There is no denying the complexity of an activated sludge process. There are so many variables including the “quality” and quantity of the organic compound in the wastewater entering a biological reactor, wastewater temperature, pH, nutrients, flow rate, the list goes on and on. So let’s acknowledge the complexity and then move on so we can break it down into a simpler model for understanding how we, the operator, can control this process.
The effectiveness of the activated sludge treatment process in reducing the organic load depends on:
Bacteria need a supply of substrate or food, predominantly carbon, for building new cellular material plus a supply of growth energy for carrying out that work. The bacteria utilizing organic material (COD/TOC/BOD5) obtain their growth energy by oxidizing about one third of the organics to inorganic products such as water, carbon dioxide, and ammonia. With this energy they convert the other two thirds to new bacterial cells.
At the same time, the bacteria suffer a continuous loss of mass due to oxidative mechanisms such as maintenance of cellular components and functions, death and reuse of bacterial cells and predation by higher organisms. Thus in the absence of an organic supply, the mass of solids declines. These mechanisms causing loss of mass result in a base oxygen demand known as endogenous respiration.
Look at Figure 1 below. When all is said and done, we only have three control points!! I have not oversimplified. We generally cannot reduce the flow coming into the plant. Though, in an industrial wastewater system, one purpose of the equalization tank is to regulate flow and, therefore, the loading. But at time point we are going to have to process the wastewater backing up in the EQ tank. The most important technique used to control the activated sludge process is controlling the solids inventory in the system with the wasting rate. The wasting of sludge affects the process more than any other process control adjustment.
Figure 1: Operator Control of the Activated Sludge Process
The waste activated sludge (WAS) removed from the process affects:
Figure 2: Microorganism Growth
You need to be aware that as the mixed liquor suspended solids (MLSS) increase, the rate at which they settle in the clarifier will decrease. This is depicted in Figure 3. There are always times when, for one reason or another, you cannot waste enough sludge. As the MLSS concentration increases beyond what you need for the oxidation of organics, your oxygen demand in the biological reactor also increases. That often leads to a drop in the dissolved oxygen concentration because the aerators can’t add enough oxygen. Now your problems are starting to compound. The increase in the MLSS concentration reduces settling in the clarifier leading to a loss of solids in the clarifier effluent and reduced effluent quality. The drop in the DO concentration in the aeration basin sets up an environment conducive to the growth of filamentous bacteria. If the filamentous population growth becomes excessive you will experience a further deterioration in solids removal in the clarifier.
Figure 3: MLSS Settleability
Sludge age or mean cell residence time (MCRT) is the key operating parameter as it directly controls nearly all other parameters of interest. Sludge age is the average solids retention time (SRT) in the process. The solids retention time equals the mass of solids in the aeration basin divided by the mass of solids leaving the system each day.
Typical Sludge Age Ranges
There is a minimum sludge age below which no bacterial growth can occur. The minimum sludge age is less than one day.