What Causes High BOD In Wastewater?


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The cause of high biochemical oxygen demand (BOD) in wastewater comes from excessive organic matter in the water. The source of the organic matter typically comes from human and animal waste like sewage and industrial processes. Other causes of high BOD include agricultural runoff, untreated wastewater, and food processing industries. In addition to organic matter, BOD levels are also influenced by pollutants (nitrates, metals, phosphates) and further organic compounds.

In 2022, the wastewater treatment market in the US accounted for $106.18 billion, expected to grow to $179.22 billion by 2030. Treating wastewater is a complex process that involves removing or treating characteristics before it is utilized for drinking water or discharged into the environment. If wastewater is not treated, it can cause irreparable damage to the environment, often resulting in companies found responsible facing serious consequences and fines. 

So, wastewater must be properly filtered and processed before it reaches the ‘clean rating’, known as effluent guidelines and wastewater discharge standards

The most prevalent pollutants requiring removal from wastewater include total suspended solids (TSS), elevated nitrogen levels (TKN/TN), and fats, oil, and grease, known as FOG. 

However, visible contaminants are not the primary issue present in untreated wastewater. Another concern in wastewater is biochemical oxygen demand (BOD), also known as biological oxygen demand. BOD serves as a standard gauge for organic water pollution by quantifying the water’s pollutants. 

What IS BOD?

Biochemical/biological oxygen demand (BOD) quantifies the level of biological substances present in water or wastewater. 

BOD measures the oxygen consumption from microorganisms during the decomposition of organic matter in the water. Essentially, the bacteria and other microorganisms in the wastewater need oxygen to degrade biosolids via oxidation. Elevated pollution levels result in excessive microbial activity and thus diminished water quality and oxygen levels in the water.

The measurement of BOD can be conducted in real-time or via sensor-equipped equipment (namely, a dissolved oxygen sensor) installed in wastewater treatment plants. In real-time a test is used to measure the amount of oxygen consumed by these organisms during a specified period of time – typically 5 days at 20°C (68°F). 

BOD has a direct impact on the dissolved oxygen levels in rivers and streams. For example, higher BOD results in faster depletion of oxygen in the stream, leading to reduced oxygen availability for aquatic life. Consequently, elevated BOD levels cause stress, suffocation, and eventually death among aquatic organisms, similar to the effects of low dissolved oxygen (DO) levels

Sources contributing to BOD include organic matter such as leaves and woody debris, decaying plants and animals, fecal waste from animals, as well as pump and paper mill effluents, wastewater treatment plants, food processing plants, feedlots, urban stormwater runoff, and failing/faulty septic systems. 

What Is Wastewater BOD?

BOD stands out as a crucial parameter in wastewater treatment, serving as a vital indicator of water quality and safety. 

In wastewater treatments, it’s important to consider the intrinsic relationship between oxygen and water at a molecular level, coupled with their utilization by naturally occurring microorganisms (comprising bacteria, yeasts, etc) in breaking down organic solids within wastewater. The contamination level they introduce is quantified in mgO2/L. 

This measurement occurs within a carefully controlled biological process, demanding orderly execution. The principle is however straightforward – higher BOD levels correspond to increased pollution rates and diminished water quality, while lower BOD levels signify reduced pollution and greater water purity, rendering the water more suitable for reuse.

Temperature influences BOD, and given that cold water generally retains more oxygen than warm water, DO levels in wastewater tend to decrease during the summer months. Consequently, wastewater treatments face heightened challenges, as BOD measurements are time-sensitive and BOD samples have a limited preservation window of only 2 days. This highlights the need for utmost caution in wastewater treatment processes, as elevated BOD levels can jeopardize water quality.

BOD Regulations In Wastewater

Regulations concerning BOD levels in wastewater differ between countries, and even within regions. However, in the United States, the Environmental Protection Agency (EPA) has implemented the National Pollutant Discharge Elimination System (NPDES) to define BOD level thresholds in wastewater. 

These limits are contingent upon the nature of the wastewater and the receiving environment. Before discharging wastewater into environments, it is imperative to consult local regulations to confirm that your facility adhered to compliance standards.

BOD’s Role In Wastewater Treatment Plants

As previously mentioned, BOD serves as a critical gauge of organic pollution within wastewater, prompting the industrial sector to discharge their final water product ONLY if it meets BOD standards.

Excessive levels of BOD in wastewater can result in regulatory fines and significant financial consequences for the respective facility, necessitating additional measures to solve the issue. Hence, wastewater treatment plants require robust systems to minimize BOD levels in the processed wastewater. 

This is accomplished through a series of treatment steps of varying intensities. Firstly, wastewater undergoes filtration to remove solids (TSS) and larger materials (like FOG), followed by UV light treatment and other processes. 

During this initial treatment phase, a large portion of BOD, TSS, and FOG are eliminated. Additionally, certain heavier chemicals such as nitrogen and phosphorus are also extracted from the wastewater, even though they may undergo further capture in the secondary stage of wastewater treatment. 

In most states in the US, the primary treatment mentioned above represents the minimal processing level necessary for wastewater irrigation. This level suffices for non-potable uses like irrigation or industrial applications. However, for higher purity requirements, secondary and tertiary treatments are key to avoid potential hazards and ensure water quality.

