1.6: Biosolids Stabilization

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Learning Outcomes

Compare and contrast the various sludge stabilization methods
Understand the different biological processes to stabilize wastewater solids
Describe the different types of bacteria used during anaerobic digestion at different temperatures

Sludge Stabilization

Once the solids have been thickened they are ready to be stabilized. At this point, the solids have only been thickened and they are the waste products of the liquid portion of the treatment process. There is a large amount of volatile organic material that needs to be stabilized. Stabilization will also help reduce odors and destroy pathogens. There are several different methods to achieve this. Digestion is the most common but stabilization can also be achieved by adding chemical or thermal stabilization by heating the sludge.

Aerobic Digestion

Aerobic decomposition is very similar to the aeration tanks discussed earlier in the activated sludge systems. The primary and/or secondary sludge is digested aerobically, meaning that aerobic bacteria will break down the organic matter. The digesters can either be rectangular or round. The bacteria are aerobic so air must be applied to the digester.

One major difference between the aeration tank in the activated sludge tanks and the aerobic digester is that there is not a continual supply of fresh BOD₅. In the digester, there is no fresh wastewater coming in only the settled solids. In the digester, the aerobic bacteria will be able to breathe but with no food source, they will undergo endogenous respiration. In this state, the bacteria begin to breakdown their own cell mass and thus reduce the amount of volatile suspended solids. Aerobic digesters will have a longer detention time, typically on the order of 30 days or more. Common volatile solids reduction can be around 45% to 70%.

Anaerobic Digestion

Recall that anaerobic means the environment has no free or combined sources of oxygen. Bacteria must find a different source of respiration. Anaerobic digestion is a two-step biological treatment process. The first step is done by a group of bacteria that breakdown solids to form volatile acids. The second step is another group of bacteria breaking down those volatile acids to form methane, carbon dioxide, and water.

When checking the operation of a digester, pH is a critical parameter. The first step of the digestion process is to create volatile acids. Excessive acids will cause a drop in pH. A pH between 6.6 and 7.6 is considered an acceptable range. If the pH drops below this, that is a sign that there are not enough methane forming bacteria to breakdown the volatile acids. Another way to examine this is by looking at the volatile acid to alkalinity ratio. Large amounts of alkalinity in the sludge will be able to buffer drastic changes in pH. If there isn’t enough alkalinity the pH could drop significantly and adversely affect the bacteria in the digester.

Mixing is also an important design and operation requirement. Good mixing will distribute the sludge evenly throughout the tank. This allows the bacteria to come in contact with the raw sludge and aid the digestion process. Proper mixing will also prevent the separation of grit and other inert solids. It will also prevent the development of a scum layer at the top of the digester.

There are three different types of anaerobic digestion based on what the operating temperature of the digester is. They are classified by the type of bacteria that is most abundant at the specified temperature ranges. The most common type is mesophilic, which operates between 85℉ and 100℉. This is the same type of anaerobic digestion that occurs in the human stomach. Psychrophilic digesters will operate between 50℉ and 68℉. The advantage of this type of digester is external heating systems are not needed to raise the temperature. However, at these colder temperatures, the bacteria are not as active. Therefore, psychrophilic digesters will require a longer detention time to achieve stabilization of the solids. Going hotter than mesophilic is thermophilic. These digesters operate between 120℉ and 135℉. At these higher temperatures, a detention time of 5 to 12 days can sufficiently stabilize the solids. However, there is an added cost to heat the sludge to these higher temperatures.

Chemical Stabilization

Chemical stabilization is achieved by adding calcium hydroxide, \(\ce{Ca(OH)2}\). It is also commonly known as slaked lime. Adding the lime will raise the pH of sludge to the point where biological activity is drastically reduced. This is much different from digestion because the organic matter is not reduced. The lime temporarily halts the biological activity and thus stabilizes the sludge. The sludge is then disposed of to a landfill. Chemical stabilization is not as common due to the high costs, regulations, and environmental impacts of handling chemicals.