Explainer
The Microbial Workforce: A Field Guide to the Functional Groups in Your Treatment System
The organisms in your treatment system don't exist as isolated species — they form functional communities, with each group performing a specific metabolic job. When we analyze your samples, we map the organisms we find to these functional groups so you can see not just who's there but what they're doing.
This guide describes each functional group in plain language: what the process is, why it matters for your system, and what it means when we report it in your results.
Nitrogen Cycling
Nitrogen removal is one of the most tightly regulated aspects of wastewater treatment. The organisms responsible for moving nitrogen through its various chemical forms — ammonia, nitrite, nitrate, and nitrogen gas — work in sequence, and the balance between them determines whether your system meets its permit.
Aerobic Ammonia Oxidation
What it is: The first step of nitrification. Specialized bacteria (such as Nitrosomonas) and some archaea convert ammonia (NH₃/NH₄⁺) to nitrite (NO₂⁻) using oxygen. These organisms are autotrophs — they get their energy from this chemical reaction rather than from consuming organic matter, and they fix CO₂ for carbon the same way plants do.
Why it matters: This is the entry point for biological nitrogen removal. If your ammonia-oxidizing population is too small or stressed, ammonia passes through the system untreated. Because these organisms grow slowly compared to heterotrophs, they're often the first population to suffer when conditions change — temperature drops, toxic loads, or insufficient SRT can all crash the ammonia oxidizers before you see ammonia in your effluent. qPCR monitoring of the amoA gene (the key enzyme gene for this process) gives you early warning of declining nitrification capacity.
Aerobic Nitrite Oxidation
What it is: The second step of nitrification. Nitrite-oxidizing bacteria — primarily Nitrospira and Nitrobacter — convert nitrite (NO₂⁻) to nitrate (NO₃⁻), again using oxygen. Like the ammonia oxidizers, these are slow-growing autotrophs.
Why it matters: If nitrite oxidizers fall behind the ammonia oxidizers, nitrite accumulates. In conventional nitrification, you want both steps running in balance. In some advanced systems (partial nitritation, deammonification), you intentionally suppress nitrite oxidizers to accumulate nitrite for anammox — in that case, monitoring this group tells you whether your suppression strategy is working.
Anammox
What it is: Anaerobic ammonium oxidation. A unique group of bacteria (such as Candidatus Brocadia and Ca. Scalindua) that convert ammonium and nitrite directly into nitrogen gas under oxygen-free conditions. Unlike conventional nitrification–denitrification, anammox doesn't require organic carbon and uses significantly less energy.
Why it matters: Anammox-based processes (deammonification, partial nitritation/anammox) are among the most promising energy-efficient alternatives to conventional nitrogen removal, particularly for high-strength sidestreams like digester centrate. These bacteria grow extremely slowly — doubling times of 10–14 days — so losing them means weeks to months of recovery. qPCR monitoring of the hzsA gene (hydrazine synthase, the signature enzyme) is essential for tracking population health in any system that depends on anammox.
Denitrification (Nitrate Respiration)
What it is: Bacteria use nitrate (NO₃⁻) as a substitute for oxygen when oxygen isn't available. They reduce nitrate step by step — nitrate → nitrite → nitric oxide → nitrous oxide → nitrogen gas (N₂) — and the nitrogen gas escapes to the atmosphere. This is an anaerobic respiratory process powered by organic carbon.
Why it matters: Denitrification is how most treatment systems actually remove nitrogen from the water. The nitrate produced by nitrification gets converted to harmless nitrogen gas. Many common wastewater bacteria can denitrify — it's a widespread capability, not restricted to specialists — so this group is usually robust. But denitrification requires a carbon source (BOD), so systems with low carbon-to-nitrogen ratios may need supplemental carbon. Monitoring denitrifier gene abundance (nirS, nirK, nosZ) helps verify that the capacity is present and can be useful when evaluating carbon dosing strategies.
Nitrite Respiration
What it is: Similar to denitrification, but using nitrite (NO₂⁻) rather than nitrate as the starting electron acceptor. Organisms reduce nitrite to nitric oxide, nitrous oxide, or nitrogen gas under oxygen-free conditions.
Why it matters: This process often operates as part of the broader denitrification pathway, but some organisms specialize in it. It's particularly relevant in systems that accumulate nitrite intentionally (partial nitritation systems) and in environments where nitrite is available as a distinct substrate. Nitrite respiration also matters for greenhouse gas accounting — incomplete reduction can release nitrous oxide (N₂O), a potent greenhouse gas, rather than going all the way to harmless N₂.
