Why the MLSS Concentration is Not Always Representative

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Ryan Hennessy
Fluroscent Viability 1000x

It is highly common and often successful to operate activated sludge treatment systems based on maintaining a targeted MLSS (mixed liquor suspended solids) concentration, however as with just about everything in wastewater, there are notable exceptions to the rule. 

What makes up the MLSS?

MLSS Components chart displaying how we would see the existing MLSS components.

As can be seen from the chart above there are several main components that constitute the mixed liquor suspended solids (MLSS) 

It is important to recognize that MLSS concentration values consist of many variables that may change rapidly depending upon environmental conditions. In our experience the most useful way to envision this is that we are generally attempting to correlate the MLSS concentration to the number of alive/viable bacteria present. Based upon fluorescent microscopy it is common for approximately 40-80% of the mixed liquor to illuminate green (indicating “alive/viable”). 40-80% is a very broad range and depending upon the plant and circumstances that are present these percentages may vary. 

Figure 1: Fluorescent Viability 1000x

The Picture above Demonstrates Alive/Viable (Green) and Dead/Non-Viable (Red)

If we think about the bacterial growth curve ideally in most activated sludge systems, optimal treatment conditions are not optimal growth conditions for the bacteria (specifically that they are on the verge of starvation/endogeny). In our experience it is common for extended aeration systems to have as low as 35-40% bacterial viability, while in highly loaded industrial wastewater processes viability % of 75% or more may be common. Each system is different depending upon the treatment goals, sludge age, temperatures present, and many other conditions. 

Explanation of Other Variables

An influx of inert materials may significantly change the MLVSS (mixed liquor volatile suspended solids) to MLSS ratio. Dead or non-viable bacteria also may possess a biological oxygen demand and if they are now available substrate they may weigh as volatile solids. Higher life form organism weight proportion to the overall MLSS concentration is also recognized to vary between 5-20%. Among other notable variables is temperature and its metabolic impact on the bacteria. In general, at higher temperatures, less bacteria are needed for treatment while when temperatures deplete higher amounts of viable bacteria are commonly needed to achieve treatment goals. 

The Bugs Don’t Lie 

It is always useful to have process control data as part of the “puzzle” to help guide operational changes, however despite what the numbers say on paper, the bacteria present will be representative of actual conditions within the plant should microscopy be performed properly by a trained professional. In summary, when the plant is not behaving as suggested by the MLSS concentration values, it is often due to a change of one of the variables above. Microscopic evaluation, along with in house process control testing, and the input of an operator familiar with the treatment plant and how it generally behaves are most often the best things to get together to help make decisions that are practical and optimize or troubleshoot the system. 

Source: https://rhwastewatermicrobiology.com/wastewater-microbiology-book/

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