Bacteria do the heavy lifting in virtually every wastewater treatment plant on the planet. They break down organic waste, strip nitrogen and phosphorus from sewage, digest sludge into biogas, and can even generate small amounts of electricity. Without bacterial communities, the biological treatment systems that protect rivers, lakes, and coastal waters from pollution simply would not function. But bacteria in wastewater are not all helpful: some carry antibiotic resistance genes, some cause operational headaches like sludge bulking, and some can regrow in treated water after it leaves the plant. Understanding which bacteria matter, what they do, and where they cause trouble gives a clearer picture of how modern sanitation actually works.
Who Lives in Activated Sludge
Activated sludge, the brownish floc-filled liquid that sits in aeration tanks, is one of the most microbially diverse engineered environments on Earth. Studies using DNA sequencing have found dozens of bacterial phyla coexisting in a single plant. Across eight activated sludge samples from different treatment plants, researchers identified 36 phyla, with four groups consistently dominating: Proteobacteria (roughly 27–49% of sequences), Bacteroidetes (19–37%), Chloroflexi (3–17%), and Actinobacteria (2–14%). Together these four phyla accounted for an average of about 85% of all bacterial sequences detected.1PubMed Central. Analysis of Bacterial Community Structure of Activated Sludge from Wastewater Treatment Plants in Winter A separate investigation of five activated sludge samples found the same dominant phyla alongside Acidobacteria and Firmicutes, while also noting that environmental factors like pH and the concentrations of phosphorus and organic carbon strongly shaped which species thrived.2Science of The Total Environment. Composition of bacterial communities in municipal wastewater treatment plant
What this means in practical terms is that no single species runs the show. Activated sludge functions because of a broad coalition of microorganisms, each occupying a slightly different metabolic niche. Some specialize in chewing through sugars and fats, others handle proteins, still others convert nitrogen compounds or store phosphorus. The diversity itself is part of the system’s resilience: if one population dips during a cold snap or a spike in industrial discharge, others can partially compensate.
Nitrogen Removal and the Two-Step Relay
Ammonia is one of the most problematic pollutants in sewage. Left untreated, it fuels algal blooms, depletes oxygen in waterways, and is toxic to fish. Bacteria remove it through a two-stage relay. First, ammonia-oxidizing bacteria convert ammonia into nitrite, and nitrite-oxidizing bacteria then convert nitrite into nitrate. This combined process, known as nitrification, requires oxygen and typically happens in aerated zones of the treatment plant.3PubMed Central. Nitrification mainly driven by ammonia-oxidizing bacteria and nitrite-oxidizing bacteria in an anammox-inoculated wastewater treatment system The two key players are bacteria in the genera Nitrosomonas (ammonia oxidizers) and Nitrobacter (nitrite oxidizers), and research on full-scale plants has shown that the ratio between these groups and the dissolved oxygen levels in the tank meaningfully affect how fast nitrification proceeds.4PubMed. Distribution of Nitrosomonas-related ammonia-oxidizing bacteria and Nitrobacter-related nitrite-oxidizing bacteria in two full-scale biological nutrient removal plants
Nitrate, however, is still a pollutant. The second stage, denitrification, takes place in oxygen-free zones where a different set of bacteria, including species of Pseudomonas, Paracoccus, and Bacillus, progressively reduce nitrate all the way to harmless nitrogen gas that bubbles out of the water.5Microbiology Research Journal International. Microbial Ecology of Denitrification Process and Its Application in Wastewater: Treatment, Challenges and Opportunities Getting the balance right between aerated and oxygen-free zones is one of the core design challenges in any treatment plant aiming for low nitrogen in its discharge.
A more recently adopted shortcut involves anammox (anaerobic ammonium oxidation) bacteria, which convert ammonia and nitrite directly into nitrogen gas without needing a full nitrification-denitrification cycle. One well-studied species, “Candidatus Brocadia sinica,” uses hydroxylamine and ammonium to synthesize an intermediate compound that it then oxidizes to nitrogen gas.6PubMed. Hydroxylamine-dependent anaerobic ammonium oxidation (anammox) by “Candidatus Brocadia sinica” Anammox-based systems use less energy because they need less aeration and less external carbon, making them attractive for plants trying to reduce their operating costs and carbon footprint.
