This article provides comprehensive standard operating procedures (SOPs) for the physicochemical testing of water quality, covering sample collection, preservation, pretreatment, analytical methods, and quality control requirements. By referencing internationally recognized standards from ISO, ASTM, and GB, laboratories can ensure accurate, reproducible, and compliant test results. The procedures apply to drinking water, surface water, groundwater, and industrial wastewater analysis.
Technical Overview
Physicochemical testing of water quality is a systematic analytical process used to evaluate the physical and chemical characteristics of water samples through quantitative measurement of parameters such as pH, turbidity, hardness, organic pollutants, nutrients, and heavy metals. A well-defined operating procedure ensures that every stage of the workflow, from sampling to reporting, is standardized, traceable, and defensible.
This procedure applies to accredited third-party laboratories, municipal water treatment facilities, environmental monitoring stations, and industrial quality control departments. The core reference standards include GB 5749 and GB/T 5750 for drinking water, GB 3838 for surface water environmental quality, GB/T 14848 for groundwater quality, GB 8978 for sewage discharge, the ISO 5667 series for water sampling and preservation, and ISO/IEC 17025 for laboratory management system requirements.
All testing personnel must be trained and technically qualified for the specific methods they perform. The laboratory environment, including temperature, humidity, dust control, and reagent storage conditions, must meet the requirements specified in the relevant method standards. Instruments such as pH meters, spectrophotometers, titration systems, and atomic spectrometers must be calibrated, verified, and maintained according to documented schedules.
Testing Samples
Drinking Water
Drinking water samples include treated tap water, bottled water, secondary water supply systems, and purified water for domestic use. Testing focuses on hygienic and safety indicators defined in GB 5749, such as turbidity, pH, total hardness, disinfection by-products, and toxic metal residues.
Surface Water
Surface water samples are collected from rivers, lakes, reservoirs, and streams. They are classified into functional zones according to GB 3838, and typical indicators include dissolved oxygen, chemical oxygen demand, ammonia nitrogen, total phosphorus, and total nitrogen.
Groundwater
Groundwater samples are obtained from wells, boreholes, and springs. Key parameters include total dissolved solids, conductivity, fluoride, chloride, sulfate, iron, manganese, and indicators of anthropogenic contamination.
Domestic and Industrial Wastewater
Wastewater samples are taken at discharge outlets, treatment plant inlets and outlets, and monitoring wells. Compliance testing typically covers chemical oxygen demand, biochemical oxygen demand, suspended solids, ammonia nitrogen, petroleum hydrocarbons, anionic surfactants, and heavy metals in accordance with GB 8978.
Recycled and Reclaimed Water
Reclaimed water used for irrigation, landscape purposes, or industrial circulation requires verification of stability and safety parameters, including residual chlorine, turbidity, and microbial support indicators such as organic carbon levels.
Testing Items
Conventional physicochemical testing items are generally organized into the following groups:
- Physical parameters: color, odor, turbidity, visible matter, suspended solids, total dissolved solids, electrical conductivity, and temperature.
- General chemical parameters: pH value, total hardness, alkalinity, acidity, total alkalinity, chloride, sulfate, fluoride, and total organic carbon.
- Organic pollution indicators: chemical oxygen demand (COD), biochemical oxygen demand (BOD5), permanganate index, ammonia nitrogen, nitrate nitrogen, nitrite nitrogen, total nitrogen, total phosphorus, petroleum hydrocarbons, and anionic surfactants.
- Disinfection-related parameters: free residual chlorine, total chlorine, chlorate, chlorite, and trihalomethanes.
- Metal elements: lead, cadmium, chromium, arsenic, mercury, selenium, copper, zinc, iron, manganese, aluminum, nickel, and antimony.
The selection of testing items depends on the water matrix, regulatory requirements, and the purpose of the assessment. For example, drinking water compliance testing follows the full hygienic indicator list in GB 5749, while wastewater discharge monitoring targets the permitted emission limits applicable to the specific industry.
Testing Methods
Method selection must follow the priority order specified in the applicable product or discharge standard. Commonly adopted analytical techniques include:
- Electrochemical methods: glass electrode potentiometry for pH measurement in accordance with ISO 10523, ASTM D1293, and GB/T 5750; ion-selective electrode methods for fluoride.
- Titrimetric methods: EDTA complexometric titration for total hardness, acid-base titration for alkalinity and acidity, argentometric titration for chloride, and iodometric titration for dissolved oxygen following ISO 5814.
- Spectrophotometric methods: Nessler or salicylate spectrophotometry for ammonia nitrogen, ammonium molybdate spectrophotometry for total phosphorus, and dichromate reflux or rapid digestion spectrophotometry for COD consistent with ISO 6060 and relevant HJ method standards.
- Dilution and inoculation methods: five-day biochemical oxygen demand (BOD5) determination in accordance with ISO 5815.
