Context:

• The water quality of the Ganga in 2018 has “improved over last year”, according to a
  written statement in the Rajya Sabha.

Details:

• The three parameters were analysed to understand the quality of water of the Ganga.
• The parameter ‘Dissolved oxygen’ showed considerable increase at 39 locations and
  ‘Biological oxygen demand’ showed demand reduction and also there was an overall
  decrease in faecal coliform.
• These three parameters are a proxy for both the presence of aquatic life as well as microbes
  that may be harmful to these biota, and are conventionally used to assess the quality of the
  river.

Dissolved oxygen:

• Presence of organic and inorganic wastes in water decreases the dissolved Oxygen (DO) content of the water. Water having DO content below 8.0 mg/L may be considered as contaminated. Water having DO content below. 4.0 mg/L is considered to be highly polluted. DO content of water is important for the survival of aquatic organisms. A number of factors like surface turbulence, photosynthetic activity, O2 consumption by organisms and decomposition of organic matter are the factors which determine the amount of DO present in water.
• The higher amounts of waste increase the rates of decomposition and O2 consumption, thereby decreases the DO content of water.

Biological oxygen demand:

• The demand for O2 is directly related to increasing input of organic wastes and is expressed as biological oxygen demand (BOD) of water.
• Water pollution by organic wastes is measured in terms of Biochemical Oxygen Demand (BOD).
• BOD is the amount of dissolved oxygen needed by bacteria in decomposing the organic wastes present in water. It is expressed in milligrams of oxygen per litre of water.
• The higher value of BOD indicates low DO content of water. Since BOD is limited to biodegradable materials Therefore, it is not a reliable method of measuring pollution load in water.

Chemical oxygen demand:

• Chemical oxygen demand (COD) is a slightly better mode used to measure pollution load in water. COD measures the amount of oxygen in parts per million required to oxidize organic (biodegradable and non-biodegradable) and oxidizable inorganic compounds in the water sample.