Mumbai Hardlook: Treating Mumbai’s wastewater

The new STPs will replace Mumbai’s outdated sewage infrastructure built in the 1980s and 1990s, which no longer meets current environmental standards. The project, aiming to reduce water pollution and address Mumbai’s daily water supply deficit, will ensure sustainable water management, improve water quality around the city, and create a healthier environment for both marine life and residents.
In the current budget, the BMC allocated Rs 5,545 crore for the STP initiative, which will treat 2,464 MLD of water daily. The facilities are expected to be operational by July 2026.

How the wastewater treated now

Out of the 3,850 million litres per day (MLD) of water consumed by Mumbai, approximately 3,040 MLD (78.96%) becomes wastewater that flows through the city’s intricate 2,070-kilometre sewage network, reaching various sewage treatment stations (STPs) for initial processing. The operational STPs at Worli, Bandra, Versova, Malad, Bhandup, and Ghatkopar handle sewage from their respective areas. After basic treatment, the wastewater is typically discha-rged into the creeks and the sea.

These new plants will replace the older, less efficient facilities and incorporate the latest technology to treat wastewater

One of the oldest STPs in Mumbai is located past the Eastern Express Highway (EEH) in Ghatkopar. This facility was built when Ghatkopar was considered the suburban fringe and was far from the urban sprawl seen today. Situated near a natural inlet of Vashi Creek, the treated sewage from Ghatkopar is disposed of into this water body. The Ghatkopar plant primarily serves the eastern suburbs, including Chembur, Mankhurd, and Govandi — areas that contain significant slum populations lacking conventional sewage systems. Additionally, these areas are close to industrial zones such as refineries and chemical factories, meaning the wastewater is often rich in both organic materials and chemical waste.

The Ghatkopar facility employs four man-made lagoons for its treatment process. The first stage removes floating waste, such as plastics, followed by the extraction of grit particles like sand and stones. The untreated water is then pumped into one of the four lagoons, where anaerobic micro-organisms break down the contaminants. Mechanical aerators further assist in the process. Once treated, the wastewater is discharged into the inlet. During dry months, Ghatkopar processes around 120 MLD of wastewater, and during the monsoon, this figure rises to 200 MLD.

The Central Pollution Control Board (CPCB) measures water pollution levels using the Biological Oxygen Demand (BOD), which indicates the amount of oxygen consumed by micro-organisms to decompose organic material in water. Higher BOD readings signify greater pollution levels. The CPCB’s permissible BOD limit for treated wastewater is 30 mg/liter. However, data from the civic body reveals that the average BOD level from Ghatkopar’s treated wastewater is 50 mg/litre, indicating that the treated water is still more polluted than desired.

The project, which aims to modernize the city’s sewage treatment system, is expected to be completed by the end of 2027

The Bandra wastewater treatment facility, operational since the mid-1990s, serves areas like Mahim, Santacruz, Bandra Kurla Complex (BKC), Dharavi, Sion, and Matunga. Unlike Ghatkopar, the Bandra plant only uses a primary treatment system, which removes floating waste and grit. After basic treatment, the wastewater is discharged into the sea through a 3.4-kilometre tunnel known as the Marine Outfall (MO). The tunnel is designed to release the wastewater far from the shore and human habitation. At present, the average BOD level for treated wastewater at the Bandra plant is 100 mg/litre, higher than other facilities.

Other operational STPs, such as those at Bhandup and Versova, also use lagoon systems for treatment, while the Worli facility discharges wastewater via MO tunnels. In contrast, the Malad plant releases wastewater directly into the sea with minimal treatment, only removing floating materials and grit.

“Areas like Worli are close to the sea, so the MO tunnels were built to ensure wastewater is disposed of far from human habitation. In places like Ghatkopar and Bhandup, where tunnels could not be constructed due to proximity to creeks, the lagoon system was implemented to provide basic treatment before discharge. Discharging water into creeks with BOD levels of 100 mg/litre would be highly detrimental to the environment,” said Ashok Mengde, deputy chief engineer of the Mumbai Sewage Disposal Project (MSDP).

Worli facility to be the largest

Among the seven proposed sewage treatment plants (STPs) in Mumbai, the Worli facility will be the largest, with a processing capacity of 500 MLD. Following Worli, the remaining plants will have capacities of 454 MLD at Malad, 418 MLD at Dharavi, 360 MLD at Bandra, 337 MLD at Ghatkopar, 215 MLD at Bhandup, and 180 MLD at Versova. Combined, these seven plants will have a total processing capacity of 2,464 MLD.

Prime Minister Modi performed the bhoomi poojan ceremony for these 7 new STP plants in January 2023. The project is being carried out by the Brihanmumbai Municipal Corporation at an estimated cost of Rs 26,000 crore

At present, the Bandra plant handles 622 MLD of wastewater from the western suburbs and central Mumbai. To manage this, the load will be redistributed between the Bandra facility and the new plant at Dharavi, which is being built from scratch. Except for Dharavi, the six remaining plants will be constructed by reclaiming the land where the existing STPs currently operate. For example, at the Ghatkopar facility, two of the four existing lagoons will be repurposed for the new plant.

Five-step treatment

In contrast to the current two-step treatment process, the new plants will adopt a more advanced, five-step treatment methodology, including the initial treatment where floating waste and grit particles, such as stone chips and sand, will be removed. Then comes pollutant removal where organic compounds and other pollutants will be extracted. Next is the oxidation treatment to increase oxygen levels in the wastewater, aiding micro-organisms in breaking down remaining dissolved pollutants. Membrane filtration is next, which will remove any remaining contaminants, and final step is gravity setting, which will ensure the treated water is free from pollutants before being discharged into natural water bodies.

