Water contains small amounts of dissolved oxygen that is vital for aquatic life. The levels of dissolved oxygen are affected by many factors and can be used to assess water health in rivers and lakes. One measure, known as biological oxygen demand can also be used to assess how well wastewater treatment plants are working to meet stringent guidelines on organic matter levels in effluent.
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.
Although water contains oxygen molecules, this is not used by aquatic life. Instead, all life in water bodies use 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, contain more oxygen than static bodies of water, such as lakes and ponds.
So, for water to be ‘healthy’ its needs a sufficient amount of dissolved oxygen.
How is it different to Chemical Oxygen Demand?
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.
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.
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 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 phosphorous-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 ways to Alabama.
A similar problem affected the Ganges River in India, but a concerted clean-up effort and higher monsoon rains have 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 very 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 it important in the water treatment industry?
Another way to look at biological oxygen demand is that is 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’.
As we have seen, BOD is a measure of dissolved oxygen in water bodies, but it can also be used as a measure in wastewater treatment plants, this time 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 therefore risk depleting oxygen levels in any body of water 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 (source: Apec Water).
How do you measure BOD?
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 form 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.
BOD levels are regulated in the European Union under the Urban Wastewater Treatment Directive (UWWTD), although some member states have set stricter controls. In the UK, the Environment Agency sets compliance levels which are limits consistent with EU regulations.
Under the UWWTD, the biochemical oxygen demand of incoming wastewater in primary wastewater treatment must be reduced by at least 20 per cent before discharge, and the total suspended solids of the incoming wastewater by at least 50 per cent.
During secondary treatment, BOD must be reduced to 25 mg/l oxygen, or a minimum reduction of 70-90 per cent (in relation to the load of the influent) must be achieved. Chemical oxygen demand must be reduced to 125 mg/l oxygen, or a minimum reduction of 75 per cent compared to influent.
At the tertiary wastewater treatment level, there are requirements for reducing nitrogen in discharge to 15 mg/l in agglomerations of 10,000-100,000 pe, and 10 mg/l in those above 100,000 pe, or a minimum reduction of 70-80 per cent.
Similarly, tight controls are imposed on phosphorous at this stage: reduced to 2 mg/l (10,000-100,000 pe) and to 1 mg/l (over 100,000 pe), or a minimum reduction of 80 per cent.