Phosphorus Removal by Polyphosphate-Accumulating Organisms
Phosphorus is the other nutrient that causes havoc in receiving waters. Biological phosphorus removal relies on a group of bacteria called polyphosphate-accumulating organisms, or PAOs. These bacteria alternate between oxygen-free and oxygen-rich conditions. In the anaerobic phase, they take up short-chain fatty acids and store them internally. In the aerobic phase, they use the stored carbon for growth and simultaneously absorb far more phosphorus from the water than they need, packing it into intracellular polyphosphate granules. The phosphorus is then removed when excess sludge containing these PAO cells is wasted from the system.
Research in high-strength wastewater reactors has identified the genus Tetrasphaera as a consistently dominant PAO. Under high organic loading, the competition between PAOs and their less helpful cousins, glycogen-accumulating organisms (which store carbon without taking up extra phosphorus), did not appear to undermine phosphorus removal performance.7PubMed Central. Partial Nitrification and Enhanced Biological Phosphorus Removal in a Sequencing Batch Reactor Treating High-Strength Wastewater That said, PAO-based phosphorus removal is notoriously sensitive to operational upsets, and many plants supplement it with chemical dosing as a safety net.
Sludge Digestion and Biogas Production
The sludge that accumulates from biological treatment still needs to be dealt with. Anaerobic digestion, where bacteria break down organic solids in sealed, oxygen-free tanks, is one of the oldest and most cost-effective methods. Four groups of microorganisms work together in sequence: fermentative bacteria first hydrolyze complex compounds like proteins and carbohydrates into simpler molecules, syntrophic bacteria and acetogens convert those intermediates into acetate and hydrogen, and finally methanogenic archaea turn acetate and hydrogen into methane and carbon dioxide.8PubMed Central. Microbial ecology of anaerobic digesters: the key players of anaerobiosis The methane-rich biogas can be captured and burned to heat the digesters, generate electricity, or both.
Researchers are actively looking for ways to speed up this process. One approach involves adding engineered materials, such as biochar combined with iron-based frameworks, to the digester. These additives promote direct electron transfer between syntrophic bacteria and methanogens, effectively shortening the chemical relay and increasing methane output.9PubMed. Biochar@MIL-88A(Fe) accelerates direct interspecies electron transfer and hydrogen transfer in waste activated sludge anaerobic digestion
Biofilms and Aerobic Granular Sludge
Not all treatment systems rely on freely floating bacteria. Biofilm-based systems, where bacteria attach to surfaces and grow in layers, are used in trickling filters, moving-bed biofilm reactors, and other configurations. A more recent innovation is aerobic granular sludge, where bacteria self-organize into dense, roughly spherical granules a few millimeters across. These granules combine different metabolic zones within a single particle: oxygen-consuming bacteria on the outer shell, oxygen-free denitrifiers deeper inside, and sometimes anammox or PAO communities in the interior. Microscopy shows that mature granules have a dense core of bacterial cells embedded in extracellular polymeric substances, surrounded by a looser fringe zone colonized by protozoa and sometimes fungi.10PubMed Central. Microbial composition and structure of aerobic granular sewage biofilms
Granular sludge settles much faster than conventional flocs, which means treatment tanks can be smaller. The biological challenge is getting granules to form reliably and stay intact. Recent work has shown that intracellular signaling molecules, specifically a compound called cyclic-di-GMP, play a role in promoting the aggregation process. When researchers engineered bacteria to overproduce a protein that modulates cyclic-di-GMP levels, the resulting aggregates were true granules rather than loose flocs and settled well, especially in carbon-rich growth conditions.11PubMed. Overexpression of a Gene That Modulates Cyclic-di-GMP Enhances Granulation in Mycobacterium smegmatis
When Bacteria Cause Problems Inside the Plant
Filamentous bulking is the most common settling problem in activated sludge plants.12Water. Strategies for Controlling Filamentous Bulking in Activated Sludge Wastewater Treatment Plants: The Old and the New Certain thread-like bacteria grow excessively and extend outward from the sludge flocs, trapping water between them. The sludge becomes fluffy, refuses to settle properly in clarifiers, and can overflow into the plant’s effluent. Microscopic examination during bulking episodes confirms a visible increase in filamentous organisms.13Environment International. Filamentous bacteria-induced sludge bulking can alter antibiotic resistance gene profiles and increase potential risks in wastewater treatment systems Operators manage bulking through strategies ranging from adjusting nutrient ratios and dissolved oxygen levels to adding chemical oxidants like chlorine.
