Introduction: A Winter Story That Starts in Our Drains

Every year, around the time of Chhath Puja, pictures of white foam covering the Yamuna near Kalindi Kunj make the news. This year was no different. Reports on 2 November 2026 described toxic foam resurfacing at the Kalindi Kunj barrage and a cleaning drive under way, and Chhath was expected between 13 and 16 November. The Central Pollution Control Board has asked the Delhi Jal Board to ensure full collection and treatment of domestic sewage in the city, noting that untreated wastewater from 22 drains led to frothing in the river.

The cause of the foam is well known. Detergents carry surfactants, which trap air and create foam, and phosphates, which feed algae and bacteria and help the foam persist. They come from households, dhobi ghats, laundries and dyeing and washing units in Delhi, Haryana and Uttar Pradesh. Seasonal chemical defoaming, as officials themselves acknowledge, hides the foam but does not remove the pollution.

For a plant owner, the lesson is practical. Phosphate is a pollutant that conventional STPs and ETPs do not always remove. A plant can have good BOD and TSS results and still discharge phosphate at several mg/L. As consents tighten, phosphate is appearing as a limit. Delhi's DPCC, for example, has set a phosphate limit of 2 mg/L as PO₄-P in its consent norms for STPs, in addition to BOD, TSS and nitrogen limits.

This guide explains where phosphate comes from, what the norms say, how to remove it with chemicals or biology, how to size a dose with a worked example and what to ask a supplier. It is written for operators and owners of STPs, hotels, hospitals, laundries and industries. Trity Enviro designs STPs and ETPs with phosphate polishing where consents require it.

Key Highlights / Table of Contents

• Where Phosphate Comes From and Why It Matters
• Detergents, sewage and industrial sources
• What the norms say
• Chemical Phosphate Removal
• Alum, ferric chloride and PAC
• A worked dosing example
• Biological Phosphate Removal and Polishing
• Enhanced biological phosphorus removal
• Filtration and tertiary polishing
• Laundries, Dyeing and Other Detergent-Heavy Sources
• Pre-treatment at the source
• Cost, Planning and Choosing a Partner
• Questions to ask before you order
• Industry Comparison & Parameter Norms
• Why Choose Trity Enviro

Where Phosphate Comes From and Why It Matters

Detergents, Sewage and Industrial Sources

Phosphorus enters sewage from human waste and, in a large share, from detergents and cleaning products. Typical raw domestic sewage carries total phosphorus in the range of a few mg/L up to around ten mg/L as P, depending on water use and detergent habits. Commercial laundries, hotels, hospital laundries, car wash and service stations, dyeing units and food processors add more. Fertilisers and industrial chemicals contribute in some catchments.

In the environment, phosphorus is the nutrient that limits algae growth in many water bodies. Too much of it causes algal blooms, oxygen depletion and fish kills, a process called eutrophication. In rivers like the Yamuna, it adds to the foam load and to the ecological stress that follows.

What the Norms Say

There is no single national phosphate limit for all STPs. The NGT order of April 2019 on STP standards, as described by commentators, set phosphate limits for discharge to ponds and lakes rather than rivers. Some SPCBs and authorities add phosphate to consents. DPCC's consent norms for Delhi STPs include phosphate of 2 mg/L as PO₄-P alongside BOD 10, TSS 10, COD 50 and total nitrogen 10 mg/L, and CPCB inspection reports have found STPs falling short of the phosphate limit. Competitor and consultancy pages often quote 5 mg/L as a general figure. Your consent decides which number binds you, so read it, and design to the stricter one if you are unsure.

Chemical Phosphate Removal

Alum, Ferric Chloride and PAC

The simplest and most dependable method is chemical precipitation. A metal salt added to the water reacts with dissolved phosphate to form an insoluble precipitate, which is then removed with the sludge in the clarifier or a filter. The common chemicals are:

• Alum (aluminium sulphate): effective at a pH of roughly 6 to 7, uses alkalinity and adds some sulphate.
• Ferric chloride: works over a wider pH range, produces dense sludge and is also useful for colour and sulphide control, but is corrosive.
• PAC (polyaluminium chloride): works over a wide pH range with less alkalinity use.

Our guide on alum vs PAC vs ferric chloride compares them in detail. The chemical can be dosed at three points: before the primary settler, directly into the aeration tank (simultaneous precipitation) or after the biological stage ahead of a clarifier or filter. Dosing after biology, with filtration, gives the lowest residual phosphate but the highest chemical cost.

Chemical removal needs more than stoichiometry. Because other compounds in the water compete for the metal, plants usually dose well above the theoretical ratio, often at a metal to phosphorus molar ratio of about 1.5 to 2.5 for modest removal and higher for very low limits.

A Worked Dosing Example

Consider a 200 KLD STP with raw total phosphorus of about 8 mg/L as P and a target outlet of 1 mg/L. The biology removes some phosphorus into the sludge, so assume about 8 mg/L reaches the dosing point and 1 mg/L must remain.

1. Phosphorus to remove: 7 mg/L × 200 m³/day = 1.4 kg P/day.
2. Moles of P: 1,400 g ÷ 31 g/mol = about 45 mol/day.
3. Ferric dose at a molar ratio of 2: 90 mol Fe/day × 55.85 g/mol = about 5.0 kg Fe/day, or 25 mg/L as Fe.
4. As ferric chloride: 90 mol × 162.2 g/mol = about 14.6 kg FeCl₃ (100 percent basis) per day, or about 73 mg/L.
5. Alkalinity used: about 0.9 mg per mg FeCl₃, so roughly 66 mg/L as CaCO₃. If raw alkalinity is 250 mg/L and nitrification is also consuming alkalinity, as explained in our guide to ammonia and nitrogen removal in STP, check that pH stays above 7.
6. Extra sludge: the iron phosphate and hydroxide precipitate adds to sludge, so plan dewatering. See ETP sludge disposal.