Carbon Cycling and Anaerobic Digestion
These functional groups handle the breakdown of organic matter — from the initial dismantling of complex molecules through fermentation, all the way to the final production of methane and CO₂ in anaerobic digesters. They also include organisms that consume methane and other single-carbon compounds.
Fermentation
What it is: The breakdown of organic compounds (sugars, amino acids, other substrates) into simpler products — acetate, propionate, butyrate, lactate, ethanol, hydrogen, and CO₂ — without using oxygen or any other external electron acceptor. The organisms generate energy through substrate-level phosphorylation, which is less efficient than respiration but doesn't depend on oxygen or nitrate.
Why it matters: Fermentation is the first major step in anaerobic digestion. Complex organic matter gets hydrolyzed and then fermented into the volatile fatty acids (VFAs) and hydrogen that feed downstream methanogens. In a healthy digester, fermentation and methanogenesis are balanced — VFAs are produced and consumed at roughly equal rates. When fermenters outpace methanogens, VFAs accumulate and pH drops, which can inhibit the methanogens further and send the digester into a downward spiral. Monitoring fermenter populations alongside VFA data helps you see this imbalance developing before it becomes a crisis.
Syntrophic Acetogenesis (SAB)
What it is: Syntrophic acetogenic bacteria oxidize intermediate fermentation products — mainly propionate, butyrate, and longer-chain fatty acids — into acetate, hydrogen, and CO₂. Here's the catch: this reaction is thermodynamically unfavorable under standard conditions. It only works when a partner organism (usually a hydrogenotrophic methanogen) continuously removes the hydrogen as fast as it's produced, keeping the hydrogen concentration low enough to make the reaction energetically feasible.
Why it matters: Syntrophs are the critical middle link in the anaerobic digestion food chain, connecting fermentation to methanogenesis. If the syntrophic community is weak or disrupted — for instance, by a sudden temperature change or a toxic load that kills their methanogenic partners — VFAs (especially propionate and butyrate) accumulate rapidly. Propionate accumulation in particular is one of the earliest and most reliable indicators of digester stress. Monitoring syntrophic populations helps you understand why VFAs are accumulating, not just that they are.
Reductive Acetogenesis / Homoacetogenesis (RAB)
What it is: Homoacetogens consume hydrogen and CO₂ and convert them into acetate via the Wood–Ljungdahl pathway. In simple terms: they do the opposite of what you might expect — instead of using hydrogen to make methane (like hydrogenotrophic methanogens do), they use hydrogen to make acetate.
Why it matters: Homoacetogens compete directly with hydrogenotrophic methanogens for the same substrate — hydrogen and CO₂. When homoacetogens win that competition, hydrogen gets funneled into acetate rather than methane, which reduces biogas yield. A system with high homoacetogen abundance and relatively low hydrogenotrophic methanogen numbers may produce more acetate than the acetoclastic methanogens can handle, leading to VFA accumulation. This is a subtler cause of digester underperformance than a simple methanogen crash, and it's essentially invisible without molecular data to reveal the competition.
Acetoclastic Methanogenesis
What it is: The direct conversion of acetate into methane and CO₂ by specialized archaea, primarily Methanosaeta (now Methanothrix) and Methanosarcina. This is one of the two main routes to methane production in anaerobic digesters.
Why it matters: In most conventional mesophilic digesters, acetoclastic methanogenesis accounts for roughly two-thirds of total methane production. Methanosaeta dominates at low acetate concentrations and is associated with stable, well-performing digesters. Methanosarcina has a faster growth rate and can tolerate higher acetate and ammonia concentrations, so it tends to dominate under stress. A shift from Methanosaeta to Methanosarcina dominance doesn't necessarily mean the digester is failing — Methanosarcina is more resilient in many ways — but it does signal changed conditions. Tracking the ratio between these two genera gives you direct insight into your digester's operating regime.
Hydrogenotrophic Methanogenesis
What it is: The production of methane from hydrogen and CO₂ by methanogenic archaea (such as Methanobacterium, Methanospirillum, and members of the order Methanomicrobiales). This is the second major route to methane, and it's the process that keeps hydrogen partial pressures low enough for syntrophic acetogenesis to function.
