Organic matter analysis is a cornerstone of modern water quality assessment, encompassing aggregate indicators such as COD, BOD, and TOC as well as trace-level pollutants including VOCs, pesticides, PAHs, and PFAS. This technical guide explains the principal laboratory methods, applicable international standards, sample types, and analytical procedures used to quantify organic contaminants in drinking water, surface water, groundwater, and wastewater. It is intended for environmental engineers, laboratory analysts, and quality managers seeking reliable, standards-compliant organic testing solutions.
Technical Overview
Organic matter in water originates from two principal sources: natural organic matter (NOM), such as humic and fulvic acids released from decaying vegetation, and anthropogenic organic pollutants, including solvents, fuels, pesticides, pharmaceuticals, surfactants, and industrial intermediates. Because most organic contaminants are present at microgram or nanogram per liter concentrations yet can exert significant toxicological, ecological, and treatment implications, their accurate quantification requires sensitive instrumentation, validated methods, and rigorous quality control.
Laboratory approaches fall into two complementary categories. Aggregate (sum-parameter) methods, including chemical oxygen demand (COD), biochemical oxygen demand (BOD), total organic carbon (TOC), and adsorbable organic halogens (AOX), measure the collective load or oxygen-consuming capacity of organic matter. Substance-specific methods, typically based on gas chromatography (GC), liquid chromatography (HPLC), and mass spectrometry (MS), identify and quantify individual compounds such as benzene, benzo[a]pyrene, atrazine, or perfluorooctanoic acid. Method selection is governed by regulatory limits defined in standards such as GB 5749 (drinking water), GB 3838 (surface water), GB 8978 (wastewater discharge), the EU Drinking Water Directive, and the U.S. EPA National Primary Drinking Water Regulations, as well as by ISO, ASTM, and EPA reference methods.
Testing Samples
Drinking Water and Treated Water
Treated tap water, bottled water, and water at each stage of the treatment train (raw water, coagulated, filtered, disinfected, distributed) are tested for regulated organic contaminants such as trihalomethanes, haloacetic acids, VOCs, pesticides, and disinfection by-product precursors measured as TOC or UV254.
Surface Water and Groundwater
Rivers, lakes, reservoirs, estuaries, and aquifers are monitored for organic pollution to classify water bodies according to environmental quality standards. Key concerns include oxygen-depleting load (COD, BOD, permanganate index), petroleum hydrocarbons, PAHs, and pesticide runoff, as well as emerging contaminants in groundwater impacted by industrial or agricultural activity.
Municipal and Industrial Wastewater
Effluents from municipal treatment plants, petrochemical facilities, pharmaceutical manufacturers, textile and dyeing operations, pulp and paper mills, and food processing plants are tested before discharge to verify compliance with discharge permits. COD, BOD5, oil and grease, phenols, anionic surfactants, and specific solvents are among the most frequently regulated parameters.
High-Purity and Process Water
Boiler feedwater, cooling water, electronics-grade ultrapure water, and pharmaceutical water (Purified Water, Water for Injection) are characterized by extremely low organic content, requiring high-sensitivity TOC methods with detection limits at the microgram-per-liter level.
Testing Items
Typical organic testing programs cover the following parameter groups:
- Aggregate organic indicators: COD, BOD5, TOC, dissolved organic carbon (DOC), permanganate index (CODMn), UV254 absorbance, AOX
- Volatile organic compounds (VOCs): benzene, toluene, ethylbenzene, xylenes (BTEX), chloroform, carbon tetrachloride, trichloroethylene, tetrachloroethylene, vinyl chloride, 1,2-dichloroethane
- Disinfection by-products: trihalomethanes (THMs), haloacetic acids (HAAs), chlorite, bromate, dichloroacetonitrile
- Semivolatile organic compounds (SVOCs): polycyclic aromatic hydrocarbons (PAHs), phthalate esters, nitrobenzenes, anilines, chlorinated benzenes, organonitrogen compounds
- Pesticides and herbicides: organochlorine, organophosphorus, triazine, and pyrethroid compounds, including atrazine, simazine, gamma-HCH, and DDT residues
- Per- and polyfluoroalkyl substances (PFAS): PFOA, PFOS, PFHxS, and related short-chain homologues
- Oil and grease and total petroleum hydrocarbons (TPH): mineral oil, animal and vegetable oils, C6–C40 hydrocarbon fractions
- Phenolic compounds: volatile phenol index, pentachlorophenol, substituted phenols
- Surfactants: anionic (methylene blue active substances, MBAS), nonionic, and cationic surfactants
- Aldehydes and carbonyl compounds: formaldehyde, acetaldehyde, glyoxal (common in ozonated water)
- Emerging contaminants: pharmaceuticals and personal care products (PPCPs), endocrine-disrupting compounds, microcystins (cyanobacterial toxins), nitrosamines such as NDMA
Testing Methods
Chemical Oxygen Demand (COD)
COD measures the mass of oxygen equivalent consumed by oxidizable organic matter under acidic dichromate reflux conditions. The classical titrimetric procedure (ISO 6060, GB 11914, HJ 828) digests the sample with potassium dichromate and sulfuric acid–silver sulfate catalyst for two hours, followed by titration with ferrous ammonium sulfate. Closed-tube spectrophotometric variants (ISO 15705, EPA 410.4, ASTM D1252) shorten digestion time and enable high-throughput analysis. Chloride interference is suppressed with mercury sulfate or managed by using alternative methods such as the chlorine-compensated titration. For low-chloride surface water, the permanganate index (CODMn, GB 11892, ISO 8467) is widely applied, particularly in drinking water surveillance.
