Fewer polluted river stretches but worst stretches unchanged

Subject :Environment

Context:

The number of polluted stretches in India’s rivers has fallen from 351 in 2018 to 311 in 2022 though the number of most polluted stretches is practically unchanged, according to a report from the Central Pollution Control Board (CPCB).

Identification of Polluted river stretch:

Biochemical Oxygen Demand (BOD) exceeding 3.0 mg/L (milligram per litre) is identified as a polluted location.
Two or more polluted locations identified on a river in a continuous sequence are considered a “polluted river stretch.”
A BOD less than 3 mg/L means the river stretch is fit for ‘Outdoor Bathing.’
Further, stretches with a BOD exceeding 30 mg/L are considered ‘Priority 1,’ meaning, the most polluted and thus needing the most urgent remediation.
There are five such categories with ‘Priority 2’ indicating a BOD of 20-30 mg/L and ‘Priority 5’ indicating 3-6 mg/L.
The success of river-cleaning programmes is measured by the number of stretches moving from 1 to 2, 2 to 3 until those in 5 (requiring the least action) to reduce.

CPCB report on the status of polluted river stretches:

The CPCB network monitors water quality at 4,484 locations in 28 States and seven Union Territories including rivers, lakes, creeks, drains and canals.
n 2018, when the CPCB published its report, there were 45 stretches categorised in Priority 1, 16 in Priority 2, 43 in Priority 3, 72 in Priority 4 and 175 in Priority 5.
The latest report counts 46 in P1, 16 in P2, 39 in P3, 65 in P4 and 145 in P5.
All of the improvements thus were in river stretches that required relatively lesser intervention.
There is no change/ slight change in the Priority I & II category of polluted river stretches.

States-wise status of polluted rivers:

While Gujarat and Uttar Pradesh had the maximum number of ‘Priority 1’ river stretches (6).
Maharashtra had the maximum number of polluted river stretches i.e. 55, followed by Madhya Pradesh (19), Bihar (18), Kerala (18), Karnataka (17) and Uttar Pradesh (17).
Every State had to ensure that at least one river stretch was “restored” to the extent that it was at least fit for bathing.

Biological Oxygen Demand:

Biological Oxygen Demand is the amount of dissolved oxygen needed by microorganisms to decompose organic matter (waste or pollutants) under aerobic reaction (in the presence of oxygen).
The more organic matter there is (e.g., in sewage and polluted bodies of water), the greater is the BOD.
Greater BOD, the lower the amount of dissolved oxygen available for higher animals such as fishes.
The BOD is therefore a reliable gauge of the organic pollution of a water body.
One of the main reasons for treating wastewater prior to its discharge into a water resource is to lower its BOD i.e. to reduce its need of oxygen and thereby lessen its demand from the streams, lakes, rivers, or estuaries into which it is released.

Dissolved Oxygen:

It is the amount of dissolved oxygen present in the water which is needed for aquatic life to survive. The quality of water increases with an increase in DO levels.

A DO level of 5 mg/l or above is the recommended level for bathing in a river.