- Turbidimetric methods: nephelometric turbidity measurement following ISO 7027.
- Gravimetric methods: determination of suspended solids, total dissolved solids, and mineral oil by weighing residues after filtration or extraction.
- Atomic spectrometry: flame and graphite furnace atomic absorption spectrometry (AAS), atomic fluorescence spectrometry (AFS), inductively coupled plasma optical emission spectrometry (ICP-OES), and inductively coupled plasma mass spectrometry (ICP-MS) for trace metals, with ICP-MS performed in accordance with ISO 17294.
Standard Operating Workflow
Sample Collection and Preservation
Sampling must follow the ISO 5667 series and GB/T 5750 sampling guidance. Containers shall be chemically inert, pre-cleaned, and appropriate for the target analytes. Representative samples are collected after adequate flushing of taps or using depth samplers for open water bodies. Preservation measures include refrigeration at approximately 4 degrees Celsius, acidification with nitric acid for metal analysis to pH below 2, and addition of chemical fixatives where the method requires. Each sample must be uniquely labeled and accompanied by a chain-of-custody record.
Sample Pretreatment
Pretreatment steps may include homogenization, filtration for dissolved fraction analysis, digestion with acid or oxidants for total metal determination, and distillation or extraction for specific organic parameters. All pretreatment procedures must be documented and consistent with the referenced method to avoid introducing systematic bias.
Instrument Calibration and Verification
Before analysis, instruments must be calibrated using certified reference materials or standard solutions traceable to national metrology institutes. Calibration curves require at least five concentration points with acceptable linearity, typically a correlation coefficient of 0.995 or higher unless the method specifies otherwise. Verification standards must be analyzed to confirm calibration validity before sample measurement.
Measurement and Recording
Samples are analyzed within the specified holding times. Instrument readings, environmental conditions, reagent batches, and any deviations must be recorded in real time. Results are calculated using the formulas defined in the method, with dilution and blank corrections applied as required.
Data Review and Reporting
Calculated results are checked against calibration verification data, quality control limits, and detection limits. Reports must clearly state the method used, the units of measurement, the detection limits, and any qualification notes. Results below the detection limit are reported accordingly and must never be replaced with zero.
Quality Control
Each analytical batch must include procedural blanks, parallel duplicate samples, and spiked recovery samples. Acceptance criteria typically require duplicate relative deviations within method-specified limits and spike recoveries between 85 and 115 percent, or the limits stated in the applicable standard. Certified reference materials are analyzed regularly to verify accuracy, and control charts are maintained to monitor long-term method stability.
Method detection limits are established and periodically re-verified. Nonconforming work must be investigated, documented, and corrected through root cause analysis and, where necessary, re-analysis of affected samples. All records, including raw data and instrument outputs, must be retained for the period specified by laboratory policy and regulatory requirements.
Applications
Physicochemical water testing procedures are applied across a wide range of scenarios, including routine compliance monitoring of municipal drinking water, environmental impact assessments for surface water bodies, groundwater contamination investigations, wastewater treatment process optimization and discharge compliance, industrial boiler feedwater and cooling water control, pharmaceutical and medical water system validation, aquaculture water quality management, and food and beverage production water verification.
Regulatory agencies rely on data generated under these procedures to enforce discharge permits and drinking water safety regulations, while enterprises use the results for process control, risk management, and demonstrating regulatory compliance through accredited third-party laboratories.
FAQ
What is the difference between COD and BOD5?
COD measures the total amount of oxygen required to chemically oxidize organic and reducible inorganic matter, delivering results within two to three hours. BOD5 measures the oxygen consumed by microorganisms during five days of aerobic decomposition and reflects the biodegradable organic fraction. The two parameters complement each other in characterizing organic pollution.
How should water samples be preserved before testing?
Most samples should be stored at approximately 4 degrees Celsius in the dark and analyzed as soon as possible. Metals require acidification with nitric acid to pH below 2, while parameters such as ammonia nitrogen and COD may require the addition of sulfuric acid. Each parameter has a defined maximum holding time that must not be exceeded.
How often should instruments be calibrated?
Instruments must be calibrated before each analytical batch or at the frequency specified by the method. Daily calibration is typical for parameters such as pH and residual chlorine, while spectrometers and titration systems require multi-point calibration curves with each batch of samples, followed by continuing calibration verification.
Which standards govern physicochemical water testing?
The principal standards include GB 5749 and GB/T 5750 for drinking water, GB 3838 for surface water, GB/T 14848 for groundwater, GB 8978 for wastewater discharge, the ISO 5667 series for sampling, and parameter-specific methods from ISO, ASTM, and national environmental protection standards.
What quality control measures are mandatory in each analytical batch?
Every batch must include at least one procedural blank, duplicate or parallel samples, and a spiked recovery sample or certified reference material analysis. Results are acceptable only when all quality control checks fall within the limits defined by the method and the laboratory quality management system.