Chlorinated and fluorinated hydrocarbons are among the most persistent contaminants in soil and groundwater. Their resistance to microbial attack is not accidental—it's rooted in chemistry, evolutionary biology, and microbial physiology. Yet, despite these barriers, cometabolic bioremediation offers a promising path forward.

Why These Compounds Resist Biodegradation

1. Extremely Strong Carbon–Halogen Bonds
The carbon–fluorine bond is one of the strongest in organic chemistry, making fluorinated compounds—especially PFAS—highly resistant to enzymatic cleavage. Microbes rarely evolve enzymes capable of breaking such stable bonds. ASM Journals 

Chlorinated hydrocarbons are somewhat more reactive, but still significantly more stable than typical natural substrates.

2. Lack of Microbial Evolutionary Pressure
Fluorinated organics are almost entirely synthetic; microbes have had little evolutionary time to develop metabolic pathways to use them as carbon or energy sources. This evolutionary gap is a major reason PFAS biodegradation is rare and slow. ASM Journals

3. Poor Chemical Reactivity and Limited Enzyme Binding
PFAS and many chlorinated solvents lack functional groups that enzymes can easily bind or oxidize. Their hydrophobicity and steric shielding further reduce microbial uptake and activation. ASM Journals

4. Missing “Weak Points” for Initial Activation
Microbial degradation typically begins with an “activation step”—oxidation, reduction, or hydrolysis at a reactive site. Many fluorinated compounds lack such sites, preventing the first metabolic step from occurring. MDPI

How Cometabolism Enables Bioremediation

Cometabolism is a process where microbes transform a contaminant unintentionally while metabolizing a different “primary” substrate (e.g., methane, toluene, propane).

They gain no energy from degrading the pollutant—but their enzymes can still modify it.

Why Cometabolism Works

• Many oxygenases and reductases are nonspecific, meaning they can attack contaminants structurally similar to their natural substrates.
• Even if the contaminant cannot support growth, these enzymes can partially oxidize or reduce it.
• This partial transformation can create new functional groups that make the molecule more biodegradable.
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Cometabolism has been shown to stimulate indigenous microbes capable of degrading both the cosubstrate and the contaminant. Springer

Cometabolic Strategies for Chlorinated Hydrocarbons

1. Methanotroph‑Driven Cometabolism
Methane monooxygenase (MMO) can oxidize:

• TCE
• TCA
• DCE
• Vinyl chloride

These transformations often produce epoxides or alcohols that downstream microbes can mineralize.

2. Toluene and Propane Oxidizers
Toluene dioxygenase and propane monooxygenase can attack chlorinated aliphatics, creating hydroxylated intermediates.

3. Reductive Dechlorination Coupled with Cometabolism
For highly chlorinated compounds (e.g., PCE), reductive dechlorinators (e.g., Dehalococcoides) remove chlorines stepwise, while cometabolic oxidizers degrade intermediates.

Cometabolic Strategies for Fluorinated Hydrocarbons (PFAS and Others)
Fluorinated compounds are far more resistant, but cometabolism can still help--indirectly.

1. Attack Non‑Fluorinated Functional Groups First
Most commercial PFAS contain:

• sulfonates
• carboxylates
• phenyl rings
• phosphonates
• occasional chlorines

Microbes can metabolize these groups, creating reactive intermediates that weaken adjacent C–F bonds. MDPI
This “activation step” is essential because direct C–F cleavage is extremely difficult.

2. Use of Strong Oxidizers or Reductants Produced by Microbes
Some microbes generate reactive oxygen species or reductive equivalents that can:

• destabilize fluorinated chains
• initiate slow defluorination

Example: Acidimicrobium sp. A6 has been shown to defluorinate PFOA under specific conditions. ASM Journals
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3. Mixed Consortia with Complementary Metabolisms
No single organism can fully degrade PFAS, but consortia can:

• activate functional groups
• cleave weakened C–F bonds
• mineralize breakdown products

This mirrors how chlorinated solvent plumes are often treated using sequential reductive and oxidative steps.

A sudden crash in dissolved oxygen (DO) can turn a stable biological system into a process emergency — fast. Whether you’re running activated sludge, SBRs, or lagoons, rapid DO loss almost always points to a sharp change in loading, toxicity, or oxygen transfer. Below are the five most common and most disruptive causes operators should watch for.

