MLSS bacteria and ways to evaluate it

Wastewater treatment plants are used across the world to treat the extensive amount of wastewater generated every day. The main aim is to protect the public from health hazards caused due to untreated wastewater. Moreover, wastewater treatment is essential to prevent environmental pollution. 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 needs to be optimized for best treatment results. 

What is MLSS in Wastewater Treatment?

Mixed Liquor Suspended Solids or MLSS in wastewater treatment is the measure of the total concentration of solids in an aeration tank. It includes non-biodegradable suspended matter, organic solids and microorganisms. Also, MLSS is considered a combination of suspended solids and influent wastewater which works as an indicator of the biomass or biology of the system. 

Optimizing MLSS in Wastewater Treatment

The traditional activated sludge wastewater treatment plants operate with MLSS concentration ranging approximately from 1500 mg/l to 5000 mg/l. According to studies, 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 is found to have advantages as well as disadvantages. It is beneficial for smaller aeration tanks where the treatment level is always the same. But sometimes with an increase in MLSS concentration, aeration efficiency can decrease. 

Higher MLSS also indicates biomass nutrient deficiency, bulking sludge, increase in BOD loading leading to excessive solid generation, high flow rates and insufficient settling times. Whereas low MLSS shows non-maintenance of DO concentration, survival of fewer organisms, decrease in flow rate due to decrease in microorganisms and decline in stalked and free-swimming ciliates. 

Thus, maintaining an optimal level of MLSS concentration is necessary for the best performance of wastewater treatment plants. It can be achieved by understanding the bacterial activity in the system. Testing and evaluation of MLSS in wastewater treatmentprocess can be done using different methods. In large plants, daily check is preferable during peak flow. 

Also Read: The Role Of Clarifiers In Wastewater Treatment Plants

Wastewater Treatment That Works Every Time

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. 

  • First step includes the collection of a grab sample of MLSS and running it through the filter.
  • Secondly, ignition of 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 purpose 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 constantly. It is helpful in the optimization of Return activated sludge (RAS) flow for a constant load during influent flow and periods of load variations. 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 analyses and assess MLSS in wastewater treatment. It includes manual collection and lab testing aimed at boosting the efficiency of the biological treatment of wastewater. Some of them are discussed. 

Oxygen Uptake Rate
Oxygen Uptake Rate or OUR is the rate at which oxygen is consumed by the biomass. It helps in measuring microbial activity as well. 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 the return of high OUR to normal in a short duration of time. It shows that MLSS concentration is low due to the lack of proper food to microorganism ratio. 

The Oxygen Uptake Rate test is conducted in three levels. 

Level 1 
These are 15-minute tests aimed at establishing bacterial health along with the organic load going into the biological system. 

Level 2 
This level involves multiple tests over a period of several hours using an aerated MLSS sample. It helps to understand if 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 is when these microorganisms start to oxidize their own cellular mass and not the organic matter in the effluent. Therefore, ideally, when the endogenous respiration of MLSS starts, treatment is said to be completed. 

Level 3
It includes a treat-ability 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 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 carried out to analyze small samples of MLSS using plate count media in labs. 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 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 in optimizing MLSS in wastewater treatment and boost 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

It is a study that can help you understand the present condition of the biological system of the wastewater treatment plant by analyzing the microbial population and the existing problems. It will further help you to take effective measures to boost the efficiency of the system.

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.

Read more: What Is Biological Wastewater Treatment And How Effective It Is?
Effective Wastewater Treatment In Speciality Chemicals Industry

To know more : 

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Post by Priyanka Khaire