When organic effluent is discharged into freshwater ecosystems, it can lead to severe environmental degradation. Under aerobic conditions, aerobic microorganisms break down this organic material, depleting the dissolved oxygen in the water. The amount of oxygen required for this aerobic biological decomposition is known as the Biological Oxygen Demand (BOD).
The table below shows typical BOD values for several agricultural and industrial effluents:
| Effluent | BOD (mg of O2 per litre of effluent\text{mg of O}_2\text{ per litre of effluent}mg of O2 per litre of effluent) |
|---|---|
| Treated paper mill discharge | 30−6030 - 6030−60 |
| Untreated paper mill discharge | 300−600300 - 600300−600 |
| Brewery wastewater | 2,000−4,5002,000 - 4,5002,000−4,500 |
| Abattoir washings | 12,000−20,00012,000 - 20,00012,000−20,000 |
| Silage effluent | 75,00075,00075,000 |
Explain which effluent is likely to have the most severe effect on the organisms in a river.
A quantity of effluent is released into a river. The overall effect on the aquatic organisms will depend on the BOD value of the effluent and other factors. Suggest one of these other factors.
Brewery wastewater is one of these effluents. Name one biological molecule found in brewery wastewater that microorganisms could feed on.
Suggest a reason for the difference between the BOD of untreated paper mill discharge and the BOD of treated paper mill discharge.
