Thank you for visiting our blog. This time we are going to make an article explaining in a simple way what it is, why it is important and the main uses of chemical oxygen demand (COD).
What is chemical oxygen demand (COD)?
Chemical Oxygen Demand (COD) is a critical indicator of water quality, especially with regard to organic matter pollution. This parameter measures the amount of oxygen required to oxidise the substances present in a water sample, which includes both organic and inorganic matter that can be oxidised by chemical means.
The COD is expressed in milligrams of oxygen per litre (mg O2/L) and is essential in wastewater treatment and in the assessment of the environmental impact of effluent discharges.
It is an excellent way to monitor the efficiency of water treatment plants.
Laboratory measurement of COD
Why is chemical oxygen demand important?
Modern societies have a high demand for water to meet a wide range of personal, health and commercial purposes. At the same time, our industrial society produces a wide range of pollutants and environmental challenges, all of which can have serious consequences for health and biodiversity if left unaddressed.
Pollutants can overwhelm natural recovery processes.
In addition to the breakdown products of natural substances (e.g. proteins, fats, carbohydrates), other potentially harmful additives such as pesticides, pharmaceuticals and cleaning products with toxic or hormonal effects accumulate.
They can also consume such large amounts of oxygen that water resources become polluted.
Elevated organic pollution in water discharged to tributaries and streams can have a number of impacts. These include:
- Toxicity of organic compounds: effects on flora and wildlife.
- Dissolved oxygen depletion and eutrophication.
- Impact on fish stocks.
To avoid the wide range of human health hazards and the continued existence of some species, it is essential that the quality of the source water is properly assessed before it is abstracted for drinking or commercial use. COD analysis is a key element of this process.
What can COD be used for?
- Determine concentrations of oxidisable pollutants in wastewater
- Analysing the effectiveness of wastewater treatment solutions
- Determine the effect of waste water disposal on the environment.
- As an index for determining overall water quality.
A higher COD in a sample indicates that it contains higher levels of oxidisable material. If so, the water will have reduced levels of dissolved oxygen. When this occurs, the effects can be detrimental to higher aquatic life forms. The aim of wastewater treatment, therefore, is to reduce the levels of COD in the water.
Monitoring COD levels allows wastewater management companies and facilities to decide on the best methods of water treatment. Without this detailed analysis and information, it can be difficult, if not impossible, to take appropriate action.
How is COD measured?
There are different methods to measure COD. These include continuous analysers and laboratory methods.
Continuous COD analysers
Modern methods of COD analysis involve the use of highly accurate equipment known as continuous analysers.
The Chemitec UV Meter 254 is a spectrophotometric analyser for the determination of absorption at 254 nm. This parameter is a measure related to the loading of many organic substances in aqueous streams and is therefore often used to determine water quality.
In many cases, the absorbance determined by the analyser at 254 nm can be related to sum organic content parameters such as TOC, COD and BOD by applying an appropriate conversion factor.
The analysis process takes less than a minute and in many cases does not require sample pre-treatment thanks to automatic turbidity compensation.
Chemitec UVMeter Continuous Analyser
General principles of the Lambert-Beer Act
The Lambert-Beer law is an empirical relationship that correlates the amount of light absorbed by a medium with the chemical nature (molar extinction coefficient), with the concentration (c) and with the thickness of the medium through which it passes.
When a light beam (monochromatic) of intensity I0 passes through a layer of thickness l of the medium, part of it is absorbed by the medium itself and part of it is transmitted with residual intensity I1.
Lambert-Beer Law
Laboratory COD analysis procedure
In the most common laboratory COD measurement process, a strong chemical oxidant, usually potassium dichromate in an acid medium, is used to oxidise the organic matter. The amount of oxidant consumed is measured and used to calculate the equivalent amount of oxygen that would have been necessary to carry out the oxidation.
This process provides a quick and reliable estimate of the amount of organic matter in water, which is essential for efficient water resource management and environmental protection.
The process of COD analysis is fast compared to traditional methods such as biochemical oxygen demand (BOD). To measure COD in a sample, a strong oxidant must be used under acidic conditions.
Typical oxidants include:
- Potassium dichromate
- Potassium iodate
- Potassium permanganate
The basic requirements of any effective COD method must be:
- Reduce the chloride concentration to a level where chloride interference is negligible - chloride interference occurs when chlorine is present at concentrations above 0.02 M
- Do not significantly alter the organic content of the sample, ensuring that there is no increase or decrease in COD.
- Make the method suitable for routine use - cheap with simplified processes.
COD analysis is carried out in two stages:
- Digestion: oxidation of the organic substances in the sample at 150 °C for 2 hours.
- Assay: measurement of COD by titrimetric or colorimetric method.
First, digestion occurs by creating a reaction that requires acid, heat and a catalyst, and then the chemical oxygen demand is measured by one of these methods:
The titrimetric analysis consists of treating the sample solution with a suitable reagent so that it reacts quantitatively with the sample solution.
The colourimetric analysis involves the use of a colour reagent and the observation of measurable colour changes in the sample solution.
Oxygen demand analysis COD
Along with COD, there are other forms of measurable oxygen demand. The most common is biochemical oxygen demand (BOD).
Chemical oxygen demand is very similar to biochemical oxygen demand in the sense that both are used to calculate the oxygen demand of a water sample.
The key difference between the two is that chemical oxygen demand measures everything that can be oxidised, whereas biochemical oxygen demand only measures the oxygen consumed by organisms.
BOD measures the amount of dissolved oxygen needed by aerobic biological organisms to break down organic matter. It is the traditional test for establishing the concentration of organic matter in wastewater and works on the principle that, if sufficient oxygen is available, the aerobic micro-organisms in the water will continue to decompose them until all organic matter is consumed.
However, COD analysis is an increasingly popular alternative to BOD because it is faster and can analyse wastewater that is too toxic for BOD. BOD analysis takes five days, but modern COD analysis methods allow COD analysis to be used as a real-time analyser, allowing wastewater operators to monitor and adjust parameters during processes. In addition, only organic compounds are consumed during BOD analysis, which results in lower concentrations than with COD analysis.
Advantages and disadvantages of chemical oxygen demand COD
The main advantage of COD is that it is a relatively rapid method of analysis that complies with APHA and ISO standards. Speed is important, but accuracy cannot be compromised. The combination of speed and accuracy of COD analysers is their main advantage in a wide range of contexts.
COD is usually higher than BOD because more organic compounds can be oxidised chemically than biologically. This includes a number of chemicals that are toxic to biological life. This makes COD tests very useful when it comes to analysing industrial wastewater, as these are not detected by BOD tests.
A potential disadvantage of this method is that it cannot differentiate between inorganic and organic carbons and may receive interferences from halides, nitrates and peroxide. Under certain circumstances, results may vary in warmer conditions or at ambient temperature, so it is important to control the temperature when measuring COD.
A rigorous and reliable process
Chemical oxygen demand is a rigorous and reliable method of measuring pollutants in a quantity of water.
The use of modern continuous analysers such as Chemitec's UV Meter can make the task quick and relatively straightforward, producing high levels of accuracy that you can rely on.
In summary, COD is a vital tool for water treatment and environmental management professionals as it provides a clear picture of the level of pollution and the effectiveness of treatments applied. Through its use, we can ensure that our waters remain clean and safe for all living things that depend on them.
At ProDos, we are the leading provider of these solutions. We help companies, technicians and engineers solve a wide range of analytical and process challenges.
