PROCESS of decreasing the concentration of a solute in a solution, USUALLY simply by mixing with more solvent like adding more WATER to the solution. To DILUTE a solution means to add more solvent without the addition of more solute.Dilution of sewage:In the sewage when the dissolved oxygen decreases then aerobic bacteria can not survive in the absence of oxygen which decomposes the organic matter, so to develop the aerobic condition of sewage large quantity of water in which dissolved oxygen is PRESENT in sufficient quantity is mixed with sewage and this process is also termed as dilution.