Impact of Sludge Concentration (MLSS) on the Operation of Biological Nitrogen and Phosphorus Removal Systems

In sewage treatment projects with high-standard discharge requirements, the stable operation of biological nitrogen and phosphorus removal processes highly depends on the refined management of process parameters. Sludge concentration (Mixed Liquor Suspended Solids, MLSS), as a core control variable, directly affects the population structure, reaction kinetics and system material balance of nitrifying bacteria (NOB/AOB), denitrifying bacteria and phosphorus accumulating bacteria (PAOs). For system integrators, IoT solution providers and EPC engineering companies, deeply understanding the influence mechanism of MLSS and implementing closed-loop control through reliable online monitoring is the key to ensuring TN and TP compliance, reducing operating energy consumption and extending equipment life cycle.

YexSensor focuses on the field of industrial process water quality monitoring and provides high-reliability YEX-S2-MLSS-A MLSS sensor along with DO, nitrate nitrogen and ORP multi-parameter sensor solutions for complex sewage treatment scenarios.

1. Influence Mechanism of Sludge Concentration on Nitrification

As the premise of biological denitrification, the nitrification process is limited by the growth characteristics of autotrophic nitrifying bacteria and is highly sensitive to MLSS.

Nitrification Rate and Positive Correlation with MLSS

Higher MLSS (typical 4000-8000mg/L) can significantly increase the number of nitrifying bacteria per unit volume and improve the nitrification reaction rate constant. In the aerobic tank, the nitrification rate (r_N) is directly proportional to the concentration of active nitrifying bacteria. Under high MLSS conditions, it can effectively shorten the aerobic hydraulic retention time (HRT) and optimize the utilization rate of tank volume.

Sludge Age (SRT) Synergistic Regulation

Nitrifying bacteria have a long generation time (about 1-3 days). In engineering, SRT≥8 days needs to be maintained to ensure they are not washed out. High MLSS extends the actual sludge age by reducing the F/M ratio (food to microorganism ratio), while reducing the BOD/TKN ratio entering the aerobic zone, inhibiting the competitive advantage of heterotrophic bacteria and strengthening the proportion of nitrifying bacteria in the microbial community.

DO Threshold Optimization

The oxygen consumption rate (OUR) of high MLSS systems increases. Under the same aeration intensity, a lower apparent DO (1.0-1.8mg/L) can be maintained, which still meets the nitrification demand. This provides operating space for processes such as oxidation ditches that are difficult to achieve high DO throughout the process. YexSensor fluorescent dissolved oxygen sensor can achieve high-precision measurement of ±0.05mg/L in low DO areas and support PID fine aeration control.

2. Influence Mechanism of Sludge Concentration on Denitrification

Denitrification is the decisive link of denitrification efficiency. High MLSS provides multi-dimensional enhancement effects on it.

Relief of DO Inhibition in Anoxic Zone

High MLSS enhances endogenous respiration and can quickly consume the dissolved oxygen carried by internal reflux. At the same time, the apparent viscosity of the mixed liquid increases, reducing the oxygen diffusion coefficient and reducing open channel reflux reoxygenation. Under high MLSS conditions, the DO in the anoxic zone can be stably controlled below 0.15mg/L, maximizing the efficiency of denitrifying bacteria using NO3⁻-N as electron acceptor.

Improvement of Denitrification Kinetics

The denitrification rate (r_DN) has a first-order reaction relationship with the concentration of denitrifying bacteria. Increasing MLSS can shorten the HRT of the anoxic section and strengthen the utilization capacity of difficult-to-biodegrade carbon sources (such as components other than VFA), which is especially suitable for projects with influent C/N ratio <4.0. By promoting the formation of anoxic microenvironment inside the bacterial floc, simultaneous nitrification and denitrification (SND) can also be achieved to improve the total nitrogen removal rate.

3. Influence Mechanism of Sludge Concentration on Biological Phosphorus Removal

Biological phosphorus removal relies on the metabolic cycle of phosphorus accumulating bacteria (PAOs) anaerobic phosphorus release - aerobic excessive phosphorus uptake, and phosphorus removal is achieved through excess sludge discharge. MLSS needs to achieve a balance between bacterial population enrichment and sludge age.

Enhancement of PAOs Activity in Anaerobic Zone

Suitable high MLSS can increase the absolute number of phosphorus accumulating bacteria in the anaerobic zone and improve the phosphorus release rate (P_release). At the same time, it strengthens the anaerobic hydrolysis and acidification effect, generating more volatile fatty acids (VFA), promoting PAOs to synthesize poly-β-hydroxybutyrate (PHB), and providing sufficient internal carbon source reserves for subsequent aerobic phosphorus uptake.

Sludge Age Constraints and Optimal Window

The efficient sludge age for biological phosphorus removal is usually controlled at 3-6 days. Excessive MLSS will lead to prolonged SRT, attenuation of PAOs excessive phosphorus uptake capacity, and decrease in sludge phosphorus content (P/VSS). Therefore, real-time MLSS monitoring with YEX-S2-MLSS-A is needed to achieve dynamic sludge discharge control and maintain the optimal operating range.

YexSensor YEX-S2-MLSS-A Online Monitoring System Integration Applications

YexSensor YEX-S2-MLSS-A adopts backscattered light principle, with strong anti-pollution ability and self-cleaning function, suitable for long-term high suspended solids working conditions in aeration tanks and return sludge lines.