Why it matters: Hydrogenotrophic methanogens are the essential partner to the syntrophic bacteria described above. Without them consuming hydrogen, the entire fatty acid degradation pathway stalls. In thermophilic digesters and in systems with high ammonia (which inhibits acetoclastic methanogens), the hydrogenotrophic pathway becomes the dominant route to methane, often operating in tandem with syntrophic acetate oxidation. qPCR monitoring of this group — alongside the acetoclastic methanogens — tells you which methanogenic pathway your digester is relying on, which directly informs how vulnerable it is to different types of stress.
Methanotrophy
What it is: The aerobic oxidation of methane (CH₄) to CO₂ by specialized bacteria (such as Methylococcus and Methylosinus). These organisms use the enzyme methane monooxygenase to break the strong C–H bond in methane, then assimilate the carbon for growth.
Why it matters: Methanotrophs are most relevant in systems where methane produced in anaerobic zones might escape to aerobic zones — landfill cover soils, the water surface above anoxic sediments, or the aerobic zones of treatment systems receiving digester return flows. In wastewater, methanotroph abundance in aeration basins can indicate dissolved methane carryover from upstream anaerobic processes. They're also relevant to greenhouse gas accounting, since they consume methane that would otherwise be emitted.
Methylotrophy
What it is: The metabolism of single-carbon compounds — methanol, methylamine, formaldehyde, formate — as both carbon and energy sources. Methylotrophs oxidize these compounds, often through formaldehyde as an intermediate, and can function under aerobic or anaerobic conditions.
Why it matters: Methylotrophs are particularly relevant in systems that receive methanol as an external carbon source for denitrification. They're the organisms actually consuming the methanol you're dosing, and their abundance and activity determine how efficiently that carbon source is used. In systems not using methanol, methylotrophs are typically a minor component but can indicate the presence of industrial discharges containing methylated compounds.
Sulfur Cycling
Sulfur-cycling organisms matter most in systems dealing with sulfide, corrosion, odor, and biofilm — cooling towers, collection systems, anaerobic processes, and industrial wastewater with sulfur-containing compounds. The balance between sulfur oxidizers and sulfur reducers determines whether your system produces or consumes sulfide.
Sulfur Oxidation
What it is: Microorganisms oxidize reduced sulfur compounds — hydrogen sulfide (H₂S), elemental sulfur, thiosulfate, or sulfite — to sulfate, using oxygen as the electron acceptor. Common sulfur-oxidizing bacteria include Thiobacillus, Acidithiobacillus, and Halothiobacillus. Some of these organisms are autotrophs (they fix CO₂ for carbon, like nitrifiers do), while others are mixotrophs that can use both organic and inorganic carbon.
Why it matters: Sulfur oxidizers play a dual role. In treatment systems, they can be beneficial — removing sulfide (and its associated odor and corrosion) by converting it to less harmful sulfate. But in the wrong context, they cause serious damage. Acidithiobacillus species produce sulfuric acid as a byproduct of sulfide oxidation and are the primary cause of microbially influenced corrosion (MIC) in concrete sewer pipes and headworks structures. In cooling towers, sulfur oxidizers in biofilms can accelerate metal corrosion. Identifying which sulfur oxidizers are present — and at what abundance — is essential for assessing corrosion risk versus beneficial sulfide removal.
Sulfate Respiration (Sulfate Reduction)
What it is: Anaerobic bacteria (such as Desulfovibrio, Desulfobulbus, and Desulfobacter) use sulfate as a substitute for oxygen in respiration, reducing it to hydrogen sulfide (H₂S). They oxidize organic carbon or hydrogen to generate energy, and the sulfide they produce is the waste product.
Why it matters: Sulfate-reducing bacteria (SRB) are responsible for most of the sulfide problems in water systems — sewer corrosion, odor, and toxicity to methanogens in anaerobic digesters. In digesters, SRB compete with methanogens for the same substrates (hydrogen and acetate), and when sulfate is abundant in the feed, SRB can divert carbon away from methane production and toward sulfide production instead. In cooling towers, SRB in biofilms produce sulfide that drives localized pitting corrosion of metal surfaces even in otherwise aerobic systems. qPCR targeting the dsrA or dsrB genes (dissimilatory sulfite reductase, the key enzyme) is one of our most-requested assays for corrosion risk assessment.
Sulfite Respiration
What it is: An anaerobic process where organisms use sulfite (SO₃²⁻) as an electron acceptor, reducing it to sulfide. This is often an intermediate step within the broader sulfate reduction pathway, but some organisms can use sulfite directly as their primary electron acceptor.
