A fundamental concept in science is the law of conservation of matter. This means that when there is no appreciable conversion of mass into energy, the sum of the masses of substances entering into a reaction must always equal the sum of the masses of products of reaction. Even if there in no chemical reaction occurring, the law of conservation underlies the concept of mass balance (also called material balance), and is useful in environmental technology
Mass balance calculations play an important role in the design and operation of water, sewage, air and solid waste treatment processes. In treatment systems, the physical, chemical, and biological processes usually occurs in vessel or tanks called reactors, and the particular reactions or processes are referred to as unit processes. In the simplest case, it can be said that the input must equal the output, or, in other words, “what goes in must go out.” If this does not occur, there must be an accumulation (depletion) of the material in the reactor equal to the difference between the input and output, or accumulation = input-output. Since, in this kind of situation, the composition of material in the reactor changes with time, it is referred to as an unsteady-state operation. In the steady state operation, it can be assumed that the rates of input and output are constant, as is the composition of the completely mixed reactor.
Suppose, for example, two pipes containing salt solutions discharge into a tank in which the two solutions are completely mixed, and the third pipe carries the mixture out of the tank