SOLUTIONS / ETP, STP AND ZLD PLANTS
ETP, STP and ZLD plants
recover more, discharge less.
We design, build and operate effluent treatment plants, sewage treatment plants and zero liquid discharge systems across India. Each project starts with a test of your wastewater in our own NABL-accredited laboratory, so the plant is sized for your water, your consent and your next stage of growth.
In short
ETP, STP and ZLD in one paragraph.
An ETP (effluent treatment plant) treats industrial process wastewater, which can carry chemicals, oil, colour, salts or metals. An STP (sewage treatment plant) treats domestic sewage from toilets, kitchens and bathrooms, which is mostly biodegradable. A ZLD (zero liquid discharge) plant goes one step further: after treatment it recovers the water with RO and evaporates what is left, so no liquid leaves the site and only dry salt remains. Which one you need is set by your wastewater and by the consent to operate your State Pollution Control Board issues under the Water Act, 1974.
- 43.9%
- Installed STP capacity against sewage generated in India (CPCB inventory, 2021)
- 17categories
- Highly polluting industries that must stream effluent data live (OCEMS)
- 4sectors
- Covered by CPCB's zero liquid discharge guidelines of 2015
- 1,498+
- Plants and turnkey projects delivered by Rispri since 1999
The basics
What are ETP, STP and ZLD plants?
All three are wastewater treatment plants. They use many of the same building blocks (screens, tanks, blowers, clarifiers, filters, membranes) but they are designed for different water and different goals. Picking the wrong one, or sizing it on a guess, is the most expensive mistake a plant owner can make.
Effluent treatment plant (ETP)
An ETP is a plant that cleans the wastewater produced by a manufacturing process: wash water, spent liquor, blowdown and floor washings. Industrial effluent varies hugely. A dairy's effluent is strong but easy for bacteria to eat; a pharma unit's may contain solvents and salts that bacteria cannot handle; an electroplating unit's carries metals that need chemistry, not biology. That is why every ETP is a custom design. Read more on our effluent treatment plant page.
Sewage treatment plant (STP)
An STP is a plant that treats domestic wastewater, also called sewage or black and grey water, from homes, offices, hotels, hospitals and factory toilets and canteens. Sewage is fairly predictable in strength, so STPs are more standardised. The main design questions are flow, space, the outlet standard and whether the treated water will be reused for flushing, gardening or cooling towers. See our sewage treatment plant page.
Zero liquid discharge (ZLD) plant
ZLD is a system in which all wastewater is treated and recovered, and nothing is discharged as liquid. A typical ZLD chain is ETP, then ultrafiltration (UF) and reverse osmosis (RO) to recover clean water, then an evaporator (MEE, MVR or non-thermal) and a dryer such as an ATFD to turn the concentrated reject into solid salt. ZLD is usually a condition in the consent of textile, tannery, distillery, pharma and some chemical units, and of units in water-stressed areas. See zero liquid discharge plants.
Where the three meet
Many sites run more than one. A factory often has an ETP for process effluent and a small STP for its colony and canteen. A ZLD plant always contains an ETP at its front end. Where the sewage share is small, the board may allow it to be treated with the effluent in one plant.
Our plants
Go straight to the plant you need.
Each page explains how the plant works, the limits it must meet, indicative costs and common problems in detail.
Effluent treatment plants (ETP)
For factories and industrial estates. Physico-chemical and biological treatment, including DAF, electro-coagulation, Fenton and ozone oxidation for hard-to-treat, high-COD streams from pharma, chemical, textile and dyeing units.
Effluent treatment plants →Sewage treatment plants (STP)
Packaged and civil STPs for apartments, townships, hotels, hospitals, institutions and factory colonies, using MBBR, SBR, MBR or extended aeration, with tertiary treatment for flushing and gardening reuse.
Sewage treatment plants →Zero liquid discharge plants (ZLD)
High-recovery RO, nanofiltration salt splitting, MEE or MVR evaporators and ATFD dryers, sized on a full mass balance so the plant is not undersized for your salt load.
Zero liquid discharge plants →Non-thermal evaporators (RCD vaporisers)
Field-proven rotating-disk modules that evaporate RO reject with air and fan power. Running cost about ₹80–160 per m³, against an estimated ₹430–1,070 in an MEE.
