Natalie Lamb and the mixed liquor suspended solids

I came across a new term recently, MLSS (mixed liquor suspended solids). Here are some of the things that I have learned about this measurement in sewage, what it actually means, how operators use it and where the Sentry sensor, a way for operators to measure microbial metabolic activity in real-time, can add value.

Introduction to MLSS

Wastewater is composed of different inorganic and organic substances, including (but not limited to) excrement, detergents, soaps, fats, greases and food particles. These large organic molecules are easily decomposed by bacteria into smaller molecules and eventually into carbon dioxide and water, but oxygen is required. The amount of oxygen required is known as the biochemical oxygen demand or BOD. BOD is often used as a measure of the “strength” of sewage (the amount of biodegradable organic material in sewage).

 For the activated sludge process to operate properly there must be a balance between the food entering the bioreactor (measured as BOD, COD, or TOC) and the microorganisms in the bioreactor (estimated using MLSS). MLSS is actually the concentration of suspended solids in the mixed liquor (mixed liquor= sewage + microbial mass) i.e. the amount of biomass in the system. In the below image, the F:M ratio (food-to-mass or food-to-microorganism) is used to express this relationship.

• F:M ratio
• Food = influent flow x influent BOD or COD concentration
• Microorganisms = aeration system volume x MLVSS or MLSS

 

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MLSS Adjustment 

Ideally operators would want to keep MLSS at a constant level in the reactor (for ASP that is generally between 2000-4000 mg/l). Operators can do this by returning part of the activated sludge to the start of the treatment process, called RAS, which would increase the “food” or MLSS, or by not returning leftover sludge, called SAS, to decrease the available “food” or MLSS. It is noted, however, that it takes a while for the new MLSS concentration to establish.

 

There is motivation to maintain MLSS levels because when MLSS is too low, the plant is not operating in an energy efficient way. Equally, when MLSS is too high, the works gets overloaded, DO decreases, organic matter is not fully degraded and the biology dies. In this case, operators would get a bulky sludge that is more difficult to settle in the final settlement tank.

 A further motivator for adjustment is that when the MLSS is increased, technically the BOD is decreased. There is motivation to do this because BOD is a parameter included in Environment Agency monitoring (as well as COD and sometimes total phosphorus and total nitrogen).

The Value of Sentry in MLSS

The value of using a Sentry probe over using MLSS is that while MLSS is the amount of biomass in the system, a normal probe would quantify everything suspended in the liquids, including mass that is not respiring, such as dead biomass. Even MLVSS, which is considered to be a more accurate estimate of the mass of microorganisms than MLSS, includes both microorganisms and organic matter.

Since Sentry quantifies the electrons emitted when microorganisms respire, only the “live” biomass is quantified. In this way, when operators are deciding whether to return part of the activated sludge to the start of the process or whether to discard it, they can get a more accurate picture of the microorganisms present within that sludge to make a more informed choice.

Summary

Thank you very much for taking the time to read this blog post today, to learn more about MLSS, ASP and the value of Sentry, brought to the UK by QCL! Please feel free to get in touch for more information.

Definitions

• MLSS- mixed liquor suspended solids
• Mixed liquor- sewage + microbial mass
• ASP- activated sludge process
• Reactor- a sewage reactor is any tank where biological reactions take place (e.g. ASP) and is not a specific process
• BOD- biochemical oxygen demand i.e. the amount of DO needed by aerobic biological organisms to break down organic material
• RAS- return activated sludge or sludge return flow rate
• SAS- surplus activated sludge
• DO- dissolved oxygen
• F:M ratio- food-to-mass or food-to-microorganism ratio
• MLVSS- mixed liquor volatile suspended solids