One promising biological alternative involves bacteriophages, viruses that specifically infect and kill bacteria. Lab studies have shown that phages can reduce foam-causing filamentous bacteria, control pathogenic species like Salmonella and E. coli, and even inhibit the biofilms that foul membranes in advanced treatment systems.14PubMed Central. Bacteriophages in sewage: abundance, roles, and applications – Section: Prospective applications of bacteriophages in wastewater treatment processes Phage therapy for wastewater is still mostly at the laboratory stage, but the specificity of phages — they target narrow groups of bacteria without wiping out the broader community — makes the approach attractive.
Antibiotic Resistance in the Treatment Process
Wastewater treatment plants receive a cocktail of antibiotics from hospitals, households, and agriculture. The dense, diverse microbial communities in treatment tanks create conditions where antibiotic resistance genes can spread between species through horizontal gene transfer. Treatment plants have been described as both reservoirs and environmental suppliers of antibiotic resistance.15PubMed. Antibiotic-Resistance Genes in Waste Water Even the filamentous bulking events described above appear to alter antibiotic resistance gene profiles in sludge, potentially increasing risk.13Environment International. Filamentous bacteria-induced sludge bulking can alter antibiotic resistance gene profiles and increase potential risks in wastewater treatment systems
Bacteriophages add another dimension. Research suggests phages may facilitate the horizontal transfer of resistance genes through transduction — packaging bits of bacterial DNA, including resistance genes, and shuttling them into new hosts. However, conjugative transfer (bacteria directly sharing genes via cell-to-cell contact) does not appear to be the main mechanism by which multidrug-resistant bacteria in these systems acquire and spread resistance.16PubMed. The role of bacteriophages in facilitating the horizontal transfer of antibiotic resistance genes in municipal wastewater treatment plants
Metagenomic surveillance is becoming a practical tool for tracking resistance. A recent framework combining antibiotic residue analysis with shotgun metagenomics across full-scale reuse-oriented plants in southern Europe found that reclaimed effluents carried lower resistance gene abundances (under half a resistance gene per bacterial cell) compared to typical secondary effluents. Yet specific clinically relevant resistance genes persisted even after advanced treatment, and the study proposed a set of sentinel markers for routine monitoring.17Environmental Science & Technology. Metagenomic Assessment of Full-Scale Wastewater Treatment Plants Identifies Sentinel Antibiotic Resistance Gene Families for Monitoring Reclaimed Wastewater and Treated Sludge
Emerging Contaminants and the Limits of Bacterial Breakdown
Conventional bacterial communities do a reasonable job with organic pollutants, but some synthetic chemicals resist biological degradation. Pharmaceuticals like ibuprofen, diclofenac, and the antibiotic sulfamethoxazole pass through many treatment plants at least partially intact. Microbial consortia — mixed communities engineered or selected for their synergistic interactions — show promise for breaking down these stubborn compounds through mechanisms like co-metabolism, where one species’ byproduct becomes another species’ substrate.18Journal of Environmental Management. Biodegradation of pharmaceutical contaminants in wastewater using microbial consortia: Mechanisms, applications, and challenges
Per- and polyfluoroalkyl substances (PFAS), the so-called “forever chemicals,” represent a harder challenge. Biological treatment does not break down the stable carbon-fluorine bonds in terminal PFAS compounds. In fact, the aeration basins where bacteria are most active can actually increase concentrations of certain PFAS by transforming precursor compounds into more persistent end products. One study found that target PFAS concentrations were highest in aeration basin effluents, with subsequent declines after clarification reflecting partitioning to solids rather than destruction.19ACS ES&T Water. PFAS in Activated Sludge Wastewater Treatment: Process-Level Insights into Transformation and Removal At a landfill leachate-impacted plant, PFAS concentrations roughly doubled in influent after an oxidation assay designed to reveal hidden precursors, confirming that a large pool of precursors exists that conventional treatment cannot address.20PubMed. Assessing PFAS and Their Precursor Transformation in a Landfill Leachate-Impacted Wastewater Treatment Plant Some microbial taxa, including members of Actinomycetia, have shown associations with elevated PFAS levels and carry genes for enzymes like haloacid dehalogenases and monooxygenases, hinting that certain bacteria may interact with PFAS precursors in ways not yet fully understood.21Ecotoxicology and Environmental Safety. Fate of per- and polyfluoroalkyl substances (PFAS) and microbial communities in wastewater treatment: Disinfection-driven changes in microbial dynamics and PFAS profiles
Bacterial Regrowth After Treatment
Even when a treatment plant does everything right, bacteria can regrow in the pipes and reservoirs that distribute reclaimed water. The loss of residual disinfectant and the presence of assimilable organic carbon in treated effluent give surviving bacteria the resources they need to multiply. Opportunistic pathogens like Aeromonas, Legionella, Mycobacterium, and Pseudomonas have been found more frequently in reclaimed water distribution systems than the traditional indicator bacteria (coliforms and enterococci) that regulations typically monitor.22PubMed Central. Regrowth of potential opportunistic pathogens and algae in reclaimed-water distribution systems
A comparison of reclaimed and potable water distribution systems found that reclaimed systems harbored distinctly different water chemistry, including elevated nutrients, alongside higher abundances of Legionella and Mycobacterium gene markers compared to potable systems.23PubMed. Microbial Ecology and Water Chemistry Impact Regrowth of Opportunistic Pathogens in Full-Scale Reclaimed Water Distribution Systems These pathogens spread primarily through aerosol inhalation rather than ingestion, which means they pose a risk even for non-potable reuse applications like landscape irrigation or industrial cooling. Current regulations were not designed with these organisms in mind, and there is growing recognition that monitoring strategies need to evolve.
