DISCUSSION

PROFESSOR HAMLIN

There are three concepts which are most important and should be remembered. One is the notion of unit processes. Each unit in a treatment plant can be conceived of as a process and for the proper design of each of them it is necessary to determine the objective of the process. Mathematically this can be defined as an objective function. In primary sedimentation the objective function may be to remove 90% of the suspended solids and there is little doubt that this could be achieved. It might necessitate the use of a chemical addition to the incoming flow but, if this objective function was stipulated, it could be met. The problem is that in setting the objective function of one unit one automatically affects what happens in subsequent units. Therefore it is necessary to take into account in setting objective functions what follows the particular unit one is designing, and one definition of the optimum design is that which, at minimum cost, produces the effluent standard required by a suitable choice of objective functions for each of the units in the design. Overall improvement of design is needed and not the improvement of a single unit so that while it is possible to conceive of various units in a process operating by themselves, they cannot be considered in isolation from the treatment process as a whole.