Process Control

Operator Control of the Activated Sludge Process

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.

Process Control

The effectiveness of the activated sludge treatment process in reducing the organic load depends on:

  • The amount of activated sludge solids in the system and,
  • The health of the microorganisms.

Requirements for Control

  • Know your  influent characteristics (TSS, COD, pH, temperature, nutrients).
  • Is the organic load to the aeration system (biological reactor)  “refractory?”
  • Know the mass of microorganisms available to provide treatment which is estimated by the mixed liquor volatile suspended solids (MLVSS) concentration.
  • Insure that sufficient oxygen is available in the aeration tank.
  • Keep your bugs happy (DO, pH, nutrients, quantity of food).

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.

System Requirements

  • An aeration basin with provisions for oxygenation and mixing of the activated sludge and wastewater, which is termed “mixed liquor.”
  • A settling basin or clarifier to separate activated sludge from the treated effluent.
  • A sludge recycling system to return settled activated sludge to the aeration basin.
  • A sludge wasting system to remove sludge at the rate that it grows, thus maintaining the desired mass of solids in the process; waste sludge can be extracted from either the aeration basin or the return sludge flow.

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:

  • Effluent quality,
  • The growth rate of the microorganisms (see Figure 2) and the types present,
  • Oxygen consumption,
  • Mixed liquor settleability (see Figure 3),
  • Nutrient quantities needed,
  • The occurrence of foaming and frothing, and
  • The possibility of nitrifying.

Figure 2: Microorganism Growth

Microorganism Growth Explained

  • Lag phase—cells grow in size, not in number
  • Log phase—cells grow in number
  • Stationary phase—microorganism population remains stable because there are as many cells dying are there are cells multiplying
  • Death phase—there are more cells dying than there are cells multiplying (also called the endogenous phase)

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.

  • Mass of solids: the mass of solids in the aeration basin increases with sludge age; at a particular sludge age the mass depends solely on the daily mass organic loading (pounds of BOD, COD, or TOC).
  • Oxygen demand: the oxygen demand also increases with sludge age; at a particular sludge age, the demand varies with the organic loading and, provided the sludge age is long enough, the TKN loading.

Typical Sludge Age Ranges

  • High rate activated sludge age of 2–3 days
  • Conventional activated sludge at 5–10 days
  • Extended aeration systems at 20 – 40 days

There is a minimum sludge age below which no bacterial growth can occur. The minimum sludge age is less than one day.

Critical Nutrients

  • Nitrogen and phosphorus are the two critical nutrients most often lacking in industrial activated sludge systems.
  • A lack of these nutrients directly affects the microorganism metabolism and removal of organics from the wastewater.
  • The required ratio is as follows: BOD/N/P:100/5/1.