Nitrous Oxide and the Climate Angle
Bacteria in treatment plants also produce nitrous oxide, a greenhouse gas roughly 265 times more potent than carbon dioxide over a hundred-year period. Nitrous oxide emissions from wastewater treatment arise during both nitrification and denitrification, and the exact mechanisms are still being worked out. Evidence points to nitrifier denitrification (where ammonia-oxidizing bacteria themselves reduce nitrite under low-oxygen conditions) and the chemical breakdown of hydroxylamine oxidation intermediates as likely pathways.24PubMed Central. Nitrous oxide emissions from wastewater treatment processes For the wastewater sector, this is a growing concern. As plants optimize for nitrogen removal by tightly controlling aeration, they risk creating the low-oxygen pockets that favor nitrous oxide production. Balancing nitrogen removal efficiency against greenhouse gas emissions is an active area of engineering research.
Temperature, Seasons, and Microbial Community Shifts
Bacteria are sensitive to temperature, and treatment performance can shift with the seasons. Research has found that wastewater treatment performs well between about 28°C and 38°C, meeting typical discharge standards within that range. But the dominant bacterial genera change with temperature: at 38°C the genus Thauera dominated nitrogen removal, while at 23°C Acidaminobacter took over. Higher temperatures promoted microbial activity and increased the abundance of key nitrogen-cycling genes, but they also reduced overall community diversity, as did very low temperatures. Both extremes create communities that are less diverse and potentially less resilient to sudden operational changes.
Bacteria as Energy Generators
Microbial fuel cells represent one of the more futuristic applications of wastewater bacteria. In these systems, bacteria colonize an electrode (the anode) and oxidize organic matter, releasing electrons that flow through an external circuit to generate electricity.25PubMed Central. Microbial Fuel Cell Construction Features and Application for Sustainable Wastewater Treatment The appeal is obvious: you clean the water and produce power simultaneously.26Energies. The Potential of Microbial Fuel Cells as a Dual Solution for Sustainable Wastewater Treatment and Energy Generation: A Case Study In practice, the power densities achieved so far are modest, and scaling up from laboratory prototypes to full-scale plants remains a significant engineering challenge. But as the cost of electrode materials drops and microbial communities are better understood, the technology could carve out a niche in decentralized or off-grid treatment applications.
Industrial Wastewater and Extremophile Bacteria
Municipal sewage is one thing; industrial wastewater is quite another. Food processing, petroleum refining, textile dyeing, and leather tanning generate wastewater loaded with organic pollutants at salt concentrations that would kill most conventional treatment bacteria. Halophilic microorganisms — bacteria that thrive in high-salt conditions — offer a way to treat these streams biologically rather than relying on expensive physical or chemical methods. These salt-loving communities can degrade aromatic hydrocarbons, phenols, and azo dyes that would otherwise require specialized treatment.27PubMed. Biodegradation of organic pollutants in saline wastewater by halophilic microorganisms: a review The broader lesson is that the bacterial toolkit for wastewater treatment extends far beyond the organisms found in a typical municipal plant. As industrial discharge regulations tighten worldwide, tailoring microbial communities to specific waste streams is becoming a practical necessity rather than a research curiosity.