In municipal wastewater treatment plants, industrial wastewater biochemical treatment stations, centralized rural wastewater treatment projects, and upgrading projects, mixed liquor suspended solids MLSS is one of the core parameters affecting the operational stability of activated sludge systems. For system integrators, environmental engineering companies, and water treatment project contractors, MLSS is not an isolated detection indicator. It is a key control variable related to nitrification efficiency, denitrification rate, biological phosphorus removal performance, sludge retention time SRT, aeration energy consumption, carbon source utilization efficiency, and excess sludge discharge strategy.

In the biological nitrogen and phosphorus removal process, nitrification is usually the prerequisite for biological nitrogen removal, and its control logic is relatively clear. Denitrification is the key link affecting nitrogen removal efficiency and is influenced by many factors such as DO, carbon source, reflux ratio, nitrate load, and sludge concentration. Biological phosphorus removal depends on the metabolic capacity of phosphorus-accumulating organisms PAOs during anaerobic phosphorus release and aerobic phosphorus uptake, and phosphorus is finally removed from the system through excess sludge discharge.

Therefore, in wastewater treatment automation upgrading projects, establishing an online monitoring system centered on MLSS and linked with DO, ORP, pH, NH4-N, NO3-N, TP, COD, or BOD-related indicators can help system integrators provide customers with more stable, traceable, and energy-efficient process control solutions.

Engineering Significance of MLSS in Biological Nitrogen and Phosphorus Removal Systems

MLSS represents mixed liquor suspended solids concentration and is commonly used to reflect the overall concentration of activated sludge in biological tanks. For A2/O, oxidation ditch, SBR, MBR, AAO, AO, and improved nitrogen and phosphorus removal processes, the level of MLSS directly affects total microbial biomass, system shock resistance, sludge age, reaction rate, oxygen transfer efficiency, and sludge settling performance.

In actual operation, MLSS is not the higher the better. A relatively high MLSS can increase the amount of microorganisms in the system, improve the potential capacity of nitrification and denitrification reactions, and enhance the buffering capacity of the system against water quality fluctuations. However, if MLSS is too high, it may also increase aeration load, increase the solid-liquid separation pressure of the secondary clarifier, cause sludge aging, increase SVI, increase effluent SS, and even affect biological phosphorus removal performance.

Therefore, the essence of MLSS control is to determine an appropriate operating range for the project site according to influent load, process type, sludge age, dissolved oxygen, reflux ratio, sludge discharge strategy, and effluent targets.

Impact of Sludge Concentration on Nitrification

Nitrification is mainly completed by nitrifying bacteria, including ammonia-oxidizing bacteria and nitrite-oxidizing bacteria. Nitrifying bacteria are autotrophic microorganisms with slow growth rates and are sensitive to DO, temperature, pH, SRT, and toxic substances. For wastewater treatment plants, the core operating parameters that can be directly regulated mainly include SRT, DO, BOD/TKN, MLSS, and sludge return and discharge strategies.

Higher MLSS Helps Increase the Total Amount of Nitrifying Microorganisms

During aerobic nitrification, higher sludge concentration means an increase in the total amount of microorganisms per unit tank volume, and the overall number of nitrifying bacteria may also increase. Under relatively stable conditions, higher MLSS can improve the potential rate of nitrification reaction, making it easier for the system to maintain stable ammonia nitrogen removal performance.

For projects with large fluctuations in influent ammonia nitrogen load, such as industrial park wastewater, food processing wastewater, and municipal wastewater plants under rainy-season shock loads, higher MLSS can improve system shock resistance and avoid ammonia nitrogen exceedance caused by short-term load changes.

MLSS and SRT Jointly Determine Nitrifying Bacteria Retention Capacity

Nitrifying bacteria grow slowly, and the system must maintain sufficient sludge retention time SRT to prevent nitrifying bacteria from being excessively discharged with excess sludge. In general, to ensure normal growth and reproduction of nitrifying bacteria, SRT usually needs to be controlled at a relatively high level. MLSS is closely related to SRT. Under certain sludge discharge volume, reflux volume, and load conditions, increasing MLSS often means increasing the sludge age of the system.