Non-thermal evaporators →Comparison
ETP vs STP vs ZLD: what is the difference?
The same tank can appear in all three. What differs is the water coming in, the target going out and who usually buys it.
| ETP | STP | ZLD plant | |
|---|---|---|---|
| What it treats | Industrial process effluent: organics, oil, colour, salts, metals, toxic chemicals | Domestic sewage from toilets, kitchens, baths and laundries | Industrial effluent (sometimes with sewage) where no discharge is allowed |
| Typical inlet | Very variable; COD from a few hundred to tens of thousands of mg/L | Fairly steady; BOD often 200–350 mg/L | ETP-treated water with high TDS (dissolved salts) |
| Typical processes | Screening, oil removal, equalisation, neutralisation, coagulation, DAF or clarifier, ASP/MBBR/SBR/MBR, UASB for strong effluent, filters | Screening, equalisation, MBBR/SBR/MBR/extended aeration/FAB, clarifier, sand and carbon filters, UV or chlorine | ETP + UF + RO (often multi-stage) + MEE, MVR or non-thermal evaporator + ATFD or crystalliser |
| Outlet target | Consent limits; general standards under EP Rules 1986 (BOD 30, COD 250, TSS 100 mg/L for inland surface water) or industry-specific standards | MoEF&CC STP standards, 2017 (BOD 20–30, TSS 50–100 mg/L), stricter where the board applies NGT-recommended values | No liquid discharge; recovered water reused in process, boiler or cooling; solid salt to authorised disposal or sale |
| Typical buyers | Factories, industrial estates, CETPs | Housing societies, townships, hotels, hospitals, schools, offices, factory colonies | Textile, tannery, distillery, pharma, chemical units; plants in water-stressed areas |
| Relative cost | Medium; driven by effluent strength | Lowest per KLD | Highest; evaporation dominates |
Values are typical and simplified. Your own consent to operate is the document that sets your limits.
Decision guide
Which plant do you need?
Answer these questions in order. Most owners know where they stand after the first three.
- 01The question
Is your wastewater only from toilets, kitchens and bathrooms?
If yesYou need an STP. Size it on population or measured flow, and add tertiary treatment if you plan to reuse the water.
- 02The question
Does any of it come from a manufacturing process, laundry, workshop or lab?
If yesYou need an ETP for that stream. Keep sewage separate unless it is a small share and your board agrees to combined treatment.
- 03The question
Does your consent, or a board or CPCB direction, say zero liquid discharge?
If yesYou need a ZLD plant: ETP, then UF and RO, then an evaporator and dryer for the reject.
- 04The question
Do you already have an MEE whose steam bill is too high, or an RO reject you cannot dispose of?
ConsiderNon-thermal evaporators beside the MEE to take load off it, or RO reject management.
- 05The question
Is fresh water scarce, metered from groundwater or bought by tanker?
ConsiderDesign for reuse from day one: MBR or tertiary filtration plus RO. The recovered water often pays for the upgrade.
- 06The question
Do you already have a plant that fails samples or keeps sending OCEMS alerts?
Start withAn audit and stage-by-stage testing, not a new plant. Retrofits such as MBBR media or DAF often fix it inside existing tanks.
Technology stack
The technologies inside ETP, STP and ZLD plants.
Every plant is a combination of a few of these. Here is what each one does, in plain words, and where it fits.
