Complete Technical Solution for Improving Accuracy and Stability of Sewage Water Quality Monitoring

This article deeply analyzes core technical methods to improve the accuracy and stability of sewage water quality monitoring, covering monitoring point layout strategies, laboratory quality control systems, online monitoring system selection and pretreatment technologies. NiuBoL provides professional water quality monitoring system integration services to help sewage treatment plants and pollutant discharge enterprises achieve true and reliable data and compliant reporting.

I. Commercial Value of Water Quality Monitoring Accuracy and Stability

For sewage treatment plants, key pollutant discharge enterprises, and environmental protection operation and maintenance units, the accuracy and stability of water quality monitoring data are not only the legal basis for compliant discharge, but also the technical cornerstone for process regulation, cost control, and environmental reputation.

Data deviation may lead to two serious consequences: first, overestimating treatment effects, triggering risks of excessive discharge and administrative penalties; second, underestimating treatment capacity, resulting in excessive chemical dosing and energy consumption waste. Therefore, establishing a scientific, rigorous, and reproducible water quality monitoring system is an inevitable choice for every environmental responsibility entity.

II. Monitoring Point Layout: The First Checkpoint for Data Representativeness

2.1 Point Layout Principles and Preliminary Investigation

Reasonable monitoring point locations are the premise for ensuring data accuracy. Before laying out monitoring points, the following work must be completed:

• On-site investigation: Comprehensive survey of sewage discharge outlet locations, pipeline directions, discharge patterns, and surrounding water body distribution

• Hydrological parameter collection: For river monitoring sections, basic data such as flow velocity, river width, and water depth must be mastered

• Pollutant preliminary analysis: Use portable detectors or laboratory rapid methods to preliminarily understand the main pollutant types and concentration ranges

2.2 Technical Specifications for Point Setting

2.3 Marking and Long-term Tracking

Physically mark the determined monitoring points (such as scales, positioning piles), and establish point archives (including latitude and longitude, on-site photos, and surrounding environment descriptions). Fixed points are the foundation for obtaining temporally comparable and spatially traceable data.

III. Laboratory Monitoring: Key Link of Quality Control

Laboratory monitoring is the traditional and main method for sewage water quality monitoring. To ensure data accuracy and stability, a full-process quality control system must be established.

3.1 Laboratory Environment and Facility Requirements

• Environmental cleanliness: Keep the laboratory tidy to avoid secondary contamination of samples by dust and aerosols

• Ventilation facilities: Must be equipped with fume hoods; operations with volatile reagents (such as potassium dichromate solution, organic extractants) must be completed inside fume hoods

• Temperature and humidity control: Precision instrument rooms are recommended to maintain 20–25℃ temperature and ≤60% relative humidity

3.2 Full-cycle Management of Instruments and Equipment

3.3 Reagent and Glassware Management

• Reagent storage: Classified storage according to properties (light-proof, refrigerated, dry), regularly check expiration dates, and immediately scrap deteriorated reagents

• Glassware cleaning: Adopt standard process of “detergent cleaning → tap water rinsing → 10% nitric acid soaking → pure water rinsing” to avoid wall residues interfering with measurement results

• Pipettes and volumetric flasks: Regularly perform metrological verification to ensure volume accuracy

3.4 Sample Collection and Preservation

• Sampling container material selection: Glass bottles for organic analysis; polyethylene bottles for heavy metal analysis

• Preservation methods: Add preservatives (e.g., sulfuric acid to pH≤2 for COD), refrigerate (4℃), and deliver for testing within time limits

• Parallel samples and blank samples: Set no less than 10% parallel samples and full-process blank samples for each batch of samples

IV. Online Monitoring System: Reliability Assurance for Real-time Data

For key pollutant discharge units, online monitoring is an inevitable choice. To obtain accurate and continuous online data, systematic control must be exercised from the following four dimensions.

4.1 Selection of Online Water Quality Analyzers

Incorrect instrument selection is a common cause of data distortion. Instruments must be selected according to sewage water quality characteristics and reporting index requirements, matching monitoring methods and ranges.

Case Illustration: COD concentration of a certain enterprise’s sewage is in the 500–600 mg/L range

Other parameter selection references:

• Ammonia nitrogen online analyzer: Salicylic acid method suitable for low concentration (≤10 mg/L); Nessler’s reagent method suitable for medium and high concentrations

• Total phosphorus online analyzer: Molybdenum-antimony anti-spectrophotometric method is mainstream, requires high-temperature digestion module

• pH meter: Select industrial-grade electrodes with temperature compensation function

4.2 Design of Water Intake Points and Water Intake Systems

Core requirements for water intake points: The taken water sample can represent the true condition of the discharged water quality.

