Explanation
When a large amount of sewage wastewater is allowed into a river, its biological oxygen demand will increase, not drop to zero.
Sewage wastewater contains a large amount of organic matter, such as human waste, food scraps, and other organic materials. When this wastewater is released into a river, bacteria in the water begin to break down the organic matter through a process called decomposition.
During the decomposition process, the bacteria consume dissolved oxygen in the water, which is necessary for their survival and the breakdown of organic matter. As the amount of organic matter in the wastewater increases, the demand for dissolved oxygen also increases, leading to a decrease in the amount of oxygen available for other aquatic organisms.
If the biological oxygen demand in a river were to drop to zero, it would indicate that all of the organic matter in the wastewater had been decomposed and there were no bacteria left in the water to consume the oxygen. However, in reality, this scenario is highly unlikely as some organic matter will always remain in the water, and there will always be bacteria present to consume the oxygen.
