In activated sludge systems, effective process control hinges on accurately assessing the biological workforce responsible for pollutant removal. Mixed Liquor Volatile Suspended Solids (MLVSS) remains a widely adopted metric for estimating biomass concentration, but its limitations as a proxy for true microbial activity are well-documented. This article critically examines the strengths and shortcomings of MLVSS and highlights advanced alternatives for practitioners seeking more granular process insights.

The Technical Merits of MLVSS
MLVSS quantifies the volatile (organic) fraction of suspended solids in the aeration tank, serving as a practical surrogate for total microbial mass. The method’s appeal lies in its operational simplicity: filtration, drying, and ignition yield a rapid estimate of organic solids, which typically comprise 60–80% of Mixed Liquor Suspended Solids (MLSS) in conventional systems. This enables routine calculation of critical ratios such as Food-to-Microorganism (F/M), supporting real-time process adjustments to maintain system stability and prevent overloads.
MLVSS is sensitive to operational perturbations. For example, toxicity events in the influent can suppress microbial growth, reflected as a drop in MLVSS—a valuable early warning for process upsets. Empirical studies have also correlated higher MLVSS with improved pollutant removal, such as enhanced decolorization and COD reduction in textile wastewater applications.

Limitations: Why MLVSS Is Not a Direct Activity Metric
Despite its utility, MLVSS is fundamentally a mass-based measurement. It does not distinguish between viable and non-viable biomass, nor does it account for extracellular polymers, inert organic debris, or plant material. As a result, elevated MLVSS may mask declining process performance if a significant fraction of the measured solids are inactive or dead. This is particularly problematic in systems with poor grit removal or aging sludge, where inert accumulation can distort the MLVSS/MLSS ratio and obscure true biological activity.
Moreover, MLVSS offers no insight into the functional diversity or metabolic state of the microbial community. For instance, nitrifiers may be present but metabolically suppressed due to suboptimal dissolved oxygen or pH, yet still contribute to the volatile solids count. In advanced processes—such as Enhanced Biological Phosphorus Removal (EBPR)—MLVSS fails to capture the dynamics of specialized functional groups, limiting its diagnostic value for troubleshooting or optimization.

Advanced Alternatives for Assessing Microbial Activity
To address these limitations, technical operators are increasingly adopting direct activity assays:

• Adenosine Triphosphate (ATP) Analysis: ATP quantification provides a real-time measure of living biomass, excluding dead cells and inert matter. ATP levels correlate strongly with actual treatment performance, offering a more precise indicator of system health.
• Oxygen Uptake Rate (OUR) and Specific OUR (SOUR): These tests directly measure microbial respiration rates, reflecting metabolic activity and enabling rapid detection of toxic inhibition or aeration inefficiencies.
• Molecular Techniques (e.g., qPCR, Next-Generation Sequencing): These methods quantify specific microbial populations or functional genes, supporting targeted diagnostics in complex or industrial wastewaters. While resource-intensive, they are invaluable for root-cause analysis and advanced process control.

A hybrid monitoring strategy—combining MLVSS with ATP, OUR, or molecular assays—yields a more comprehensive understanding of system performance and resilience.

Summary
MLVSS remains a foundational tool for routine process control due to its accessibility and cost-effectiveness. However, its indirect nature and inability to discriminate active from inactive biomass limit its utility for advanced monitoring and optimization. By integrating direct activity measurements, technical teams can achieve more accurate diagnostics, optimize operational parameters, and enhance effluent quality. Regularly auditing and updating monitoring protocols is essential for maintaining robust and efficient wastewater treatment operations.

Postbiotics are one of those under‑used but high‑leverage tools in biological wastewater treatment. They don’t add new microbes the way bioaugmentation does—instead, they add the biochemical signals, enzymes, metabolites, and structural components that high‑performing microbial communities naturally produce. Think of them as microbial performance boosters rather than microbes themselves.
Here’s how they elevate system performance in a way operators can actually feel in the basin.
🌱 What Postbiotics ArePostbiotics include:

• Enzymes (proteases, lipases, amylases, cellulases)
• Organic acids (lactic, acetic)
• Peptides and biosurfactants
• Cell wall fragments and signaling molecules
• Metabolites that modulate microbial behavior

They’re essentially the “working molecules” of a healthy microbial community—delivered directly, without needing the microbes that made them.
🚀 How Postbiotics Improve Biological Treatment Performance1. Faster Hydrolysis of Complex WastesHydrolysis is the rate‑limiting step in many biological treatment. Postbiotic (free) enzymes:

• Break down fats, oils, and grease into usable fatty acids
• Split proteins into amino acids
• Convert starches and cellulose into simple sugars

This gives the existing biomass more accessible food, reducing lag time and improving COD removal.

