The pollution body said untreated wastewater from 22 drains led to frothing in the Yamuna river.
The Central Pollution Control Board (CPCB) has asked the Delhi Jal Board (DJB) to ensure 100 per cent collection and treatment of domestic sewage generated in the city, saying untreated wastewater from 22 drains led to frothing in the Yamuna river.
Data collected in October shows that the Yamuna river water quality doesn't meet the criteria for bathing standards in the "entire Delhi stretch" except Palla village, the CPCB said in a communication to the DJB.
The pollution watchdog said continuous discharge of partially treated and untreated wastewater from 22 drains resulted in frothing downstream of ITO and Okhla barrage.
"Therefore, the DJB shall ensure 100 per cent collection of domestic wastewater generated in Delhi and also provide treatment as per the consented norms of the Delhi Pollution Control Committee," it said.
The CPCB also asked the DJB to ensure that no untreated domestic wastewater is discharged into any of the drains.
The water utility has been asked to adhere to the timelines set by the National Green Tribunal in September last year with respect to the Interceptor Sewer Project (ISP) and to rectify deficiencies of the existing sewage treatment plants (STPs).
Earlier, sewage water from unauthorised colonies would run directly into the three main drains in the city -- Najafgarh, Supplementary and Shahdara -- that flow into the river.
The ISP blocks waste water from unauthorised colonies and diverts it to nearby STPs that releases the treated effluent into the main drains, reducing pollution in the Yamuna.
Visuals of toxic froth floating on the surface of the Yamuna river near Kalindi Kunj in Delhi made their way back to social media recently, with experts citing detergents as one of the major reasons behind the pollution.
Majority of the detergents in the country don't have a certification by the ISO (International Organization for Standardization), which has capped the concentration of phosphates in the chemical substance, an official of the CPCB had said.
The primary reason behind the formation of the toxic foam was high phosphate content in the wastewater because of detergents used in dyeing industries, dhobi ghats and households, according to the official.
"A large number of unbranded detergents are also used in households and dyeing industries. The wastewater containing high phosphate content reaches the river through untapped drains," he had said.
The level of dissolved oxygen (DO) -- the amount of oxygen available to living aquatic organisms -- is "nil" at seven of the nine ghats along the banks of the Yamuna in Delhi, according to tests conducted by the Delhi Pollution Control Committee (DPCC) in early November.
The biological oxygen demand (BOD), which should be 3 milligrams per litre or less, was as high as 45 mg/l at some places, the readings show.
Aquatic life is put under stress if dissolved oxygen levels in water drop below 5 milligram per litre, according to experts.
DO levels were 7.5 mg/l at Palla, where the Yamuna enters Delhi, and 6.3 mg/l at Surghat (downstream of Wazirabad barrage).
At rest of the places - including Khajori Paltoon Pool, Kudesia ghat, ITO bridge, Nizamuddin, Agra canal (Okhla), after meeting Shahdara drain and Agra canal (Jaitpur), the DO levels were "nil", according to DPCC''s "water quality status of river Yamuna" report.
This means the river was relatively clean at Palla, but the water quality deteriorated significantly by time it reached Khajori Paltoon Pool, which falls downstream of Najafgarh drain.
However, in October, the DO levels were "nil" only at three places -- Khajori Paltoon Pool, Kudesia ghat, ITO bridge. This means the Yamuna water quality deteriorated over the last month despite no immersion taking place.
The BOD was the highest at Khajori Paltoon Pool (45 mg/l) in November and at Kudesia ghat (50 mg/l) in October.
Biochemical oxygen demand is the amount of dissolved oxygen used by microorganisms in the biological process of metabolizing organic matter in water.
High BOD levels mean there is a high level of microorganisms in the water, and a high content of organic material that is broken down by the organisms.
The greater the BOD, the lower the amount of dissolved oxygen available for fishes and other aquatic life.
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