This is a design estimate. Run a jar test to find the actual dose, and fit pH monitoring and dosing control so the dose tracks the load.

Biological Phosphate Removal and Polishing

Enhanced Biological Phosphorus Removal

Some bacteria, called phosphorus accumulating organisms, take up more phosphorus than they need if they are first stressed in an oxygen-free, nitrate-free zone with easy food in the form of volatile fatty acids, and then moved into an aerobic zone. They store the phosphorus inside their cells, and when excess sludge is wasted, the phosphorus leaves with it. This is enhanced biological phosphorus removal, or EBPR.

EBPR needs an anaerobic zone at the start of the plant, enough readily degradable BOD, careful control of nitrate and dissolved oxygen and sludge handling that does not release the phosphorus again. It saves chemicals but is more sensitive and harder to operate, and it rarely achieves very low limits on its own. Many plants combine EBPR with a small chemical dose as a safety net. It fits SBR and MBBR designs when planned from the start, and it is easier in new plants than in retrofits.

Filtration and Tertiary Polishing

Phosphate precipitates are fine, so low outlet limits need solids removal. Pressure sand filters, tube settlers, ultrafiltration and membrane bioreactors capture them. Our tertiary treatment plant page describes filtration and disinfection stages, and our guide to tertiary water treatment for industrial reuse explains when more than filtration is needed. For very low limits, adsorption media and polishing precipitation can follow.

Laundries, Dyeing and Other Detergent-Heavy Sources

Pre-Treatment at the Source

Cutting phosphate at the source is cheaper than removing it at the end of the pipe. Industrial laundries, hospital laundries, denim and dyeing units and car wash stations should have pre-treatment before discharging to a sewer or STP. A typical train includes screening for lint and fibres, equalization, pH correction, coagulation with a metal salt, flotation or settling and filtration. Dissolved air flotation works well on detergent and fibre loaded water, as covered in our DAF system selection guide. Our guide to industrial wastewater pre-treatment before the ETP lists the stages.

Units can also reduce phosphate by switching to low-phosphate or phosphate-free detergents, controlling dosing and reusing rinse water. Societies and hotels can choose products carefully, and educate staff and residents.

Foam inside a plant is a related problem. Detergent-laden sewage can foam in the aeration tank, as explained in our guide to STP problems and solutions, and good grease and surfactant management upstream reduces it.

Cost, Planning and Choosing a Partner

Chemical dosing is the lowest capital cost route, but it adds running cost and sludge. EBPR lowers chemical use but needs design changes and skilled operation. Tertiary filtration or UF adds capital cost but gives the lowest and most reliable outlet values. The right mix depends on your outlet limit, plant type and budget.

Start with data. Test inlet and outlet total phosphorus and orthophosphate for a few weeks, along with alkalinity, BOD and TSS, so that the design fits your plant. If your consent has no phosphate limit today, ask your SPCB whether one is likely, and leave space for dosing and filtration in the layout.

Questions to Ask Before You Order

• What phosphate limit does my consent set, and what data did you design from?
• Which removal route do you propose: chemical, biological or both, and why?
• What outlet phosphate do you guarantee, and what are the chemical and sludge quantities?
• How is the dose controlled, and what happens to pH and alkalinity?
• Is filtration or UF included to capture fine precipitate?
• What pre-treatment is proposed for laundry or detergent-heavy streams?
• What operation, maintenance and training do you provide?

Trity Enviro's sewage treatment plant and effluent treatment plant designs include chemical dosing, clarification and filtration, and an annual maintenance contract keeps dosing pumps, probes and filters in order.

Industry Comparison / Operational Parameter Table

Why Choose Trity Enviro

Trity Environ Solutions is an ISO 9001:2015 certified and QCI-approved sewage treatment plant and effluent treatment plant manufacturer and supplier in India, delivering robust, high-efficiency systems engineered for CPCB and SPCB regulatory compliance.

Every installation is supported by pan-India Annual Maintenance Contract and Operation and Maintenance services.

5. FAQs (Frequently Asked Questions)

• Q1: Why does the Yamuna foam, and is phosphate responsible?
• Ans: Foam forms when detergent surfactants trap air in polluted, untreated water. Phosphates from detergents and sewage feed algae and bacteria and add to the pollution load. Officials and experts link the foam to untreated sewage and industrial discharges.

• Q2: What is the phosphate limit for STPs in India?
• Ans: There is no single national limit for all STPs. Delhi's DPCC consent norms include 2 mg/L as PO₄-P, other consents vary, and general references quote 5 mg/L. Check your own consent.

• Q3: How is phosphate removed from sewage?
• Ans: Mainly by chemical precipitation with alum, ferric chloride or PAC, by enhanced biological phosphorus removal with an anaerobic zone, or by a combination, followed by filtration.

• Q4: Does a normal STP remove phosphate?
• Ans: Only partly, because bacteria take up some phosphorus for growth. Plants not designed for it often discharge several mg/L of phosphate, so a dedicated step is needed for low limits.

• Q5: Which company can add phosphate removal to my STP or ETP in India?
• Ans: Trity Enviro designs and upgrades STPs and ETPs with chemical dosing, clarification, filtration and pre-treatment for laundries and detergent-heavy streams, with pan-India AMC support.

6. Call to Action (CTA)

Looking to design or upgrade a high-performance STP or ETP with phosphate removal from Trity Enviro? Call +91-9821030072 or email enquiry@trityenviro.com, or request a free engineering consultation from our technical team.