1. Shock Organic Loading (High BOD, FOG, or Industrial Slugs)
Why it happens:
A sudden surge of high-strength wastewater — tank cleanouts, production dumps, FOG slugs — sends oxygen uptake rate (OUR) skyrocketing.

2. Toxic or Inhibitory Compounds Entering the System
Why it happens:
Solvents, disinfectants, surfactants, metals, or cyanide-containing waste can stun or kill key microbial groups.

3. Temperature Spikes
Why it happens:
Warm industrial discharges or seasonal heat raise basin temperature.

4. Hydraulic Surges and Short-Circuiting
Why it happens:
Stormwater inflow, pump failures, or EQ bypasses push large volumes through the system quickly.

5. Aeration System Failures (Blowers, Valves, or Diffusers)
Why it happens:
Mechanical or air-delivery issues reduce actual oxygen transfer.
 
Sudden DO loss is rarely random — it’s a warning. Whether the cause is loading, toxicity, temperature, hydraulics, or mechanical failure, fast diagnosis protects nitrification, prevents filament outbreaks, and keeps effluent quality stable.
 
 
 

Biological foaming is one of those problems that feels like it shows up overnight. One day your aeration basin looks normal; the next, it’s buried under a thick, sticky, chocolate‑milk‑colored foam that refuses to go away. For many plants, the usual suspect is Nocardia — along with its Actinomycete relatives — and understanding why it forms foam is the key to stopping it.

Why Nocardia Creates Such Persistent Foam
Nocardia species are built differently. Their cell walls contain mycolic acids, long-chain fatty acids that make the cells extremely hydrophobic. This water‑repelling surface drives two behaviors that matter in wastewater:

• They migrate toward the air–water interface.
  
• ​They cling to rising air bubbles instead of staying in the mixed liquor.

Unlike compact floc-formers, Nocardia grows as branched filaments.

• Mechanism: Hydrophobic filaments attach to aeration bubbles as they rise.
• Outcome: The filaments interlock, forming a mesh that stabilizes the bubble structure.

This mesh prevents bubbles from popping. Instead of collapsing, the foam becomes thicker, more stable, and more resistant to normal aeration turbulence.

EPS and Biosurfactant
Nocardia doesn’t stop at hydrophobicity. It also produces extracellular polymeric substances (EPS) that behave like natural surfactants.
These EPS compounds:

• Lower surface tension
• Help bubbles form more easily
• Bind foam into a cohesive, sticky layer
  
  

The combination of hydrophobic cells + filament structure + biosurfactants is why Nocardia foam is so stubborn.

What Triggers a Nocardia Bloom?

Nocardia is a specialist. It thrives when the system shifts in ways that disadvantage faster-growing floc-formers.

• Long Sludge Age (MCRT) - Slow growers like Nocardia accumulate when MCRT drifts too high. Long sludge ages give them the time they need to dominate.
  
• High FOG LoadingGrease, oils, and long-chain fatty acids are their preferred carbon source. A surge of restaurant grease or industrial oil is essentially a growth accelerator.
  
• Low F/M Ratio - When food is scarce, Nocardia outcompetes other bacteria.

 How Operators Can Get Control Back

• Reduce MCRT - Increase wasting to wash out slow-growing filaments. This is often the single most effective long-term correction.
  
• Surface Wasting - Because the biomass is floating, bottom wasting alone won’t remove it. Surface skimmers, foam collectors, or selector basins help remove the problem where it accumulates.
  
• Chlorine Sprays - A light, controlled hypochlorite mist can:
  • ​Collapse foam
  • Kill exposed filaments
  • Avoid harming the underlying mixed liquor.
    
• Manage Upstream FOG - Tighten pretreatment. Inspect grease traps. Work with industrial users. Cutting off the carbon source removes the fuel that drives Nocardia blooms.

Fatty acids enter lagoon wastewater systems from a wide range of sources — food processing, slaughterhouses, dairies, municipal FOG, industrial cleaners, and biological breakdown of lipids. Whether they are short‑chain, medium‑chain, or long‑chain, these compounds share a dangerous trait:

They form surface films that dramatically reduce oxygen transfer from the atmosphere.

Operators often focus on BOD loading, aerator performance, or seasonal temperature swings when DO drops. But in many overloaded or industrial lagoons, the real culprit is a thin, persistent fatty‑acid film that suffocates the system from the top down.

