Biochemical Oxygen Demand (BOD) is defined as the amount of oxygen consumed by bacteria in water during the aerobic decomposition of organic matter at a specified temperature.
Jargon aside, BOD is how we quantify the amount of oxygen used by bacteria during the breakdown of organic matter present in a water body.
Almost all natural waters will have some organic compounds in varying quantities. Aquatic microorganisms will use the dissolved oxygen in the water to breakdown these organic compounds in order to use them for food to fuel growth and reproduction. As such, we term this as the organisms having an ‘oxygen demand’ on the water – hence ‘Biochemical Oxygen Demand’.
First and foremost, BOD is usually a measure of organic pollution. In water systems such as rivers and oceans, oxygen is vital in maintaining aquatic environments. Different features of the aquatic environment will have different oxygen requirements, if these requirements are not met, it can cause the destabilisation of an environment both temporarily and sometimes permanently.
Generally, the microbial metabolism (i.e. the microorganisms consuming oxygen) is related to the number of consumable organic compounds and the system will exist in a balance. In situations where in-stream oxygen is reduced or the number of microorganisms rises without a corresponding rise in organic compounds, the system can become oxygen starved which will impact all aquatic life and functioning.
Low BOD levels are generally associated with good quality water whereas higher BOD levels and increases in BOD are associated with pollution.
Why not just measure Dissolved Oxygen (DO)?
DO is also an important water quality parameter in its own right, a basic measure of how much oxygen is in the system; however, it is a simplistic descriptive measurement and when it comes to a pollution metric, it is lacking in context. BOD focuses on all the oxygen consuming constituents of the water whereas DO simply tells us the amount of oxygen in the water. Unless you have prior data, a single DO reading won’t immediately inform you of the pollution status whereas BOD has strongly defined categories where unpolluted, pristine rivers typically have below 1-2 mg/L BOD, heavily polluted rivers are categorised as such when in exceedance of 8 mg/L BOD. In contrast, a DO reading can be the same between two different rivers in vastly different pollution levels.
BOD is an extremely valuable measure of the impact of pollution as it is the closest to replicating the natural oxidation and recovery conditions of a water body. BOD focuses on all the oxygen consuming constituents in the water and their requirements of oxygen as opposed to dissolved oxygen which is a descriptive parameter for the level of oxygen in the water.
BOD is also an important parameter used in the treatment of wastewater, with many regulators worldwide having permits for the minimum level of BOD reduction to be achieved during treatment. Wastewaters with a high BOD can be extremely problematic to the receiving waters, causing oxygen depletion and potential anoxia.
Polluted water will see an increase in the BOD, meaning there is a far greater demand on the oxygen available in the water; an example would be nutrient pollution, more commonly referred to as algal blooms. One of the main man-made causes of algal blooms is a sudden increase in nutrients in a water body, such as from agricultural run off or spillages. The increase in nitrates and phosphates, creating the conditions for favourable growth, causes the algae population to increase rapidly as they consume these nutrients. As oxygen is used by the algae to consume the nutrients, the oxygen depletes rapidly leaving oxygen poor conditions or anoxia. Generally algal blooms are self-limiting as once the oxygen has run out they will start to die off however they can do substantial harm to aquatic life during their formation as the rest of the system becomes starved of oxygen. Some algal blooms also carry significant risk to humans and mammals due to some species of algae and cyanobacteria producing toxins.
The BOD5 test – a traditional stalwart in WQ surveys
The more ‘traditional’ method of measuring BOD is enshrined in water quality standards worldwide, based on a 5-day incubation test. The test involves measuring the amount of oxygen consumed by bacteria decomposing organic matter over a 5-day period at a standard incubation temperature of 20o Consumption of oxygen is split into two stages, carbonaceous and nitrogenous, defined by the constituents involved in the conversion.
The ‘five day’ rule is not as closely rooted in science as the rest of the test as it is based on the amount of time as when the test was created it was based on the time taken for sewage deposited in English rivers to reach the sea. Since then, science has moved on somewhat and while 5 days has been accepted as use for a pollution metric, it is also known that for a full biochemical oxidation of organic matter, 20 days is more appropriate.
New Frontiers – Transforming laboratory waits into real-time field data.
Given that in terms of pollution monitoring, a five day wait for results that confirm pollution can allow time for substantial damage to an ecosystem, Proteus Instruments has used the intrinsic fluorescence properties of organic matter to streamline the process, allowing for real-time, instantaneous monitoring.
The Proteus BOD method is a scientifically-proven method whereby the fluorescence of organic matter in the system is correlated to a range of BOD readings. This is then fed into an algorithm that can compute the relationship, taking into account both temperature and turbidity factors, to produce a real-time output of BOD.
Each probe is calibrated locally to the water source to ensure that the unique features of each aquatic system are considered in the output, so it can be deployed in-situ. There are also more general, global calibrations based on water source which allows The Proteus to also be used as a spot measurement device.
The main advantages apart from the removal of the five-day wait is that where laboratory samples are costly and are generally a once/twice a day phenomenon, the Proteus can log continuously if required, or be set up to a specified time interval from seconds to hours making it far less likely for pollution events to be missed.
