Guest Column | November 8, 2022

How To Lower BOD In Wastewater

By Emily Newton

Lowering BOD in wastewater is the key to making polluted water clean, benefiting people and the environment.

Reducing biochemical oxygen demand (BOD) in wastewater is a key challenge for treatment facilities, but some solutions can help. BOD is a crucial factor in monitoring water quality and safety, and achieving a low BOD is one of the top goals of any plant. These six strategies can improve the BOD reduction process, making it more efficient and effective.

Strategies For Lowering BOD In Wastewater

BOD is a critical metric in wastewater treatment. As long as biodegradable organics exist in wastewater, microorganisms, and bacteria can continue to thrive and reproduce. Maintaining a low BOD ensures water leaves treatment facilities clean and safe.

Several strategies can be used to lower BOD in wastewater throughout the standard primary, secondary, and tertiary filtration phases. These methods include physical and chemical processes as well as some equipment considerations that can aid in lowering BOD. Ideally, these strategies should help reduce BOD from hundreds of parts per million to one or less.

1. Lower TSS First

The first step to lowering BOD more efficiently is focusing on total suspended solids (TSS) first. TSS is closely related to BOD. It will be difficult to reduce BOD if it remains high. Some people may only concentrate on lowering TSS in the primary filtration phase, but it is important to remember that a suspended solid can be as small as 2 microns in width or diameter. These particles will feed into BOD measurements as long as they remain in the water, even if they aren’t easy to see.

There are several ways to reduce TSS in wastewater. For example, physical filtration equipment like rotary strainers or rod sieves is often used to filter out suspended solids. This machinery is vital to low TSS, so treatment facilities must maintain it well. A broken, damaged, or poorly maintained filter can lead to clogging elsewhere from solids that don’t get filtered out.

Chemicals may also be added to wastewater digesters to break down solids and prevent them from settling. This can reduce the amount of solid waste a treatment plant has to dispose of since the solids are chemically broken down rather than simply filtered out.

2. Use The Correct Size EQ Tank

The EQ tank is an important equipment consideration to remember when trying to lower BOD. It may seem arbitrary at first, but the size of the EQ tank can have a major impact on BOD in wastewater. The size of the equalization tank determines flow fluctuations, which result in aeration changes in the water. Aeration can raise or lower BOD depending on how it is controlled.

To determine the proper size EQ tank, wastewater treatment professionals must understand the ideal loading rate of the water being treated. This will differ from one plant to another, but it is primarily determined by the type of water and any organic substrates being added to aid in the process.

An EQ tank with the proper volume will result in balanced flow fluctuations and loading rate, ensuring water is moving in the most efficient way to reduce BOD.

3. Increase Aeration For Activated Sludge Processes

One of the best ways to reduce BOD in wastewater is to increase aeration in treatment basins and lagoons. Aeration is key to using activated sludge, one of the most popular methods worldwide for filtering pollution out of wastewater. Activated sludge uses beneficial bacteria to digest harmful sewage and clean water through biological processes.

Oxygen is needed for activated sludge to work. This may sound ironic at first since BOD partly relies on starving harmful bacteria and microorganisms of oxygen. However, in this case, air diffusers provide oxygen to beneficial bacteria in the activated sludge so it stays alive long enough to break down the waste.

Activated sludge is highly effective in reducing BOD in wastewater. It essentially works by building two teams in the aeration basin and secondary clarifier: the beneficial bacteria team and the waste sludge team. Mixing them through aeration allows the beneficial bacteria to slowly but surely convert pollutants into waste sludge that’s filtered out and disposed of in the secondary clarifier.

4. Implement Coagulation And Flocculation

Two important parts of optimizing the activated sludge process are coagulation and flocculation. Implementing these two processes is crucial to reducing BOD in wastewater using the activated sludge tactic. Coagulation is the process of combining particles or “clumping” them together. One larger particle is easier to identify and catch than many smaller ones.

Flocculation is the process of getting those aggregated, coagulated particles to settle at the bottom of the tank. Chemical flocculants are often added to the secondary clarifier basin to speed up the process. This allows activated sludge to be created and begin eliminating undesired sewage. A wastewater treatment facility experiencing slow flocculation or trying to improve the efficiency of its activated sludge process should add more flocculants to speed up the process.

Various flocculants are available today, including aluminum-based versions like aluminum sulfate as well as iron-based types like ferric chloride and ferrous sulfate. The best flocculant depends on a specific wastewater treatment facility’s unique characteristics. For instance, incoming suspended solids and pollution may have chemical features that make iron-based flocculants more effective.

5. Consult Additional Experts

When choosing flocculants and other chemicals for water treatment, it is worth considering bringing in additional experts to refine the treatment process. Chemists and water treatment specialists are crucial components of designing an effective water treatment strategy. However, other scientists may be able to provide knowledge that leads to more efficient BOD reduction based on the unique properties of a treatment facility’s water, land, and region.

For example, it may be worth consulting a hydrogeologist, a scientist specializing in interactions between water and the Earth. This expert can give insights into what will happen to the waste and water after it leaves the treatment facility. A hydrogeologist may be able to recognize how a certain flocculant or treatment chemical could negatively impact groundwater in a particular region or environment.

Pedologists are often similarly helpful in this part of wastewater treatment. These scientists specialize in understanding different types of soils, so they may be able to give insights into incoming wastewater from sources like agriculture and mining. Understanding of the water’s origins allows for the strategic selection of chemicals for efficiently reducing BOD in wastewater unique to different regions and environments.

6. Keep Temperatures Low

It is important to remember that BOD and dissolved oxygen (DO) levels are inversely related. Ideally, wastewater treatment facilities want water to have a low BOD and a high DO. This is because dissolved oxygen is critical for beneficial bacteria to break down the organic matter that causes high BOD in wastewater.

One element that can negatively impact DO levels is high temperatures. This factor often goes unnoticed amid attempts to concentrate on trying new chemicals or flocculants. However, paying attention to the temperature of the water as it moves through the treatment process can reveal opportunities to lower BOD more efficiently in wastewater.

Lowering the temperature may boost beneficial bacteria if it seems to take a long time for BOD to go down in water treatment basins. Remember: lower temperatures cause higher DO levels and lower BOD levels. However, significantly low temperatures, such as near freezing, can result in a slower activated sludge process. The key is to find the sweet spot where temperatures are cool enough to maintain a high DO level but high enough for efficient activated sludge processes.

Generating Low BOD In Wastewater

Lowering BOD in wastewater is a complex process, but a few key strategies can make it more efficient and effective. Understanding the unique nature of wastewater and conditions in a treatment facility’s particular region allows chemicals and treatment processes to be fine-tuned and customized. Temperature control and properly sized equipment can set plants up for success. A strategic approach can reduce BOD in wastewater quickly and effectively.

Emily Newton is an industrial journalist. She regularly covers stories for the utilities and energy sectors. Emily is also Editor-in-Chief of Revolutionized