This post breaks down how fatty acids behave in lagoons, why they form films, and how those films disrupt the oxygenation that lagoon biology depends on.

Fatty Acids in Lagoons: Not Just “Grease”Fatty acids vary widely in structure:

• Short‑chain (C2–C6) – volatile, odorous, often produced during anaerobic fermentation
• Medium‑chain (C8–C12) – oily, moderately soluble
• Long‑chain (C14–C18+) – waxy, hydrophobic, prone to crystallization
  
  

Despite these differences, all fatty acids share two behaviors that matter in lagoon systems:

They migrate to the surface

Fatty acids have hydrophobic tails that orient away from water, causing them to float and spread into monolayers or multi‑layer films.

They resist natural dispersion

Even short‑chain fatty acids, which are more soluble, can accumulate at the surface when production outpaces mixing and biodegradation. The result is a surface layer that behaves like a flexible, self‑healing membrane.

How Fatty Acid Films Suppress Atmospheric Oxygen TransferAtmospheric reaeration is the backbone of lagoon oxygenation. Wind, waves, and surface turbulence constantly renew the air–water interface, allowing oxygen to dissolve into the water column.

Fatty acid films disrupt every part of that process.

• Physical Barrier to Gas Exchange
  A fatty acid film reduces the effective gas–liquid interface. Even a monolayer only a few molecules thick can cut oxygen transfer by 50% or more.
  
  
• Damping of Surface Turbulence
  Fatty acids reduce surface tension gradients, flattening waves and eliminating the micro‑turbulence that drives oxygen diffusion.
  
  
• Increased Surface Viscosity
  The film behaves like a semi‑solid sheet. Instead of breaking apart under wind or aeration, it stretches — preventing the surface renewal events needed for oxygen transfer.
  
  
• Bubble Interference in Aerated Lagoons
  If mechanical aeration is present, fatty acid films:
  • cause bubbles to coalesce
  • reduce bubble breakup
  • trap bubbles under the film
  • lower oxygen dissolution efficiency
    
    
• Creation of Anaerobic Microzones
  Under the film, oxygen demand continues but supply collapses. This leads to:
  • localized anaerobic conditions
  • sulfide and VFA production
  • odor episodes
  • filamentous growth

The lagoon becomes oxygen‑stratified from the surface downward.

System‑Level Impacts of Fatty Acid Films

• Chronic Low DO Even with Aeration Running
  Operators often misdiagnose this as aerator failure or high BOD load.
  
  
• Odor Generation
  Short‑chain fatty acids (butyric, propionic, valeric) are potent odorants.
  Anaerobic zones also produce H₂S and mercaptans.
  
  
• Reduced BOD and Ammonia Removal
  Low DO slows heterotrophic and nitrifying bacteria, causing:
  • higher effluent BOD
  • ammonia breakthrough
  • solids carryover
    
    
• Foaming and Filamentous Growth
  Hydrophobic filaments (Nocardia, Gordonia) thrive in fatty‑acid‑rich, low‑DO environments.
  
  
• Thermal and Chemical Stratification
  The film prevents natural mixing, increasing the risk of turnover events.
  
  

Why Fatty Acid Films Are So PersistentFatty acids — especially medium‑ and long‑chain — are:

• hydrophobic
• poorly soluble
• slow to biodegrade under low DO
• capable of forming crystalline or gel‑like structures

Even when broken by wind or aeration, the film rapidly reforms. Short‑chain fatty acids don’t crystallize, but they accumulate at the surface due to their amphiphilic nature and can still form monolayers that inhibit oxygen transfer.

Strategies to Reduce Fatty Acid Film Formation1. Improve Upstream FOG and Lipid Capture

• DAF optimization
• grease trap maintenance
• pretreatment enforcement
  
  

2. Enhance Biological Fatty Acid Degradation
Aerobic bacteria such as Pseudomonas, Acinetobacter, and Rhodococcus degrade fatty acids efficiently — when DO is adequate.

3. Increase Surface Disturbance

• surface aerators
• splash aeration
• wind‑enhancing baffles
  
  

4. Add High‑Oxygen Contact Zones
Selectors or inlet contact chambers oxidize fatty acids before they reach the lagoon surface.
​
5. Use Biological/Enzymatic Additives
Some formulations emulsify or accelerate fatty acid breakdown, reducing film persistence.
 
