What is chemical oxygen demand (COD)?

Chemical oxygen demand, or COD, is the measure of the capacity of water to consume oxygen during the decomposition of organic matter in the water.  In other words, it’s the amount of oxygen that’s needed to oxidise the organic matter present in a quantity of water.

COD analysis is used as an indirect measure of pollutants (organics) in a water sample. It is an important parameter in water quality analysis, helping to reduce risk to humans and the environment.

COD is an excellent way of monitoring the efficiency of water treatment plants. If water is left untreated, or partially treated, discharged water contains effluent organics that can compete with downstream organisms for oxygen.  This oxygen demand can kill or inhibit life downstream of the discharge area.  It should be clear, therefore, that anything that can help gather accurate information about water quality, such as COD, has a key role to play in reducing the likelihood of pollutants causing any environmental damage.

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 produce serious health and biodiversity outcomes if left untreated.  Pollutants can overwhelm natural processes of recovery.

In addition to products arising from the decomposition of natural substances (e.g. proteins, greases, carbohydrates) there is a build-up of other, potentially harmful, additives such as pesticides, effluents and garbage, which contaminate drinking water supplies with their toxic or hormonal effects.

They may also consume such large quantities of oxygen that water resources become fouled.

High organic contamination in water discharged to tributaries and streams can have a range of impacts. These include:

  • Toxicity of organic compounds: health effects on plants and wildlife
  • Decreasing dissolved oxygen and eutrophication
  • Impact on fish populations

To prevent the wide range of health dangers to humans, and the continued existence of some species, it’s essential that water source quality is properly assessed before it’s drawn off for consumption or commercial use.  COD testing is a key part of this process.

COD can be used to:

  • Determine concentrations of oxidisable pollutants in wastewater
  • Analyse the effectiveness of wastewater treatment solutions
  • Determine the effect of wastewater disposal on the environment.
  • As an index for determining overall water quality.

COD measures the amount of oxygen necessary to break down the organic substances that are pollutants in water.  A higher COD in a sample indicates that it contains higher levels of oxidisable material. If this is the case, then the water will have reduced dissolved oxygen levels.  Where this happens, the effects can be environmentally damaging to higher aquatic lifeforms.  The aim of wastewater treatment, therefore, is to reduce levels of COD in water.

Monitoring COD levels enables wastewater management companies and facilities to decide on the best methods for water treatment. Without this detailed analysis and information, it can be challenging if not completely impossible to take the correct action.

How is COD measured?

There are different methods that can be used when it comes to measuring COD. These include online testing and offline laboratory methods using  environmental analysers.

The principle behind the COD testing method is that, under acidic conditions, a strong oxidising agent will oxidise almost any organic compound to carbon dioxide. COD analysis will measure the equivalent amount of oxygen that is required to chemically oxidise organic compounds in water.

Modern methods of COD testing involve the use of highly accurate scientific instruments known as environmental analysers..  A leading example of this technology is the EasyPREP COD-200. This mixes, heats, cools and then analyses COD samples. The device automates the COD analysis process, making it more streamlined. The COD-200 also offers turbidity detection for added accuracy. In COD analysis, the presence of turbidity can cause deviations of up to 30% from the real sample value due to interference with a sample’s measured absorbance. This capability allows the end user to confidently analyse samples knowing that the turbidity level is adequate for COD analysis.

What is the COD Analysis Process?

The COD analysis process is rapid when compared with traditional methods such as biochemical oxygen demand (BOD). To measure COD in a sample you must use a strong oxidant under acidic conditions.

Typical oxidants include:

  • Potassium dichromate
  • Potassium iodate
  • Potassium permanganate
  • Ceric sulphate

The basic requirements of any effective COD method should be to:

  • Reduce the chloride concentration to a level at which chloride interference is insignificant – chloride interference occurs where there is the presence of chlorine in concentrations greater than 0.02 M
  • Not alter the organic content of the sample to a significant extent – ensuring there’s no increase or decrease in COD
  • Make the method suitable for routine use – inexpensive with simplified processes.

COD analysis takes place in two stages:

  1. Digestion – oxidising the organic substances in the sample
  2. Determination – measuring COD using either the titrimetric or colourimetric method.

You first achieve digestion by creating a reaction, which requires acid, heat and a catalyst.

You then measure chemical oxygen demand by one of these methods:

  • Titrimetric analysis involves treating the sample solution with a suitable reagent to react quantitatively with it.
  • Colourimetric analysis involves the use of a colour reagent and then observing measurable colour changes in the sample solution.

Analysis of Oxygen Demand

Along with COD, there are other forms of measurable oxygen demand. The most common of these is biochemical oxygen demand (BOD).

Chemical oxygen demand is broadly similar to biochemical oxygen demand in that they are both 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 oxidized, whereas biochemical oxygen demand only measures the oxygen demanded by organisms.

BOD measures the amount of dissolved oxygen that aerobic biological organisms require to break down organic material. It is the traditional test for establishing the concentration of organic matter in wastewater and it works on the principle that if sufficient oxygen is available then aerobic microorganisms in water will continue to decompose until all the waste is consumed.

However, COD analysis is an increasingly popular alternative to BOD because it’s faster, and it can test wastewater that is too toxic for BOD.  The BOD test takes five days, but modern COD testing methods mean that this method can be used as a real-time analyser, enabling wastewater operatives to monitor and adjust parameters during processes. Additionally, only organic compounds are consumed during BOD testing, which gives lower concentration results than COD testing.

What are the advantages and disadvantages of Chemical Oxygen Demand

COD’s chief advantages are that it is a relatively rapid testing method which complies with APHA and ISO standards. Speed matters, but accuracy can’t be compromised. COD analysers combination of speed and precision are it’s key advantage in a wide-range of contexts.

COD will normally be higher than BOD because more organic compounds can be chemically oxidised than biologically oxidised. This includes a range of chemicals that are toxic to biological life. This makes COD tests highly useful when it comes to testing industrial sewage as these will not be captured by BOD testing.

One potential disadvantage to this method is that it cannot differentiate between inorganic and organic carbons and that it can receive interference from halides, nitrates and peroxide. In certain circumstances, results may vary in warmer conditions or at room temperature so it is important to monitor 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. Using modern environmental analysers such as the EasyPREP COD can make the task rapid and relatively straightforward, producing high-levels of accuracy that you can rely upon