Chemical Oxygen Demand (COD) Analysis Tutorial

Chemical Oxygen Demand (COD) measures the oxygen required to oxidize all organic and inorganic matter in water, expressed as mg/L O2. The EPA accepts the dichromate reflux method as the standard procedure, using potassium dichromate as the oxidizing agent in acidic medium with a catalyst. COD measurement indicates treatment efficacy by comparing influent and effluent values, expressed as percentage removal. Applications span municipal and industrial wastewater treatment, power plants, chemical manufacturing, commercial laundry, pulp mills, agriculture, environmental studies, and education. Color does not interfere with COD measurements because chromophoric groups are fully oxidized. Modern automated systems process up to 25 samples simultaneously with safety features preventing accidents.

This section covers BOD and COD parameters and treatment efficiency calculations. BOD (Biochemical Oxygen Demand) measures oxygen consumed by microorganisms decomposing organic matter over 5 days at 20°C. The measurement procedure involves: (1) Collecting a 300 mL sample, (2) Measuring initial dissolved oxygen, (3) Incubating at 20°C for 5 days, (4) Measuring final dissolved oxygen, (5) Calculating BOD as the difference. The BOD curve shows oxygen consumption over time, with the initial steep portion representing carbonaceous BOD from biodegradable organic matter. COD measures all oxidizable organic matter through chemical oxidation, typically completed in 2 hours. COD values are higher than BOD because they include non-biodegradable organic compounds. The BOD/COD ratio indicates biodegradability, with higher ratios indicating easier biological treatment. Treatment efficiency is calculated as: (1) Total solids removal = (influent TSS - effluent TSS) / influent TSS × 100%, (2) BOD removal = (influent BOD - effluent BOD) / influent BOD × 100%, (3) COD removal = (influent COD - effluent COD) / influent COD × 100%. Primary treatment achieves 60% TSS removal and 30-40% BOD removal. Secondary treatment achieves 85-95% BOD removal.

BOD (Biochemical Oxygen Demand) measures the oxygen consumed by bacteria to biologically decompose organic matter in water under aerobic conditions, while COD (Chemical Oxygen Demand) measures the oxygen required to chemically oxidize all organic and inorganic matter using strong oxidizers like potassium dichromate; COD values are typically higher than BOD because they account for both biodegradable and non-biodegradable organics, whereas BOD only measures biodegradable organics; the BOD/COD ratio for untreated domestic wastewater ranges from 0.5 to 0.7, decreasing to 0.1 to 0.2 after biological treatment, making COD measurement more practical for monitoring wastewater treatment plant performance due to its faster chemical analysis process.

Wastewater treatment includes oxidation ponds (aerobic process with algae-bacteria cycle) and activated sludge (aerobic with air supply). BOD measures oxygen required for organic matter decomposition, while COD measures both organic and inorganic oxidizable matter. COD is always greater than BOD. Treatment selection depends on available technology and effluent requirements.

The COD/BOD ratio (Chemical Oxygen Demand divided by Biochemical Oxygen Demand) is a critical parameter in effluent treatment plants that indicates the biodegradability of organic matter in wastewater; a low ratio (e.g., 2) suggests wastewater is well-suited for biological treatment methods like activated sludge, while a high ratio (e.g., 4) indicates significant non-biodegradable organic matter requiring advanced treatment such as chemical oxidation or filtration before or alongside biological processes, enabling engineers to design efficient treatment systems, optimize operational parameters, and ensure environmental compliance.

Mercury sulfate (HgSO4) is added first in COD testing at 0.25 grams. It prevents chloride ion interference in wastewater samples. Chloride ions in wastewater can oxidize and produce incorrect COD values. Mercury sulfate reacts with chloride ions to form stable AgCl2, which does not participate in further reactions.

Chloride ions can interfere with COD testing by reacting with dichromate. To eliminate this interference, mercuric sulfate is added to the sample. This prevents chloride from participating in the oxidation reaction and ensures accurate COD measurement.

COD determination employs redox chemistry where potassium dichromate acts as the oxidizing agent and ferrous ammonium sulfate as the reducing agent. Silver sulfate catalyzes oxidation of organic compounds including alcohols and hydrocarbons. Chloride interference is prevented by mercuric sulfate forming mercuric chloride complexes. The phenanthroline indicator exhibits blue-green color in oxidizing environments and red color in reducing environments, enabling visual endpoint detection. The reaction produces chromic sulfate with evolution of oxygen, which is quantified through back titration with standard FAS solution.

Three main types of interferences affect COD analysis: (1) Chloride interference - removed by adding mercury sulfate (0.4g per 2000mg/L chloride); (2) Nitrite/nitrate interference - removed by adding sulfuric acid (120mg per 6mg/L nitrate); (3) Reducing organic species (sulfur compounds) - removed by oxidation during digestion. These interferences can cause higher results if not properly addressed.

Chloride ions interfere with COD measurement by reacting with dichromate to produce toxic chlorine gas and reducing dichromate to Cr³⁺. Additionally, chloride reacts with silver sulfate catalyst to form insoluble silver chloride precipitate. Mercury sulfate is added to complex chloride ions, forming soluble mercury-chloride complexes that do not interfere with subsequent measurements. This complexation prevents both the formation of toxic chlorine gas and the precipitation of silver chloride, ensuring accurate COD determination in chloride-containing samples.

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