Why it matters: Sulfite respiration is most relevant as a component of the sulfate reduction pathway. Elevated sulfite-reducing activity can indicate active sulfate reduction even when sulfate concentrations don't appear to be dropping significantly, because sulfite is an intermediate that gets produced and consumed rapidly. In industrial systems where sulfite is present as a chemical additive (such as boiler water treatment or dechlorination), sulfite-reducing organisms can cause unexpected sulfide production.
Sulfur Respiration
What it is: Anaerobic bacteria use elemental sulfur (S⁰) as an electron acceptor, reducing it to hydrogen sulfide. This occurs in environments where elemental sulfur is available — often as a byproduct of partial sulfide oxidation.
Why it matters: Elemental sulfur can accumulate in systems where sulfide is being partially oxidized — for instance, in micro-aerobic desulfurization of biogas, or in the transition zones between aerobic and anaerobic regions of biofilms. Organisms that reduce this sulfur back to sulfide can create a sulfur cycle within the system that regenerates sulfide faster than your treatment process removes it. Identifying this group helps explain situations where sulfide levels remain stubbornly high despite active sulfur oxidation.
Thiosulfate Respiration
What it is: An anaerobic process where organisms use thiosulfate (S₂O₃²⁻) as an electron acceptor. The primary respiratory product is sulfide. Some organisms can also disproportionate thiosulfate — splitting it into both sulfide and sulfate simultaneously — which is a distinct metabolic strategy from true respiration but can occur in the same environments.
Why it matters: Thiosulfate is a common intermediate in sulfur cycling and is also used as a chemical additive in some industrial processes (dechlorination, photographic processing, certain cooling water treatments). Organisms that respire or disproportionate thiosulfate can produce sulfide in systems where you wouldn't expect it based on sulfate concentrations alone. If your system uses thiosulfate chemically and you're seeing unexpected sulfide or corrosion, this functional group may be responsible.
Phosphorus Cycling
Biological phosphorus removal depends on a specific competitive dynamic between organisms that store phosphorus and organisms that don't. The balance between these two groups determines whether your EBPR process works reliably or drifts toward chemical backup.
Phosphorus-Accumulating Organisms (PAOs)
What it is: PAOs — primarily Candidatus Accumulibacter and related lineages — are the bacteria responsible for enhanced biological phosphorus removal (EBPR). Their metabolism has two distinct phases. Under anaerobic conditions, they take up volatile fatty acids (primarily acetate and propionate) and store them internally as polyhydroxyalkanoates (PHA), using energy from breaking down their stored polyphosphate (which releases phosphorus into solution). Under subsequent aerobic or anoxic conditions, they consume the stored PHA for growth and energy, and in the process take up phosphorus from the water in large excess — far more than they need for basic cell function — storing it again as polyphosphate.
Why it matters: PAOs are the engine of biological phosphorus removal. The net effect of their two-phase metabolism is that more phosphorus gets taken up in the aerobic zone than was released in the anaerobic zone, producing a net removal from the wastewater. When the biomass is wasted from the system, that stored polyphosphate leaves with it. If PAO populations decline — due to insufficient VFAs in the anaerobic zone, competition from GAOs, high temperatures, or loss of the anaerobic selector — phosphorus removal deteriorates, often rapidly. qPCR targeting the ppk1 gene (polyphosphate kinase) gives you a direct measure of PAO population size. Accumulibacter has multiple clades (IA, IIA, IIB, IIC, and others) with different substrate preferences and denitrification capabilities, so community profiling can reveal which clade your system has selected and what that means for robustness.
Denitrifying PAOs (DPAOs)
What it is: A subset of PAOs — primarily certain Accumulibacter clades — that can use nitrate or nitrite instead of oxygen as the electron acceptor during the PHA-consumption and phosphorus-uptake phase of their metabolism. They perform denitrification and phosphorus uptake simultaneously.
Why it matters: DPAOs are operationally valuable because they perform two jobs at once: removing nitrogen (as denitrifiers) and removing phosphorus (as PAOs) using the same carbon that was stored as PHA. This means they use less oxygen and less external carbon than systems where nitrification–denitrification and phosphorus removal are performed by separate organisms. If your BNR configuration includes anoxic phosphorus uptake (as in a University of Cape Town or modified Bardenpho configuration), the proportion of your PAO population that can denitrify directly affects performance. Community profiling can distinguish DPAO-capable clades from those that require aerobic conditions for phosphorus uptake.
