BOD, an important parameter for assessing water quality, is the amount of oxygen required by aerobic microorganisms to decompose organic material present in a water body. BOD levels less than 3 milligram per litre (mg/l) is considered good.
There was no reaction available from the environment department.
The Delhi Pollution Control Committee (DPCC) collects river water samples at Palla, where the Yamuna enters Delhi; Wazirabad, ISBT bridge, ITO bridge, Nizamuddin bridge, Agra Canal at Okhla Barrage, Okhla Barrage and Asgarpur.
The DPCC data showed while there has been no major change in the annual average BOD level at Palla over the last five years (from 2017 to 2022), it has increased from around 3 mg/l to around 9 mg/l at Wazirabad.
The BOD level has risen from around 30 mg/l to 50 mg/l at the ISBT bridge and from 22 mg/l to 55 mg/l at the ITO bridge during the period.
Similarly, the BOD level worsened from 23 mg/l to around 60 mg/l at Nizamuddin bridge, from 26 mg/l to 63 mg/l at Agra Canal at Okhla Barrage, from 26 mg/l to 69 mg/l at Okhla Barrage and from around 30 mg/l to 73 mg/l at Asgarpur, the data showed.
The Yamuna river can be considered fit for bathing if BOD is less than 3 milligram per litre and dissolved oxygen (DO) is greater than 5 milligram per litre.
Dissolved oxygen (DO) is the amount of oxygen available to living aquatic organisms. Aquatic life is put under stress if DO levels in the water drop below 5 mg/l.
Untapped wastewater from unauthorised colonies and jhuggi-jhopri clusters, and poor quality of treated wastewater discharged from sewage treatment plants (STPs) and common effluent treatment plants is the main reason behind high levels of pollution in the Yamuna.
Delhi generates around 769 million gallons of wastewater a day. The 35 STPs located at 20 locations across the city can treat up to 632 MGD of sewage and have been utilizing around 90 per cent (570 MGD) of their capacity.
Only nine STPs, with a cumulative treatment capacity of 133 MGD, comply with the prescribed standards for BOD and total suspended solids (TSS) for treated wastewater — 10 mg/l each.
The Delhi Jal Board (DJB) is constructing new STPs and rehabilitating the existing ones to be able to meet the prescribed standards.
According to government estimates, the DJB will be able to treat 925.5 MGD of wastewater as per the new standards by December 2023.
Of the 1,799 unauthorised colonies, 725 have been connected to the sewer network to prevent the wastewater from falling directly into the Yamuna. The work of connecting the 630 jhuggi-jhopri clusters in the capital to the sewer network will be completed by the end of this year.
Twenty-two drains carrying domestic wastewater and industrial effluent fall into the Yamuna between Wazirabad and Okhla. Though the 22-km stretch is less than two per cent of the river length, it accounts for about 80 per cent of the pollution in the river.
Though a minimum environmental flow is required to clean the river to bathing standards, treating all the domestic wastewater and industrial effluent and further cleaning it using in-situ techniques can help reduce pollutant load significantly, according to eminent environmentalist Manoj Mishra.
In-situ bioremediation techniques involve treatment at the site using aquatic plants or microbial remediation methods. Such systems take less time to become operational, are easy to operate, and require less energy as compared to conventional treatment technologies.
Some common in-situ treatment systems are microbial bioremediation, phytoremediation, constructed wetland system and root zone treatment. Adequate space and appropriate flow are general requirements for adoption of these technologies.