August 10, 2025

Wastewater

MLSS Bacteria and Ways to Evaluate It

Blogs / MLSS Bacteria and Ways to Evaluate It

Wastewater treatment plants are used worldwide to treat the vast amount of wastewater generated daily.

Table of Contents

The primary objective is to safeguard the public from health hazards resulting from untreated wastewater.

Moreover, wastewater treatment is crucial in preventing environmental pollution.

According to a 2025 report by the UN, 80% of the world’s wastewater is discharged untreated, contributing to severe water pollution and scarcity.

Biological wastewater treatment is the most important process at any treatment facility, where living microorganisms play a vital role in degrading organic waste.

Among the various components of the system, MLSS is an operational control parameter that must be optimized for optimal treatment results.

What is MLSS in Wastewater Treatment?

Mixed Liquor Suspended Solids (MLSS) in wastewater treatment refers to the total concentration of solids in an aeration tank.

It includes non-biodegradable suspended matter, organic solids, and microorganisms.

Additionally, MLSS is considered a combination of suspended solids and influent wastewater, serving as an indicator of the system’s biomass or biological activity.

A study by the Water Environment Federation (2024) highlights that optimizing MLSS levels can improve effluent quality by 25%, reducing overall operational costs.

Optimizing MLSS in Wastewater Treatment

The traditional activated sludge wastewater treatment plants operate with MLSS concentrations ranging approximately from 1,500 mg/L to 5,000 mg/L.

According to studies, the MLSS concentration is even higher in Membrane Bioreactors (MBRs), ranging from 10,000 to 12,000 mg/L.

However, it has been observed that a higher concentration of MLSS has both advantages and disadvantages.

It is beneficial for smaller aeration tanks where the treatment level remains constant.

However, with an increase in MLSS concentration, aeration efficiency can sometimes decrease.

Higher MLSS also indicates a biomass nutrient deficiency, bulking sludge, and an increase in BOD loading, resulting in excessive solid generation, high flow rates, and insufficient settling times.

Whereas low MLSS indicates non-maintenance of DO concentration, fewer organisms survive, a decrease in flow rate occurs due to a reduction in microorganisms, and a decline in stalked and free-swimming ciliates is observed.

Therefore, maintaining an optimal level of MLSS concentration is crucial for the optimal performance of wastewater treatment plants.

It can be achieved by understanding the bacterial activity in the system.

Testing and evaluation of MLSS in wastewater treatment processes can be done using different methods.

In large plants, a daily check is preferable during peak flow.

Evaluation Techniques of MLSS in Wastewater Treatment

The standard procedure for evaluation of MLSS in wastewater treatment was introduced in the late 1800s and continues to be followed today.

The first step involves collecting a grab sample of MLSS and running it through the filter.
Secondly, the ignition of the filter should be carried out in a muffle furnace (550 degrees ) for about 15-20 minutes. It is done to dry the residue.
In the third step, the residue should be transferred to a desiccator for cooling purposes once the filter is cooled.
Finally, weigh the residue left on the filter. Thereafter, you can calculate the volatile solids in the sample using the following formula: VSS, mg/l=Weight of Material Lost by burning(g)sample volume (ml)

In the 1970s, with the advancement of technology, online instrumentation was developed to monitor MLSS continuously.

It is helpful in optimizing the Return-activated sludge (RAS) flow for a constant load during influent flow and periods of load variation.

However, manual assessment using a sample is still considered a better option, as online instrumentation is costly and requires constant calibration.

In the past decades, several methods have been developed to analyze and assess MLSS in wastewater treatment.

It includes manual collection and laboratory testing aimed at enhancing the efficiency of biological wastewater treatment.

Some of them are discussed.

Oxygen Uptake Rate

Oxygen Uptake Rate or OUR is the rate at which the biomass consumes oxygen. It also helps in measuring microbial activity.

When OUR is lower than normal, it indicates that MLSS is affected by toxicity. Higher levels show organic overloads.

Another scenario that can occur is a return to normal levels of high OUR within a short period.

It indicates that the MLSS concentration is low due to an inadequate food-to-microorganism ratio.

The Oxygen Uptake Rate test is conducted in three levels:

Level 1

These are 15-minute tests designed to assess bacterial health and the organic load entering the biological system.

Level 2

This level involves multiple tests over several hours using an aerated MLSS sample.

It is helpful to understand whether the biological system is properly sized to handle the incoming organic load.

Extended OUR tests help in assessing how long it will take to complete biological treatment based on the endogenous respiration of MLSS bacteria.

Here, the endogenous respiration of the bacteria occurs when these microorganisms start to oxidize their cellular mass, rather than the organic matter in the effluent.

Therefore, ideally, when the endogenous respiration of the MLSS begins, treatment is considered complete.

Level 3

It includes a treatability study, which is generally conducted to determine if an incoming wastewater stream will be helpful or highly toxic to the microorganisms inside a bioreactor.

One of the main components involves MLSS bacteria attaining endogenous respiration.

Settleability Test

Biomass efficiency depends on the density and how it settles in a clarifier. Thus, a Settleability test is done using a Settleometer.

Recording the Settled Sludge Volume (SVV) of the biomass allows operators to assess the age of the sludge.

Solids settling at a faster rate may indicate older sludge, while solids settling too slowly may get washed out during high hydraulic loads.

Plate Counts and ATP analysis

In the Plate Counts method, tests are conducted to analyze small samples of MLSS using plate count media in the laboratory.

It helps in determining the microbial population in wastewater.

In ATP analysis, a luminometer that produces light during ATP reactions is used to analyse ATP, which is stored energy in microbes.

Flow Cytometry

It involves the use of fluorescent dyes to evaluate the viability of individual bacterial cells in an effluent sample.

Molecular Testing

It allows quantification of the DNA of the MLSS bacteria and specific higher life forms of bacteria, such as nitrifiers and Nocardia.

This testing has the advantage of being highly accurate and rapid.

Organica Biotech is one of the leading companies with advanced solutions for wastewater treatment.

It also helps optimize MLSS in wastewater treatment and enhances the efficiency of the biological treatment process in wastewater treatment plants through its biological solutions.

Organica Biotech also offers two Wastewater treatment-related studies:

BioCheck- Bacteriological Wastewater Analysis

This study can help you understand the current state of the biological system in a wastewater treatment plant by analyzing the microbial population and identifying existing problems.

This will further help you take effective measures to boost the system’s efficiency.

BioSure- Wastewater Treatability Study

BioSure will help you identify the right microbial population needed for your biological system by confirming its suitability and sustainability, depending on the type of industrial wastewater treatment plant.

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