Clean Zone Decomposition Zone Septic Zone Recovery Zone Clean Zone 人 人 人 Normal clean water organisms Trash fish Fish absent,fungi Trash fish Normal clean water organisms (Trout,perch,bass (carp,gar sludge worms carp,gar (Trout,perch,bass mayfly.stonefly) leeches) bacteria eeches） mayfly,stonefy) (anaerobic) 8 ppm 8 ppm Dissolved oxygen Oxygensag Biological oxygen demand 2 ppm Direction of flow Point of waste or heat discharge- Time or distance downstream-> Figure19-Diltion and decay of degradable.oxygen-demanding wastes and heatinhxy gen sag curve(orange)and the curve of oxygen demand(blue).Depending on flow rates and the amount of pollutants,streams recover from oxygen-demanding wastes and heat if they are given enough time and are not overloaded. Figure 2 Self-Purification processes in Rivers(3)Figure 2 Self-Purification processes in Rivers(3)