Chemical Oxygen Demand (COD): Measuring Oxidisable Load
Use this guide alongside the BOD explainer, the ISO 80000-9 reference, and the water efficiency calculator to harmonise analytical data, process modelling, and investment planning.
Introduction
Chemical oxygen demand (COD) quantifies the equivalent oxygen required to chemically oxidise organic and oxidisable inorganic substances in water. Unlike BOD, COD does not rely on microbial activity; strong oxidants convert the sample’s organics to carbon dioxide and water within hours. Laboratories and treatment operators favour COD for its speed, reproducibility, and correlation with organic loading in many waste streams.
COD results, typically expressed in milligrams of oxygen per litre (mg·L⁻¹ O₂), underpin discharge permits, industrial pretreatment design, and pollution prevention programmes. This article explains dichromate-based methods, historical milestones, quality assurance practices, and the interplay between COD, BOD, and total organic carbon (TOC) in contemporary monitoring. It also surveys emerging technologies such as online spectrophotometers and microwave-assisted digestion that streamline COD reporting.
Definition and Analytical Methods
Standard COD analysis uses potassium dichromate (K₂Cr₂O₇) in concentrated sulfuric acid, with silver sulfate catalysis and mercuric sulfate to suppress chloride interference. During a two-hour reflux at 150 °C, dichromate oxidises organic matter while itself reducing from Cr₂O₇²⁻ to Cr³⁺. The equivalent oxygen demand is computed from the amount of dichromate consumed, determined via titration with ferrous ammonium sulfate or spectrophotometric absorption of Cr³⁺.
Closed Reflux Colorimetric Method
Small-volume sealed digestion vials enhance safety and throughput by minimising acid handling. After digestion, COD concentration is measured photometrically at 600 nm or 420 nm depending on the kit. Calibration curves correlate absorbance with mg·L⁻¹ O₂, enabling rapid batch processing for routine compliance monitoring.
Open Reflux Titrimetric Method
For high-strength samples (> 900 mg·L⁻¹), open reflux digestion with subsequent titration maintains accuracy and allows greater sample volumes. Ferroin indicator marks the end point as the solution shifts from blue-green to reddish-brown. Analysts compute COD using COD = (A − B) × M × 8000 / V, where A is the titrant volume for the blank, B for the sample, M the molarity of ferrous ammonium sulfate, and V the sample volume in millilitres.
Historical Context
COD testing gained prominence in the mid-twentieth century as environmental regulations accelerated industrial wastewater control. Early approaches used permanganate oxidants, but dichromate offered superior oxidation power and reproducibility, prompting its adoption in Standard Methods in 1955. Automated digestion blocks, introduced in the 1960s, expanded COD monitoring capacity for municipal laboratories handling rapidly urbanising service areas.
Recent decades emphasised safety and waste minimisation; mercury-free reagents and low-volume sealed vials reduce hazardous waste generation. Regulators worldwide continue to harmonise COD protocols through ISO 6060 and regional adaptations, preserving comparability across jurisdictions even as instrumentation evolves.
Conceptual Relationships with BOD and TOC
COD typically exceeds BOD₅ because chemical oxidants attack compounds recalcitrant to five-day biological degradation. The COD/BOD ratio indicates biodegradability: values near 1 suggest readily treatable wastes, while ratios above 3 imply refractory organics requiring advanced oxidation, adsorption, or incineration. Correlating COD with TOC, measured in mg·L⁻¹ C, supports source identification and real-time control when COD is unavailable.
Engineers develop site-specific regression models linking COD, BOD₅, TOC, and dissolved oxygen to accelerate decision-making. Online UV–Vis sensors calibrated against laboratory COD can provide continuous data for supervisory control, alarm management, and load equalisation. Integrating COD with BOD₅ and nutrient measurements ensures comprehensive oxygen balance evaluations in receiving waters.
Applications Across Sectors
Municipal wastewater plants use COD to monitor influent variability, detect toxic shocks, and verify primary clarifier performance. Industrial facilities—petrochemical, pharmaceutical, food processing—apply COD tracking to optimise pretreatment, size equalisation tanks, and document compliance for trade effluent agreements. Environmental agencies assess COD in rivers, lakes, and estuaries to gauge pollution loads and prioritise remediation.
Manufacturing sites integrate COD data into digital twins that combine hydraulic residence time models with aeration demand forecasting. Sustainability teams convert COD reductions into avoided greenhouse gas emissions when aeration energy or chemical usage declines. Finance and ESG reporting frameworks increasingly require transparent disclosure of COD trends to demonstrate responsible water stewardship.
Importance and Future Directions
COD remains a critical control parameter because it captures total oxidisable load, including compounds not amenable to biological treatment. As industries adopt novel solvents, additives, and process chemistries, rapid COD screening identifies potential treatment challenges before discharge. Online COD analysers paired with supervisory control systems improve resilience during climate-driven storm surges or supply chain disruptions.
Future developments focus on reagentless optical methods, machine-learning calibration, and integration with thermodynamic property models that predict oxidation kinetics. Embedding COD trends into corporate sustainability dashboards supports transparent stakeholder communication and encourages investment in circular water strategies. Maintaining rigorous QA/QC—method blanks, duplicates, standard checks—ensures COD data continues to underpin reliable environmental decision-making.