1.6: Biological Treatment Overview
Learning Outcomes
- Describe different methods to biologically treat wastewater
- Examine what target contaminants are being removed by the different biological processes
- Compare and contrast different biological processes and the type of bacteria and environmental conditions needed
Aerobic, Anaerobic, and Facultative 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 (\(\ce{CO2}\)).
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. Common examples of combined oxygen in wastewater is nitrate (NO3). In anaerobic conditions, there is absolutely no oxygen available to the bacteria for respiration. Anaerobic bacteria have more sophisticated means of respiration by using other chemical reactions instead of using oxygen directly. However, similar to aerobic bacteria, when anaerobic bacteria breakdown the chemicals in wastewater for respiration and feed off other organics, they subsequently remove the contaminants from the wastewater. 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.
Stabilization Ponds
Stabilization ponds are a simple way to reduce the amount of organic material in wastewater and require low maintenance. However, they require a lot of land and are mainly used for smaller rural communities. The concept of ponds is pretty simple. You have a large pond filled with wastewater. The size of the pond allows for a very long detention time. The pond creates an ecosystem for bacteria to thrive and those bacteria breakdown the organic matter. Since the detention time is very long, typically 45 days or longer, solids will also settle to the bottom and be removed from the effluent. Eventually, the solids will have to be removed but a well-operated pond can go a few decades before that is needed to be done.
As discussed earlier there are several different types of ponds depending on what type of bacteria are expected to be prevalent. Aerobic ponds utilize aerobic bacteria and require oxygen. These ponds are typically more shallow, around 2ft to 5ft, since the oxygen is provided by the atmosphere and algae that is produced on the surface of the pond. Ponds have a complex relationship between how much dissolved oxygen is available to the bacteria. This is driven by sunlight over a 24-hour period and photosynthesis. Photosynthesis is where plants such as algae use sunlight and carbon dioxide (CO2) to thrive. During the day when there is plenty of sunlight, the algae will utilize the CO2 in the water and produce oxygen. This will cause the pH in the pond to decrease and the dissolved oxygen concentration to increase. This is a good thing because, in an aerobic pond, the bacteria need oxygen to live and breakdown the organic waste. During the night when the sun goes down, the reverse happens. Without sunlight, photosynthesis is reduced and the algae will utilize the oxygen and produce CO2. This causes the pH to increase and the dissolved oxygen will decrease. Because sunlight is an important factor in pond performance, there will also be seasonal variations in addition to daily fluctuations. During the winter when the sun isn’t as prevalent, the amount of oxygen produced via algae and photosynthesis will be reduced. If more oxygen is required than what can be produced naturally, it can be provided by mechanical means. Surface aerators and a mixing system can be installed. However, the mechanical systems in these aerated ponds will require more maintenance. They will also require more electricity to run the aeration equipment.
Anaerobic ponds utilize anaerobic bacteria which require no oxygen. To create this environment in a pond system, the depth is increased. Typically depths can range from 6 ft to 16 ft. At these increased depths, oxygen from the atmosphere is unable to penetrate the water, and a no oxygen environment, ideal for the anaerobic bacteria to thrive, is created. In these types of ponds, the bacteria are breaking down the more complex solids and organics that are found in the wastewater. It can provide sufficient removal of nitrogen, phosphorus, and BOD5. The solids are further stabilized, broken down, and collected as sludge at the bottom of the pond. Like the aerobic ponds, these solids will eventually need to be removed.
Facultative ponds are a mix of aerobic and anaerobic ponds. Their depth is on the order of 3ft to 8ft. Recall, that facultative bacteria can switch from living in anaerobic conditions to aerobic conditions. This can be useful during seasonal variations where little oxygen is available. The facultative bacteria can then focus on breaking down organics by anaerobic decomposition. When sufficient oxygen is available they will switch mechanisms and break down the organic matter by aerobic decomposition.
Pond Performance
To improve pond performance, it is very common to have multiple types of ponds in series. Anaerobic ponds can handle higher influent BOD5 loading, so they would be the first pond. Then a facultative pond would follow, where further BOD5 reduction is achieved. Lastly would be an aerobic pond that would reduce organic wastes and remove pathogenic organisms.
Pond performance is going to be dependent on the loading of organic wastes and detention time. Typically a max of 30lbs of BOD5/day/acre is the desired loading rate. However, ponds are able to handle fluctuations if loading rates are exceeded for short periods of time. The wastewater must remain in the pond long enough so the bacteria can come in contact with the organic matter and consume it. Anaerobic ponds will maintain a detention time of 1 to 7 days. Facultative ponds have a detention time of 5 to 30 days and aerobic ponds are usually 30 days or greater.