6. The Bottom Line: Fatty Acid Films Are a Hidden Oxygen Crisis
Fatty acids — not just long‑chain ones — can quietly cripple lagoon performance by blocking the system’s primary oxygen source. The film may be invisible from shore, but its impact is unmistakable:

• low DO
• odors
• poor treatment performance
• filamentous growth
• unstable lagoon biology

Recognizing fatty acid films early and addressing their root causes can restore oxygen transfer, stabilize microbial communities, and dramatically improve effluent quality.

Biological odor control focuses on managing the microbial processes that generate odorants — primarily hydrogen sulfide (H₂S), mercaptans, amines, and volatile fatty acids. These compounds form when wastewater becomes anaerobic, allowing sulfate‑reducing and fermentative bacteria to dominate.
Biological solutions aim to shift the microbial ecology toward organisms that prevent or outcompete odor‑forming pathways.

Common biological odor‑control strategies

• Bioaugmentation — adding specialized microbial cultures that oxidize sulfides, degrade VFAs, or prevent anaerobic niches.
• Biostimulation — adding nutrients, electron acceptors, or alkalinity to favor beneficial microbial pathways.
• Biofiltration — using biologically active media to oxidize H₂S and VOCs in foul air streams (high‑speed biofiltration can remove 87–99% of H₂S).
• Liquid‑phase biological dosing — preventing sulfide formation in force mains and collection systems by maintaining aerobic or nitrate‑reducing conditions.
  
  

Why Odors Form in Headworks & Collection Systems
Odors spike where wastewater becomes stagnant, anaerobic, or highly loaded with organics:

• Headworks: turbulence + high organic load + reduced sulfur compounds = intense odor release.
• Collection systems: long retention times, warm temperatures, and low DO create ideal conditions for sulfate‑reducing bacteria.
• Lift stations & force mains: intermittent pumping creates anaerobic pockets.
• Primary clarifiers & sludge handling: fermentation and protein breakdown generate amines and VFAs.
  
  

Why Tracking Biology & Environmental Conditions Is Essential
Biological odor control only works when it’s matched to the actual microbial and chemical conditions in the system. Without data, plants end up overdosing chemicals, underdosing biologicals, or treating the wrong source.

The key variables to track
1. Microbial community structure
Environmental Genomics™ tools identify:

• Sulfate‑reducing bacteria (SRB) abundance
• Fermenters producing VFAs
• Nitrifiers/denitrifiers that influence redox balance
• Biofilm vs. suspended growth dynamics
  
  

2. Environmental conditions

• Dissolved oxygen (DO) — low DO drives sulfide formation
• ORP (oxidation‑reduction potential) — predicts anaerobic zones
• pH & alkalinity — influence sulfur chemistry and microbial pathways
• Temperature — accelerates odor‑forming metabolism
• Retention time & flow patterns — determine anaerobic exposure
  
  

3. Chemical indicators

• H₂S in air and liquid
• Sulfide, sulfate, VFA levels
• Ammonia and amines
• COD/BOD loading
  
  

How Monitoring Enables Efficient Biological Solutions

1. Pinpoint the true odor source
Odors often originate upstream of where they’re detected. Tracking biology + ORP + sulfides reveals the exact point where anaerobic conditions begin.

2. Select the right biological pathway to target

• High SRB → promote nitrate‑reducing bacteria
• High VFAs → add cultures that degrade volatile fatty acids
• Low DO zones → adjust aeration or add oxygen‑releasing compounds
• Biofilm‑driven sulfide → target biofilm disruptors or surface‑active microbes
  
  

3. Optimize dosing and reduce chemical costs
Biological solutions become predictable when tied to real‑time conditions. Using ORP meters in collection systems, allows for adjustments in real-time.

4. Prevent corrosion and infrastructure damage
Controlling sulfide biologically reduces sulfuric acid formation in concrete and metal structures — a major benefit in headworks and collection systems.
​
5. Improve community relations
Consistent odor control reduces complaints and regulatory pressure.
 