| Technology | What it is | Used in | Best for | Watch-outs |
|---|---|---|---|---|
| ASP (activated sludge process) | Air is blown into a tank of wastewater and bacteria; the bacteria eat the organic matter and settle in a clarifier, and part is returned to the tank. | ETP, STP | Steady loads, larger plants with land | Large footprint; sludge bulking if loads swing |
| MBBR (moving bed biofilm reactor) | Small plastic carriers float in the aeration tank; bacteria grow as a film on them, so more biology fits in less volume. | ETP, STP | Tight sites, upgrades of old tanks, variable loads | Screens must hold the media in; needs a clarifier after |
| SBR (sequencing batch reactor) | One tank fills, aerates, settles and decants in timed cycles, doing the work of an aeration tank and clarifier together. | STP, ETP | Intermittent flows, nitrogen removal | Depends on PLC, valves and level sensors working |
| MBR (membrane bioreactor) | Biological tank with submerged ultrafiltration membranes (flat sheet or hollow fibre, around 0.04 µm) instead of a clarifier. | STP, ETP before RO | Reuse, smallest footprint; permeate TSS below 1 mg/L and BOD below 5 mg/L | Higher power; membrane cleaning and replacement |
| UASB (upflow anaerobic sludge blanket) | Effluent flows up through a bed of granular anaerobic sludge that breaks down organics without air and makes biogas. | ETP | Strong organic effluent: food, dairy, distillery, starch | Slow start-up; needs an aerobic polish after |
| DAF (dissolved air flotation) | Micro-bubbles lift oil, grease and fine flocs to the surface, where they are skimmed off. | ETP | Oily, fatty or light solids before biology | Needs correct coagulant and polymer dosing |
| UF and RO | Ultrafiltration removes fine solids and bacteria; reverse osmosis pushes water through a membrane that holds back dissolved salts. | ETP reuse, ZLD, STP reuse | Recovering process, boiler or cooling water | RO makes a salty reject (typically 20–40% of feed) that must go somewhere |
| MEE (multiple-effect evaporator) | Boiler steam boils the reject in the first effect; its vapour heats the next effect, and so on, reusing the heat several times. | ZLD | Large reject flows with steam available | Steam cost; scaling and corrosion of tubes |
| MVR (mechanical vapour recompression) | A compressor squeezes the vapour leaving the evaporator so it can heat the same evaporator again, so electricity replaces most of the steam. | ZLD | Sites with costly steam; steam requirement cut by up to 90% against MEE | High capital cost; power typically tens of kWh per m³ |
| ATFD (agitated thin film dryer) | A heated drum with rotating blades spreads the thick concentrate into a thin film and dries it to powder. | ZLD | Final step from slurry to dry salt | Steam use; sticky, organic-rich concentrates |
| Non-thermal evaporator (RCD vaporiser) | Slowly rotating textured disks carry thin films of reject into an air stream, which evaporates the water without steam. | ZLD, RO reject | Cutting MEE load or replacing it where heat is free and the climate is hot and dry | Needs space and air flow; output depends on weather or waste heat |
Other units used where needed: electro-coagulation, Fenton and ozone oxidation to crack hard-to-degrade COD; nanofiltration to split sodium sulphate from sodium chloride so a saleable salt can be recovered; pressure sand and activated carbon filters; UV and chlorination.
Our NABL-accredited laboratory tests BOD, COD, TSS, TDS, oil and grease, nutrients and metals, and runs treatability trials, so the technology is chosen on evidence.
Turnkey delivery
How a turnkey ETP, STP or ZLD project runs.
Typical timings for a mid-size ETP or ZLD plant are about 12 to 16 weeks from design to commissioning. Packaged STPs are faster; large civil plants take longer.
- 1
Sample and treatability
Weeks 1–2. Composite samples tested in our NABL lab for BOD, COD, TDS, silica and organics; jar tests and treatability trials.
- 2
Design
Weeks 3–5. Mass and heat balance, process flow diagram, P&ID, civil layout, piping and control panel design.
- 3
Build
Weeks 6–10. Fabrication of skids, RO and MBR frames and evaporator equipment, with factory hydro-testing.
- 4
Install and commission
Weeks 11–15. Erection, PLC/SCADA programming, sensor calibration, a continuous 72-hour trial run, outlet testing and OCEMS connection where required.
- 5
Operate and maintain
Operator training, O&M contracts, chemicals, spares and monthly lab reports for your board.
Rules
The rules your plant is designed to.
These are the regulations auditors and board officers check. The detailed limits are on each plant's own page.
| Rule | What it covers |
|---|---|
| Water (Prevention and Control of Pollution) Act, 1974 | Consent to establish and consent to operate from the SPCB for any unit that discharges wastewater |
| Environment (Protection) Rules, 1986 | General discharge standards (Schedule VI) and industry-specific standards (Schedule I) |
| MoEF&CC STP notification, 2017 | Discharge standards for sewage treatment plants: pH, BOD, TSS and faecal coliform |
| CPCB ZLD guidelines, 2015 | Zero liquid discharge for textiles, tanneries, distilleries and pulp and paper |
| CPCB OCEMS directions | Live effluent data to CPCB and SPCB servers for the 17 highly polluting categories, CETPs and others named by boards |
| National Framework on Safe Reuse of Treated Water, 2022 | Guidance on reusing treated water in industry, cooling, construction and landscaping |
| Hazardous and Other Wastes Rules, 2016 | Storage and disposal of ETP sludge and evaporator salts through authorised facilities |
| CGWA guidelines, 2020 | Groundwater abstraction permits, metering and conditions that push units towards reuse |
Cost
What do ETP, STP and ZLD plants cost?