However, sludge age cannot be increased indefinitely. If sludge remains aged for a long time, it may lead to reduced activity, poorer settling performance, increased endogenous respiration, and an impact on biological phosphorus removal. Therefore, in system integration solutions, MLSS, DO, NH4-N, and sludge discharge volume should be monitored simultaneously to avoid focusing only on high sludge concentration while ignoring sludge activity.

High MLSS Can Maintain Nitrification Performance Under Lower DO Conditions

DO is an important control indicator in the nitrification stage. In traditional operation, the DO in the aerobic zone is often controlled at about 2 mg/L or above. However, in some oxidation ditch, A2/O, or improved biological tank systems, even when the average DO is maintained at around 1 mg/L, the system may still maintain good nitrification performance. One important reason is that the biological tank has relatively high MLSS, a large total microbial biomass, and enhanced effective reaction volume and biological reaction capacity.

From an engineering perspective, increasing MLSS can reduce the unit microbial load to a certain extent, allowing the system to maintain nitrification capacity under lower DO conditions. However, it should be noted that high MLSS also increases oxygen consumption. Under the same aeration volume, the value displayed by the DO meter may decrease. Therefore, online DO data must be evaluated together with MLSS, ammonia nitrogen, nitrate, and aeration intensity. Nitrification status should not be judged based only on the DO value.

Effect of BOD/TKN on the Competitive Relationship of Nitrifying Bacteria

The proportion of nitrifying bacteria in activated sludge is strongly related to BOD/TKN. When the influent organic matter concentration is high, heterotrophic bacteria reproduce rapidly and preferentially compete for dissolved oxygen, making it difficult for slow-growing nitrifying bacteria to become dominant, ultimately reducing the nitrification rate.

Higher MLSS may consume more biodegradable organic matter in the anaerobic or anoxic stage, resulting in relatively lower BOD/TKN entering the aerobic zone and improving the competitive environment for nitrifying bacteria. This is of great significance for wastewater treatment plants that need stable ammonia nitrogen effluent.

Impact of Sludge Concentration on Denitrification

Denitrification is a process in which denitrifying bacteria use oxygen in nitrate or nitrite as electron acceptors under anoxic conditions to oxidize and decompose organic matter and reduce nitrate nitrogen to nitrogen gas. Most denitrifying bacteria are heterotrophic facultative microorganisms and are widely present in wastewater treatment systems.

Denitrification efficiency is affected by pH, temperature, DO, carbon-nitrogen ratio, nitrate load, reflux ratio, and sludge concentration. In actual projects, the carbon-nitrogen ratio is usually limited by influent water quality and is difficult to change directly, while DO, reflux ratio, and MLSS are more common adjustment objects in operation control.

High MLSS Helps Reduce DO Interference in the Anoxic Zone

Denitrification requires an anoxic environment. If the internal reflux liquid carries too much DO into the anoxic zone, denitrifying bacteria will preferentially use molecular oxygen for respiration, thereby reducing nitrate reduction efficiency and consuming limited carbon sources.

A high-MLSS system can appropriately reduce the DO control value in the nitrification stage while still maintaining nitrification performance. This helps reduce the DO content carried by the reflux liquid at the end of nitrification and reduces the inhibition of DO on the denitrification process in the anoxic zone. In addition, the endogenous respiration oxygen consumption capacity of a high sludge concentration system is relatively strong, which can further consume dissolved oxygen in the reflux liquid and anoxic section.

In some treatment processes that use open channels as reflux passages, high MLSS may also change the viscosity of the mixed liquor, increase diffusion resistance, and reduce oxygenation during reflux falling, thereby creating a more stable anoxic environment for denitrification.

High MLSS Can Increase the Total Amount of Denitrifying Bacteria and Reaction Rate

The denitrification reaction rate is closely related to the concentration of denitrifying bacteria. Since denitrifying bacteria are widely present in wastewater treatment systems, increasing MLSS can increase the total amount of denitrifying bacteria per unit tank volume, thereby shortening the time required for denitrification or improving nitrate removal capacity under the same anoxic tank volume.

This is especially important for nitrogen and phosphorus removal projects with insufficient carbon sources. When the anoxic tank volume is fixed and external carbon source addition is restricted by cost, higher MLSS can improve the system’s ability to utilize refractory organic matter and endogenous carbon sources, improving denitrification efficiency.

