How To Reduce BOD In Wastewater


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There are many easy ways to reduce biochemical oxygen demand (BOD) in wastewater. These include reducing total suspended solids (TSS) via physical filtration and chemical processes. Another method is aeration in activated sludge treatment facilities, which reduces BOD by supporting beneficial bacteria. Maintaining optimal temperatures is also essential in reducing BOD in wastewater. Alternative methods such as coagulation-flocculation and moving bed biofilm reactors (MBBR) are also used to enhance BOD reduction in wastewater. 

Reducing BOD in wastewater is challenging for wastewater treatment facilities, but there are many methods that work. BOD is a vital factor in monitoring water quality and safety, and therefore, achieving and maintaining low BOD levels is a top priority. 

What Is BOD In Wastewater?

Biochemical oxygen demand serves as an indicator of the presence of biological substances in water or wastewater. BOD quantifies the oxygen that is utilized by microorganisms when organic matter is decomposed in the water. 

BOD measurements are typically taken in real-time or via sensor-equipped equipment like dissolved oxygen (DO) sensors, spanning a 5-day period at 20°C (68°F). 

Because BOD directly influences DO levels in rivers and streams, impacting aquatic organisms, various sources can contribute to high BOD, thus highlighting why it is important to reduce BOD levels when they are undesirable. 

Issues With High BOD In Wastewater

If BOD levels become too high in wastewater treatment plants, it becomes a serious problem. When BOD is high, the opposite occurs to DO; when BOD is high, DO is low. When there isn’t enough oxygen dissolved in the water, it cannot sustain aquatic life. So, if this water is released into the environment, it puts a strain on life that relies on the discharged water. This oxygen depletion due to high BOD results in suffocation and the death of aquatic life.

High BOD can also indicate a high carbon dioxide (CO2) which can become corrosive, damaging infrastructure and systems within the wastewater treatment facility. 

Causes of High BOD

Before we get into ways to reduce BOD in wastewater, it is important to understand the different causes of high BOD in the water

  • Excessive organic loading 
  • Chemical pollutants 
  • Agricultural runoff
  • Urban area runoff
  • Household waste
  • Sewage from domestic wastewater
  • Increased temperatures
  • Landfills
  • Nutrient enrichment from nitrogen and phosphorus
  • Sedimentation (when silt, clay, and organic matter settle 

How To Reduce BOD Levels In Wastewater?

Microorganisms and bacteria thrive in wastewater due to the presence of dissolved organics. In water, keeping the biochemical oxygen demand (BOD) low is important for ensuring that water is treated properly for cleanliness and safety. There are many ways that can be applied across the first, second, and third filtration stages to reduce BOD in wastewater.

These methods combine chemical and physical processes, in addition to equipment adjustments, focused on reducing BOD from a high PPM (parts per million) value, to only one or lower, enhancing wastewater treatment effectiveness. 

Lowering Total Suspended Solids (TSS)

The initial step in reducing BOD in wastewater is to focus on TSS. Total suspended solids are very closely related to BOD, and therefore, it would be challenging to reduce BOD in wastewater if the TSS level remains high. When lowering TSS during the first filtration phase, it is critical to know that TSS can be tiny, as tiny as two microns. Despite being so small and not easy to see, tiny TSS particles are included in BOD measurements until they are removed from the water. 

The good news is, there are many ways to reduce total suspended solids in wastewater. The first is to use mechanical filtration equipment such as rod sieves or rotary strainers. These are both vital for lowering TSS and reducing BOD, so wastewater treatment plants must maintain this machinery well. If the machinery becomes broken or is poorly maintained, clogging can occur elsewhere and therefore, TSS will not be filtered out from the wastewater.

Another method to lower TSS is to add chemicals to the wastewater digesters, which are used to break apart the solids and stop them from settling in the water. This process reduces the number of solid waste in wastewater treatment plants as the solids are efficiently chemically removed, rather than being manually filtered out. 

Utilizing The Correct Equalization Tank

The equalization tank (EQ tank) is an essential piece of equipment when reducing BOD in wastewater. While seemingly random to take into consideration the proportions of the EQ tank, it can in fact have a massive influence on BOD in wastewater. 

