View More

High MLSS: Not Always Good in Sewage Treatment Systems

High MLSS (Mixed Liquor Suspended Solids) in the reactor, when coupled with a low organic load, creates conditions that strongly favor the overgrowth of certain filamentous bacteria, leading to bulking.

Here's a breakdown of why this happens:

1. The Concept of Food-to-Microorganism (F/M) Ratio:

The F/M ratio is a crucial operational parameter in activated sludge. It represents the amount of organic food (BOD or COD) available per unit of biomass (MLSS) per day.

  • F/M = (Influent BOD or COD, kg/day) / (MLSS, kg * Reactor Volume, m³)

When you have:

  • High MLSS: More microorganisms in the system.

  • Low Organic Load (low influent BOD/COD): Less food coming into the system.

...the result is a very low F/M ratio.

2. The Role of Sludge Age (MCRT):

A low F/M ratio is directly correlated with a high Sludge Age (Mean Cell Residence Time or MCRT). This means the sludge is kept in the system for a longer period of time before being wasted.

  • High MLSS with low organic load means the system needs to retain biomass for a longer time to ensure adequate treatment of the limited food.

3. How Low F/M (High Sludge Age) Favors Filamentous Bacteria:

Certain types of filamentous bacteria are more competitive than floc-forming bacteria under conditions of nutrient limitation and low food availability.

Here's why they thrive in a low F/M/high sludge age environment:

  • Lower Maintenance Energy Requirements: Filamentous bacteria often have lower "maintenance energy" requirements. This means they can survive and even slowly grow when food is scarce, whereas floc-formers, which grow rapidly when food is abundant, struggle to maintain their numbers.

  • Better Nutrient Scavengers: Some filamentous bacteria are more efficient at scavenging and storing limited nutrients from the bulk liquid, giving them an advantage when food is low.

  • Surface Area to Volume Ratio: Their filamentous structure provides a high surface area to volume ratio, which can be advantageous for nutrient uptake in dilute environments. They can spread out to find sparse food more effectively.

4. The Mechanism of Bulking:

As these filamentous bacteria proliferate due to the low F/M conditions:

  • They grow outward from the floc structure, forming long chains or filaments that extend into the bulk liquid.

  • These long filaments interfere with the natural aggregation and compaction of the activated sludge flocs in the secondary clarifier. Instead of the flocs settling tightly, the entangled filaments create an open, voluminous structure that traps water.

  • This leads to a high Sludge Volume Index (SVI), where the sludge occupies a much larger volume after settling, and eventually overflows the clarifier weirs, leading to high effluent TSS.

Scenario in Operation:

This situation often arises when:

  • A plant is designed for a much higher load than it is currently receiving (e.g., new development with few residents initially, or an industrial plant with reduced production).

  • Operators try to maintain a very high MLSS concentration (perhaps thinking "more bugs = better treatment"), but the actual organic load is low.

  • Over-aeration can also contribute by promoting a very long sludge age.

In summary, high MLSS coupled with low organic load creates a "starvation" environment (low F/M, high sludge age) that selectively favour the growth of filamentous bacteria that outcompete the desired floc-forming bacteria, ultimately leading to poor sludge settling and bulking.