High MLSS Is Beneficial to Simultaneous Nitrification and Denitrification

Under higher MLSS conditions, microbial floc diameter is often larger. When DO in the aerobic zone is relatively low, nitrification can occur on the outer part of the floc, while a micro-anoxic environment may form inside the floc, promoting denitrification. This phenomenon is commonly known as simultaneous nitrification and denitrification SND.

For oxidation ditch systems, low-DO operation systems, and some energy-saving wastewater treatment projects, reasonably increasing MLSS and combining it with precise DO control can help reduce aeration energy consumption while improving total nitrogen removal performance.

Impact of Sludge Concentration on Biological Phosphorus Removal

The core of biological phosphorus removal is that phosphorus-accumulating organisms PAOs release phosphorus and absorb volatile fatty acids VFA under anaerobic conditions, excessively absorb phosphorus under aerobic conditions, and finally remove phosphorus from the system through excess sludge discharge.

Therefore, biological phosphorus removal performance depends not only on MLSS, but also on sludge age, anaerobic zone environment, VFA supply, reflux nitrate interference, DO control, and sludge discharge strategy.

Appropriate MLSS Is Beneficial to Increasing the Total Amount of Phosphorus-Accumulating Bacteria

Under reasonable sludge age and sludge discharge conditions, increasing MLSS can increase the concentration of phosphorus-accumulating bacteria in the anaerobic zone and increase the amount of microorganisms involved in phosphorus release. After entering the aerobic stage, the amount of microorganisms capable of phosphorus uptake also increases accordingly, thereby improving the overall phosphorus removal capacity of the system.

For projects that need to meet both TN and TP effluent targets, MLSS control must be coordinated with anaerobic phosphorus release, anoxic denitrification, and aerobic phosphorus uptake.

Excessively High MLSS May Reduce Biological Phosphorus Removal Efficiency

Biological phosphorus removal relies on excess sludge discharge to remove phosphorus from the system. If MLSS is too high and leads to excessively long SRT and insufficient sludge discharge, phosphorus-accumulating bacteria may absorb phosphorus, but the phosphorus cannot be discharged in time through excess sludge, ultimately affecting overall phosphorus removal efficiency.

Biological phosphorus removal usually requires a relatively moderate sludge age. Under certain influent SS and load conditions, MLSS and SRT are often positively correlated. When MLSS exceeds a reasonable range, excessively long sludge age may lead to reduced phosphorus removal performance. Therefore, in nitrogen and phosphorus removal systems, MLSS cannot be controlled only according to nitrification requirements; it must also take into account the sludge discharge requirements of biological phosphorus removal.

High MLSS in the Anaerobic Zone Can Promote Hydrolysis and Acidification of Some Organic Matter

In the anaerobic zone, high MLSS can enhance the hydrolysis and acidification of some macromolecular refractory organic matter in the system and improve the potential generation of VFA. The energy released by phosphorus-accumulating bacteria during phosphorus release can be used to actively absorb acetate, H+, and other substances and form PHB stored in the cells, providing the basis for subsequent aerobic phosphorus uptake.

This process is valuable for low-carbon-source wastewater, industrial mixed wastewater, and some municipal wastewater treatment plant upgrading projects. By reasonably controlling MLSS, anaerobic retention time, and reflux nitrate, the biodegradability and carbon source utilization efficiency of the system can be improved.

Application Value of YexSensor Online Monitoring Solution in MLSS Control

For system integrators, MLSS control should not rely on manual experience judgment or intermittent laboratory testing. A more reasonable solution is to combine MLSS online sensors with DO, ORP, pH, ammonia nitrogen, nitrate, total phosphorus, COD, and other online monitoring equipment to build an automated sensing layer suitable for wastewater treatment process optimization.

YexSensor can provide online water quality monitoring equipment suitable for engineering integration in wastewater treatment projects. It supports industrial communication methods such as RS485 Modbus RTU and can be easily connected to PLCs, RTUs, data loggers, IoT gateways, SCADA systems, and cloud platforms.

