Organic impurities may be of vegetable or animal origin. The former, unless present in large quantities, are
not harmful. Animal impurities, which are far more dangerous, are derived from sewage and find their way
into the water by soaking through the soil from manure heaps, sewers, etc., or they may be washed into the
water during heavy rains. Sewage is frequently ‘poured directly into rivers. Hence river water is liable to contamination and great care is necessary in using it as drinking water.