2. Stabilized Microbial CommunitiesPostbiotics contain quorum-sensing molecules and cell wall fragments that:

• Encourage beneficial guilds (e.g., floc formers, nitrifiers)
• Reduce stress responses
• Improve floc structure and settling

You get denser, more resilient biomass.

3. Improved Nitrification and Nutrient RemovalOrganic acids and cofactors in postbiotics:

• Support nitrifier metabolism during shock loads
• Reduce pH micro‑gradients inside flocs
• Enhance electron transfer efficiency

This leads to more stable ammonia oxidation and better total nitrogen removal, especially in cold or variable conditions.

4. Reduced Filamentous PressureSome postbiotic compounds:

• Disrupt filamentous quorum signals
• Strengthen EPS production by floc-formers
• Improve shear resistance of flocs

The result is less bulking, better SVI, and tighter sludge.

5. Enhanced Sludge ReductionPostbiotic enzymes accelerate endogenous respiration by:

• Breaking down dead cell material
• Increasing solubilization of bound organics
• Supporting fermentative pathways

This can translate into lower sludge yield and reduced hauling costs.

6. Better Resilience to Toxic or Variable LoadsPostbiotics often include:

• Antioxidants
• Stress‑response metabolites
• Chelators that bind inhibitory metals

These help the biomass recover faster from toxicity, pH swings, or hydraulic shocks.

🧪 Why Operators Like Postbiotics

• They work immediately—no lag for microbial growth
• They’re stable and easy to dose
• They don’t compete with existing biomass
• They enhance what’s already there

They’re especially powerful in systems with:

• High FOG
• Industrial variability
• Cold weather nitrification issues
• Filamentous instability
• High sludge production

In composting—as in activated sludge, biosolids stabilization, and other biological treatment systems—the real work is performed by a dynamic microbial consortium. What looks like “rotting” is actually a structured succession of microbial guilds responding to substrate availability, temperature, moisture, and oxygen transfer.

Two functional groups dominate this process: bacteria and fungi. Their interaction is not static; it shifts through phases of competition, inhibition, and cooperation as the organic matrix changes. Understanding this succession is key to optimizing any aerobic stabilization process, whether in a backyard pile or a biosolids windrow.
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1. Functional Guilds: Rapid Responders vs. Structural Degraders

​This division mirrors what wastewater professionals see in RBCs, trickling filters, and aerobic digesters: bacteria handle the fast, soluble load; fungi and actinomycetes step in when the substrate becomes more complex.
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2. Temperature-Driven Succession: Three Distinct Phases
Microbial dominance in composting is governed largely by temperature, which itself is a byproduct of metabolic heat release.

Act I: Mesophilic Phase — Bacterial Bloom
Typical Range: ~20–40°C
Fresh nitrogen-rich inputs trigger a surge of mesophilic heterotrophic bacteria. Their rapid oxidation of soluble organics increases temperature and oxygen demand.

• Dynamics: Bacteria outcompete fungi due to faster growth rates and access to soluble substrates.
• Outcome: Heat accumulation pushes the system into thermophilic conditions, suppressing most fungal activity.
  
  

Act II: Thermophilic Phase — High-Rate Oxidation
Typical Range: ~45–70°C
As temperatures rise, mesophiles decline and thermophilic bacteria take over. This phase is analogous to high-rate aerobic digestion.

• Fungal Status: Most fungi are inhibited or sporulate; activity is concentrated in cooler outer zones.
• Notable Group: Actinobacteria, filamentous bacteria often mistaken for fungi, thrive here and contribute to lignocellulose breakdown and geosmin production (the “earthy” odor).
  
  

Act III: Curing Phase — Fungal Recolonization
Typical Range: Cooling to ambient
Once readily biodegradable organics are depleted, metabolic heat drops and the system cools.

• Dynamics: Fungi re-establish dominance, targeting lignin, the most recalcitrant fraction of plant biomass.
• Importance: Lignin degradation exposes cellulose and hemicellulose, enabling secondary bacterial activity and driving humus formation—the stable end product.
  
  

This curing phase is analogous to the endogenous respiration period in biosolids stabilization, where slow-growing organisms finish the job.

