Organic matter determination is one of the core pillars of water quality analysis, providing essential indicators such as COD, BOD, TOC, and compound-specific organic parameters for pollution assessment and regulatory compliance. This article explains the fundamental principles behind the measurement of organic substances in water, covering oxidation-based aggregate parameters, spectrophotometric indicators, and chromatographic techniques, together with sample types, applicable standards, and practical applications.
Technical Overview
The determination of organic matter in water is fundamentally based on the fact that organic compounds, regardless of their individual identity, can be converted into simple, measurable end products through oxidation, microbial degradation, or combustion. Because natural waters and wastewaters contain thousands of individual organic substances at trace levels, it is impractical to identify and quantify each one. Instead, analytical science has developed a family of aggregate (sum) parameters that respond to the total organic load, supplemented by compound-specific chromatographic methods for targeted substances of toxicological or regulatory concern.
Five principal measurement concepts underpin virtually all organic matter testing in water:
- Oxidation stoichiometry (COD and permanganate index): A strong oxidant, typically potassium dichromate in acidic medium or potassium permanganate in boiling conditions, quantitatively oxidizes organic compounds to carbon dioxide and water. The amount of oxidant consumed, determined by titration or spectrophotometry, is stoichiometrically proportional to the chemically oxidizable organic content.
- Microbial respiration (BOD): Aerobic microorganisms metabolize biodegradable organic matter over a defined incubation period (conventionally 5 days at 20 °C), and the dissolved oxygen consumed reflects the biochemical oxygen demand of the sample.
- Combustion or wet-chemical oxidation to CO2 (TOC): All organic carbon is oxidized, either catalytically at high temperature or by UV-persulfate treatment, and the evolved carbon dioxide is quantified by non-dispersive infrared (NDIR) detection, giving a direct mass-based measure of organic carbon.
- Spectrophotometric indication: Aromatic and unsaturated organic structures absorb ultraviolet light; the absorbance at 254 nm (UV254) serves as a rapid surrogate parameter for humic substances and disinfection by-product precursors.
- Chromatographic separation with selective detection: Volatile and semi-volatile organics, pesticides, phenols, and oils are isolated from the water matrix by headspace equilibration, purge-and-trap, or solid-phase extraction, separated by gas or liquid chromatography, and quantified with detectors such as FID, ECD, or mass spectrometers.
These principles are formalized in international and national standards, including ISO 15705 and ISO 8467 for chemical oxygen demand, ISO 5815-1 for biochemical oxygen demand, ISO 8245 and EN 1484 for total organic carbon, ASTM D1252 and ASTM D2579, and the Chinese national standard series GB 11892, GB 11914, GB 7488, and GB 13193, among others. Accredited third-party laboratories routinely apply these standardized methods to ensure comparability and legal defensibility of results.
Testing Samples
Natural Water
River water, lake water, reservoir water, and groundwater generally contain low-to-moderate levels of naturally occurring organic matter, chiefly humic and fulvic acids derived from plant decay. These matrices are typically analyzed for permanganate index, TOC, UV254, and specific contaminants such as pesticides or petroleum hydrocarbons.
Municipal Wastewater
Raw influent, primary effluent, secondary biological effluent, and final discharges from wastewater treatment plants contain high concentrations of biodegradable organics from domestic sources. COD, BOD5, and anionic surfactants are the key organic parameters for process control and discharge permit verification.
Industrial Wastewater
Effluents from petrochemical, pharmaceutical, pulp and paper, textile dyeing, food and beverage, and chemical manufacturing industries carry complex organic loads, often including toxic or poorly biodegradable compounds. Depending on the sector, testing may target COD, TOC, oil and grease, phenols, VOCs, PAHs, or specific process chemicals.
Drinking Water and Pure Water Systems
Treated drinking water, bottled water, and high-purity process water (such as boiler feed water or electronics-grade water) demand extremely low detection limits. TOC and specific regulated organics such as trihalomethanes are the dominant parameters.
Marine and Estuarine Water
Seawater and brackish water require special analytical considerations due to high chloride content, which interferes with several oxidation-based methods; method selection and matrix correction are therefore critical.