Biochemical Oxygen Demand (BOD5)
BOD5 quantifies the dissolved oxygen consumed by aerobic microorganisms during five days of incubation at 20 °C. The dilution-and-seed method (ISO 5815-1, EPA 5210B, HJ 505) requires careful seeding with adapted microbial inocula, blank correction, and validation that residual dissolved oxygen remains above 1 mg/L with at least 2 mg/L depleted. Nitrification inhibition with 2-chloro-6-(trichloromethyl)pyridine (TCMP) may be used to determine carbonaceous BOD (CBOD). Manometric respirometry (EPA 405.1) offers an automated alternative for routine monitoring.
Total Organic Carbon (TOC)
TOC directly measures the carbon bound in organic compounds. The combustion–catalytic oxidation method with non-dispersive infrared detection (NDIR), standardized in ISO 8245, GB 13193, HJ 501, EPA 415.1, and EPA 9060A, oxidizes organic carbon to CO2 at 680–950 °C over a platinum or alumina catalyst and reports TOC as the difference between total carbon and inorganic carbon, or via acidification–sparging of the sample before oxidation (NPOC approach). Wet-chemical persulfate–UV oxidation instruments provide adequate sensitivity for purified water systems, achieving detection limits down to a few micrograms per liter, and support online monitoring at pharmaceutical and ultrapure water facilities.
Volatile Organic Compounds by GC-MS
VOCs are determined by purge-and-trap or headspace extraction coupled with gas chromatography–mass spectrometry. Purge-and-trap GC-MS (EPA 524.2/524.3, EPA 8260, ISO 15680, HJ 620) achieves sub-microgram-per-liter detection limits and is the reference technique for regulated solvents in drinking water. Static headspace GC-MS (ISO 17943, HJ 639) simplifies automation for higher-concentration wastewater and groundwater samples. Internal standard calibration with compounds such as fluorobenzene and 1,2-dichlorobenzene-d4, surrogate recovery checks, and confirmation by qualified ion ratios are mandatory quality control elements.
Semivolatile Organic Compounds, PAHs, and Phthalates
SVOCs are extracted from water by liquid–liquid extraction or solid-phase extraction (SPE), concentrated, and analyzed by GC-MS (EPA 525.2, EPA 8270) or HPLC with fluorescence detection for PAHs (EPA 550.1, ISO 17993, HJ 478). SPE cartridges containing C18 or polymeric sorbents allow liter-scale samples to be concentrated, enabling nanogram-per-liter detection of benzo[a]pyrene, di(2-ethylhexyl) phthalate, and nitroaromatic compounds. Deuterated surrogate standards are added before extraction to monitor recovery, with typical acceptance criteria of 70–130 percent.
Pesticides and Herbicides
Organochlorine and pyrethroid pesticides are determined by GC with electron capture detection (ECD) or GC-MS/MS after hexane or SPE extraction (EPA 508, EPA 8081, ISO 6468), while polar organophosphorus and triazine compounds are analyzed by HPLC-MS/MS or GC with nitrogen–phosphorus detection. Modern triple-quadrupole LC-MS/MS platforms permit multi-residue screening of dozens of pesticides in a single injection with detection limits below 0.01 microgram per liter, satisfying the stringent 0.1 microgram-per-liter single-substance limits applied in many jurisdictions.
PFAS Analysis
Per- and polyfluoroalkyl substances are extracted by weak-anion-exchange SPE and quantified by LC-MS/MS operating in negative electrospray mode (EPA 537.1, EPA 533, ISO 21675). Strict cross-contamination controls are essential: polytetrafluoroethylene-containing labware and tubing must be eliminated, isotope-dilution calibration (for example, 13C4-PFOA) is applied, and method blanks are monitored at sub-nanogram-per-liter levels. Reporting limits of 1–2 ng/L or lower are routinely achievable, meeting drinking water advisory values in the low single-digit ng/L range.
