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Filamentous Microbes in Wastewater Treatment - Not so bad after all

It's easy to focus on the negative aspects of filamentous microbes in wastewater treatment, primarily because their overgrowth causes the notorious problems of sludge bulking and foaming. However, in controlled and moderate amounts, filamentous microbes (which are primarily filamentous bacteria, but can also include some fungi) are actually beneficial and play crucial roles in maintaining a healthy and efficient activated sludge process.

Here are the key benefits of filamentous microbes in sewage treatment:

1. Floc Formation and Structure ("Floc Backbone"):

  • Mechanism: Filamentous bacteria act as a "backbone" or "skeleton" for the activated sludge floc. Their long, thread-like strands intertwine to create a mesh-like network.

  • Benefit: This mesh provides a support structure onto which other floc-forming bacteria (the desirable, compact-growing ones) can attach and grow. Without this fibrous network, the flocs would be much smaller, weaker, and more prone to shearing apart.

  • Result: A well-structured floc is essential for efficient solids separation in the secondary clarifier, as it allows the overall mass to become denser and settle more effectively.

2. Improved Floc Strength and Resistance to Shear:

  • Mechanism: The filamentous backbone reinforces the floc.

  • Benefit: This makes the activated sludge flocs more robust and less susceptible to being broken apart by the turbulent mixing and hydraulic shear forces present in the aeration tank and pumping systems.

  • Result: Stronger flocs lead to better settling and less turbidity in the final effluent, as fewer fine, non-settleable particles are produced.

3. Enhanced Entrapment of Fine Particulates:

  • Mechanism: The mesh-like structure created by the filaments can act like a net.

  • Benefit: As the flocs settle in the clarifier, this network helps to physically entrap and sweep down very fine, non-colloidal suspended solids that might otherwise remain suspended in the treated effluent.

  • Result: This leads to a clearer effluent with lower Total Suspended Solids (TSS), improving overall treatment quality.

4. Reduced Pin Floc Formation:

  • Mechanism: "Pin floc" refers to very small, dispersed, poorly settling flocs that can lead to turbid effluent, even if overall sludge settling (SVI) is good. This often occurs when there are too few filaments.

  • Benefit: A moderate amount of filamentous growth helps to bind these smaller particles into larger, more settleable flocs, preventing the formation of pin floc.

  • Benefit: A moderate amount of filamentous growth helps to bind these smaller particles into larger, more settleable flocs, preventing the formation of pin floc.

5. Degradation of Specific/Recalcitrant Pollutants:

  • Mechanism: Some types of filamentous bacteria are specialized in degrading specific, often challenging, organic compounds, including some long-chain fatty acids, oils, and greases, or even certain complex industrial chemicals.

  • Benefit: This can contribute to a more comprehensive removal of a wider range of pollutants from the wastewater, especially in systems receiving industrial inputs.

  • Example: Some filamentous organisms, like Thiothrix spp., can oxidize sulfides (H2S), which is beneficial as sulfides are toxic to other microorganisms (including nitrifiers) and cause odour issues.

6. Nutrient Cycling and Adaptability:

  • Mechanism: Filamentous bacteria are part of the diverse microbial community that performs various functions within the activated sludge. Some species have unique metabolic capabilities or can thrive under conditions that might challenge other bacteria (e.g., low substrate concentrations, specific nutrient limitations).

  • Benefit: Their presence contributes to the overall robustness and adaptability of the activated sludge ecosystem, helping it cope with variations in influent wastewater characteristics.

The "Too Much of a Good Thing" Principle

It's crucial to reiterate that these benefits are only realized when filamentous microbes are present in moderate and controlled amounts. The problem arises from excessive or uncontrolled growth, which then leads to:

  • Sludge Bulking: The filaments extend too far from the floc, causing "interfloc bridging" where they physically intertwine and prevent the flocs from compacting. This dramatically increases the settled volume and leads to poor settling (high SVI) and solids carryover.

  • Foaming: Certain hydrophobic filamentous bacteria (e.g., Nocardia spp., Microthrix parvicella), often in combination with fats, oils, and greases (FOG) and surfactants, can stabilize air bubbles, leading to thick, persistent foam layers on the aeration tanks and clarifiers. This reduces oxygen transfer, traps solids, and creates operational and aesthetic problems.

Therefore, the goal should be not to eliminate filamentous microbes entirely (which would lead to "pin floc" and turbid effluent), but to manage their growth through process control to maintain them at their beneficial optimal levels.