP?

Depends on the state and the sewer. In Karnataka, in areas without underground drainage, projects above 20 units need an in-situ STP (CSIS); where sewers exist the 2024 amendment raised the threshold to over 120 units. In other states the trigger is your consent conditions — check them, not a rumour.

We have an STP but it barely runs. Are we compliant?

No. An STP that is installed but non-functional still discharges untreated sewage, and boards treat it the same way — the Bengaluru notices of ₹5 lakh went mostly to apartments whose STPs underperformed (Citizen Matters). Keep monthly lab reports and an O&M contract.

What quality must our treated water meet?

The notified discharge standard for STPs is BOD 30 mg/L, and 20 mg/L in metro cities. Where you reuse the water on site, CPCB's reuse guideline prescribes quality requirements per end use such as flushing and landscaping (CPCB). Your consent may set stricter limits — it wins.

Can the board disconnect our water or power?

Boards have used closure and utility-disconnection notices as coercive measures, and courts have required due process — proper notice and a hearing — before such steps (Citizen Matters). Treat any board notice as urgent regardless; the stay bought time, not exemption.

Who in the society is legally responsible?

The occupier — for a society, the managing committee through the association. Board consent conditions bind the premises, so the association is the respondent in any notice. Put STP compliance in the facility manager's job description and budget for it in the maintenance charges.

Do we need environmental clearance for a retrofit?

A small retrofit inside an existing building usually needs only State PCB consent. Larger integrated redevelopments may additionally trigger environmental clearance requirements. Confirm the classification with the board or your consultant for your specific case.

What a committee should do this quarter

Compliance is a chain — consent, plant, operation, records — and boards now inspect every link. If your building never had an STP, the cheapest route is the planned one: a correctly sized plant with reuse plumbing, consented before the first notice arrives. Navbharat Water builds and operates STPs for residential and infrastructure projects from 10 KLD to 10 MLD — see our sewage treatment solutions and how we work with builders and infrastructure projects, or get a quote for a retrofit assessment for your society.

Sources

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