Six Causes Of High BOD In Wastewater

There are 6 main causes of high BOD in wastewater:

  • Chemical Pollutants
  • Sewage Discharges
  • Temperature
  • Landfills
  • Nutrient Enrichment
  • Sedimentation

Chemical Pollutants

Chemical pollutants, including detergents, pesticides, and industrial wastes, can significantly elevate BOD levels in wastewater. These pollutants infiltrate the wastewater system via multiple sources, including industrial operations, household waste disposal, and agricultural runoff. Their presence introduces high levels of organic material into the water, consequently heightening the demand for oxygen by microorganisms.

Also, chemical pollutants can escalate nitrogen and phosphorus concentrations in the water. One particular concern is phosphorus, which has been a common component of cleaning products since the 1960s. Its prevalence in natural water bodies is linked to algal blooms, known as eutrophication, wherein algae populations experience exponential growth, disrupting the ecosystem balance, and releasing toxins. 

Sewage Discharges

Domestic wastewater sewage is a significant contributor to wastewater BOD.  Laden with organic matter from human waste, sewage increases BOD levels upon discharge. Microorganisms degrade the organic matter in sewage, exacerbating oxygen demand in the water.

Additionally, sewage may contain an array of other pollutants, such as detergents, industrial wastes, and metals, further amplifying BOD levels in the wastewater. 


Temperature represents another factor that influences BOD levels in wastewater. Elevated temperatures accelerate the rate at which microorganisms degrade organic matter, intensifying oxygen demand. That being so, leads to higher BOD levels in the wastewater. 

Furthermore, heightened temperatures foster an increase in microbial population, thereby amplifying oxygen demand and contributing to higher BOD levels. 


Landfills emerge as significant sources of high BOD levels in wastewater. Landfill sites harbor a variety of organic materials, including food and yard waste, subject to microbial breakdown. Therefore, this process elevates oxygen demand, resulting in increased BOD levels in the water. 

Additionally, landfills house an abundance of pollutants, such as metals and chemicals, further increasing BOD levels in wastewater.

Nutrient Enrichment

Nutrient enrichment stands out as a key catalyst for increased BOD levels in wastewater. As mentioned, nutrients like nitrogen and phosphorus stimulate algae growth, increasing oxygen demand by microorganisms and therefore sky-high BOD levels.

Nutrient enrichment can also trigger eutrophication, depleting the water of oxygen further, and exacerbating BOD levels in the wastewater. 


Sedimentation constitutes another factor contributing to elevated BOD levels in wastewater. This process occurs when particles such as clay, silt, and organic matter settle at the bottom of the water’s body. Microbial breakdown of these particles intensifies oxygen demand and thus increases BOD levels. Sedimentation additionally releases pollutants like metals and chemicals, also contributing to high BOD in wastewater. 

How To Lower BOD In Wastewater?

In a typical case where BOD levels are elevated in untreated wastewater, it is imperative to employ suitable treatment approaches, including aerobic and anaerobic treatment approaches, including aerobic and anaerobic biological treatments, filtration as well as coagulation and flocculation, to effectively remove organic matter from the water. 

Also, it is essential to subject treated effluent to BOD testing to verify that it poses no threat to receiving ecosystems and water bodies prior to discharge. 

Improving BOD In Wastewater Systems

To improve BOD in wastewater treatment plants, several methods and strategies can be employed. 

Lowering the TSS in the wastewater is crucial, as TSS is closely related to BOD. This reduction can be achieved through the utilization of physical filtration equipment or chemical additives aimed at breaking down or eliminating the solids. Utilizing the appropriate size of equalization (EQ) tank is essential, as it helps regulate flow fluctuations and aeration levels in the wastewater. Aeration plays a significant role in affecting BOD by altering the oxygen availability for microorganisms. Increasing aeration in activated sludge processes, can enhance the biological degradation of organic matter, subsequently lowering the BOD level in the wastewater. Aeration methods increase mechanical aerators, diffused air systems, or surface aerators.

Coagulation and flocculation are chemical processes that effectively remove colloidal and dissolved organic matter from the wastewater. Coagulation involves the addition of chemicals to neutralize the electrical charges of particles, causing them to clump together. Flocculation involves adding chemicals that bind these clumps together into larger flocs, facilitating their separation via sedimentation or filtration. 

When unsure, consulting additional experts such as engineers, consultants, or researchers, will help provide specialized advice and solutions for enhancing BOD in wastewater treatment plants. 

These experts can assist in optimizing the design, operation, and maintenance of wastewater treatment processes and equipment. 


High levels of biochemical oxygen demand (BOD) in wastewater stem primarily from excessive organic matter, with sewage, industrial waste, agricultural runoff, and urban runoff serving as major contributors. These contaminants, full of pollutants and nutrients, escalate oxygen demand by microorganisms, thus increasing BOD levels.

Addressing high BOD in wastewater necessitates employing effective treatment methods such as aerobic and anaerobic biological treatments, filtration, and coagulation-flocculation processes.

If you would like to know more about BOD or what testing kits we have to prevent high BOD in wastewater, contact the world-class team at Atlas Scientific.

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