The proportions of the EQ tank determine the fluctuations of flow, which directly affect changes in aeration, in the water. For example, aeration can either lower or raise the biochemical oxygen demand, depending on how well it is managed and controlled. 

When selecting and determining the size of the EQ tank, it’s important to recognize the ideal loading rate of the wastewater that is being treated.

This can vary from one wastewater plant to another, yet it is initially determined by the type of water and if any organic substances are added to the water to aid the treatment process. If the EQ tank has the correct volume, it will lead to flow fluctuations becoming balanced and the loading rate stabilizing, thus ensuring that water is flowing in the best way possible to lower BOD. 

Increasing Aeration For ASP (Activated Sludge Processes)

One of the most successful ways to lower BOD in wastewater is to increase aeration. Aeration uses activated sludge processes (ASP), and this is one of the most accepted ways to filter pollution from wastewater systems.

Activated sludge utilizes beneficial bacteria to break down harmful sewage and use biological processes to clean wastewater. For the activated sludge to operate, oxygen is required. Air diffusers are used to supply oxygen to the beneficial bacteria in the activated sludge, allowing the bacteria to survive long enough to decompose the waste in the wastewater. This aeration process allows the conversion of pollutants into sludge waste that can be later filtered out and removed in the secondary clarifier. 

Flocculation And Coagulation

Flocculation and coagulation are the two foremost parts of maximizing the activated sludge process and reducing BOD in wastewater. 

Coagulation involves combining particles and clumping them together. This makes it much easier to remove and catch one larger particle than lots of smaller ones.

Flocculation involves aggregating the coagulated particles which settle at the bottom of the EQ tank. Chemical flocculants are added to the secondary clarifier basin in the tank to help hurry up the process. During this time, the activated sludge is created and allows the elimination of undesired sewage in the wastewater.

If the flocculation process is too slow, the wastewater treatment facility can add more flocculants to expedite the process even more. The same applies if the wastewater treatment facility is trying to improve the efficiency of the activated sludge process. 

There are various flocculants available, the most common being:

  • Aluminum sulfate (aluminum-based)
  • Ferric chloride (iron-based)
  • Ferrous sulfate (iron-based)

The best flocculant depends on the characteristics of the wastewater treatment facility. 

Consulting Experts

When selecting chemicals and flocculants for wastewater treatments, it is always suggested that you consult with an additional expert. These chemists and wastewater treatment experts are crucial parts of having an effective treatment strategy to reduce BOD in wastewater. Specialists can also provide invaluable knowledge on efficient BOD reduction based on the wastewater facility’s land, water, and region. 

Different specialists can help with the reduction of BOD in several ways:

  • Hydrogeologist: Provide insights into what occurs to the waste and water after leaving the wastewater treatment facility. They can also recognize how different flocculants or chemicals could negatively impact the groundwater in the surrounding environment. 
  • Pedologists: Provide insights into incoming wastewater sources such as agricultural and mining sources. By knowing the water’s origin, the selection of chemicals becomes more efficient in reducing BOD in wastewater. 

Keeping Temperatures Low

One parameter that can negatively affect DO levels is increased temperatures. This is often disregarded due to focusing too much on new chemicals and flocculants. However, monitoring temperature in wastewater systems reveals opportunities to lower BOD more efficiently. Reducing the temperature can enhance beneficial bacteria if the process of reducing BOD is taking too long in the EQ tank and basin.

When adjusting the temperature, remember that very low temperatures (close to freezing) can cause a reduced activated sludge process. And so you must work closely with a temperature sensor to find the perfect temperature warm enough for an efficiently activated sludge process, yet low enough to maintain a high DO level. 


Reducing BOD in wastewater can be a complex process. By understanding the nature of the wastewater by consulting specialists, familiarizing yourself with flocculation and coagulation processes, utilizing the correct EQ tank size, lowering the total suspended solids, keeping the temperature low, and increasing aeration during the activated sludge process, you can efficiently and effectively reduce BOD in wastewater. 

If you want to know more about reducing BOD in wastewater or what testing kits we have (such as temperature sensors and DO sensors), feel free to contact the world-class team at Atlas Scientific

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