An ever-present challenge for building managers and water treatment specialists is managing Biochemical Oxygen Demand (BOD) levels in their effluent to be compliant with local regulations. Failure to meet permissible BOD limits can result in fines and damage to the environment, making it a hot button issue in the water treatment business. But what does BOD mean?
What is BOD?
Biochemical Oxygen Demand (BOD) is the amount of oxygen consumed by bacteria and other microorganisms such as algae while they decompose organic matter in water. BOD occurs in all bodies of water with the amount depending on conditions. All lakes and streams contain small amounts of oxygen needed for fish and plants to survive. The decay of organic matter in water can lessen the amount of oxygen, which is harmful to the environment.
Biochemical oxygen demand essentially measures how much oxygen is required to remove organic waste from water through decomposition.
Sources of BOD
Organic matter can enter bodies of water from a number of sources, both natural and man-made. Leaves, dead plants and animals, and manure are examples of natural waste that is a source of BOD. Some man-made sources include wastewater treatment plants, food-processing plants, and urban stormwater runoff. The amount out of BOD in effluent can vary by source as well as by day, so water treatment specialists are frequently testing BOD levels to make sure they are compliant with local regulations.
How is BOD determined?
The most common method for calculating the biological oxygen demand needed to break down organic matter is the five-day BOD test or BOD5.
The BOD5 test method measures the difference in dissolved oxygen from a test sample for five days. The amount of dissolved oxygen is measured before the test sample is incubated at 20°C for five days, where it is measured again. The BOD value is then calculated from the amount of dissolved oxygen consumed and measured in milligrams per liter. Once the BOD level is measured, water treatment specialists compare the value to local permissible limits for BOD in wastewater.
What is the permissible limit of BOD in wastewater?
The amount of BOD allowed in effluent prior to being discharged into a river, stream, or municipality will vary depending on local ordinances. Permits based on concentration are typically around 250-300 mg per liter for discharge to sewer systems, while direct environmental wastewater discharge is significantly lower, commonly 10 mg per liter.
Depending on your location you may require different methods for reducing BOD levels to be compliant with local regulations to make sure you’re being a good partner to the environment and reducing your chances of hefty fines. But how do you lower the BOD value in wastewater?
How BOD is lowered by wastewater treatment plants
BOD is lowered by wastewater treatment plants in a number of ways depending on the levels of BOD and the source. Common methods of lowering BOD in wastewater treatment plants include:
- Wave action from wind in the facultative pond: Wave action allows for the addition of oxygen to ponds for natural bacterial processes to occur.
- Aerated ponds agitating: Similar to a facultative pond, but with the addition of air. Air addition can be in the form of large or fine bubbles, which help bacteria to naturally break down the BOD. The addition of air can also help by agitating the water, which creates an effect similar to wave action.
- Activated sludge: After primary treatment is utilized for the removal of insoluble BOD and other constituents in the water, activated sludge can be used for the removal of soluble BOD. The water to be treated is sent to an aeration basin where oxygen levels are maintained so biological processes can be maintained and reduce BOD. Naturally occurring bacteria combined with oxygen reduce soluble BOD through cellular uptake and digestion. For every pound of BOD removed, this process produces approximately 0.4 to 0.5 of cellular biological sludge. This biological matter is removed through sludge wasting, known as waste activated sludge (WAS). Some of the biological material is recycled back into the aeration to maintain a biological balance, known as return activated sludge (RAS).
- MBBR (Moving Bed Bioreactor): A very similar process to activated sludge, but utilizes the use of a media in the aeration basin. The media greatly increase the surface area, which allows greater bacterial growth in a smaller footprint. This also removes the need to return activated sludge.
- Coagulation step: Coagulation is used to make insoluble constituents in the water-soluble. Inorganic coagulants (such as aluminum and iron-based metal salts) and organic coagulants (such as polyamines) can be used independently or in combination to neutralize charges on particles. Once neutralized, the particles are in a soluble form that can be flocculated, and then removed through primary treatment, such as a DAF.
Managing BOD levels is a key step in the wastewater treatment process. For facility managers and water treatment specialists, it is important to stay diligent and be proactive in both treating water and maintaining a functioning system.
Remember, the wastewater experts at HOH can help you determine your BOD levels and develop a plan to better manage your wastewater.
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