Surface Water Chemical Oxygen Demand (COD) Testing: Methods, Standards, and Applications

Surface water chemical oxygen demand (COD) testing quantifies the amount of oxygen required to chemically oxidize organic and oxidizable inorganic matter in rivers, lakes, and reservoirs, making it one of the most important indicators of organic pollution. This technical guide explains COD testing principles, sampling procedures, recognized standard methods, and the regulatory limits used to classify surface water quality.

Technical Overview

Chemical Oxygen Demand (COD) is a cumulative water quality parameter that measures the mass of oxygen equivalent to the amount of oxidizable matter, both organic and inorganic, dissolved or suspended in a water sample. In surface water monitoring, COD serves as a primary indicator of the degree of organic pollution entering rivers, lakes, reservoirs, and other natural water bodies from municipal discharge, industrial effluent, agricultural runoff, and natural decomposition processes.

Surface water bodies with elevated COD levels experience depleted dissolved oxygen as microorganisms consume oxygen to degrade organic material. This oxygen depletion stresses or kills aquatic organisms, accelerates eutrophication, and degrades the ecological, recreational, and potable value of the water resource. For this reason, COD (and its related parameter, the permanganate index, COD_Mn) is a mandatory monitoring item under surface water quality standards such as GB 3838-2002, Environmental Quality Standards for Surface Water, and corresponding international norms including ISO 6060 and ASTM D1252.

Accredited third-party laboratories routinely perform surface water COD testing to support government monitoring programs, pollution source apportionment, discharge permit compliance, environmental impact assessments, and academic research. Reliable COD data depend on representative sampling, correct preservation, appropriate method selection, and rigorous quality control.

Testing Samples

River Water

Samples collected from rivers and streams represent flowing surface water influenced by upstream discharges, tributary inputs, and watershed land use. Sampling points are typically established upstream and downstream of suspected pollution sources, at hydrological monitoring sections, and at cross-border or transboundary control sections specified by regulatory authorities.

Lake and Reservoir Water

Still or slow-moving water bodies are sampled at multiple depths and horizontal positions to capture spatial variability in organic load. Lakes and reservoirs used as drinking water sources are subject to more frequent COD monitoring because organic matter affects treatment requirements and disinfection by-product formation.

Estuarine and Tidal Water

In estuarine zones where freshwater mixes with seawater, chloride interference becomes a critical consideration for COD analysis. Samples with high salinity require specialized digestion conditions or chloride masking techniques to prevent silver catalyst precipitation and chlorine oxidation artifacts.

Canal, Pond, and Wetland Water

Irrigation canals, landscape ponds, constructed wetlands, and urban waterways are monitored to verify ecological restoration outcomes, stormwater management performance, and compliance with local water function zone targets.

Testing Items

The core testing portfolio for surface water oxygen demand assessment typically includes the following parameters:

• Chemical Oxygen Demand (COD_Cr) — determined by dichromate oxidation under acidic, catalyzed conditions; applicable to polluted or moderately polluted surface water.
• Permanganate Index (COD_Mn) — determined by alkaline or acidic potassium permanganate oxidation; the preferred regulatory parameter for relatively clean surface water such as Class I–III water bodies.
• Biochemical Oxygen Demand (BOD₅) — often measured alongside COD to evaluate the biodegradable fraction of organic matter and calculate the BOD/COD ratio.
• Total Organic Carbon (TOC) — used as a supplementary or alternative organic matter indicator with faster analysis and no toxic reagents.
• Dissolved Oxygen (DO) — measured in the field to interpret the ecological significance of COD concentrations.

Depending on the project scope, COD results may be interpreted together with ammonia nitrogen, total phosphorus, total nitrogen, and suspended solids to provide a complete picture of surface water quality status.

Testing Methods

Dichromate Reflux Method (GB 11914-89 / HJ 828-2017 / ISO 6060)

The classic dichromate method digests the sample with a known excess of potassium dichromate in strongly acidic medium, with silver sulfate as a catalyst and mercuric sulfate added to complex chloride interference. Remaining dichromate is titrated with standardized ferrous ammonium sulfate using ferroin indicator. The method offers a broad measurement range (approximately 10–700 mg/L) and high oxidation efficiency for most organic compounds, though it uses hazardous reagents and requires a two-hour reflux period. HJ 828-2017 is the current Chinese national standard for COD determination by dichromate titration and includes provisions for low-level and high-chloride samples.

Rapid Digestion Spectrophotometric Method (HJ/T 399-2007)

This streamlined technique digests the sample in a sealed tube at 165 °C for approximately 20 minutes using dichromate in a concentrated sulfuric and phosphoric acid medium. Hexavalent chromium consumption, or the formation of trivalent chromium, is measured spectrophotometrically at 600 nm or 440 nm respectively. The method is faster, generates less waste, and is well suited to high-throughput screening of surface water samples, with a typical working range of 10–1,000 mg/L when validated against the titration method.