Flow matters, but so does strength and where the water must go. These indicative ranges help you set a budget; detailed tables are on each plant's page.
| Plant | Example size | What sets the price | Indicative plant cost* |
|---|---|---|---|
| STP (packaged) | 10–50 KLD | Process (MBBR, SBR, MBR), reuse level, automation | ₹5 to 20 lakh |
| STP | 100 KLD | Process, tertiary treatment, civil or steel tanks | ₹20 to 40 lakh |
| ETP (conventional biological) | 50 KLD | Organic load, discharge route, pre-treatment | ₹15 to 35 lakh |
| ETP (conventional biological) | 100 KLD | Organic load, toxicity, oil, colour | ₹25 to 60 lakh |
| ZLD (ETP + RO + evaporator) | 100 KLD | TDS, recovery, evaporator type, salt handling | Several crore; quoted after a mass balance |
*Indicative 2026 ranges for equipment, electricals and installation, excluding land, civil works and taxes. Evaporation usually takes the largest share of a ZLD plant's capital and running cost. We give a firm price after testing your wastewater and visiting the site.
Capital cost drivers
- Strength of the wastewater, not only the flow
- Discharge route: drain, sewer, land, reuse or ZLD
- Civil tanks or steel and packaged units
- Space, soil and how far the outlet must travel
- Automation, SCADA and OCEMS
Running cost drivers
- Power, mostly aeration blowers and RO pumps
- Steam for MEE and ATFD in ZLD plants
- Chemicals: pH, coagulants, antiscalants, cleaning
- Sludge and salt disposal
- Operators, lab tests, membranes and media
Ways to cut cost
- Segregate strong and weak streams
- Recover water to offset fresh water purchase
- Move evaporation load to non-thermal evaporators
- Tune blowers with dissolved oxygen control
- Recover saleable salt by NF salt splitting
Our non-thermal RCD vaporisers evaporate RO reject at about ₹80–160 per m³, roughly 80% below a typical MEE, and can run beside your existing evaporator.
Troubleshooting
Common plant problems, solved at the design stage.
- 01The problem
Loads swing with CIP washes, batch dumps and seasonal production.
How we solve itEqualisation and treatability tests in our lab size the plant for real peaks, not averages.
- 02The problem
High-TDS reject cannot be discharged anywhere.
How we solve itRO with MEE, MVR or non-thermal evaporation, sized on a full mass balance so the ZLD plant is not undersized.
- 03The problem
The online monitor keeps sending breach alerts.
How we solve itAnalysers, PLC control and automatic dosing hold COD, TSS and pH steady at the outlet, and we check the analysers are calibrated.
- 04The problem
An old ASP plant is overloaded and there is no land to expand.
How we solve itMBBR or MBR retrofits add capacity inside the existing tanks.
- 05The problem
Evaporator tubes scale and the ZLD plant keeps shutting down.
How we solve itSoftening and silica control upstream, correct antiscalants, cleaning schedules, and moving part of the load to scale-tolerant rotating-disk evaporators.
Where it fits
Guides by sector.
Each sector's wastewater behaves differently. These guides explain what to expect and which plant usually fits.
Hotels and resorts STP
Compact STPs with reuse for flushing, gardens and cooling.
Hotels guide →Metalworking and coolants
Oily emulsions that must be broken before treatment.
Coolant guide →Buying guide
What to ask any wastewater plant supplier.
Use this list for an ETP, STP or ZLD plant, with any supplier, including us.
- Was the design based on tests of my water? Composite samples, and treatability trials for difficult effluent.
- Which outlet values are guaranteed in writing? Against my consent, with the test method named.
- For ZLD, where is the mass balance? Flow, TDS and salt load at every stage, including the reject.
- What does a cubic metre cost to treat? Power, steam, chemicals and labour, shown separately.
