Essential Guide: Biochemical Oxygen Demand - BOD testing and measurement in water

What is Biological Oxygen Demand?

When microorganisms like bacteria break down organic matter under aerobic conditions in bodies of water, they consume oxygen. The amount of oxygen being consumed is known as biological oxygen demand (BOD), although it is sometimes also referred to as biochemical oxygen demand.

Why is dissolved oxygen important?

Although water contains oxygen molecules, this is not used by aquatic life. Instead, all life in water bodies uses dissolved oxygen, which is present in small volumes – approximately 10 molecules of oxygen per million of water. 

Oxygen mainly enters bodies of water via the atmosphere, but also through groundwater discharge. Generally, fast-moving water, like rivers, contains more oxygen than static bodies of water, such as lakes and ponds. 

So, for water to be ‘healthy’, it needs a sufficient amount of dissolved oxygen.

Chemical Oxygen Demand vs Biological Oxygen Demand: Key Differences

Chemical oxygen demand (COD) is an alternative method for estimating how much oxygen would be consumed by the decomposition of organic matter by bacterial action. The test uses a strong oxidising agent to chemically oxidise organic material in a wastewater sample under heat and strong acid conditions.

The figures derived from COD will be higher than through BOD due to the processes involved. The advantage of COD over BOD is that results can be ready in hours rather than days. However, the use of chemicals yields hazardous waste.

What factors affect biological oxygen demand?

Levels of dissolved oxygen naturally fluctuate but can be affected by intense heat and by what humans put into the water, for example, fertilizers and other organic matter.

• Extreme temperatures: Organic matter is decomposed in water by microorganisms like bacteria, which consume oxygen in the process. When levels of organic matter increase, dissolved oxygen levels fall. This has a greater effect on static bodies of water than rivers. When you add in hot summers, or periods of drought, it is easy to see how eutrophic conditions prevail. Generally, cold water has greater capacity for storing dissolved oxygen. The resulting oxygen-depleted water is the perfect environment for algae blooms, which are then consumed by bacteria, leading to further oxygen depletion, making it very hard for aquatic life to survive, effectively leading to the body of water dying.
• Human intervention and pollution: Nutrient-rich discharges in water bodies is of particular concern. The US Geological Survey (USGS) highlights the Gulf of Mexico ‘dead zone’ as a prime example of what can happen if BOD becomes too great for life to exist. This seasonal zone occurs in the waters south of Louisiana, where discharge from the Mississippi and Atchafalaya Rivers are nitrogen and phosphorus-rich. The subsurface water becomes depleted in oxygen leading to hypoxia, which makes what is normally a productive fishing ground effectively dead. The zone can extend from the waters of Texas all the way to Alabama. A similar problem affected the Ganges River in India, but a concerted clean-up effort and higher monsoon rains contributed to dissolved oxygen levels increasing and biodiversity improvements.

What are the main sources of BOD?

Biological oxygen demand comes from organic materials that end up in a body of water. Some of these are natural products of the environment, for example, fallen leaves and woody debris, such as twigs and branches, plants, seeds, grass, animal faeces and manure, and even dead animals.

Increasingly, in the modern world, BOD sources also come from human activity: fertilisers (both agricultural and from urban lawns), effluent from industrial processes, wastepaper, food processing plants, animal feed, septic tanks, urban storm water run-off, and other forms of pollution.

Why is BOD important in the water treatment industry?

Another way to look at biological oxygen demand is that it measures how much oxygen is needed to remove waste organic matter from water. This means how much oxygen is needed by microorganisms in the water to make the organic matter ‘safe’, or as the USGS states, ‘unobjectionable’.

BOD can also be used as a measure in wastewater treatment plants, i.e. of the levels of organic pollution in water before it is discharged. Testing BOD levels at different stages of the wastewater treatment plant also offers a test of how effectively each process is working. 

If organic material levels are too high when released, the discharged treated water will contribute to biological oxygen demand and risk depleting oxygen levels in any body of water it eventually reaches.

If dissolved oxygen levels fall to 5 parts per million or lower, all freshwater fish species such as trout and salmon will have died and even those species that can withstand lower oxygen levels, such as catfish and carp, will be at risk. 

How do you measure BOD in water?

Essentially, determining biological oxygen demand involves measuring how dissolved oxygen levels change over a five-day period. One of the most common methods used is known as Standard Methods 5210B. Once collected in a sample bottle, the water needs to be tested in a controlled environment for BOD levels within 48 hours to determine a baseline figure.

Another test is performed after five days to find out how the levels have changed. The higher the BOD rating, measured in parts per million, the more organic matter is available as ‘food’ for bacteria, which leads to oxygen depletion. The lower the BOD level, the better the water quality. 

What regulation affects BOD levels?

Most countries will be subject to regulation on BOD levels in wastewater effluent. Regulation will include effluent concentration limits, maximum levels permittable per ‘population equivalent’, minimum reduction levels from influent to effluent, and the number of times samples need to be taken.

Factors that can be taken into account include unusual weather conditions and abnormal operating conditions. There may also be separate rules governing nitrogen and phosphorous limits.

• Europe: BOD levels are regulated in the European Union under the Urban Wastewater Treatment Directive (UWWTD), although some member states have set stricter controls.
• United Kingdom: In the UK, the Environment Agency sets compliance levels. These are determined by a calculation based on the 'population equivalent' of the 'agglomeration' a WWTP serves. This means that for any WWTP, the total population count for residential and commercial activities (including tourists) determines the BOD levels permitted. Population equivalents are banded into thresholds for the purpose of treatment and monitoring. Industrial processes also have regulations governing BOD levels. BOD level monitoring tests either pass or fail; the frequency of testing is determined by the population equivalent banding.
• United States: The US EPA sets national limits, but BOD limits are also set locally, with permits based on concentration. These are typically set at around 250-300 mg/l for discharge to sewer systems, while direct environmental wastewater discharge is significantly lower, commonly 10 mg/l.
• Australia: Guidelines for BOD levels and monitoring requirements are determined by the National Water Quality Management Strategy, especially the National Guidelines for Water Recycling: Managing Health and Environmental Risks, which is an authoritative reference for the supply, use and regulation of recycled water schemes.

In many countries, national regulations are often supplemented by local state guidelines on acceptable BOD levels.