MUMBAI, July 23 -- The seven sewage treatment plants (STPs) being built across the city will turn grey into gold. The civic administration says the plants, at various stages of construction, will save millions of litres of potable water a day. The grey water produced from sewage treatment will be used in tertiary activities, such as deep cleaning of roads and watering public gardens, thus saving potable water, which currently meets these needs. According to Rajesh Tamhane, deputy municipal commissioner (environment), the total savings would be 1,200 MLD of potable water a day from all seven plants. Improving the quality of sewage discharged into the Arabian Sea will also enhance the health of the marine ecosystem, according to the Brihanmumbai Municipal Corporation (BMC). The seven STPs are being constructed in Worli, Bandra, Dharavi, Versova, Malad, Bhandup and Ghatkopar, at a cost of Rs.17,000 crore. They have a combined treatment capacity of 2,464 MLD. Of the seven new STPs, the plants in Worli, Bandra and Dharavi are expected to be completed by July 2026. The Worli facility is designed to handle 500 MLD of sewage, half of which will be treated and discharged into the sea, while the remaining 250 MLD will be regenerated for reuse in non-potable activities such as gardening and deep cleaning. Tamhane said that, currently, sewage with high biological oxygen demand (BOD) levels is being directly released into the sea. "The initial water treatment will bring water to 10 BOD levels and it will be released into the sea. Marine pollution will decrease. After secondary treatment, we will use ultraviolet treatment or chlorination, and that water will be used for tertiary purposes," Tamhane said. As an example of what can be achieved, the Colaba STP, which is not one of the seven being constructed, is already treating 37 MLD of sewage water, of which 3 MLD is being reused for cleaning and gardening in Navy Nagar and at the Willingdon Club. Treated water is supplied to Willingdon Club at Rs.20 per kilolitre and to Navy Nagar at Rs.1 per kilolitre. The Ghatkopar STP, work on which is more than 40% complete, has received interest from BPCL and HPCL, which plan to use the treated grey water as a cooling agent for their machinery. The Bandra STP will be the only plant to feature open public spaces, including a two-hectare garden, viewing gallery and knowledge centre. The Dharavi STP will boast the tallest sequential batch reactor in Asia (where processing of sewage water takes place), a ground-plus-three storey building, the only multi-storey SPT. The Malad STP was covered with mangroves and BMC took requisite permission from Bombay high court for this plant. Ashok Mengde, deputy chief engineer, BMC, said each plant will incorporate three levels of treatment-primary, secondary, and tertiary. Additionally, biogas produced during the process will be used to generate electricity. As part of a broader vision, the BMC is also planning to mandate that all future redevelopment and high-rise construction projects include in-built sewage treatment systems in line with the Development Control Rules. These mini-STPs within buildings will enable recycling of grey water on-site, further boosting the city's water efficiency....