Testing Items
Common organic determinands in water quality testing programs include the following, expressed in their conventional units:
- Chemical Oxygen Demand (CODCr), mg/L — total chemically oxidizable organic load
- Permanganate Index (CODMn), mg/L — oxidizable organics under milder conditions, used for drinking and surface water
- Biochemical Oxygen Demand (BOD5), mg/L — biodegradable organic fraction
- Total Organic Carbon (TOC) and Dissolved Organic Carbon (DOC), mg/L
- UV254 absorbance, cm-1 — surrogate for aromatic organic matter
- Mineral oil and petroleum hydrocarbons (C10–C40), mg/L
- Phenolic compounds (volatile phenol), mg/L
- Volatile organic compounds (benzene, toluene, ethylbenzene, xylenes, chlorinated solvents), µg/L
- Semi-volatile organics (PAHs, phthalates, nitrobenzenes), µg/L
- Pesticide residues (organophosphorus, organochlorine, pyrethroids), µg/L
- Anionic surfactants (MBAS), mg/L
- Specific organics such as formaldehyde, acrylamide, and chlorobenzenes, µg/L
Testing Methods
Chemical Oxygen Demand — Dichromate Reflux Method
In the reference method (ISO 15705, GB 11914, ASTM D1252), the sample is refluxed with a known excess of potassium dichromate in strongly acidic sulfuric acid medium, with silver sulfate as a catalyst for the oxidation of straight-chain organics. The remaining dichromate is back-titrated with standardized ferrous ammonium sulfate using ferroin indicator, and the difference corresponds to the oxygen equivalent of the oxidized organics. Chloride interference is masked with mercury sulfate or corrected mathematically. A widely used variant is the rapid digestion spectrophotometric method, in which the sample is digested in sealed tubes and the green Cr3+ reduction product is measured colorimetrically at about 600 nm.
Permanganate Index
For relatively clean waters (ISO 8467, GB 11892), the sample is boiled with a known quantity of potassium permanganate in acidic medium for a fixed time. Because permanganate is a weaker oxidant than dichromate, it oxidizes only the more readily oxidizable fraction, which is advantageous for monitoring trace organics in source and drinking water. Excess permanganate is quantified iodometrically by titration with sodium thiosulfate.
Biochemical Oxygen Demand — Dilution and Seeding Method
Following ISO 5815-1, GB 7488, and ASTM D888, the sample is diluted with aerated, nutrient-fortified water, seeded with adapted microorganisms when necessary, and incubated in the dark at 20 °C for five days. The dissolved oxygen concentration is measured before and after incubation by the iodometric Winkler method or by electrochemical or optical sensors; the depletion, corrected by blank controls, yields BOD5. Alternative rapid techniques include manometric respirometry, in which oxygen consumption is tracked via pressure change in a closed vessel, and microbial sensor-based rapid BOD analyzers that measure the respiratory activity of immobilized bacteria within minutes.
Total Organic Carbon — Combustion and Wet Oxidation with NDIR Detection
Per ISO 8245, EN 1484, ASTM D2579, and GB 13193, the sample is acidified and sparged to remove inorganic carbon (carbonates and bicarbonates). Organic carbon is then oxidized either catalytically at 680–950 °C or by UV-activated persulfate at lower temperature, converting all organic carbon to carbon dioxide. The CO2 stream is carried by high-purity air or nitrogen into a non-dispersive infrared detector, whose signal is proportional to the carbon mass. TOC can be measured directly (NPOC approach) or by difference (TC minus IC), and online TOC analyzers enable continuous monitoring at water treatment plants.
UV254 Absorbance
Filtered sample is placed in a quartz cuvette and its absorbance at 254 nm is read against ultrapure water. Because aromatic rings and conjugated double bonds—characteristic of humic substances—absorb strongly at this wavelength, UV254 provides a fast, reagent-free indicator of organic content and correlates well with TOC and disinfection by-product formation potential.
Oil and Grease
Depending on the regulatory framework, mineral oil in water is determined by infrared spectrophotometry after extraction with carbon tetrachloride or tetrachloroethylene (GB 16488), by gas chromatography with flame ionization detection after n-hexane extraction and silica cleanup, which quantifies the petroleum hydrocarbon range C10–C40 (ISO 9377-2), or gravimetrically for total oil and grease.
