MLSS vs. MLVSS

Attribute	MLSS	MLVSS
Definition	MLSS stands for Mixed Liquor Suspended Solids. It represents the total concentration of suspended solids in a mixed liquor sample.	MLVSS stands for Mixed Liquor Volatile Suspended Solids. It represents the concentration of volatile suspended solids in a mixed liquor sample.
Measurement	Measured by filtering a mixed liquor sample through a filter paper and drying the retained solids at a specific temperature.	Measured by filtering a mixed liquor sample through a filter paper and burning off the volatile solids at a specific temperature.
Composition	Includes both organic and inorganic suspended solids present in the mixed liquor.	Includes only the organic fraction of the suspended solids present in the mixed liquor.
Application	Used to assess the overall solids concentration in a wastewater treatment process.	Used to assess the concentration of biologically active organic solids in a wastewater treatment process.
Importance	Indicates the total solids content and settling characteristics of the mixed liquor.	Indicates the concentration of microorganisms responsible for biodegradation in the wastewater treatment process.

Introduction

MLSS (Mixed Liquor Suspended Solids) and MLVSS (Mixed Liquor Volatile Suspended Solids) are two important parameters used in wastewater treatment processes. They provide valuable insights into the concentration and composition of solids present in the mixed liquor. Understanding the differences and similarities between MLSS and MLVSS is crucial for optimizing treatment processes and ensuring efficient operation of wastewater treatment plants. In this article, we will explore the attributes of MLSS and MLVSS, their significance, and how they can be used to monitor and control wastewater treatment processes.

Definition and Calculation

MLSS refers to the total concentration of suspended solids, both organic and inorganic, in the mixed liquor. It includes both volatile and non-volatile solids. MLVSS, on the other hand, represents the concentration of only the volatile suspended solids in the mixed liquor. It provides an estimate of the organic content of the solids present.

The MLSS and MLVSS concentrations are typically measured in milligrams per liter (mg/L) or parts per million (ppm). These values are determined through laboratory analysis of samples collected from the treatment process. The samples are usually taken from the aeration tank or the final clarifier.

To calculate MLSS, a known volume of the mixed liquor sample is filtered through a pre-weighed filter paper. The filter paper with the retained solids is then dried and re-weighed to determine the mass of the suspended solids. The mass is divided by the volume of the sample to obtain the MLSS concentration.

For MLVSS determination, the sample is first heated to a high temperature to volatilize the organic solids. The remaining non-volatile solids are then filtered and weighed to calculate the MLVSS concentration.

Significance in Wastewater Treatment

Both MLSS and MLVSS play crucial roles in wastewater treatment processes. They provide valuable information about the biological activity, solids retention, and treatment efficiency of the system.

MLSS is an indicator of the total solids concentration in the mixed liquor. It helps in assessing the overall biomass concentration and the potential for solids settling in the clarifier. High MLSS values may indicate a high biomass concentration, which can lead to poor settling and carryover of solids in the effluent. On the other hand, low MLSS values may suggest insufficient biomass for effective treatment.

MLVSS, being a measure of the volatile organic solids, is particularly important in assessing the biological activity and treatment efficiency. It represents the portion of the solids that can be readily consumed by microorganisms during the treatment process. Higher MLVSS concentrations indicate a higher concentration of readily biodegradable organic matter, which can enhance treatment performance. Monitoring MLVSS levels can help operators optimize aeration and nutrient dosing to achieve desired treatment goals.

Factors Affecting MLSS and MLVSS

Several factors can influence the MLSS and MLVSS concentrations in a wastewater treatment plant. These factors include the influent characteristics, treatment process design, operational parameters, and solids retention time.

The influent characteristics, such as the organic loading rate and the presence of toxic substances, can significantly impact the MLSS and MLVSS concentrations. High organic loading rates may result in higher MLSS and MLVSS concentrations due to increased biomass growth. Toxic substances, on the other hand, can inhibit microbial activity and reduce the MLVSS concentration.

The treatment process design, including the type of activated sludge system and the configuration of the aeration tanks and clarifiers, can also affect the MLSS and MLVSS concentrations. Proper design considerations are essential to ensure optimal solids retention and minimize solids loss in the effluent.

Operational parameters, such as the sludge wasting rate, aeration intensity, and solids retention time, play a crucial role in maintaining desired MLSS and MLVSS levels. Proper control of these parameters is necessary to prevent excessive biomass growth or washout of valuable microorganisms.

Lastly, the solids retention time, which represents the average time solids spend in the treatment system, can impact the MLSS and MLVSS concentrations. Longer solids retention times generally result in higher MLSS and MLVSS concentrations, as more biomass accumulates in the system.

Monitoring and Control

Regular monitoring of MLSS and MLVSS concentrations is essential for effective control and optimization of wastewater treatment processes. By tracking these parameters, operators can identify deviations from desired levels and take appropriate corrective actions.

Monitoring MLSS and MLVSS can be done through grab sampling or continuous online measurement systems. Grab sampling involves collecting periodic samples from the treatment process and analyzing them in the laboratory. Online measurement systems, on the other hand, provide real-time data by continuously monitoring the suspended solids concentration in the mixed liquor.

Based on the MLSS and MLVSS measurements, operators can adjust operational parameters, such as the sludge wasting rate, aeration intensity, and nutrient dosing, to maintain optimal biomass concentrations and treatment performance. Regular monitoring also helps in detecting process upsets, such as excessive biomass growth or washout, and allows for timely corrective actions to be taken.

Conclusion

MLSS and MLVSS are important parameters in wastewater treatment processes that provide insights into the concentration and composition of solids in the mixed liquor. While MLSS represents the total suspended solids concentration, MLVSS specifically indicates the concentration of volatile organic solids. Both parameters play crucial roles in assessing treatment efficiency, biomass concentration, and solids settling characteristics.

Understanding the factors influencing MLSS and MLVSS concentrations, as well as monitoring and controlling these parameters, is essential for optimizing wastewater treatment processes. By maintaining appropriate MLSS and MLVSS levels, operators can ensure efficient operation, minimize solids loss in the effluent, and achieve desired treatment goals.