Bacteria in Wastewater Treatment: Key Roles and Impacts

Wastewater treatment relies on biological processes to remove contaminants and protect environmental health. Bacteria play a central role in breaking down organic matter, cycling nutrients, and neutralizing harmful substances, ensuring wastewater is safely treated before being released or reused.

Major Bacterial Groups

Different bacterial groups contribute to pollutant breakdown and waste stabilization. Their metabolic capabilities determine how efficiently organic materials, nutrients, and contaminants are processed. The three main categories—aerobic, anaerobic, and facultative bacteria—function under varying conditions, influencing treatment outcomes.

Aerobic Bacteria

These microorganisms require oxygen and thrive in aeration tanks, where they decompose organic matter by converting carbon-based compounds into carbon dioxide, water, and biomass. Species such as Nitrosomonas and Nitrobacter facilitate ammonia oxidation, a key step in nitrogen removal. Activated sludge systems rely on aerobic bacteria, with oxygen supplied through mechanical aeration or diffused air systems. Maintaining dissolved oxygen levels between 1.5–2.5 mg/L ensures effective organic degradation and nitrification. If oxygen levels drop, treatment efficiency declines, leading to incomplete processing and regulatory non-compliance.

Anaerobic Bacteria

These bacteria thrive in oxygen-deprived environments like anaerobic digesters and sludge treatment units. They break down complex organic compounds through hydrolysis, acidogenesis, and methanogenesis. Species such as Clostridium and Methanosarcina degrade proteins, lipids, and carbohydrates into volatile fatty acids, which methanogenic bacteria convert into methane and carbon dioxide. This biogas can be captured for energy use, enhancing wastewater treatment sustainability. Temperature affects anaerobic digestion efficiency, with mesophilic conditions (30–40°C) being the most common. Lower temperatures slow bacterial metabolism, reducing gas production and organic matter breakdown.

Facultative Bacteria

These adaptable microorganisms function in both aerobic and anaerobic environments, making them valuable in systems with fluctuating oxygen levels. Enterobacter and Pseudomonas species contribute to organic matter degradation and denitrification under low-oxygen conditions. Their ability to switch metabolic pathways helps maintain treatment performance during oxygen depletion, preventing system failure. In activated sludge systems, facultative bacteria support phosphorus removal by storing excess phosphorus under aerobic conditions and releasing it under anaerobic conditions, improving treatment stability.

Nitrification And Denitrification

Nitrogen transformation in wastewater treatment depends on nitrification and denitrification. These microbial processes regulate nitrogen levels, preventing harmful accumulation and ensuring compliance with discharge standards. Nitrification converts ammonia into nitrate under aerobic conditions, while denitrification reduces nitrate to nitrogen gas under anoxic conditions.

Nitrification starts with ammonia oxidation by ammonia-oxidizing bacteria (AOB) like Nitrosomonas, which convert ammonia (NH₃) into nitrite (NO₂⁻). This step requires oxygen and is sensitive to dissolved oxygen levels, temperature, and pH. Optimal conditions include oxygen concentrations above 2 mg/L, temperatures between 25–35°C, and a pH range of 7.5–8.5. Nitrite-oxidizing bacteria (NOB) like Nitrobacter then oxidize nitrite into nitrate (NO₃⁻), completing the process. Maintaining a balance between AOB and NOB populations prevents nitrite accumulation, which can cause toxicity and regulatory issues.

Denitrification removes nitrate from the system under anoxic conditions. Bacteria such as Pseudomonas, Paracoccus, and Thiobacillus use nitrate as an electron acceptor, reducing it to nitrogen gas (N₂), which escapes into the atmosphere. A carbon source like methanol or acetate is essential for sustaining denitrifying bacteria. Without sufficient carbon, denitrification may be incomplete, leading to nitrous oxide (N₂O) accumulation, a potent greenhouse gas.

Wastewater treatment facilities optimize nitrogen removal through technologies like sequencing batch reactors (SBRs), moving bed biofilm reactors (MBBRs), and integrated fixed-film activated sludge (IFAS) systems. These systems create alternating aerobic and anoxic conditions, supporting both nitrifiers and denitrifiers. Some facilities use simultaneous nitrification-denitrification (SND), where oxygen gradients within biofilms or flocs enable both processes in a single reactor, improving efficiency while reducing aeration energy demands.