Volatile Organic Compounds — Headspace and Purge-and-Trap GC-MS
VOCs are partitioned from the water into the gas phase either by static headspace equilibration at controlled temperature (ISO 11423) or by purging with inert gas and trapping the analytes on an adsorbent (ISO 15680, EPA-style purge-and-trape protocols standardized in national methods). The concentrated analytes are thermally desorbed, separated on a capillary GC column, and identified and quantified by mass spectrometry in selected-ion mode, achieving detection limits at the sub-microgram-per-liter level.
Semi-Volatile Organics, Phenols, and Pesticides
Semi-volatile substances such as PAHs, phthalate esters, and pesticides are isolated by liquid-liquid extraction or solid-phase extraction, cleaned up through silica or Florisil cartridges, and analyzed by gas chromatography with ECD, FID, or MS detection, or by high-performance liquid chromatography with fluorescence detection for PAHs (ISO 17993). Volatile phenols are determined by distillation followed by the 4-aminoantipyrine colorimetric method (ISO 6439, GB 7490), in which phenol reacts with 4-aminoantipyrine under alkaline oxidative conditions to form a colored antipyrine dye measured at 500–510 nm. Anionic surfactants are quantified as methylene blue active substances through ion-pair extraction and spectrophotometry (ISO 16265).
Applications
Organic matter determination underpins a wide spectrum of environmental, industrial, and public-health activities:
- Wastewater treatment process control: Daily COD and BOD monitoring guides aeration intensity, sludge loading, and dosing decisions in biological treatment plants.
- Regulatory discharge compliance: Permitted effluent limits for COD, BOD5, oil, and phenols are verified through accredited third-party laboratories for legal reporting.
- Drinking water safety assessment: TOC and UV254 control the risk of disinfection by-product formation, while VOC and pesticide screening confirms source water protection.
- Environmental impact assessment and remediation: Baseline surveys and contaminated-site investigations use hydrocarbon and PAH data to delineate pollution plumes and verify cleanup effectiveness.
- Industrial water management: High-purity TOC monitoring protects boilers, cooling circuits, and semiconductor process water from organic fouling and corrosion.
- Online water quality networks: Continuous TOC, COD, and UV254 analyzers provide early-warning signals of accidental organic spills in rivers and distribution systems.
FAQ
What is the essential difference between COD and BOD?
COD measures the total chemically oxidizable organic matter using a strong chemical oxidant within about two hours, capturing both biodegradable and non-biodegradable substances. BOD measures only the fraction that aerobic microorganisms can biologically degrade over a five-day incubation. COD is therefore always greater than or equal to BOD for the same sample.
What does the BOD/COD ratio indicate?
The ratio reflects biodegradability. A BOD/COD ratio above roughly 0.3 to 0.4 suggests that the wastewater is readily treatable by biological processes, whereas a low ratio indicates a high proportion of refractory or toxic organics that may require advanced oxidation, adsorption, or other physicochemical treatment.
Why is TOC increasingly preferred over COD in modern monitoring?
TOC is a direct, mass-based measurement of organic carbon that avoids hazardous reagents such as dichromate and mercury salts, generates minimal hazardous waste, achieves high precision at low concentrations, and is fully automatable for online, real-time monitoring.
How should samples for organic analysis be preserved?
Samples are collected in glass containers, filled without headspace when VOCs are targeted, acidified to pH below 2 with sulfuric acid for COD and TOC (except when analyzing readily volatile components), and stored at approximately 4 °C. Holding times vary by parameter—typically 24 to 48 hours for BOD and VOCs and up to seven days for COD and TOC—so prompt analysis or proper preservation is essential for valid results.
How is chloride interference in COD determination handled?
Chloride ions are oxidized by dichromate and consume the oxidant, causing falsely elevated results. The classical remedy is the addition of mercuric sulfate to complex chloride; alternatively, chloride-free digestion chemistry, correction formulas based on measured chloride concentration, or the standard addition approach can be applied for high-salinity samples such as seawater.