3. Carbon–Nitrogen Balance Through a Microbial Lens
The classic “Browns vs. Greens” rule maps directly to microbial ecology:

• Excess Nitrogen (Greens):
  High soluble organics → bacterial overgrowth → oxygen depletion → anaerobic pockets → odor generation (VFAs, reduced sulfur compounds).
• Excess Carbon (Browns):
  High lignocellulose → limited bacterial activity → insufficient heat generation → fungal-dominant, low-temperature composting.
  

Just as in wastewater treatment, substrate balance dictates microbial selection.

4. Operational Practices That Influence Microbial Performance

1. Limit late-stage turning:
   Excessive agitation disrupts fungal hyphae during the curing phase. Similar to biofilm systems, structural integrity matters for lignin degradation.
2. Particle size reduction:
   Shredding carbon-rich materials increases surface area, improving fungal colonization and oxygen penetration.
3. Moisture control:
   Bacteria require thin-film water for substrate transport; fungi tolerate lower moisture.
   Over-drying shifts the system toward fungal dominance and uneven decomposition.
   
   

Summary

• Bacteria drive the high-rate oxidation of soluble organics and thermophilic heat generation.
• Fungi are essential for degrading lignin and completing humification.
• Succession—not a single microbial group—is what stabilizes organic matter effectively.
• Operational controls (moisture, aeration, particle size, C:N ratio) determine which guild dominates and when.

 

As city managers and engineers responsible for maintaining efficient and reliable wastewater infrastructure, you're aware that the collection system is the backbone of urban sanitation. However, one persistent threat often lurks unseen until it causes major disruptions: fats, oils, and grease (FOG). Commonly entering the system from food service establishments, households, and industrial sources, FOG can solidify and accumulate, leading to a cascade of operational headaches. In this article, we'll dive into why grease poses such significant problems in collection systems, with a particular emphasis on lift stations—critical components that pump wastewater uphill or across long distances. We'll explore the formation of hydrogen sulfide (H2S) in anaerobic zones, the damage to pumps and floats, and the heightened risk of sanitary sewer overflows (SSOs). Understanding these issues is key to proactive management and cost savings.

How Grease Infiltrates and Accumulates in Collection Systems
Grease doesn't just disappear down the drain. When hot FOG from cooking or cleaning cools in pipes, it congeals into solid deposits, often combining with other debris like wipes or solids to form blockages. In the U.S., FOG deposits are responsible for over 45% of sewer pipe occlusions, leading to increased maintenance costs and environmental risks. These accumulations reduce pipe capacity, slow flow, and create ideal conditions for further problems downstream. While main sewer lines bear the brunt initially, the effects are amplified in lift stations, where wastewater pools and gravity can't always do the work.

The Unique Vulnerabilities of Lift Stations
Lift stations are engineered to handle variable flows, but they're particularly susceptible to grease buildup due to their design. Wastewater collects in a wet well, where pumps activate based on level sensors or floats. Over time, FOG forms a "grease cap" or scum layer on the surface, which can harden and adhere to sidewalls, floats, and other components. This accumulation accounts for roughly 30% of sewage backups in lift stations, turning routine operations into emergencies. The confined space and stagnant conditions exacerbate issues, making lift stations a hotspot for grease-related failures.

H2S Formation: The Corrosive Byproduct of Anaerobic Zones
One of the most insidious effects of grease in lift stations is the creation of anaerobic environments. Beneath a thick grease cap, oxygen levels plummet, allowing sulfate-reducing bacteria to thrive. These microbes convert sulfates in the wastewater into hydrogen sulfide (H2S) gas—a toxic, corrosive compound with a rotten-egg odor. H2S not only poses health risks to maintenance crews entering confined spaces but also reacts with moisture to form sulfuric acid. This acid attacks concrete walls, metal pipes, and equipment, accelerating structural degradation. In force mains and downstream structures, H2S can drive down pH levels, leading to crown corrosion in manholes and influent lines. For city managers, this means higher repair bills and shorter asset lifespans; engineers might see this as a call for enhanced ventilation or chemical dosing strategies to mitigate anaerobic conditions.

Damage to Pumps and Floats: Operational Nightmares
Grease doesn't just corrode—it clogs and encrusts. Pumps in lift stations can become fouled as FOG adheres to impellers and seals, reducing efficiency and causing overheating or outright failure. Floats and sensors, which detect water levels to trigger pumps, often get coated in a greasy layer, preventing proper actuation. When floats stick, pumps may run dry or fail to start, leading to overflows or backups. In severe cases, this requires frequent pump-outs—sometimes more than twice a year—to clear solids and grease, adding to operational costs. The combination of mechanical wear and corrosion from H2S can shorten pump life dramatically, turning a reliable system into a maintenance sinkhole.

