(a) If a large volume of untreated sewage is discharged into a river then the Biological Oxygen Demand (BOD) will get higher which would lead to the death of organisms such as fishes due to low availability of oxygen in the water. Also, large volumes of untreated sewage in the water would cause water borne diseases like cholera and dysentery as people drink water directly from the river.
(b) Anaerobic sludge digestion is important in sewage treatments because it decreases the Biological Oxygen Demand (BOD) and the sludge digestion occurs in the presence of anaerobic bacteria and when the sludge is digested the anaerobic bacteria releases a mixture of gases like methane(CH3), carbon dioxide(CO2) and hydrogen sulphide(H2S). These are the main constituents of biogas.
Thus, anaerobic sludge digestion is important as it not only helps in sewage waste water treatment and reducing the BOD levels to suitable levels for drinking and aquatic animals respiration but it also produces biogas.
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