- Where do sludge and salts go? Dewatering, drying and the authorised disposal route.
- Is OCEMS included if my unit needs it? Analysers, data logger and server connectivity.
- Who operates it after handover? Training, O&M contract, chemicals and spares.
- Is there room to grow? Space and design margin for more flow or tighter limits.
Why Rispri
One accountable partner, from lab bench to running plant.
Lab and engineering under one roof
Our NABL-accredited laboratory tests the water, sizes the process and verifies results after start-up. No hand-offs between vendors.
1,498+ plants and projects
Designs proven on real sites since 1999, from compact STPs for hotels to industrial ETP, RO and ZLD systems.
Research that lowers running cost
Proven non-thermal evaporators and other technologies, in collaboration with IIT Bombay, IIT Kharagpur and ICT Mumbai.
We stay after handover
Chemicals, genuine spares, operator training and O&M contracts keep the plant performing for years.
Questions
ETP, STP and ZLD plant FAQs.
What is the difference between an ETP, an STP and a ZLD plant?
An ETP treats industrial process effluent, an STP treats domestic sewage, and a ZLD plant treats and recovers all wastewater so that nothing is discharged as liquid. A ZLD plant contains an ETP at its front end, followed by RO and an evaporator.
Do I need an ETP or an STP?
If your wastewater comes only from toilets, kitchens and bathrooms, you need an STP. If any of it comes from a manufacturing process, laundry, workshop or laboratory, you need an ETP for that stream. Many factories run both.
When is zero liquid discharge mandatory?
ZLD is required when your consent to operate or a CPCB or board direction says no effluent may leave the site. It is common for textiles, tanneries, distilleries, pulp and paper, pharma and units in water-stressed areas.
Can an ETP and an STP be combined into one plant?
Sometimes. When the sewage share is small and the board agrees, sewage can be treated with process effluent. Where the effluent contains toxic chemicals or metals, keeping sewage separate is usually simpler and lets the treated sewage be reused.
What is the difference between MEE and MVR evaporators?
An MEE uses boiler steam and reuses its heat across several effects. An MVR compresses the vapour it produces and reuses it as the heating medium, so electricity replaces most of the steam, cutting steam need by up to about 90% compared with an MEE, at a higher capital cost.
What is a non-thermal evaporator?
A non-thermal evaporator removes water without boiling it. Rispri's field-proven RCD vaporisers spread RO reject over slowly rotating disks and let air or low-grade waste heat evaporate it, at a running cost of about ₹80–160 per m³ against an estimated ₹430–1,070 in an MEE.
Which is better for an STP: MBBR, SBR or MBR?
MBBR is compact and forgiving, SBR suits variable flows and nitrogen removal, and MBR gives the best water for reuse in the smallest space but costs more to run. The right choice depends on your space, outlet standard and reuse plans.
How much does an ETP, STP or ZLD plant cost?
As an indication, a packaged 10–50 KLD STP costs about ₹5–20 lakh and a 100 KLD conventional ETP about ₹25–60 lakh for equipment and installation, excluding civil work. ZLD plants cost several times more because of RO and evaporation. We price after testing your wastewater.
How long does a turnkey plant take to build?
A mid-size ETP or ZLD plant typically takes 12 to 16 weeks from detailed engineering to commissioning, depending on capacity. Packaged STPs can be supplied in a few weeks, while large civil plants take longer.
Can treated wastewater be reused?
Yes. Treated sewage is widely reused for flushing, gardening and cooling towers, and treated effluent can feed cooling towers, boilers and processes after UF and RO. The National Framework on Safe Reuse of Treated Water, 2022 encourages this.
Do your plants include OCEMS?
Where your unit must run online continuous effluent monitoring, we include analysers for parameters such as pH, flow, COD, BOD and TSS, a data logger and the connection to CPCB and SPCB servers.
Can an existing plant be upgraded instead of replaced?
Usually, yes. We convert old aeration tanks to MBBR or MBR, add DAF or tertiary filters, and add RO and evaporation to move an existing ETP towards ZLD, often without new civil tanks.
Do you operate and maintain plants?
Yes. We run ETPs, STPs and ZLD plants under O&M contracts with trained operators, supply chemicals and spares, and provide NABL-accredited test reports you can submit to your board.
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