The Escalating Risk of Sanitary Sewer Overflows
Perhaps the most alarming consequence of grease buildup is the increased likelihood of SSOs—events where untreated sewage spills into streets, properties, or waterways. Blockages from FOG reduce flow capacity, and in lift stations, encrusted sensors can allow levels to rise unchecked until overflow occurs. These spills not only carry pathogens like typhoid but also violate environmental regulations, potentially incurring fines and public backlash. Globally, FOG is linked to a significant portion of SSOs; in the U.S., it's a factor in 40-50% of annual blockage-related incidents. For engineers, modeling tools like CMOM (Capacity, Management, Operations, and Maintenance) plans can help predict and prevent these risks, but prevention starts upstream with grease interceptors and public education.

Strategies for Mitigation and Why It Matters
To combat these issues, consider implementing robust FOG management programs. Regular wet well cleanings, biological treatments to break down grease, and upstream controls like properly maintained grease interceptors can make a difference. Monitoring H2S levels and investing in corrosion-resistant materials for lift stations can extend infrastructure life. As city leaders, prioritizing these measures not only safeguards public health and the environment but also optimizes budgets—preventing a single SSO can save thousands in cleanup and repairs.
In summary, grease in collection systems isn't just a nuisance; it's a multifaceted threat that undermines the reliability of lift stations through anaerobic H2S production, equipment damage, and overflow risks. By staying informed and proactive, city managers and engineers can keep their systems flowing smoothly, ensuring sustainable urban wastewater management for years to come.
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Technologies for Addressing FOG in Collection Systems

• Grease trap maintenance programs
  Routine inspection and maintenance can be improved with the use of biological treatment. Microbes have proven effective in reducing FOG and BOD5 entering the municipal collection systems. Unlike solvents and surfactants treatments, the microbial treated grease does not solidify downstream.
  
  
• Mixing/aeration in lift stations
  Adding mixing/aeration in a lift station wet well reduced anaerobic conditions preventing H2S formation and keeps grease from forming a solids cap.
  
  
• Biological treatment in gravity lines and lift stations
  The same biological treatment process used in grease traps can also be adapted for collection system maintenance. Benefits include reduced SSO, H2S, and lower BOD5/FOG entering the treatment plant.

Wastewater treatment might not be the most glamorous topic, but it's essential for protecting our environment and public health. Behind the scenes of these complex systems, tiny microorganisms work tirelessly to break down pollutants and purify water. While bacteria often steal the spotlight, protozoa—single-celled eukaryotic organisms—play a crucial supporting role that's often overlooked. In this blog post, we'll dive into what protozoa do in wastewater treatment systems, why they're important, and how they help keep things running smoothly.

What Are Protozoa?

Protozoa are microscopic, animal-like organisms that thrive in aquatic environments, including the activated sludge processes commonly used in wastewater treatment plants. Unlike bacteria, which are prokaryotes, protozoa are more complex and can move around using structures like cilia, flagella, or pseudopods. In wastewater systems, they act as predators in the microbial food chain, feeding primarily on bacteria and organic matter.

Common types found in these systems include:

• Amoebae
  These shape-shifting protozoa dominate in young sludge or under stressful conditions like low dissolved oxygen (DO) or high nutrient loads.
• Flagellates
  They appear when there's a high food-to-microorganism (F:M) ratio, competing with bacteria for soluble nutrients.
• Ciliates
  The most diverse group, divided into free-swimming, crawling, and stalked varieties. Free-swimmers handle dispersed bacteria, crawlers improve floc structure, and stalked ciliates indicate mature, stable sludge.

These eukaryotic organisms are visible under a light microscope, making them easier to monitor than smaller bacteria.

How Protozoa Contribute to Wastewater Treatment

Protozoa's main function is predation—they consume free-floating bacteria and particles that bacteria alone can't handle efficiently. This grazing helps control bacterial populations, preventing overgrowth that could lead to cloudy effluent or poor settling. By feeding on dispersed bacteria, protozoa promote the formation of flocs (clumps of bacteria and organic matter), which settle out more easily during clarification stages.

In biological treatment processes like activated sludge or aerobic granular sludge (AGS), protozoa enhance nutrient removal. While bacteria do the heavy lifting in breaking down organic compounds and removing nitrogen and phosphorus, protozoa support this by maintaining a balanced ecosystem. For instance, in AGS systems, sessile ciliates like Epistylis and Vorticella dominate and boost pathogen removal, such as E. coli, through filter-feeding. Predation can account for an extra 0.5 to 2.5 log reduction in pathogens, making the water safer for discharge or reuse.

