Steps for Determining Organic Matter Content in Water: Professional Testing Procedures

Determining the organic matter content in water is a fundamental procedure in environmental monitoring, wastewater treatment, and drinking water quality control. This technical guide details the complete testing workflow—from sample collection and preservation to COD, BOD5, permanganate index, and TOC analysis—in accordance with internationally recognized standards such as ISO, ASTM, EN, and GB methods. Following these steps, whether in an in-house laboratory or an accredited third-party laboratory, ensures accurate and reproducible results for regulatory compliance and process optimization.

Technical Overview

Organic matter content is one of the most critical indicators used to evaluate water quality. Organic compounds in water originate from natural sources such as humic substances and plant decay, as well as anthropogenic inputs including domestic sewage, agricultural runoff, and industrial effluents. Elevated organic loads consume dissolved oxygen, degrade aquatic ecosystems, interfere with disinfection processes, and can produce harmful by-products such as trihalomethanes during water treatment.

Because individual organic compounds are too numerous to measure one by one in routine practice, the organic matter content of water is quantified through aggregate surrogate parameters. These include chemical oxygen demand (COD), biochemical oxygen demand (BOD5), total organic carbon (TOC), the permanganate index, and ultraviolet absorption at 254 nm (UV254). The complete determination workflow consists of seven sequential stages: sampling plan definition, sample collection and preservation, transport and storage, pretreatment, wet-chemical or instrumental analysis, quality control verification, and data calculation with reporting. Standard references include ISO 6060 and GB 11914 for dichromate COD, ISO 15705 for rapid digestion COD, ISO 8467 and GB 11892 for the permanganate index, ISO 5815-1 for BOD5, and ISO 8245, EN 1484, and HJ 501 for TOC.

Testing Samples

Surface Water and Groundwater

Rivers, lakes, reservoirs, and groundwater typically contain low to moderate organic loads. These samples are commonly analyzed for the permanganate index, TOC, and UV254 to assess natural organic matter levels, eutrophication trends, and the formation potential of disinfection by-products. Representative sampling requires consideration of depth, flow conditions, and seasonal variation.

Drinking Water and Treated Water

Treated drinking water and process water exhibit very low organic concentrations, generally below 5 mg/L as TOC. High-sensitivity TOC analysis is the preferred method because COD and BOD5 lack sufficient sensitivity at trace levels. TOC monitoring also serves as a key control point for coagulation optimization and disinfection by-product precursor removal.

Municipal and Industrial Wastewater

Domestic sewage and industrial effluents carry high organic loads, often ranging from 50 to several thousand mg/L as COD. These samples usually require dilution before analysis and may contain interfering substances such as chlorides, sulfides, ferrous iron, or oil, which must be addressed during pretreatment. Effluent samples are analyzed to verify compliance with discharge permits and treatment performance.

High-Salinity and Special-Matrix Samples

Marine water, brines, and certain chemical industry wastewaters contain elevated chloride concentrations that severely interfere with dichromate COD digestion. Such matrices require the addition of mercuric sulfate as a complexing agent or the selection of alternative methods such as TOC or the permanganate index. Samples containing floating oil or grease must be thoroughly homogenized before subsampling.

Testing Items

The following parameters are routinely determined to characterize the organic matter content of water, with results expressed in the indicated reporting units:

• Chemical oxygen demand, COD-Cr, expressed as mg O2/L, determined by dichromate reflux titration or rapid digestion spectrophotometry.
• Permanganate index, COD-Mn, expressed as mg O2/L, suitable for low-chloride surface water and drinking water.
• Biochemical oxygen demand, BOD5, expressed as mg O2/L, representing biodegradable organic matter over a five-day incubation.
• Total organic carbon, TOC, and dissolved organic carbon, DOC, expressed as mg C/L, including non-purgeable organic carbon (NPOC).
• UV254 absorbance, expressed as cm-1, as a rapid surrogate indicator of aromatic organic matter.
• Oil and grease, expressed as mg/L, for samples contaminated with petroleum hydrocarbons or fats.
• Biodegradability ratio, BOD5/COD, used to evaluate the biological treatability of wastewater.