Glycogen-Accumulating Organisms (GAOs)
What it is: GAOs — primarily Candidatus Competibacter and Defluviicoccus — use a metabolism superficially similar to PAOs: they take up VFAs under anaerobic conditions and store them as PHA. The critical difference is the energy source. Instead of breaking down polyphosphate to fuel VFA uptake (as PAOs do), GAOs use internally stored glycogen. This means they compete with PAOs for the same carbon substrates in the anaerobic zone but don't cycle phosphorus at all.
Why it matters: GAOs are the primary biological competitors to PAOs in EBPR systems. Every molecule of VFA a GAO takes up in the anaerobic zone is a molecule a PAO doesn't get — and since GAOs don't remove phosphorus, that carbon is wasted from a treatment perspective. GAOs are favored by warmer temperatures (above 25°C), propionate-poor influent, and longer SRTs. The GAO:PAO ratio is one of the most important metrics for predicting EBPR stability: a rising GAO fraction is an early warning of declining phosphorus removal, often weeks before effluent phosphorus starts increasing. qPCR monitoring of Competibacter and Defluviicoccus alongside Accumulibacter quantifies this competitive balance directly.
Iron Cycling
Iron-cycling organisms are most relevant in systems dealing with groundwater, corrosion, distribution system water quality, or environments with significant redox transitions where iron moves between its oxidized (ferric, Fe³⁺) and reduced (ferrous, Fe²⁺) forms.
Iron Oxidation
What it is: Iron-oxidizing bacteria (such as Gallionella, Leptothrix, Sphaerotilus, and Mariprofundus) oxidize dissolved ferrous iron (Fe²⁺) to ferric iron (Fe³⁺), which precipitates as iron oxides and hydroxides — the orange-brown deposits commonly called rust, ochre, or iron fouling. Some are chemolithoautotrophs (they use the iron oxidation reaction as their sole energy source), while others are heterotrophs that catalyze the reaction incidentally.
Why it matters: Iron-oxidizing bacteria cause some of the most visible and costly biofilm problems in water systems. In drinking water distribution, they produce tubercles — hard, mound-like deposits on pipe walls that reduce flow capacity and harbor other bacteria. In groundwater wells, they form the characteristic ochre slime that clogs screens and pumps. In cooling water systems, iron-oxidizing bacteria in biofilms create localized corrosion cells by depositing iron oxides on metal surfaces, concentrating oxygen differentials, and creating the conditions for under-deposit corrosion. Identifying and quantifying these organisms helps distinguish biologically driven iron fouling from purely chemical iron precipitation — a distinction that determines whether the fix is biological (biocide, cleaning) or chemical (pH adjustment, sequestration).
Iron Reduction
What it is: Iron-reducing bacteria (such as Geobacter, Shewanella, and Geothrix) use ferric iron (Fe³⁺) as an electron acceptor in anaerobic respiration — essentially using rust the way other organisms use oxygen or nitrate. They reduce insoluble ferric iron to soluble ferrous iron (Fe²⁺), often mobilizing iron that was locked up in sediments or deposits.
Why it matters: Iron reduction is relevant in several contexts. In anaerobic digesters receiving iron-rich substrates (or iron-dosed for sulfide control), iron-reducing bacteria can compete with methanogens for organic substrates, potentially diverting carbon away from methane production. In water distribution systems, iron reducers in the anoxic interior of biofilms can dissolve the protective iron oxide scale on pipe walls, causing red water events and accelerating corrosion from the inside out. In natural and constructed wetlands, iron reduction mobilizes not just iron but also arsenic and phosphorus that co-precipitate with iron oxides, which can be either a treatment mechanism or a water quality problem depending on context.
Filamentous Organisms
Filamentous bacteria aren't a single metabolic functional group — they span multiple phyla and metabolisms. What unites them is their growth form: long, thread-like cells or chains of cells that extend from or between flocs in activated sludge. We track them as a distinct category because of their outsized impact on solids separation.
Filamentous Bulking Organisms
What it is: A diverse group of bacteria — including Candidatus Microthrix parvicella, Thiothrix, organisms classified as Type 0041 (Chloroflexi-affiliated), Type 0092, Type 021N, and many others — that grow as long filaments in activated sludge. Different filament species are favored by different conditions: low dissolved oxygen, low F:M ratio, long SRT, sulfide in the influent, grease and fat, or nutrient deficiency.