Oil and Grease and Total Petroleum Hydrocarbons
Oil and grease is determined gravimetrically after n-hexane extraction and distillation (EPA 1664, HJ 637, ISO 9377-2), which reports hexane extractable material (HEM) and silica-gel-treated mineral oil fractions. Infrared spectrophotometry measuring C–H stretch absorbance at 2930, 2960, and 3030 cm-1 offers higher sensitivity for mineral oil in surface water, while GC-FID fractionation provides hydrocarbon banding from C6 to C40 for site characterization.
Phenols, Surfactants, and AOX
Volatile phenol index is measured by distillation followed by the 4-aminoantipyrine spectrophotometric method (ISO 6439, HJ 503), with detection limits near 0.1 microgram per liter after chloroform extraction. Anionic surfactants are quantified as methylene blue active substances (MBAS) by spectrophotometry (ISO 7875-1, ISO 16265, GB 7494). Adsorbable organic halogens, an indicator of chlorinated organics including bleach-plant effluents, are determined by activated carbon adsorption, combustion, and microcoulometric chloride detection (ISO 9562).
Sample Preservation and Quality Assurance
Correct sampling and preservation are decisive for data validity. Samples for COD are acidified to pH below 2 with sulfuric acid and stored at 4 °C for up to 28 days; BOD5 samples are refrigerated unacidified and analyzed within 24 hours; VOC vials are filled without headspace, acidified, and held at 4 °C with a 14-day limit; TOC samples are acidified and held up to 28 days; PFAS samples are collected in HDPE bottles with 28-day holding time. Quality assurance in accredited third-party laboratories includes method blanks, laboratory control samples, matrix spikes, duplicate analyses, calibration verification, and control charting, consistent with ISO/IEC 17025 requirements.
Applications
Organic matter testing supports a wide range of regulatory, industrial, and scientific objectives:
- Drinking water compliance monitoring against GB 5749, EU, and WHO guideline values for disinfection by-products, solvents, and pesticides
- Wastewater discharge verification and pollutant release inventories under discharge permit programs
- Surface water quality classification and total maximum daily load development for river basins
- Groundwater contamination assessment and remediation performance monitoring at industrial and fuel-release sites
- Treatment process optimization, including coagulant dosing control via TOC and UV254, and ozone/BAC performance evaluation
- Pharmaceutical and electronics water system validation with online and offline TOC
- Bottled water, beverage, and food safety certification programs
- Environmental impact assessments, research studies, and forensic source tracking of organic pollution
FAQ
What is the difference between COD and BOD?
COD measures the chemically oxidizable fraction of organic matter using dichromate and is complete within about two hours, capturing both biodegradable and non-biodegradable compounds. BOD5 measures only the fraction biodegraded by microorganisms over five days and better reflects the ecological oxygen demand exerted on receiving waters. The BOD/COD ratio is commonly used to assess wastewater biodegradability, with ratios above 0.3 generally indicating good biological treatability.
When should TOC be used instead of COD?
TOC directly measures organic carbon without oxidation-interference from chloride or reduced inorganic species, offers faster analysis, and achieves far lower detection limits, making it the preferred parameter for purified water, drinking water, and TOC removal requirements. COD remains the regulatory basis for most wastewater discharge permits, so the choice depends on the applicable standard and matrix.
What detection limits can be achieved for VOCs and PFAS?
Purge-and-trap GC-MS routinely achieves VOC reporting limits of 0.1–0.5 microgram per liter in drinking water. LC-MS/MS methods for PFAS achieve reporting limits of 1–2 nanograms per liter or lower, sufficient for current health advisory levels.
How should samples for organic analysis be collected and preserved?
Use amber glass VOA vials with PTFE-lined caps for VOCs, fill without headspace, acidify to pH below 2, and hold at 4 °C for a maximum of 14 days. Collect SVOC and pesticide samples in amber glass bottles and preserve per method requirements. PFAS samples require HDPE containers and avoidance of fluoropolymer-containing materials. Chain-of-custody documentation should accompany every sample.
Can one method measure all organic pollutants?
No. Aggregate parameters such as COD, BOD, and TOC quantify bulk organic load but cannot identify individual substances, while compound-specific chromatographic methods target defined analyte lists. A comprehensive assessment therefore combines sum parameters for screening with GC-MS, LC-MS/MS, and spectrophotometric methods for regulated and site-specific compounds, selected according to the applicable standard and the concentration range of interest.