1.8: Activated Sludge
Learning Outcomes
- Compare and contrast the difference between fixed film and suspended growth biological treatment systems
- Describe how the activated sludge process can more efficiently reduce organic wastes
- Understand how different process control strategies work in an activated sludge system
- Examine how a secondary clarifier is utilized in an activated sludge system
Suspended Growth Processes
Unlike the fixed film biological systems discussed in Chapter 7, activated sludge uses a suspended growth process. This means that there is no media for the bacteria to become fixed to. The microorganisms are suspended in the tanks either by a mixer or air diffusers and are then mixed in with the incoming wastewater. This mixture of wastewater and microorganisms is referred to as the Mixed Liquor Suspended Solids (MLSS). A sample can be taken from the activated sludge system and analyzed in a laboratory to determine how many mg/L there are of MLSS in the system. This laboratory result will be a critical design and operational parameter that will need to be continually monitored to ensure effective treatment.
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 that 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.
To achieve this on a large scale, conventional activated sludge systems are comprised of an aeration tank followed by a secondary clarifier. In the secondary clarifier, the microorganisms will separate from the now treated wastewater. A majority of those microorganisms will be returned back to the reactor and are now activated to treat more of the incoming wastewater. In the aeration tanks, the bacteria are in an environment with dissolved oxygen readily available and organic matter to consume. Like the other biological processes, this is how the reduction of BOD5 occurs, through aerobic decomposition. What makes activated sludge unique is that in the secondary clarifier, the bacteria become stressed because their food source has been drastically decreased and there is no more dissolved oxygen. They begin to go through endogenous respiration. The bacteria basically become so starved that they begin to breakdown their own cellular structure. Before the bacteria completely cannibalize themselves, they are sent back to the aeration tank where they are re-introduced to readily available dissolved oxygen and organic matter. The starved bacteria are then able to rapidly re-start aerobic decomposition and quickly reduce the BOD5 in the wastewater.
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.
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 (WAS)
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. After anaerobic digestions, the solids will be dewatered and ultimately sent to a landfill for disposal.
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.
Process Control
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”.
Alternative Process Configurations
There are many different types of process configurations of activated sludge systems that differ by how the wastewater enters the aeration tanks. A plug flow reactor has all of the wastewater and RAS entering at the beginning of the tank. This will have a large organic loading at the beginning of the tank and as the wastewater moves through the tank, the load will lessen. Often, tapered aeration will be used in plug flow reactors. Tapered aeration will have lots of air diffusers at the beginning of the tank so more oxygen is available to the bacteria to handle the increased organic loading. As the wastewater flows through and the BOD5 is reduced, the air diffusers are also reduced.
An alternative is a step feed reactor where the incoming wastewater is split and sent to different portions of the tank. For example, 25% of the flow is sent to the first ¼ of the tank, another 25% to the second ¼ of the tank, and so on. This allows for a more even distribution of the organic loading as well as a more uniform demand for the air diffusers. Typically, a steep feed system will use less air overall than a plug flow reactor.
Secondary Clarification
A secondary clarifier is a key component of the activated sludge system. Not only does it separate out the microorganisms from the now treated wastewater, but it will also concentrate them through the sedimentation process. A secondary clarifier works exactly as discussed previously in primary sedimentation. The only difference is that in a primary sedimentation tank the main goal is to remove unwanted solids. In a secondary sedimentation (or clarifier) tank, the goal is to concentrate the MLSS so it can be returned to the aeration tanks.
How the biomass is settling in the clarifier can be difficult to see in real-time as the tanks are typically below-grade and made of concrete. A “sludge judge” is used to determine how many feet of MLSS are settled on the bottom and how much clear water there is on top. The tool is simply a clear PVC pipe with a ball check valve at the bottom. As the pipe is inserted into the water, the ball check valve opens, letting liquid in. When the stick hits the bottom, the operator will pull the PVC pipe out of the water forcing the ball check to close producing a cross-sectional sample of MLSS on the bottom and clear water on top. There are markings on the PVC pipe for every foot. So as the operator pulls it out of the clarifier, they can see where the clear water stops and where the MLSS is settling. Typically a sludge blanket of 1 to 3 feet on the bottom of the clarifier is optimal.
The settleability of the MLSS can also be seen in the laboratory by taking a sample of the MLSS and putting it into a 1000 mL beaker. After 30-minutes have gone by, the graduations of where the MLSS has settled will be recorded. Dividing the settled sludge volume in mLl/L, dividing by the MLSS in mg/L, and multiplying by 1,000 mg/g will yield the Sludge Volume Index (SVI) in mL/g. The SVI is utilized to gauge how well the MLSS will settle in the clarifier and will also shed light on how the activated sludge plant is operating.
Typical SVI is around 100 to 200 mL/g. Less than 100 mL/g will often show rapid settling of the sludge in the clarifier. When the MLSS settles too quickly, smaller “pin floc” particles will remain suspended in the middle of the clarifier. This will also be seen in the sludge judge test where there is clear water on top of the sludge judge, murky water in the middle, and darker solids on the bottom. This is usually caused by having a higher MCRT. An SVI greater than 200 mL/g will have sludge settling very slowly in the clarifier. This can be caused by having a low MCRT.
The water leaving the secondary clarifier has now gone through all of the previous treatment steps from preliminary screening, to primary sedimentation, and activated sludge treatment. In some areas with less stringent regulations, this water is clean enough to be discharged to the ocean or a nearby waterbody. However, some additional treatment like filtration and disinfection may be needed to meet regulatory compliance. Also, the solids that were removed from the primary and secondary process will still need to be treated and disposed of.