Testing Methods

Step 1: Sample Collection and Preservation

Correct sampling is the foundation of reliable results. The following practices apply across all organic matter parameters:

• Collect samples in clean glass or chemically resistant plastic bottles that have been pre-washed with detergent, tap water, dilute hydrochloric acid, and reagent water.
• Rinse bottles two to three times with the sample before filling, leaving minimal headspace for COD and TOC samples.
• For COD and TOC analysis, acidify samples to pH below 2 with concentrated sulfuric acid to inhibit microbial degradation.
• For BOD5 analysis, do not acidify; instead refrigerate the sample and begin analysis as soon as possible, preferably within 6 hours and no later than 24 hours after collection.
• Store all samples at 2 to 5 degrees Celsius in the dark. The maximum holding time is 7 days for the permanganate index, 14 days for acidified COD, and 28 days for acidified TOC.
• Record sampling location, time, temperature, pH, and any field observations on a chain-of-custody form.

Step 2: Sample Pretreatment

Before analysis, samples are brought to room temperature and homogenized with a magnetic stirrer or blender. Highly concentrated samples are diluted quantitatively with reagent water so that the measured value falls within the calibrated range. For COD determination on chloride-rich samples, add mercuric sulfate at approximately ten times the expected chloride mass to complex the chloride. Suspended solids are removed by 0.45-micrometer membrane filtration when DOC rather than TOC is required. Turbid or colored samples analyzed by spectrophotometry may require the standard addition approach or confirmation by titrimetry.

Step 3: COD Determination by Dichromate Reflux Titration

The dichromate method per ISO 6060, GB 11914, and HJ 828 is the reference procedure for COD. The operating sequence is as follows:

• Pipette a measured volume of well-mixed sample (typically 20.0 mL) into a reflux flask.
• Add mercuric sulfate to mask chloride interference, followed by silver sulfate catalyst solution.
• Add a known excess of standard potassium dichromate solution, then slowly add concentrated sulfuric acid while cooling the flask.
• Attach the flask to a condenser and reflux the mixture at approximately 150 degrees Celsius for 2 hours.
• Allow the solution to cool, rinse down the condenser, dilute with reagent water, and add a few drops of ferroin indicator.
• Titrate the excess dichromate with standardized ferrous ammonium sulfate until the color changes from yellow-green to reddish brown.
• Run a reagent blank through the identical procedure and calculate COD from the difference in titrant consumption, applying the dilution factor.

Step 4: Rapid Digestion Spectrophotometric COD

For high-throughput screening, the sealed-tube digestion method per ISO 15705 and HJ/T 399 is widely used. The sample is mixed with pre-dosed dichromate reagent in a sealed vial, digested at 165 degrees Celsius for 15 to 20 minutes in a block heater, cooled, and measured spectrophotometrically at 440 nm or 600 nm against calibration standards prepared from potassium hydrogen phthalate. Results should be periodically verified against the reflux titration method.

Step 5: Permanganate Index Determination

For relatively clean waters, the permanganate index per ISO 8467 and GB 11892 is applied. The steps include:

• Add a measured sample volume to a flask with dilute sulfuric acid and a known excess of standardized potassium permanganate solution.
• Place the flask in a boiling water bath and digest for exactly 30 minutes.
• Add an accurately measured excess of standard sodium oxalate solution to decolorize the permanganate.
• Back-titrate the residual oxalate with potassium permanganate standard solution until a faint pink color persists for 30 seconds.
• Calculate the permanganate index from the permanganate consumed, reporting results as mg O2/L.

Step 6: BOD5 Determination

The five-day biochemical oxygen demand is determined by the dilution and seeding method per ISO 5815-1 and HJ 505:

• Determine the sample COD first to estimate the required dilution ratio so that residual dissolved oxygen after incubation remains above 1 mg/L.
• Prepare dilution water saturated with dissolved oxygen, buffered at pH 7.2, and seeded with settled domestic sewage or a commercial microbial inoculum when the sample lacks adequate microorganisms.
• Fill airtight BOD bottles completely with the diluted sample, ensuring no air bubbles remain.
• Measure the initial dissolved oxygen in one bottle immediately by the iodometric Winkler method or a membrane electrode probe.
• Incubate duplicate bottles at 20 plus or minus 1 degree Celsius in darkness for 5 days.
• Measure the final dissolved oxygen and calculate BOD5 from the oxygen depletion, correcting for blank depletion and the dilution factor.
• Verify method performance using a glucose-glutamic acid check standard.

