Around 25% of Mumbai’s domestic waste, which comes from slums, flows straight into nullahs and creeks, polluting the marine ecosystem. But that doesn’t mean that the remaining 75% waste, which enters the city’s 1,915km sewer network, is properly treated before being released into the sea.

The Maharashtra Pollution Control Board’s 2017 review found that six of the eight sewage treatment plants (STPs) in Mumbai released treated waste with high levels of BOD (Biochemical oxygen demand) and COD (Chemical Oxygen Demand), which are wastewater quality indicators. These six STPs treat over 80% of the waste that enters the sewer network.

In all, the eight STPs—at Worli, Colaba, Versova, Bandra, Ghatkopar (two plants), Malad and Charkop—treat around 2,600 million litres of sewage—human and kitchen waste—per day.

“BOD outlet values for all STPs except those at Versova and Ghatkopar II are beyond the standards provided by MPCB. Outlet values for COD at all STPs are beyond the standards,” said a recent MPCB report presented to the legislative assembly in the winter session.

According to sources, the major reason is that the sewage receives only preliminary treatment before it is pumped 3km into the sea. The BMC’s Malad STP, which handles the waste of 40 lakh people, is perhaps the worst, as it is limited to preliminary treatment and the waste is discharged in the Malad creek. The dissolved oxygen (DO) level in the Malad creek has thus reached lowest levels ever.

When asked about the issue, a senior BMC official claimed that all the STPs have been treating sewage as per the base norms prescribed by MPCB. BMC has a Rs 12,000-crore plan ready to set up seven new plants at the same locations where the existing ones stand. The plants will include tertiary treatment. According to BMC sources, the treated water will then be recycled and reused for industrial gardening and supplied to construction sites.

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