Why it matters: Filaments are the most common cause of poor settling and thickening in activated sludge systems. When abundant, they extend from floc surfaces and physically bridge between particles, preventing compaction in the clarifier and driving SVI upward. The key insight from molecular identification is that different filaments require different operational responses. Ca. Microthrix responds to SRT reduction and selector operation. Thiothrix responds to sulfide control and increased DO. Chloroflexi-type filaments have different ecology entirely. Microscopy lumps morphologically similar organisms together; molecular identification tells you which species is actually causing the problem so you can target the response. See our article "Connecting Filaments to Clarifier Behavior" for a detailed treatment of this topic.
Foam-Forming Organisms
What it is: A subset of filamentous and branching bacteria with hydrophobic cell surfaces that cause them to accumulate at the air–water interface, stabilizing persistent biological foam. The primary foam-formers are Ca. Microthrix parvicella (which also causes bulking) and the nocardioform actinomycetes — a morphological group that includes the genera Gordonia, Rhodococcus, Mycobacterium, Dietzia, and Williamsia.
Why it matters: Biological foaming reduces treatment capacity, creates housekeeping problems, can carry solids over weirs and into the effluent, and in severe cases fills enclosed spaces with foam that creates safety hazards. Molecular identification is particularly important for foam-formers because the nocardioform group — which microscopy identifies as a single morphological category — actually comprises several genera with different growth rates, foaming potency, and susceptibility to control strategies. Gordonia is a particularly aggressive foam-former; Rhodococcus species vary widely in foaming potential. Knowing which genus dominates your foam guides whether wasting, SRT reduction, selector operation, or targeted treatment will be effective.
Non-Filamentous (Viscous) Bulking
Not all settling problems are caused by filaments. Viscous bulking is a distinct mechanism driven by organisms that look perfectly normal under the microscope — no threads, no bridging — but produce so much extracellular slime that flocs become waterlogged and unable to compact.
Viscous Bulking Organisms
What it is: Non-filamentous bulking — also called viscous bulking or zoogloeal bulking — is caused by bacteria that overproduce extracellular polymeric substances (EPS), primarily exopolysaccharides, creating a gel-like matrix that traps water within the floc structure. The organisms most commonly responsible belong to the genera Zoogloea, Thauera, Azoarcus, and related members of the Rhodocyclaceae family. Unlike filamentous bulking, which is caused by long thread-like cells physically bridging between flocs, viscous bulking is caused by otherwise normal-looking bacteria producing so much EPS that the floc becomes swollen, diffuse, and unable to compact.
Why it matters: Viscous bulking is frustrating because it doesn't present the way operators expect a settling problem to look. Microscopy may show no filaments at all — flocs appear large and irregularly shaped, sometimes with a translucent, gelatinous halo, but there are no obvious threads extending between them. SVI climbs, blanket depth increases, and the sludge takes on a slimy, viscous texture that resists thickening and dewatering. Because the traditional diagnostic reflex is to look for filaments, viscous bulking often goes misidentified or unidentified for weeks while operators chase a filament problem that doesn't exist.
Molecular identification cuts through this ambiguity. When community profiling shows Zoogloea, Thauera, or Azoarcus at elevated relative abundance — particularly above 5–10% of the total community — in a system with high SVI and no significant filament population, viscous bulking becomes the working diagnosis. These organisms are commonly denitrifiers (particularly Thauera and Azoarcus), so they tend to thrive in BNR systems with anoxic zones and readily available carbon. They're also favored by high soluble BOD in the influent, particularly simple sugars and volatile fatty acids, and by low dissolved oxygen conditions that promote EPS production.
The operational response to viscous bulking differs from the response to filamentous bulking in important ways. SRT reduction can help by washing out the slow-compacting biomass faster, but it has to be balanced against the needs of nitrifiers and PAOs. Reducing soluble BOD loading to the biological reactor — through better primary clarification or equalization — removes the substrate that fuels EPS overproduction. Increasing DO in the aerobic zone can reduce EPS production in some cases, since some of these organisms overproduce EPS under oxygen-limited conditions. Cationic polymer addition to the secondary clarifier can temporarily improve compaction by neutralizing the negatively charged EPS matrix, but it's a bandage, not a fix.
qPCR monitoring of Zoogloea and Thauera gives you a direct, quantitative way to track whether your interventions are reducing the responsible population — something that SVI alone can't distinguish from other causes of poor settling. Over time, correlating EPS-producer abundance with SVI, influent BOD fractionation, and DO data reveals the thresholds specific to your system where viscous bulking becomes a problem.
If your system involves functional groups or target organisms not listed here — including organisms relevant to specific industrial processes, emerging contaminants, or antibiotic resistance genes — let us know. We can customize our analysis to target what matters most for your treatment objectives.