Protozoa also act as "filter feeders" and "grazers." Stalked ciliates anchor to flocs and remove free bacteria from the water column, while crawling types nibble at the outer layers of flocs, reducing excess extracellular polymers (EPS) and keeping the biomass active and aerobic. This not only improves floc density and settling but also prevents issues like bulking sludge, where poor settling leads to operational headaches.

Why Protozoa Are Key Indicators of System Health

Beyond their direct contributions, protozoa serve as bioindicators, revealing the overall health of the treatment system. Their populations shift based on factors like food availability, DO levels, sludge age, and toxicity. For example:

• A dominance of amoebae or flagellates signals young sludge, high bacterial loads, or low oxygen—indicating potential instability.
• An abundance of free-swimming ciliates suggests improving conditions but still high food loads.
• Crawling and stalked ciliates point to a mature, healthy system with good clarification.

Operators can use microscopic analysis to monitor these shifts and make proactive adjustments, such as tweaking aeration or nutrient inputs, before problems escalate. This biological monitoring complements chemical tests, providing a more holistic view of the process.

In high-rate systems or advanced setups like AGS, protozoan communities are especially important for efficient pathogen control, with larger granules showing higher predation rates due to species like Epistylis.

Protozoa may be tiny, but their role in wastewater treatment is massive. From predation and floc improvement to serving as system sentinels, they ensure cleaner water and more efficient operations. Next time you think about wastewater, remember these unsung heroes working behind the microscope. If you're in the industry, consider incorporating routine protozoan monitoring— it could be the key to optimizing your plant.

In wastewater treatment systems, bacteria produce extracellular polymeric substances (EPS)—complex mixtures of proteins, polysaccharides, lipids, and other compounds—as a protective mechanism to form biofilms, flocs, or granules that aid in pollutant removal and sludge settling. However, excess EPS can lead to issues like non-filamentous bulking, poor settleability, and operational inefficiencies. While nutrient deficiencies or imbalances (such as high carbon-to-nitrogen ratios) are often cited as triggers, they are far from the only factors; numerous environmental stresses prompt bacteria to overproduce EPS as a survival strategy.
 
Common environmental triggers include:
 

• Nutrient Imbalances - lack of vital macro and micronutrients slows microbial metabolism and excess organics are stored in EPS. This is most commonly observed in industrial wastewaters.
  
  
• Temperature fluctuations - rapid changes in temperature destabilize microbial communities, prompting bacteria to secrete more EPS for protection and adaptation. For instance, sudden swings can increase EPS output in activated sludge systems.

 

• pH variations - shifts in pH create unfavorable conditions, leading bacteria to produce excess EPS to maintain cellular stability and survive stress.

 

• Low dissolved oxygen (DO) or hypoxic conditions - oxygen starvation hinders complete organic metabolism, causing bacteria to store excess organics in EPS layers. Low DO in aerobic-facultative bacteria or hypoxic setups promotes higher EPS production, while higher DO can also boost bound EPS (especially carbohydrates) in oxic zones.

 

• High salinity - elevated salt levels induce osmotic stress on bacterial cells, triggering increased EPS secretion as a protective barrier. This is particularly evident in hyperhaline wastewater, where total EPS content rises with sodium concentrations, altering composition toward more proteins and polysaccharides.

 

• Presence of toxic substances or shocks - influxes of heavy metals, industrial wastes, pharmaceuticals (e.g., ciprofloxacin), dyes, phenols, surfactants, microplastics, or persistent organic pollutants stress bacteria, leading to defensive EPS overproduction for adsorption, exclusion, or degradation of toxins.

 

• Substrate type and operational conditions - easily biodegradable carbon sources (e.g., glucose) under aerobic or anaerobic conditions stimulate excess protein-rich EPS for microbial aggregation. Factors like shear forces, fluid flow, sludge retention time, or high food-to-microorganism ratios also encourage EPS secretion by altering metabolism and promoting attachment.

 

• Other stressors - Mechanical forces (e.g., surface roughness), high ammonia nitrogen, or signaling molecules can further enhance EPS as bacteria adapt to harsh environments.

 
These triggers often interact; for example, anaerobic conditions combined with saline wastewater can amplify EPS in granular sludge. Managing them involves monitoring influent quality, optimizing aeration, and using bioaugmentation to mitigate excess EPS without relying solely on nutrient adjustments.