Foam on the Aeration Basin: Why Nocardia Takes Over — and How Operators Can Take Back Control

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.