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1.2: Biological Treatment Overview

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    Learning Outcomes

    • Understand which environments the different types of bacteria thrive
    • Explain how activated sludge treatment process is more efficient than other methods
    • Describe the nitrogen cycle and how it’s used to remove nitrogen from wastewater

    Aerobic, Facultative, and Anaerobic Organisms

    Aerobic bacteria require an environment that has free dissolved oxygen. The bacteria use that oxygen for respiration to live. They feed on the organic matter and other nutrients in the wastewater. As the bacteria consume these materials, they are removed from the wastewater, making it less contaminated. The byproduct of the aerobic decomposition of organic matter is carbon dioxide (CO2).

    Anaerobic bacteria require an environment that has no free or combined oxygen. Free oxygen is when there is excess oxygen dissolved in the water and is available as O2. Combined oxygen is when the oxygen molecule is bound to another element. A common example of combined oxygen in wastewater is nitrate (NO3). In anaerobic conditions, there is absolutely no oxygen available to the bacteria for respiration. The byproduct of anaerobic decomposition is methane gas.

    Facultative bacteria have the ability to thrive in either aerobic or anaerobic conditions. While they prefer aerobic conditions, they have the ability to adapt when no oxygen is available and survive in anaerobic conditions.

    Activated Sludge Treatment

    The activated sludge process was developed by two scientists, Edward Arden and William Lockett, in England in 1914. Their experiments showed that by taking the microorganisms that were already established from aerobic decomposition and introducing them to fresh wastewater, it would speed up the decomposition of the new organic wastes. Instead of relying on the 30 to 45 days it would normally take to breakdown these organic wastes, the activated sludge process can achieve the same level of treatment in less than one day. This means that an activated sludge wastewater treatment system will be able to handle higher flow rates and higher organic loading than pond treatment or fixed film systems.

    Activated sludge treatment systems consist of an aeration tank followed by a secondary clarifier. These two tanks each provide a unique function but also work together to reduce the BOD5 in the wastewater. In the aeration tanks, oxygen is diffused into the water so the bacteria can survive in aerobic conditions. The air diffusers also keep the bacteria in suspension so it can continually be in contact with the incoming wastewater. In the secondary clarifiers, the bacteria will be separated from the treated wastewater. The treated wastewater will continue on to the tertiary treatment step. A majority of the settled bacteria will be sent back to the beginning of the aeration tanks. In the clarifier, the bacteria were stressed due to a lack of food and oxygen. They begin to undergo endogenous respiration and are so starved they begin to breakdown their own cells to survive. When the stressed bacteria are reintroduced into the aeration tanks they are now in an environment with plenty of oxygen and food. These bacteria will now quickly begin to breakdown the organic wastes in the wastewater much faster than before.

    Return Activated Sludge

    The return activated sludge, or RAS, are the bacteria that have settled in the secondary clarifier and are being sent back to the aeration tank. The rate of speed at which the microorganisms are returned is something that the operator can control.

    Waste Activated Sludge

    Like the other biological process discussed, the bacteria, overtime will grow and their population will increase. To control the amount of bacteria in the system, a portion of the RAS will not be returned to the aeration tank but instead is directed to a separate solids handling treatment unit. Typically, these solids are combined with the settled solids from the primary sedimentation tanks and sent to an anaerobic digester.

    Food to Microorganism Ratio

    A key parameter to determine the effectiveness of activated sludge systems is the food to microorganism ratio, or F/M. The food is determined by the amount of BOD5 in the incoming wastewater and the amount of microorganisms available to consume that food is determined by the amount of MLSS in the aeration tanks. Since it’s a ratio, the units of these two laboratory results must be the same. The mg/L concentration of the laboratory results is converted into a mass with the units of lbs. While each treatment plant will determine which level of F/M has historically given effective treatment, a common range is around 0.2 to 0.5.

    Mean Cell Residence Time

    How much bacteria remains in the system and how much is wasted can be determined by the mean cell residence time or MCRT. The MCRT is a theoretical calculation of the average time a single bacteria will stay within the activated sludge system before being wasted. To calculate the MCRT an operator would determine how many pounds of MLSS there is in the system and divide that by how many pounds were removed from the system; typically over a 24 hour period.

    Review of Basic Principles of Operation

    The F/M ratio, MCRT, RAS, WAS, and dissolved oxygen concentrations can all be manipulated by the operators of a wastewater treatment facility to optimize the effectiveness of treatment. In fact, those parameters are the only things that can be easily controlled. The amount of incoming wastewater is going to be what it is and it will fluctuate throughout a 24 hour period as well as vary by season. The incoming BOD5 loading is what it’s going to be and the operators can’t control it.

    If the F/M is too low, it means that there are more bacteria than what is needed to consume the available food. This is inefficient because supplying oxygen to the bacteria requires a significant amount of energy. If there is too much bacteria in the system and not enough food, the bacteria will still be consuming oxygen but the BOD5 won’t be further reduced. To increase the F/M ratio operators can only control the “M” portion of the equation. By increasing the wasting rate, the MLSS will be reduced causing the F/M ratio to increase.

    If the F/M ratio is high, then there is not enough bacteria to consume the large amounts of incoming BOD5. This will lead to poor treatment and the effluent will have a high BOD5 concentration. Operators cannot decrease the amount of BOD5 coming into the plant so they will have to increase the amount of MLSS in the system. They can do this by decreasing or stopping the wasting rate. Decreasing the wasting rate will cause the MLSS to increase and the F/M ratio will be reduced.

