• Aeration time

• Wastewater toxicity

To obtain the desired level of performance in an activated sludge system, a proper balance must be maintained among the amount of food (organic matter), organisms (activated sludge), and dissolved oxygen (DO). The majority of problems with the activated sludge process result from an imbalance among these three items.

To fully appreciate and understand the biological process taking place in a normally functioning activated sludge process, the operator must have knowledge of the key players in the process: the organisms. This makes a certain amount of sense when you consider that the heart of the activated sludge process is the mass of settleable solids formed by aerating wastewater containing biological degradable compounds in the presence of microorganisms. Activated sludge consists of organic solids plus bacteria, fungi, protozoa, rotifers, and nematodes.


To understand the microbiological population and its function in an activated sludge process, the operator must be familiar with the microorganism growth curve. In the presence of excess organic matter, the microorganisms multiply at a fast rate. The demand for food and oxygen is at its peak. Most of this is used for the production of new cells. This condition is known as the log growth phase (see Chapter 8, Figure 8.15). Over time, the amount of food available for the organisms declines. Floc begins to form, while the growth rate of bacteria and protozoa begins to decline. This is referred to as the declining growth phase (Figure 8.15). The endogenous respiration phase occurs as the food available becomes extremely limited and the organism mass begins to decline (Figure 8.15). Some of the microorganisms may die and break apart, thus releasing organic matter that can be consumed by the remaining population.

The actual operation of an activated sludge system is regulated by three factors: (1) the quantity of air supplied to the aeration tank, (2) the rate of activated sludge recirculation, and (3) the amount of excess sludge withdrawn from the system. Sludge wasting is an important operational practice because it allows the operator to establish the desired concentration of MLSS, food-to-microorganisms ratio, and sludge age.

Note: Air requirements in an activated sludge basin are governed by: (1) biochemical oxygen demand (BOD) loading and the desired removal effluent, (2) volatile suspended solids concentration in the aerator, and (3) suspended solids concentration of the primary effluent.

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