Which of the following is the least important in the successful operation of an activated sludge plant?

In the context of an activated sludge plant, the biological oxygen demand (BOD) measurement is crucial for understanding the organic load entering the treatment system and evaluating the effectiveness of the treatment process. BOD indicates the amount of oxygen that micro-organisms will consume while decomposing organic matter in the water. It is essential for assessing the health of the treatment process and ensuring that the effluent meets regulatory standards.

However, when comparing BOD to the factors of food entering the process (solids), sludge age, and the presence of organisms that treat the wastes, it is the food solids, sludge age, and organisms that are more directly influential in determining the operational success. The actual treatment mechanism relies on the interaction of microorganisms with the incoming organic material, along with adequate residence time (sludge age) for effective treatment.

In essence, BOD is an important metric, yet it serves more as an indicator of plant operation rather than a direct operating parameter itself. The core factors that directly affect the treatment process—such as the characteristics and amount of incoming solids, the age of the sludge, and the adequacy of the microbial population—are fundamental to maintaining an efficient activated sludge system.