Potassium Permanganate Method — Permanganate Index (GB 11892-89 / ISO 8467)

For relatively unpolluted surface water, the permanganate index is the standard regulatory measure. The sample is boiled with a known excess of potassium permanganate under acidic (or alkaline, for high-chloride water) conditions, and the remaining oxidant is back-titrated with sodium oxalate. Because permanganate oxidizes organic matter less completely than dichromate, the permanganate index yields lower values and is specifically referenced in the Class I–III water quality criteria of GB 3838-2002.

ASTM D1252 Test Methods

ASTM D1252 (Types I, II, III, and Type II-B low-level variants) describes dichromate-based COD procedures with titrimetric or colorimetric finish, widely used in international projects where ASTM methods are contractually specified.

Sample Collection and Preservation

Representative grab or composite samples are collected in clean glass containers, avoiding headspace and floating debris. For COD analysis, samples are acidified with concentrated sulfuric acid to pH below 2 immediately after collection, cooled to approximately 4 °C, and analyzed promptly — generally within the holding time specified by the applicable method (commonly up to 14 days for properly acidified dichromate COD samples, while permanganate index samples should be analyzed as soon as practicable, typically within 48 hours). Field blanks, duplicate samples, and matrix spikes are incorporated into the quality assurance plan.

Quality Control and Interferences

Key interferences include high chloride concentrations, which consume oxidant and inflate results, and volatile organic compounds, which may be lost during open digestion. Laboratories control these effects through mercuric sulfate masking, sample dilution, sealed digestion, and the analysis of certified reference materials. Method detection limits, calibration verification, blank control, and spike recovery criteria are documented to ensure data defensibility for regulatory submission.

Applications

Surface water COD testing supports a wide range of environmental management and regulatory objectives:

• Compliance monitoring against surface water quality standards such as GB 3838-2002, which assigns COD and permanganate index limits to water function classes (for example, COD_Mn of 2 mg/L for Class I up to 15 mg/L for Class V, and COD_Cr of 15 mg/L for Class I–II up to 40 mg/L for Class V).
• National and regional water quality monitoring networks, pollution census programs, and watershed pollution load calculations.
• Environmental impact assessment and post-construction verification for infrastructure, industrial parks, and municipal wastewater projects.
• Discharge permit supervision, including mixing zone studies downstream of outfalls and total pollutant load allocation.
• Ecological restoration and black-odor waterbody remediation performance evaluation, where COD trend data demonstrate treatment effectiveness.
• Drinking water source protection, helping operators anticipate organic load changes that affect coagulant dosing and disinfection by-product risk.
• Academic research, hydrological modeling, and environmental big-data platforms.

FAQ

What is a good COD level for surface water?

Acceptable levels depend on the designated use of the water body. Under GB 3838-2002, Class I and Class II source water (suitable for drinking after conventional treatment) requires a permanganate index of no more than 2–4 mg/L, while Class V water, applicable to general landscape use, permits up to 15 mg/L. Local or international water quality objectives should always be consulted for project-specific criteria.

Why are both COD and the permanganate index measured in surface water?

The dichromate COD method oxidizes nearly all organic matter and is best suited to polluted water, whereas the permanganate index oxidizes only the more reactive fraction and is better suited to clean surface water where dichromate values approach detection limits. Regulatory frameworks specify which parameter applies to each water quality class, so many monitoring programs report both.

How do chlorides affect surface water COD testing?

Chloride ions are oxidized by dichromate and permanganate, causing positively biased results. In estuarine or saline samples, laboratories apply mercuric sulfate complexation, use the alkaline permanganate procedure, dilute the sample, or select a chlorine-insensitive method to obtain accurate results.

How quickly must surface water samples be analyzed?

Samples for COD should be acidified to pH below 2 with sulfuric acid at the time of collection and kept cold and dark. Properly preserved dichromate COD samples are generally stable for up to 14 days, while permanganate index samples should be analyzed as soon as possible, preferably within 48 hours, in accordance with the governing method.

Can COD results be compared directly with BOD results?

No. COD reflects the total chemically oxidizable load, while BOD reflects only the biodegradable portion degraded over five days. The BOD/COD ratio is itself a useful diagnostic: values above roughly 0.3–0.5 indicate readily biodegradable organic matter, whereas low ratios suggest industrial organics or toxic conditions inhibiting biological oxidation.

Which laboratories can perform accredited surface water COD testing?

Testing should be entrusted to accredited third-party laboratories holding relevant accreditation (for example, CMA or ISO/IEC 17025) with validated methods, documented quality control, and experience in surface water sampling protocols. Method selection should match the expected concentration range and the chloride content of the water body.