The treated water will be classified into two categories — secondary treated water that will be discharged into creeks and the sea and tertiary treated water that will be recycled for non-potable purposes.

Once operational, these plants are expected to significantly reduce water pollution in natural bodies. The biochemical oxygen demand (BOD) of the secondary treated water will be reduced to 10 mg/litre — five times lower than the current discharge levels at Bhandup, Ghatkopar, and Versova, and 10 times lower than those at Bandra, Worli, and Malad.

Though these sites were originally chosen for their location on the outskirts of residential areas, urban growth has led to habitation clusters developing around these facilities. As a result, the new plants will be entirely odour-free, unlike the existing facilities, where the smell of wastewater is noticeable, especially near the periphery.

Additionally, the new plants will incorporate chlorination, UV treatment, and disinfection before directing the tertiary-treated water into a separate chamber, where it will be recycled for non-potable purposes.

“All seven facilities are designed to enable 100 per cent of the treated wastewater (2,464 MLD) to be recycled for non-potable or tertiary use. Initially, we’ve earmarked 50 per cent of the total capacity for recycling, but over time, we plan to expand this and reduce the volume of water discharged into creeks and natural bodies,” said Ashok Mengde, deputy chief engineer of MSDP.

These new STPs will not only treat the city’s sewage more effectively but also make the treated water reusable, addressing the growing need for water conservation in the megacity

“Multiple industrial and petroleum organisations have approached us for purchasing tertiary-treated water for non-potable uses in their operations. As the plants reach full capacity, we anticipate more interest, which will help reduce water discharge into natural bodies and lower dependency on the city’s potable water supply for commercial purposes,” he added.

According to the BMC’s timeline, the STPs at Ghatkopar, Bhandup, and Versova will be operational by 2026, while the Worli, Bandra, and Dharavi facilities are expected to start in 2027, with the Malad plant to be completed by 2028. To ensure timely completion, each STP will be managed by a single contractor.

“The Malad facility experienced delays due to lengthy land acquisition processes, but civil work commenced last year. Despite the delay, we’ve already completed 12.04 per cent of the physical work,” Mengde said.

Bridging Mumbai’s water supply gap

As an island city, Mumbai lacks natural water resources and rely heavily on external sources to meet its water needs. The Brihanmumbai Municipal Corporation (BMC) supplies 3,800 MLD of water daily to the city, sourced from seven lakes — five located in the satellite districts of Thane, Palghar, and Nashik, and two in Mumbai. The water is sent to the BMC’s filtration complex in Bhandup, from where it is distributed to households and commercial establishments across the city.

At present, Mumbai’s water demand stands at 4,250 MLD, while the BMC can only supply 3,850 MLD, resulting in a deficit of 400 MLD. This shortfall translates to a 15 per cent water shortage, particularly during the peak summer when delayed monsoons exacerbate the problem.

The new sewage treatment plants (STPs) will play a pivotal role in addressing this gap. Once operational, the plants will generate 1,232 MLD of tertiary-treated water, nearly three times the city’s current water deficit. The treated water will be recycled for non-potable uses, significantly reducing the pressure on Mumbai’s potable water supply.

The BMC is also planning to construct a water service tunnel that will transport 970 MLD of tertiary water daily from the STPs at Ghatkopar, Dharavi, and Bhandup to the BMC’s filtration complex. The BMC has allocated Rs 840 crore for this project in this year’s budget. Additionally, Rs 450 crore has been set aside for the development of a new 2,000 MLD water treatment plant at the Bhandup filtration complex.

“While there is a stigma surrounding recycled treated water, particularly for drinking purposes, it’s important to recognise that the 1,232 MLD of tertiary water generated by the STPs will be equivalent to the water supplied from at least four lakes today,” said Abhijit Bangar, additional municipal commissioner (projects). “By investing in this infrastructure, we are ensuring Mumbai is prepared for its future water needs,” added Bangar

Bangar also emphasised that before any tertiary-treated water is used for potable purposes, a process of public consultation will take place, allowing citizens to voice their opinions and concerns. “In countries such as Singapore, tertiary water is mixed with raw water in dams, then filtered before being supplied to houses. We will adopt a similar approach in Mumbai, which is why we are expanding the capacity at our filtration complex,” he added.

With these developments, the STP project will not only help alleviate Mumbai’s current water crisis but also pave the way for sustainable water management in the city’s future.

New open spaces

In addition to addressing Mumbai’s water supply challenges and reducing pollution in natural water bodies, the STP project is set to provide Mumbaikars with much-needed accessible open spaces and recreational areas.

The Bandra STP, in particular, will feature a bio-reactor unit and a multi-layered design, with the basement of the reactor located at least 10 metres below the ground level. This innovative design will create an additional two-acre space on the terrace, which will be transformed into a lush green zone and recreational area for the public.

Given the sea-facing location of this STP, the BMC also plans to build a viewing gallery adjacent to the plant, offering visitors an unobstructed view of the Arabian Sea. In addition, a 2,500-square-meter knowledge center will be established at the facility. This centre will cater to students and science enthusiasts, providing an opportunity to learn about water recycling and sustainability.

“The Bandra STP is situated near the Bandra-Worli Sea Link (BWSL), which imposed a height restriction to preserve the skyline’s view. As a result, we designed the facility to be multi-level, with the top portion dedicated to public green spaces,” according to an official on the condition of anonymity.

“Since the facility will be completely odour-free, visitors will not even be aware that an STP is located beneath the garden. To ensure better access, a service road will also be constructed by the roads department,” the official added.