What is Mixed Liquor (MLSS) in an Sewage Treatment Plant?

In an Sewage Treatment Plant, Mixed Liquor is a combination of sewage and activated sludge in the aeration tank during the biological treatment of wastewater. The role of the mixed liquor is to degrade organic matter of wastewater through microorganisms. Biological operation is of utmost importance while treating wastewater before its release into the environment or for reuse. Here in this blog, we will use the term importance of mixed liquor in an STP, its composition, and its role in the sewage treatment process.

Composition of Mixed Liquor in STP

Mixed liquor is a suspension of suspended microorganisms (protozoa, bacteria, and other microorganisms) in wastewater. It consists of two large components:

· Activated Sludge: This is the biomass of the microorganisms, e.g., bacteria, used for the breakdown of organic matter in the wastewater. The activated sludge is recycled constantly through the aeration tank to facilitate aeration and the biological treatment process.

· Wastewater: Wastewater or treatment effluent to be treated is mixed with the activated sludge in the aeration tank. Both are mixed and referred to as mixed liquor. Activated sludge microorganisms decompose organic pollutants in the wastewater into simple substances easily removable.

The Function of Mixed Liquor in STP

Mixed liquor is also vital in proper sewage treatment. Here's how it functions within the biological treatment process:

· Biological Degradation of Organic Matter: Microorganisms within the activated sludge break down the organic components of wastewater. They metabolize the contaminants such as oils, fats, proteins, and carbohydrates by breaking them down into lower toxic compounds such as carbon dioxide, water, and methane. The biological breakdown is the major function of mixed liquor in STPs.

· Supply of Oxygen to Microorganisms: Aeration tank is continuously aerated (either mechanically or through diffusers) to provide the necessary oxygen to microorganisms. Such oxygen serves their metabolic requirements and makes them live comfortably, thereby increasing the rate of decomposition of organic pollutants in wastewater. Mixed liquor that is continuously circulated is caused to maintain oxygen uniformly distributed throughout so that nutrients present in the wastewater are supplied to all the microorganisms.

· Floc Formation: Over time, the microorganisms in the activated sludge grow into flocs, which are collections of bacteria and organic material bound together. The flocs settle on the bottom of the aeration tank in the settling or clarifier zone, where the treated water is discharged. Floc formation is an important part of the biological treatment process, and mixed liquor is tasked with forming and maintaining flocs.

Importance of Monitoring Mixed Liquor Parameters

Control of mixed liquor parameters in an STP is essential to the attainment of maximum treatment performance. Some of these parameters are:

Mixed Liquor Suspended Solids (MLSS):

MLSS is solids density (microbes) in suspended liquor mixture. Concentration of high MLSS is increased density of microbes, resulting in more effective treatment. Increased MLSS levels, on the other hand, can cause inadequate aeration or solid settling in the clarifier. Maintaining MLSS levels through periodic monitoring enables control of the flow rate and aeration to achieve optimum treatment conditions.

Mixed Liquor Volatile Suspended Solids (MLVSS):

MLVSS is suspended solids of the mixed liquor and is composed mainly of live microorganisms. The ratio of MLVSS to MLSS is a critical parameter to determine the efficiency and health of the biological treatment process. As the concentration of MLVSS increases, the concentration of active microorganisms also increases, which is required to break down the pollutants.

Dissolved Oxygen (DO):

Yet another important parameter to be observed is the mixed liquor dissolved oxygen concentration. The activated sludge contains microorganisms which require oxygen in order to destroy organic contaminants. Below some value, treatment efficiency will not be good and if DO values are low because at low oxygen level microorganisms will not function in a well-suited way.

How to Maintain and Manage Mixed Liquor in STP?

Regular maintenance and care of the mixed liquor are important in sustaining the efficiency of the biological treatment process.

· Adjusting Aeration Levels: Adjusting Aeration Levels is some of the management and maintenance measures that can be employed to take care of mixed liquor in STPs.Good aeration is critical in the supply of the volume of oxygen in the mixed liquor. By controlling the aerators or diffusers to control the amount of oxygen supplied, the operators can be able to provide active microorganisms and treat wastewater efficiently.

· Controlling MLSS and MLVSS Levels: Constant monitoring of MLSS and MLVSS levels ensures microorganism levels are kept at an optimum level to ensure effective treatment. Sludge in excess has to be wasted out of the system during high MLSS levels. Excess sludge needs to be added in the form of activated sludge when MLSS levels are low to provide effective treatment efficiency.

· Sludge Wasting: Sludge wasting refers to the release of a portion of the mixed liquor to avoid solid accumulation. It assists in maintaining the MLSS and the correct microbial balance. Sludge wasting on a regular basis allows only the healthy, active microbes to stay in the system for effective treatment.

· Control of the Hydraulic Retention Time (HRT): Hydraulic retention time is the duration of wastewater residence in the aeration tank. HRT is controlled to balance the period that microorganisms have to degrade organic pollutants. A longer HRT will improve the treatment performance at the cost of tanks and energy. Appropriate control of the HRT is required in achieving a balance between efficiency in operation and performance.

Conclusion

Mixed liquor is one of the natural components of biological treatment process of a Sewage Treatment Plant (STP). It is a mixture of wastewater and activated sludge wherein the latter gives protection to microorganisms for breaking down organic pollutants. Proper management of mixed liquor through control of parameters such as MLSS, MLVSS, and dissolved oxygen level is of prime importance to maintain STP efficiency. By operating STPs properly in mixed liquor, operators are able to guarantee efficient sewage treatment along with low operational costs and environmental regulations compliance.

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