The Municipal Corporation of Greater Mumbai (MCGM) is constructing seven wastewater treatment facilities (WWTFs) in Mumbai with a combined treatment capacity of around 2,500 million litres per day (mld). Of these, the Worli WWTF is the largest facility, having a capacity of 500 mld, which can further be extended up to 800 mld to accommodate extra water flow during monsoons. The other six facilities are being developed at Malad (454 mld), Bandra (360 mld), Ghatkopar (337 mld), Dharavi (418 mld), Bhandup (215 mld) and Versova (180 mld).
Worli WWTF: Project snapshot
Objectives
The Worli WWTF is being built to enhance the wastewater treatment capacity and improve disposal processes in order to produce high-quality effluents and biosolids. It is being developed under the Master Plan (2022) and the Mumbai Sewage Disposal Project Stage II – Priority Works on a design, build and operate basis. The project’s contractor is Suez India, while Sweden-based company IVL Svenska Miljöinstitutet is the consultant.
There are four main objectives of the project, which are:
- Improve the quality and reliability of wastewater treatment and disposal.
- Restore the quality of water in the Arabian Sea by ensuring 100 per cent compliance of the secondary treated effluent with the latest pollution control standards set by the National Green Tribunal (NGT).
- Produce tertiary quality effluent that complies with norms set by the Central Public Health and Environmental Engineering Organisation (CPHEEO) for non-potable urban and industrial standards.
- Achieve 25 per cent dryness and produce Class A quality sludge in accordance with the standards set by the United States Environmental Protection Agency (USEPA).
In keeping with the above objectives and standards, the treated secondary effluent will be released into the Arabian Sea and thus help in the restoration of water quality. The discharged water will strictly adhere to the latest pollution control standards set by the NGT, ensuring a biochemical oxygen demand of less than 10, total suspended solids not exceeding 20, chemical oxygen demand below 50, total coliforms less than 10, phosphorus levels below 1 and faecal coliforms not exceeding 100 mpn (most probable number) per hundred ml. Furthermore, 50 per cent of the wastewater will undergo tertiary treatment, equivalent to 250 mld, tailored for non-potable urban and industrial usage in alignment with the norms outlined by the CPHEEO of the Ministry of Housing and Urban Affairs.
For the residual sludge after treatment of the wastewater, the project aims to produce biosolids of USEPA Class A quality, which will ensure the project’s commitment to both environmental sustainability and meeting stringent quality standards throughout the wastewater treatment and disposal processes.
Treatment technologies in place
The Worli WWTF features several advanced in-house state-of-the-art technologies developed by Suez India, including high-rate primary clarifier, membrane bioreactors, advanced sludge digestion process and advanced heat recovery units.
- Primary treatment
For primary treatment, the Worli WWTF utilises a high-rate primary clarifier. It is a compact system that incorporates stages of coagulation/flocculation, water clarification and settling/thickening of sludge. For the wastewater treatment plants, the grit removal and degreasing stages are performed within the clarifier. This technology enables fast physico-chemical settling, clarification and sludge thickening within a single unit. The plates on the surface of the clarification zone enhance the surface area, improve circularity and facilitate the continuous recycling of sludge, thereby increasing sludge concentration. The Suez primary clarifier has a footprint 8 to 10 times smaller than conventional clarifiers. It offers benefits such as efficient treatment for multiple applications (wastewater, drinking water etc.); fast and easy operation even in the event of significant variations in flow and/or load; no abrasive ballasting material to be handled, removed or replaced; flexible fitting options that can be tailored to any plant size; and compactness of the equipment due to its modular and lamellar design.
- Biological treatment
This process combines biological treatment using activated sludge and clarification by immersing ultrafiltration membranes, producing superior-quality treated effluent. This method employs the membrane bioreactor technology. It is a process of wastewater treatment through hollow fibre ultrafiltration membranes, adapted to all installation sizes, combining biological treatment with membrane filtration. This refined biological treatment enables discharges in protected zones, protecting biodiversity. The membrane bioreactor technology offers various other benefits such as conserving resources due to the reuse of effluent for specific needs (irrigation, watering city parks and gardens, etc.) and reducing environmental footprint owing to its compact size. It is a fully automated operation, ensuring the safety of operating partners, and is the first step in the treatment of micropollutants. At the Worli WWTF, filters with a pore size of 0.35 micrometres (µm) have been used, creating a physical barrier against bacteria.
- Sludge treatment
The final step involves treating the residual sludge after the filtration of treated water. This step involves two processes – advanced sludge digestion and advanced heat recovery unit. The advanced sludge digestion process aims to maximise biogas production and optimise sludge hygienisation. It incorporates a two-step anaerobic digester, with the first phase involving thermal hydrolysis and the second phase consisting of a mesophilic digester with a heat recovery system. Suez’s two-phase digester offers advantages such as a 40 per cent reduction in volume compared to conventional digesters, offering a simple, low-tech solution that is easy to operate. It does not require precise vessel specifications, height, or temperature, leading to increased biogas production with less digested sludge.
In the advanced heat recovery unit process, sludge is used to recover heat from a flash-cooling recycle system, combining a vacuum cooler and condenser. This system by Suez requires approximately 50 per cent less heating surface area compared to conventional heat exchange systems, recovering about 80 per cent of heat, unlike the conventional systems. Additionally, this system requires low maintenance since the sludge is not in contact with the heat exchanger, resulting in minimal heat loss and significant footprint reduction. It can easily adapt to temperature variation requirements due to seasonal changes.
Challenges
The new treatment plant is being constructed in place of an existing sewage treatment facility, which will remain operational during the construction period. This is a significant challenge for the contractor, who must keep the existing plant in operation while constructing the new facility. Consequently, the project is being executed in a phased manner. In the initial phase, an identical treatment facility will be built before the old facility is demolished. Another challenge that Suez faces is the limited area of only 8.5 hectares, characterised by an irregular geometry. This presents a unique challenge in designing and constructing a treatment plant with a required capacity of 800 mld. Thus, Suez is committed to maximising the self-sustainability of the plant while minimising the need for tree-cutting.
Key takeaways
The project is expected to have a positive impact on the climate and water resource preservation. It is projected to mitigate around 31,800 tonnes per annum of carbon dioxide emissions. Further, the Worli facility is expected to generate 39 gigawatt-hours of electrical energy from biogas annually and is expected to be 76 per cent self-sufficient in energy. With this, the facility aims to recycle around 250 mld of treated effluent for non-potable purposes, advancing on the path towards a circular economy.
Upon completion, the Worli WWTF will stand out as the largest membrane bioreactor technology-based sewage treatment plant in India. With comprehensive operation and maintenance services planned for 15 years, it aims to achieve the highest quality water and sludge. This facility will serve as a showcase for Suez’s compact footprint technologies.
Based on a presentation by Punit Singh, General Manager, SUEZ India, at a recent India Infrastructure conference