In conventional wastewater treatment, various bacteria play a crucial role in breaking down organic matter, nitrogen, phosphorus, and other pollutants. These bacteria are categorized based on their oxygen requirements and the processes they are involved in. Below is a detailed review of the types of bacteria used in wastewater treatment, their names, and their mechanisms of digestion:

1. Aerobic Bacteria

Aerobic bacteria require oxygen to survive and function. They are typically found in the aerobic section of wastewater treatment plants, such as in activated sludge processes, trickling filters, and aerated lagoons.
Key Aerobic Bacteria:
• Pseudomonas spp.: A versatile group that breaks down various organic compounds, including complex hydrocarbons.
• Nitrosomonas spp.: A nitrifying bacterium that oxidizes ammonia (NH₃) to nitrite (NO₂⁻) in the nitrification process.
• Nitrobacter spp.: Converts nitrite (NO₂⁻) to nitrate (NO₃⁻) in the second step of nitrification.
Digestion Mechanism:
• Organic Matter Breakdown: Aerobic bacteria metabolize organic matter by using oxygen as the electron acceptor. This process is referred to as aerobic respiration, and it results in the conversion of organic pollutants into carbon dioxide (CO₂), water (H₂O), and biomass.
• Nitrification: Nitrosomonas and Nitrobacter work together to convert toxic ammonia into less harmful nitrate. This two-step oxidation process helps in reducing ammonia toxicity in treated wastewater.
2. Anaerobic Bacteria
Anaerobic bacteria thrive in environments devoid of oxygen. These bacteria are primarily used in sludge digestion, anaerobic lagoons, and some advanced treatment processes like Upflow Anaerobic Sludge Blanket (UASB) reactors and anaerobic digesters.
Key Anaerobic Bacteria:
• Methanosaeta (formerly Methanothrix spp.): Key methanogen that converts acetate into methane (CH₄) in anaerobic digestion.
• Methanosarcina spp.: A versatile methanogen capable of metabolizing both acetate and hydrogen to produce methane.
• Clostridium spp.: Fermentative bacteria that break down complex organic compounds into simpler substances like fatty acids, hydrogen, and alcohol.
• Bacteroides spp.: Involved in the hydrolysis of proteins and carbohydrates in sludge.
Digestion Mechanism:
• Hydrolysis: Complex organic compounds like carbohydrates, fats, and proteins are broken down into simpler soluble compounds such as sugars, fatty acids, and amino acids by hydrolytic bacteria like Clostridium.
• Acidogenesis: Soluble compounds are further converted into volatile fatty acids (VFAs), alcohols, carbon dioxide (CO₂), and hydrogen (H₂) by acidogenic bacteria like Bacteroides.
• Acetogenesis: VFAs are converted into acetate, H₂, and CO₂ by acetogenic bacteria.
• Methanogenesis: Finally, methanogens like Methanosaeta and Methanosarcina convert acetate, H₂, and CO₂ into methane (CH₄) and CO₂, the end products of anaerobic digestion.
3. Facultative Bacteria
Facultative bacteria can survive in both aerobic and anaerobic environments, making them adaptable to varying conditions within a wastewater treatment plant. They are commonly found in facultative lagoons or in environments where oxygen availability fluctuates.
Key Facultative Bacteria:
• Escherichia coli (E. coli): Known for its role in indicating fecal contamination, E. coli can break down simple organic compounds in both aerobic and anaerobic conditions.
• Enterobacter spp.: Common in facultative lagoons, breaks down sugars and produces acids and gases.
• Paracoccus denitrificans: A denitrifying bacterium that reduces nitrate (NO₃⁻) to nitrogen gas (N₂) under anoxic conditions.
Digestion Mechanism:
• Aerobic Respiration: Facultative bacteria, when oxygen is present, use it to oxidize organic matter, similar to strict aerobic bacteria.
• Anaerobic Respiration: In the absence of oxygen, they switch to processes like denitrification, where nitrate (NO₃⁻) is used as the terminal electron acceptor instead of oxygen. This leads to the conversion of nitrate to nitrogen gas (N₂), which is released into the atmosphere.
4. Anammox Bacteria
Anammox (Anaerobic Ammonium Oxidation) bacteria are a relatively recent discovery in wastewater treatment and are used in advanced processes for nitrogen removal.
Key Anammox Bacteria:
• Brocadia spp.
• Kuenenia spp.
Digestion Mechanism:
Anammox bacteria oxidize ammonia (NH₄⁺) under anoxic conditions using nitrite (NO₂⁻) as the electron acceptor to produce nitrogen gas (N₂) and water (H₂O). The overall process is energy-efficient and does not require the addition of an external carbon source, making it a cost-effective method for nitrogen removal in wastewater treatment.
5. Phosphorus-Accumulating Organisms (PAOs)
These bacteria are involved in enhanced biological phosphorus removal (EBPR) processes. PAOs take up excess phosphorus under certain conditions and store it as polyphosphate within their cells.
Key PAOs:
• Candidatus Accumulibacter phosphatis: The most studied PAO responsible for phosphorus removal.
• Tetrasphaera spp.: Another group of PAOs, playing a role in phosphate storage.
Digestion Mechanism:
• Phosphate Uptake: Under anaerobic conditions, PAOs release phosphate into the surrounding environment and take in volatile fatty acids (VFAs). During subsequent aerobic conditions, PAOs take up phosphate in excess, which is then stored as polyphosphate inside the cell. The biomass is later removed from the system as waste activated sludge (WAS), thereby reducing phosphorus levels in treated wastewater.
6. Denitrifying Bacteria
Denitrifying bacteria are involved in the reduction of nitrate (NO₃⁻) to nitrogen gas (N₂) in anoxic conditions, helping to remove nitrogen from wastewater.
Key Denitrifying Bacteria:
• Paracoccus denitrificans
• Pseudomonas spp.
• Thiobacillus denitrificans
Digestion Mechanism:
In the absence of oxygen, denitrifying bacteria use nitrate as the terminal electron acceptor. This process is part of anaerobic respiration, where nitrate is reduced to nitrogen gas, which escapes into the atmosphere. This is a key step in biological nitrogen removal in wastewater treatment.
Summary of Processes:
• Aerobic Respiration: Bacteria metabolize organic matter with oxygen to produce CO₂, water, and new bacterial biomass.
• Anaerobic Digestion: Complex organics are broken down into methane and CO₂ in the absence of oxygen.
• Denitrification: Nitrate is reduced to nitrogen gas under anoxic conditions, removing nitrogen from wastewater.
• Phosphorus Removal: PAOs take up phosphorus and store it as polyphosphate under alternating anaerobic and aerobic conditions.
• Anammox Process: Ammonia is oxidized to nitrogen gas in anoxic environments without requiring external carbon sources.