Recommended Monitoring Parameter Combination

Recommended System Integration Architecture

In wastewater treatment automation projects, the MLSS online sensor can be installed at key locations in the biological tank and output real-time data through RS485 Modbus RTU. After field data enters the PLC or RTU, it can participate in control logic together with DO, ORP, pH, ammonia nitrogen, nitrate, and total phosphorus data.

The typical system architecture is as follows:

The sensor layer includes MLSS, DO, ORP, pH, NH4-N, NO3-N, TP, and other online monitoring equipment.

The data acquisition layer consists of PLCs, RTUs, or industrial data loggers.

The control execution layer includes blowers, aeration valves, internal reflux pumps, sludge return pumps, excess sludge pumps, and carbon source dosing pumps.

The platform layer can connect to SCADA, HMI, local servers, or cloud platforms for trend analysis, alarm records, remote operation and maintenance, and process optimization.

Through this architecture, system integrators can upgrade MLSS from a single detection parameter to a process control variable for biological nitrogen and phosphorus removal, achieving more refined process management.

Typical Project Application Scenarios

Municipal Wastewater Treatment Plant Upgrading

In projects with stricter TN and TP discharge indicators, MLSS online monitoring can help operators determine whether the sludge concentration in biological tanks meets the requirements of nitrification, denitrification, and phosphorus removal, and optimize aeration, reflux, and sludge discharge strategies together with online ammonia nitrogen, nitrate, and total phosphorus data.

Industrial Park Wastewater Treatment Stations

The water quality of industrial park wastewater fluctuates greatly and is prone to shock loads. Through online MLSS monitoring, the change trend of microbial biomass in the system can be judged, assisting in adjusting sludge return and sludge discharge and improving system shock resistance.

MBR Membrane Bioreactor Systems

MBR systems usually operate under relatively high MLSS conditions. Online MLSS data can help determine membrane tank load, sludge concentration changes, and membrane fouling risks, providing data support for stable membrane system operation.

Centralized Rural Wastewater Treatment and Smart Water Platforms

Rural wastewater stations are scattered, and manual inspection costs are high. Through the combination of MLSS with DO, ORP, pH, and other sensors, remote monitoring, abnormal alarms, and unattended operation and maintenance can be achieved, improving the operational stability of stations.

Selection Guide: What Conditions Need to Be Confirmed for MLSS Online Monitoring Projects

1. Process Type

Different processes have different MLSS control ranges. A2/O, oxidation ditch, SBR, MBR, and AO processes have different operating targets, and sensor installation points and control logic should also be different.

2. Installation Position

MLSS sensors can be installed in the anaerobic zone, anoxic zone, aerobic zone, membrane tank, return sludge pipeline, or excess sludge discharge pipeline according to project requirements. During selection, it should be confirmed whether the installation is immersion type, pipeline type, or flow-through type.

3. Communication Protocol

For automation system integration projects, it is recommended to select online sensors that support RS485 Modbus RTU output, making it easy to connect to PLCs, RTUs, data loggers, and SCADA systems.

4. On-Site Maintenance Conditions

Wastewater sites are prone to attachment, bubbles, fiber entanglement, and sludge deposition. Sensors should have structural designs suitable for long-term operation, and cleaning and maintenance solutions should be configured according to site conditions.

5. Whether Multi-Parameter Linkage Is Required

Monitoring MLSS alone can only reflect changes in sludge concentration and cannot fully determine nitrification, denitrification, and phosphorus removal status. For nitrogen and phosphorus removal projects, it is recommended to link at least DO, ORP, pH, NH4-N, and NO3-N.

Integration Considerations

Avoid Using MLSS as the Only Control Indicator

MLSS is an important parameter, but it is not the only basis for judgment. System operation should be comprehensively analyzed together with SRT, SVI, DO, ORP, NH4-N, NO3-N, TP, influent load, and sludge discharge volume.

Set Alarm Thresholds Reasonably

MLSS alarm thresholds should be set according to process type and historical operating data. Low-level alarms can indicate sludge loss or excessive sludge discharge, while high-level alarms can indicate sludge aging, settling risk, or increased aeration load.

Link with Aeration Control

An increase in MLSS will increase oxygen consumption in the system. In aeration control, MLSS and DO data should be combined to adjust blower frequency or aeration valve opening, avoiding control lag caused by relying solely on DO.

