Microthrix parvicella, filamentous foaming: Causes and Control Strategies

Filamentous foam on wastewater treatment plant with high FOG influent.

Microthrix parvicella is a filamentous bacterium frequently encountered in wastewater treatment processes, especially in activated sludge systems. It plays a significant role in causing operational issues such as bulking and foaming. These problems arise because Microthrix parvicella's filamentous structure interferes with the settling ability of activated sludge, leading to poor performance in secondary clarifiers and reduced effluent quality.

Key Characteristics of Microthrix parvicella

• Lipophilic Nature - thrives on lipid-rich substrates, making environments with high fats, oils, and grease (FOG) ideal for its growth
  
• Slow Growth Rate - has a slower growth rate compared to other bacteria, but it can dominate in certain conditions
  
• Temperature Tolerance - flourishes at lower temperatures and is often more prevalent in temperate climates.
  
  

Control Strategies
Managing Microthrix parvicella requires a multifaceted approach aimed at reducing its dominance while maintaining overall process efficiency:

• Improve Aeration - enhancing aeration levels can help control its growth as it thrives in low dissolved oxygen conditions.
• Reduce FOG - implementing pre-treatment processes to minimize fats, oils, and grease in the influent can limit the substrates that promote its proliferation.
  
• Disinfection of filaments - chlorine or other oxidizing agents can be used to selectively target filamentous bacteria, though care must be taken to avoid harm to beneficial microbial populations.
  
• Use antifoam/defoamer chemistry - do not use fatty acid or oil based defoamers as these offer a substrate (food) source for M. parvicella and other Nocardia type bacteria. The preferred chemistry is based on silicone formulations.
  
• Reducing sludge retention time or optimizing process parameters can create conditions unfavorable to its proliferation.
  
• Some facilities have reported a reduction in Microthrix bulking/foaming by adding Polyaluminum Chloride (PAX 14). https://onlinelibrary.wiley.com/doi/abs/10.1002/aheh.200400574