Anaerobic Digestion

Anaerobic digestion stabilizes sludge and generates biogas by breaking down organic matter without oxygen. Specialized bacterial communities convert complex materials into simpler molecules through distinct biochemical stages. Maintaining optimal environmental conditions ensures microbial populations function efficiently, maximizing waste reduction and energy recovery.

Hydrolytic bacteria initiate the process by secreting enzymes that break down proteins, lipids, and carbohydrates into amino acids, fatty acids, and simple sugars. Acidogenic bacteria then ferment these monomers into volatile fatty acids, alcohols, hydrogen, and carbon dioxide. Imbalances in acid production can disrupt the process and reduce methane yield.

Acetogenic bacteria convert fermentation products into acetic acid, hydrogen, and carbon dioxide, which methanogenic archaea, such as Methanosarcina and Methanobacterium, use to produce methane and carbon dioxide in a roughly 65:35 ratio. The methane can be captured for renewable energy, reducing treatment plant costs. Mesophilic conditions (30–40°C) are preferred for stability, while thermophilic conditions (50–60°C) accelerate degradation but require higher energy inputs.

Biological Breakdown Of Organic Compounds

Microbial activity drives organic matter degradation in wastewater treatment, reducing biochemical oxygen demand (BOD), a key parameter indicating how much oxygen microorganisms need to decompose organic material. High BOD in untreated wastewater depletes oxygen in receiving water bodies, leading to environmental problems like hypoxia and fish kills. Efficient microbial breakdown prevents these issues and ensures regulatory compliance.

Bacteria secrete exoenzymes that hydrolyze complex macromolecules into smaller, more metabolizable units. Proteases break down proteins into amino acids, while lipases convert fats into glycerol and fatty acids. These molecules are then absorbed and metabolized, providing energy for bacterial growth. The efficiency of this process depends on temperature, pH, and the presence of inhibitory substances like heavy metals or toxic organics, which can disrupt microbial function.

Microbial Interactions With Chemical Contaminants

Bacteria in wastewater treatment interact with contaminants like heavy metals, pharmaceuticals, and industrial pollutants. Some contaminants are transformed into less harmful forms, while others can disrupt microbial activity, affecting treatment efficiency and effluent quality.

Heavy metals such as lead, mercury, and cadmium are not biodegradable and can accumulate in biomass. Certain bacteria, including Pseudomonas and Bacillus, use biosorption, bioaccumulation, and enzymatic transformation to reduce metal toxicity. Some microbes produce extracellular polymeric substances (EPS) that bind metals, preventing cellular entry, while others convert toxic metal ions into less soluble forms. However, excessive metal concentrations can still inhibit microbial activity.

Pharmaceutical residues and personal care products raise concerns due to their persistence and potential environmental impact. Bacteria like Rhodococcus and Sphingomonas degrade antibiotics, hormones, and analgesics, but incomplete degradation can produce bioactive metabolites with residual environmental risks. Prolonged antibiotic exposure in wastewater can also contribute to antibiotic-resistant bacteria. Advanced treatment strategies, such as bioaugmentation with specialized microbial strains and integrating biological with chemical or physical treatments, enhance contaminant removal while preserving microbial function.

Factors That Affect Bacterial Performance

Several environmental and operational factors influence bacterial efficiency in wastewater treatment. Maintaining stable conditions is essential, as fluctuations in temperature, pH, nutrient availability, and toxic substances can disrupt microbial activity.

Temperature regulates bacterial metabolism, with mesophilic bacteria functioning best between 30–40°C and thermophilic organisms thriving at 50–60°C. Deviations slow enzymatic activity, reducing organic matter degradation and nutrient removal. Similarly, most wastewater treatment bacteria perform optimally within a pH range of 6.5–8.5. Extreme pH conditions can denature enzymes and disrupt metabolic pathways.

Nutrient balance is also critical, as bacteria require adequate carbon, nitrogen, and phosphorus. A recommended carbon-to-nitrogen-to-phosphorus (C:N:P) ratio of 100:5:1 supports microbial activity in biological treatment systems. Deviations can lead to excessive sludge production or nutrient deficiencies that hinder bacterial performance. Additionally, high concentrations of ammonia, sulfides, or industrial chemicals can inhibit enzyme activity or damage cell structures. Process control strategies, such as gradual bacterial acclimation to toxic compounds and maintaining proper aeration and mixing, help mitigate these effects and ensure stable treatment outcomes.