Water purification in rivers

Originally mankind depended on the fundamental process of self-purification in rivers. Self-purification is a well-known process in nature to restore and preserve the water quality of a river. This natural process is composed of the following steps ( Fig. 5.1 ). Figure 5.1 a) Natural biological cycle in streams, b) food chain in the ecological system A flowchart illustrating the natural biological cycle in streams and the food chain in the ecological system. https://www.w3.org/1999/xlink" xlink:href="#long-desc34"/> https://www.w3.org/1999/xlink" xlink:href=" https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781003760962/4c83110b-3e32-4fe7-abf4-b91ad93bae27/content/pg130_1.tif "/> Reaeration. Mass transfer through the interface between the surface of a stream and the atmosphere increases the dissolved oxygen concentration (DO) in the stream. The transfer rate is proportional to the deficiency in DO. Oxidation of organic waste. With discharge of organic matter into a river, the bacteria begin to grow and consume the DO so as to oxidize the organic waste. The biochemical process of the oxidation is described in the following section. The bacterial population and then the protozoa and fish populations also increase in accordance with the ecological food-chain pyramid if the amount of dissolved oxygen is sufficient. Production of CO 2 . Increased fauna population (bacteria, protozoa and fish) contributes to the decrease in DO and to the increase in the amount of carbon dioxide (CO 2 ) dissolved in the water that is necessary for algal photosynthesis. The deficiency in DO causes the process of reaeration in 1. Supply of oxygen by algal photosynthesis. Algal growing on the surface of gravel and pebbles on a river bed supply oxygen as a result of photosynthesis. For algal photosynthesis to occur, the stream transparency must be high; i.e. the water should be clean enough for the sun’s rays to reach the bottom.