Bringing It All Together
Biological odor control is most effective when it’s data‑driven. Tracking microbial populations and environmental conditions transforms odor management from reactive to proactive:

• You understand why odors form
• You intervene before they spike
• You apply the right biological tools
• You reduce chemical use and operational cost
• You protect infrastructure and community trust

In activated sludge systems, floc formation isn’t just a microbiological curiosity—it directly drives clarifier performance, sludge compaction, dewatering efficiency, and overall plant stability.
Operators often focus on familiar levers like filament control, F/M ratio, or polymer dosing. But one of the most powerful—and most overlooked—drivers of floc structure lives at the microscopic level the chemistry of extracellular polymeric substances (EPS).

At Aster Bio, we spend a lot of time looking beyond “good sludge” and asking why certain systems form dense, resilient floc while others struggle. One answer consistently rises to the surface: bacteria that produce cationic capsular EPS.
These organisms don’t just make slime—they fundamentally change how floc forms, strengthens, and settles.

EPS 101: Not All Polymers Are Created Equal
EPS in activated sludge exists in several forms:

• Loosely bound EPS (LB‑EPS) – diffuse, highly hydrated
• Tightly bound EPS (TB‑EPS) – more structured, cohesive
• Capsular EPS – a dense layer directly attached to the cell surface
  
  

Capsular EPS is the most structurally important of the three. It forms the “glue” that holds floc together under shear and determines how compact or fluffy the biomass becomes.
What makes certain bacteria truly stand out is this:

Their capsular EPS carries a net positive (cationic) charge.

That’s unusual in wastewater systems, where most EPS is neutral or negatively charged due to carboxyl and phosphate groups.

Why Positive Charge Changes Everything
Mixed liquor is full of negatively charged material, including:

• Fine clays and silt
• Organic colloids
• Humic substances
• Other microbial cells
• Dissolved organics with carboxyl groups
  
  

A cationic EPS capsule acts like a biological magnet, pulling these particles together through electrostatic attraction. The result is faster aggregation, stronger bonding, and more compact floc—without adding a pound of polymer.
In other words, the biology starts doing the job operators usually rely on chemicals to perform.

How Cationic EPS Improves Floc and Clarifier Performance

1. Faster and Stronger Floc FormationPositively charged capsules bind dispersed particles into stable microflocs that rapidly grow into mature floc. This reduces colloidal turbidity and improves solids capture.
2. Higher Shear Resistance
   Capsular EPS is tightly bound and structurally rigid. Floc formed around it resists breakage under sheer and stress.
   This translates directly into lower SVI and more stable clarifier blankets.
3. Improved Settling Velocity
   Cationic EPS increases floc density by incorporating inorganic fines and reducing bound water
   The floc behaves less like fluffy sludge and more like a compact particle—settling faster and more predictably.
4. Better Dewaterability
   Because capsular EPS is less hydrated than loosely bound EPS, it holds less bound water & polymer demand decreases. Plants frequently see higher cake solids when these organisms are well established.
   
   

The Microbes That Make It Happen
Not all EPS producers are equal. Research across wastewater, soil, and environmental systems points to several key groups known for producing amine‑rich or cationic capsular EPS.

Most Relevant to Activated Sludge Systems

• Paenibacillus – natural bioflocculant producers with strongly cationic EPS
• Rhodococcus – amine‑rich capsules with strong colloid‑binding capacity
• Acinetobacter (select strains) – compact microfloc builders under nutrient‑rich conditions
• Bacillus (select strains) – peptide‑linked, lysine‑rich EPS
• Paracoccus – strong floc formers in denitrifying environments
• Thauera (specific strains) – conditionally cationic EPS under certain operating regimes

These organisms are commonly associated with dense, well‑settling sludge and low effluent turbidity when system conditions favor their growth.

Can You Encourage Cationic EPS Producers?
Creating conditions where these beneficial organisms thrive:

• Maintain moderate F/M ratios (not chronically low)
• Avoid persistent low DO
• Provide balanced carbon and micronutrients
• Prevent toxic or hydraulic shocks
• Keep pH near neutral to slightly alkaline
• Avoid excessive wasting that strips slow‑growing EPS producers

This is where microbial insight matters. DNA‑based tools like 16S rRNA sequencing allow plants to track these populations and directly correlate biology with settling and dewatering performance.

The Takeaway
Strong floc isn’t just about controlling filaments or adding polymer—it’s about who’s building the matrix.
Bacteria that produce cationic capsular EPS create denser, stronger, more resilient floc that settles faster and dewaters better. When these organisms are present and supported, plants often see:

• Lower SVI
• Improved clarifier stability
• Reduced chemical demand
• Better solids handling performance