• Location selection: Appropriate distance from discharge outlet (avoid dead zones), fully mixed water flow areas

• Depth setting: Take water 0.5–1.0 m below the surface, avoiding surface oil and bottom sediment

• Water intake system: Recommend “submersible pump + self-priming pump” dual-stage water intake scheme to ensure continuity and reliability

• Pipeline design: Shorten distances, reduce elbows, maintain appropriate flow velocity (1–1.5 m/s) to prevent deposition

4.3 Water Sample Pretreatment System: Purification Without Sacrificing Representativeness

The goal of pretreatment is “remove interference, protect instruments, while not changing water sample representativeness”.

Engineering suggestion: Pretreatment system should adopt multi-stage series method, automatically or manually selecting pretreatment depth according to water sample impurity conditions. COD, ammonia nitrogen, and total phosphorus analyzers generally require only Level 2 pretreatment; heavy metal analyzers may require Level 3.

4.4 System Maintenance and Data Review

• Regular cleaning: Backwash water intake pipelines once a week; clean sampling cups and filters every half month

• Reagent replacement: Replace according to manufacturer recommended cycles, immediately stop using deteriorated reagents

• Data review: Set upper and lower limit alarms, mutation prompts, and conduct daily manual review of abnormal data

• Comparison verification: Monthly comparison of laboratory and online data (relative error ≤±10%)

FAQ

Q1. What could be the reasons for frequent fluctuations in water quality monitoring data?

Possible reasons include: improper water intake point location (affected by turbulence or dead zones); pretreatment system blockage or instability; instruments not calibrated; actual fluctuations in discharged water quality. It is recommended to troubleshoot the water intake system one by one.

Q2. If online monitoring instrument data does not match laboratory data, which one shall prevail?

Laboratory methods are usually standard methods (such as HJ 828-2017), while online instruments use rapid methods. Differences within acceptable ranges (e.g., COD ±10%) are normal. If the difference is too large, calibrate the online instrument or investigate the representativeness of the pretreatment system.

Q3. How to select the range for online COD analyzers?

The principle is that the upper limit of the range is 1.5–2 times the normal concentration. For example, if the normal concentration is 500 mg/L, select 1000 mg/L range. Too narrow easily triggers over-limit; too wide has insufficient resolution.

Q4. Will water sample pretreatment cause data to be lower?

Yes. Any pretreatment will remove some substances to varying degrees. The key is “controllable and consistent” — after fixing the pretreatment method, establish a stable conversion relationship between data and actual field values or use the same method for standard comparison.

Q5. Must monitoring points be fixed? Why?

They must be fixed. Only fixed points can yield temporally comparable data. Moving points will make data sequences incomparable and prevent judgment of water quality change trends.

Q6. What is the impact of incomplete laboratory glassware cleaning?

It will cause cross-contamination. For example, digestion tubes used for COD not thoroughly cleaned will leave residual organic matter, causing the next sample result to be higher. It is recommended to implement standardized cleaning processes and regularly spot-check cleaning effectiveness.

Q7. How often does the online monitoring system need calibration?

For conventional items such as pH, COD, and ammonia nitrogen, calibration is recommended every 7–15 days; heavy metal analyzers can be appropriately extended to 30 days. Recalibration is required after each reagent replacement.

Q8. What water quality monitoring system services can NiuBoL provide?

NiuBoL provides full-chain services from monitoring point design, instrument selection, system integration, installation and commissioning to operation and maintenance training, covering parameters such as COD, ammonia nitrogen, total phosphorus, total nitrogen, pH, and flow, supporting data networking upload and environmental protection platform docking.

Summary

Improving the accuracy and stability of sewage water quality monitoring is a systematic project that runs through the entire chain of “point layout → sampling → pretreatment → analysis → calibration → review”.

Three Core Principles:

1. Representativeness first: Whether in point layout or pretreatment methods, the premise must be to ensure water samples represent the real water body.

2. Standardized execution: All operations from glassware cleaning to instrument calibration must be evidence-based and traceable.

3. Systematic thinking: Laboratory monitoring and online monitoring complement each other; manual comparison and automatic calibration support each other.

Water Quality Sensor Data Sheet

NBL-WQ-CL Water Quality Sensor Online Residual Chlorine Sensor.pdf    

NBL-WQ-DO Online Fluorescence Dissolved Oxygen Sensor.pdf    

NBL-WQ-NHN Ammonia Nitrogen Water Quality Sensor.pdf    

NBL-WQ-COD Online Water Quality COD Sensor.pdf    

NBL-WQ-PH Online pH Water Quality Sensor.pdf    

NBL-WQ-EC water quality conductivity sensor.pdf    

NBL-WQ-BOD-4A Online BOD Sensor.pdf    

NBL-WQ-TH-4S online total hardness sensor.pdf