Step 7: TOC Determination

Total organic carbon is determined per ISO 8245, EN 1484, ASTM D2579, and HJ 501, using either high-temperature catalytic combustion or UV-persulfate wet oxidation coupled with non-dispersive infrared detection of carbon dioxide:

• Acidify the sample to pH 2 or below with hydrochloric acid and purge with carrier gas to remove inorganic carbon as carbon dioxide.
• Inject a measured aliquot into the combustion furnace at 680 to 950 degrees Celsius over a platinum catalyst, or into the wet oxidation reactor.
• Convert all organic carbon to carbon dioxide, which is carried to the non-dispersive infrared detector.
• Quantify the carbon dioxide signal against a multi-point calibration curve prepared from potassium hydrogen phthalate standards.
• Report results as NPOC in mg C/L; alternatively determine TOC by the difference between total carbon and inorganic carbon.

Step 8: Quality Control and Data Reporting

Every analytical batch must include procedural blanks, calibration verification standards, duplicate samples, and matrix spikes. Spike recoveries should fall within 80 to 120 percent, and duplicate relative deviations within 10 percent for concentrations above ten times the method detection limit. Control charts are maintained to track trends. Final results are reported with units, dilution factors, detection limits, and measurement uncertainty, and are reviewed and signed by authorized personnel before release.

Applications

Organic matter determination supports a wide range of regulatory, operational, and scientific purposes:

• Compliance monitoring of municipal and industrial wastewater discharge against national and local effluent standards.
• Process control in biological treatment plants, where COD and BOD5 loading govern aeration capacity, sludge age, and nutrient dosing.
• Drinking water treatment optimization, using TOC removal as a performance indicator for coagulation, adsorption, and membrane processes.
• Environmental impact assessment of receiving water bodies and long-term trend monitoring of rivers, lakes, and reservoirs.
• Biodegradability evaluation of industrial effluents through the BOD5 to COD ratio prior to pretreatment design.
• Verification of remediation performance at contaminated sites and validation of water reuse systems.

FAQ

What is the difference between COD and BOD5?

COD measures the total chemically oxidizable organic matter using a strong oxidant, delivering results within about three hours. BOD5 measures only the portion of organic matter biodegraded by microorganisms over five days. BOD5 is always lower than COD, and the ratio between them indicates how readily a wastewater can be treated biologically.

How do chlorides interfere with COD determination, and how is this corrected?

Chloride ions are oxidized by dichromate and consume the oxidant, producing falsely elevated results. The interference is suppressed by adding mercuric sulfate to complex the chloride, or by using a chloride-resistant digestion formulation. For samples with extremely high salinity, TOC analysis is recommended instead.

Which method should be chosen for low-level organic matter in drinking water?

TOC analysis is the preferred technique for trace organic levels because it offers detection limits at the microgram-per-liter level, whereas COD and BOD5 lack the sensitivity needed below a few milligrams per liter. UV254 can serve as a rapid supplementary indicator.

What are the maximum holding times for organic matter samples?

Unacidified samples for the permanganate index should be analyzed within 7 days, acidified COD samples within 14 days, and acidified TOC samples within 28 days, all stored at 2 to 5 degrees Celsius. BOD5 samples must be tested within 24 hours of collection and should never be chemically preserved.

How often should calibration and quality control checks be performed?

A multi-point calibration should be established at the start of each analytical batch or whenever the instrument response drifts beyond control limits. Each batch must include a blank, a continuing calibration verification standard, duplicates, and matrix spikes, with acceptance criteria defined in the laboratory quality manual aligned with ISO/IEC 17025 requirements.