Link with Sludge Discharge Strategy

MLSS is closely related to SRT and excess sludge discharge. It is recommended to combine MLSS online data with sludge discharge pump operating time, sludge flow rate, and sludge concentration to optimize sludge discharge cycles.

Pay Attention to Sensor Cleaning and Maintenance

The environment of wastewater biological tanks is complex, and long-term sensor operation may be affected by sludge attachment and bubbles. It is recommended to regularly check the probe surface, configure automatic cleaning if necessary, or establish an on-site maintenance plan.

FAQ

Q1: Why is MLSS important for biological nitrogen and phosphorus removal?

MLSS reflects the overall concentration of activated sludge in the biological tank. It directly affects the quantity of nitrifying bacteria, denitrifying bacteria, and phosphorus-accumulating bacteria, and also affects SRT, DO consumption, carbon source utilization, and sludge discharge strategy. Therefore, it is a core parameter in nitrogen and phosphorus removal control.

Q2: Does higher MLSS always mean better nitrification performance?

Not necessarily. Properly increasing MLSS helps increase the total amount of nitrifying bacteria and improve system shock resistance. However, if MLSS is too high, it may cause sludge aging, increased aeration energy consumption, and poorer settling performance. It needs to be judged together with SRT, DO, and ammonia nitrogen data.

Q3: Why is high MLSS beneficial to denitrification?

High MLSS can increase the total amount of denitrifying bacteria and enhance the oxygen consumption capacity of the system, helping reduce DO interference in the anoxic zone. At the same time, when carbon sources are insufficient, high MLSS can improve the utilization capacity of endogenous carbon sources and refractory organic matter.

Q4: Will excessively high MLSS affect biological phosphorus removal?

Yes. Biological phosphorus removal relies on excess sludge discharge to remove phosphorus from the system. If MLSS is too high and causes excessively long SRT and insufficient sludge discharge, it may affect the renewal of phosphorus-accumulating bacteria and the system discharge of phosphorus, thereby reducing phosphorus removal efficiency.

Q5: Which wastewater treatment processes are suitable for MLSS online sensors?

MLSS online sensors are suitable for A2/O, AO, oxidation ditch, SBR, MBR, AAO, industrial wastewater biochemical treatment, rural wastewater treatment, and other activated sludge processes. They can be used for biological tanks, membrane tanks, return sludge, and excess sludge monitoring.

Q6: Can MLSS data be directly used for automatic sludge discharge control?

It can be used as an important reference, but it is recommended to combine it with SRT, sludge flow rate, reflux ratio, effluent indicators, and historical operating trends for comprehensive control. Automatic sludge discharge should not rely only on single-point MLSS values.

Q7: Why is it recommended to select RS485 Modbus RTU sensors for system integration?

RS485 Modbus RTU is a commonly used communication method in industrial sites. It is compatible with PLCs, RTUs, data loggers, IoT gateways, and SCADA systems, and is suitable for batch deployment and later maintenance in engineering projects.

Q8: Is the YexSensor online monitoring solution suitable for system integrators?

Yes. YexSensor is oriented toward engineering integration applications and can provide sensor solutions for wastewater treatment online monitoring. It helps system integrators build a complete water quality monitoring system from field sensing and data acquisition to control linkage and platform display.

Conclusion

MLSS sludge concentration is a key operating parameter in biological nitrogen and phosphorus removal systems. It affects nitrifying bacteria retention capacity, denitrifying bacteria reaction rate, phosphorus-accumulating bacteria phosphorus removal performance, system sludge age, aeration energy consumption, and excess sludge discharge strategy. Properly increasing MLSS can enhance system shock resistance and nitrogen removal potential, but excessively high MLSS may also cause sludge aging, settling risks, reduced phosphorus removal efficiency, and increased energy consumption.

For wastewater treatment plant upgrading, industrial wastewater biochemical treatment, MBR systems, and smart water projects, MLSS online monitoring should be used in linkage with DO, ORP, pH, NH4-N, NO3-N, TP, and other parameters. YexSensor can provide online water quality monitoring solutions suitable for engineering sites for system integrators, environmental engineering companies, and project contractors. It supports industrial communication methods such as RS485 Modbus RTU, helping projects achieve stable operation, automatic control, and data-based operation and maintenance.