Chemical Oxygen Demand
Calculate the chemical oxygen demand (COD) of a water sample using the standard dichromate method with FAS titration. Free online COD calculator for environmental water quality testing, wastewater treatment, and lab analysis.
About This Calculator
The Chemical Oxygen Demand (COD) Calculator helps environmental scientists, water quality technicians, students, and wastewater treatment professionals determine the oxygen demand of a water sample using the standard closed-reflux dichromate titration method with ferrous ammonium sulfate (FAS).
COD is a critical water quality parameter that measures the amount of oxygen required to chemically oxidize organic and inorganic pollutants in water using a strong oxidizing agent (potassium dichromate) under acidic conditions and high temperature. The result is expressed in milligrams per liter (mg/L) of oxygen consumed.
The calculation uses the formula: COD = ((A - B) × N × 8000) / Sample Volume, where A is the volume of FAS consumed by the blank titration, B is the volume consumed by the sample titration, N is the normality of FAS, and the sample volume is the original water sample volume in milliliters. The factor 8000 converts the result to mg/L, accounting for the equivalent weight of oxygen and unit conversions.
Regional Notes
India: The Central Pollution Control Board (CPCB) prescribes COD limits of 250 mg/L for inland surface water discharge and 100 mg/L for sewers under the Environmental Protection Rules. Industrial effluents must comply with these standards for environmental clearance.
United States: The EPA regulates COD under the Clean Water Act through National Pollutant Discharge Elimination System (NPDES) permits. Typical effluent limits range from 30-100 mg/L depending on the industry and receiving water body. Method 5220D (closed reflux, colorimetric) is the standard EPA-approved method.
United Kingdom: The Environment Agency sets COD consent conditions under the Water Resources Act and Environmental Permitting Regulations. Trade effluent discharges typically require COD below 125 mg/L for sewer discharge and stricter limits for direct watercourse discharge.
Frequently Asked Questions
What is chemical oxygen demand (COD)?
Chemical oxygen demand (COD) measures the capacity of water to consume oxygen during the decomposition of organic matter. It indicates how much oxygen is needed to oxidize the organic pollutants present in a water sample, expressed in milligrams per liter (mg/L).
How is chemical oxygen demand calculated?
COD is calculated using the formula: COD = ((A - B) × N × 8000) / Sample Volume, where A is the volume of FAS consumed by the blank run, B is the volume of FAS consumed by the sample, N is the normality of FAS, and sample volume is the volume of the original water sample in milliliters.
What is a good COD level for drinking water?
The preferred chemical oxygen demand (COD) for drinking water is 10-20 mg/L. Low COD levels indicate water with fewer organic pollutants and better suitability for consumption. However, COD alone does not guarantee water safety.
What is the difference between BOD and COD?
Biochemical oxygen demand (BOD) measures the oxygen that microorganisms require to break down organic materials biologically. COD measures the oxygen required to break down organic materials by chemical oxidation using a strong oxidant like potassium dichromate. COD tests are faster (hours vs 5 days for BOD) and measure a broader range of organic compounds.
What does high COD indicate in water?
High COD in water indicates the presence of significant organic pollutants such as decaying vegetation, human waste, or industrial effluents. It suggests poor water quality and potential environmental contamination that may harm aquatic life.
Why is COD testing important for wastewater treatment?
COD testing is crucial for wastewater treatment as it helps monitor treatment efficiency by measuring organic pollutant levels before and after treatment. It ensures compliance with environmental regulations and helps protect aquatic ecosystems from untreated effluent discharge.
What are the units of chemical oxygen demand?
Chemical oxygen demand is expressed in milligrams per liter (mg/L), which indicates how many milligrams of oxygen are consumed per liter of water sample. It may also be reported as parts per million (ppm), which is numerically equivalent to mg/L for dilute aqueous solutions.
Can COD give a negative result?
A negative COD result indicates an error in the testing procedure. Possible causes include instrument miscalibration, contaminated reagents, incorrect sample dilution, or calculation errors. The test should be rerun with fresh reagents and proper technique.