    The MCRT is another process control tool that is used to determine the wasting rate by manipulating the equation to calculate the MCRT. Often the desired MCRT rate is determined by the design of the treatment facility or from historical data. By taking the pounds of MLSS in the system and dividing it by the desired MCRT, you will determine what the wasting rate needs to be to achieve that MCRT. However, the MCRT and F/M methods for determining the wasting rate can often conflict with each other. Operators need to look at the changes in MCRT and F/M over time and make minor adjustments to the process so the bacteria aren’t “shocked”.

    Toxic Substances

    Toxic substances can be detrimental to the efficacy of activated sludge systems. Activated sludge treatment relies on living bacteria to feed off the organic wastes, thus removing it from the wastewater. If toxic substances are introduced to the activated sludge system, the bacteria can die off. Without a significant population of bacteria, the organic wastes will not be consumed and the effluent of the treatment plant will have high amounts of BOD5.

    Toxic substances can include heavy metals, pesticides, high concentrations of salts, cyanide, PCBs, and other chemicals. These toxic substances, if introduced in the wastewater collection system, will be sent to the wastewater treatment facility and can destroy the population of bacteria that treats organic wastes. To prevent toxic substances from entering the collection system, municipalities will have pre-treatment programs. These programs will have a public outreach component to inform the community of its wastewater infrastructure and the harm it can cause if chemicals and other toxic materials are dumped down the drain. In addition to household waste, pre-treatment programs will also work closely with manufacturing and processing facilities to ensure they are not discharging toxic materials into the wastewater collection system.

    Pure Oxygen Treatment

    Many treatment plants utilize air blowers or surface aerators to provide oxygen to the bacteria in the aeration tanks. These systems provide oxygen that’s in the atmosphere and diffuses it into the wastewater in the tank. One drawback of these methods is that our atmosphere is only around 21% oxygen. A majority of the atmosphere is nitrogen. Having a pure oxygen system is more efficient because the oxygen concentrations can be as high as 99% pure oxygen. However, there are many drawbacks to pure oxygen plants. First off, extra equipment will be needed to create pure oxygen. This equipment will have electrical, maintenance, and operational costs associated with it. Secondly, pure oxygen can be extremely hazardous to deal with. At higher purities oxygen can be explosive. Extra caution must be taken to ensure no sparks, oil, or other contaminants are near the pure oxygen generators.

    Enhanced Biological Treatment

    If the treated wastewater leaving a treatment facility is being discharged into an impaired waterbody, the NPDES permit will most likely have limits on ammonia, nitrogen, and phosphorus. These nutrients at high concentrations can cause eutrophication in a waterbody. Eutrophication occurs when there is an excess of nutrients in a waterbody that spurs algae growth. The algae population will become out of control and consume dissolved oxygen in the waterbody to the point where fish and other aquatic life can not survive. Nitrogen and phosphorus can be removed from the treatment plant by utilizing biological nutrient removal (BNR) processes.

    Nitrogen Removal

    Nitrogen can be removed from the wastewater by making slight modifications to the activated sludge treatment process. Nitrogen enters the treatment plant as ammonia (NH3). In the aeration tank, aerobic bacteria will nitrify this ammonia to create nitrite (NO2) and nitrate (NO3). In a well-operated plant, a majority of the nitrogen formed is nitrate. The wastewater is then conveyed to an anoxic tank. Anoxic means that the environment where the bacteria are living contains no free dissolved oxygen but there is combined oxygen. The combined oxygen is due to the NO3 sent to the tank from the aeration tanks. In this anoxic condition facultative bacteria break apart the NO3 bond and use the oxygen for respiration. The nitrogen molecules combine to form nitrogen gas (N2) which is vented back to the atmosphere. Recall that 78% of the Earth’s atmosphere is N2 gas.

    The anoxic tanks in a BNR system can be placed in a number of different configurations. Since the nitrification process is required first to create the NO3 one would expect to have the anoxic tanks following the aeration tanks. While there are treatment plants that operate in this manner, it may require additional chemicals as the denitrification process requires a certain amount of carbon for the bacteria to feed off of. The amount of carbon available in the wastewater can be measured by the CBOD5 or carbonaceous biochemical oxygen demand. At the end of the aeration tank, the aerobic bacteria have significantly reduced the amount of CBOD5 in the wastewater. A process alternative is having the anoxic tank prior to the aeration tank. The wastewater at the end of the aeration tank that has been nitrified is then recycled back to the anoxic tank. Here, fresh wastewater with high amounts of CBOD5 is mixed with the NO3 and the conditions will be just right for denitrification to occur. This will also lessen the aeration demands since some of the CBOD5 will be reduced in the anoxic tanks before it enters the aeration tank. One drawback is that not all of the nitrate will be captured and sent back to the anoxic tank. Therefore, another process set up is to have alternating zones of anoxic, aerobic, anoxic, aerobic.


    Phosphorus can also be reduced in a well-operated wastewater treatment facility. The amount of phosphorus coming into the facility will determine the level of treatment required. Much of the phosphorus can be removed during the primary and secondary sedimentation processes. If more phosphorus removal is required then treatment plants can utilize enhanced biological phosphorus removal (EBPR). EBPR uses polyphosphate accumulating bacteria (PAO) that, under anaerobic conditions, will accumulate phosphorus in their cells and remove it from the wastewater. A typical configuration of an EBPR process is the RAS and influent wastewater will be mixed in an anaerobic tank where phosphorus reduction occurs. Then the wastewater enters the anoxic tank where the RAS, influent wastewater, and nitrified effluent from the aeration tanks create the ideal environment for denitrification. Then everything is conveyed to the aeration tanks where free dissolved air is added. The incoming ammonia is nitrified to NO3 and the remaining BOD5 is reduced.

    This page titled 1.2: Biological Treatment Overview is shared under a CC BY license and was authored, remixed, and/or curated by Nick Steffen (ZTC Textbooks) .