Typical Process Integration Solutions:

- A²/O Process: Segmented deployment of YEX-S2-MLSS-A + DO + NO3-N combined monitoring in anaerobic/anoxic/aerobic zones, realizing decoupled control of internal reflux ratio, external reflux ratio and aeration volume through PLC.

- Oxidation Ditch Process: Multi-point MLSS gradient monitoring in the ditch, combined with ORP to achieve precise division of zonal oxidation-reduction environment and support SND process enhancement.

- MBR Process: High-concentration MLSS monitoring in membrane tank for membrane fouling early warning and automatic sludge discharge/backwashing strategy optimization.

Communication Protocols and System Compatibility:

Supports 4-20mA, RS485 (Modbus RTU), Modbus TCP, Profibus DP/PA and MQTT protocols, and can seamlessly connect with Siemens, Rockwell, ABB and domestic mainstream PLC/DCS and IoT platforms.

Engineering Application Cases:

In a 200,000 m³/d municipal sewage treatment project, after adopting YexSensor YEX-S2-MLSS-A monitoring system, MLSS was stably controlled at 5800±400mg/L, TN removal rate increased to 87.3%, TP removal rate reached 93.5%, and unit power consumption decreased by 14.8%.

Selection Guide

Measurement Principle Selection:

YEX-S2-MLSS-A uses backscattered light method, suitable for 0-20 g/L (0-20000 mg/L) range with excellent stability in high sludge concentration environments.

Key Parameter Comparison (YEX-S2-MLSS-A):

Integration Considerations

- Installation location should select representative areas with uniform mixing, avoiding dead corners, aeration disturbance zones and inlet impact points.

- Regular (1-3 months) laboratory MLSS (drying weighing method) comparison calibration is recommended, and on-site calibration protocol should be established.

- Connect YEX-S2-MLSS-A signal to advanced process control (APC) or fuzzy control algorithm to achieve multi-variable coupling optimization.

- 1+1 redundant configuration is recommended for key control points to improve system availability.

- Data processing needs to configure digital filtering and outlier elimination mechanisms to prevent control oscillation.

FAQ

Q1 What negative effects will excessive MLSS concentration have on nitrification and phosphorus removal?

Excessive MLSS will lead to excessive prolongation of SRT. Although nitrifying bacteria can be maintained, the excessive phosphorus uptake capacity of phosphorus accumulating bacteria will decay, and the sludge P/VSS ratio will decrease. At the same time, it may induce filamentous bacteria expansion and affect mud-water separation efficiency. It is recommended to control MLSS within the optimal process design window through online monitoring.

Q2 In oxidation ditch process, how to maintain efficient nitrification under lower DO conditions?

By increasing MLSS to increase the absolute amount of nitrifying bacteria and endogenous oxygen consumption rate, combined with low DO operation strategy (0.8-1.5mg/L), the balance between nitrification efficiency and aeration energy saving is achieved. YexSensor multi-parameter sensors can provide real-time data support.

Q3 What is the quantitative improvement of high MLSS on denitrification carbon source utilization efficiency?

High MLSS can increase the concentration of denitrifying bacteria and enhance the utilization capacity of medium and low biodegradable organic matter. Under low C/N ratio conditions, it can increase the denitrification rate by 20-35% and reduce the amount of additional carbon source dosing.

Q4 How to ensure long-term stability of MLSS sensors in high SS or oily wastewater environments?

YexSensor YEX-S2-MLSS-A adopts anti-pollution sensitive elements and intelligent self-cleaning systems, which can achieve a maintenance-free cycle of ≥12 months in most industrial wastewater projects. It is recommended to conduct adaptability verification tests in the early stage of the project.

Q5 How to use MLSS monitoring to achieve aeration system energy saving?

Real-time MLSS data can dynamically adjust aeration intensity to avoid excessive oxygen supply. Through OUR estimation and DO-MLSS joint control, typical projects can reduce aeration energy consumption by 12-18%.

Q6 What technical indicators should be focused on for MLSS monitoring in MBR process?

Wide range (8000-18000mg/L) and high anti-pollution models should be selected, and associated with TMP (transmembrane pressure difference) monitoring to achieve early membrane fouling warning and sludge concentration optimization control.

Q7 What is the integration compatibility of YexSensor sensors with mainstream automation systems?

It supports various protocols such as Modbus, Profibus, Ethernet/IP and MQTT, and can be directly connected to Siemens TIA Portal, Rockwell Studio 5000 and various SCADA platforms, providing complete SDK and technical support.

Q8 What is the main basis for determining the optimal MLSS operating range of the project?

It needs to comprehensively consider factors such as influent BOD5, TN, TP load, process type, target effluent quality and temperature, and be determined through on-site debugging and model verification. YexSensor can provide technical consultation and joint debugging services.

Summary

Sludge concentration (MLSS) is a key process parameter for achieving efficient and stable operation of biological nitrogen and phosphorus removal processes. Through scientific regulation of MLSS with YexSensor YEX-S2-MLSS-A, nitrification kinetics, denitrification completeness and phosphorus accumulating bacteria metabolic efficiency can be simultaneously optimized, ultimately improving system nitrogen and phosphorus removal performance and reducing comprehensive operating costs.

As a manufacturer focusing on industrial process monitoring, YexSensor is committed to providing high-precision and highly compatible online monitoring solutions for system integrators and engineering companies. We welcome partners to conduct in-depth exchanges on specific project needs and jointly promote the intelligent upgrading of sewage treatment.

Contact Information: Welcome to visit the official website or contact technical engineers to request product specification sheets, application white papers and project reference cases.