Wastewater quality is routinely assessed through a series of measurements designed to quantify the concentration of various pollutants. Among these metrics, Biological Oxygen Demand (BOD) serves as a fundamental indicator of the organic strength of the wastewater stream. High BOD values signify that the water contains a large amount of biodegradable organic material, which represents significant pollution. Understanding the causes of elevated BOD is the first step in managing and treating this pollution before it can affect natural water systems.
Defining Biological Oxygen Demand
Biological Oxygen Demand is a measure of the amount of dissolved oxygen required by aerobic microorganisms to decompose the organic matter present in a water sample. This process is a natural function of the microbial population, which consumes oxygen as it biologically oxidizes organic compounds into more stable forms like carbon dioxide and water. The greater the amount of organic material in the water, the more oxygen the microbes will need to consume.
The standard laboratory procedure for quantifying this pollutant load is the BOD5 test, which involves incubating a water sample for five days at a constant temperature of 20°C. The result is expressed in milligrams of oxygen consumed per liter of water (mg/L). A high BOD value, such as 300 mg/L for typical untreated sewage, directly correlates to a high concentration of organic pollutants. This oxygen consumption rate is a reliable surrogate for organic pollution in a water body.
Primary Sources of Organic Load
The primary cause of high BOD is the discharge from municipal and residential sewer systems. This domestic wastewater, often referred to as sewage, is rich in readily degradable organic compounds. The largest single contributor is human waste, or fecal matter, which is composed primarily of organic solids and proteins that rapidly exert an oxygen demand.
Food waste is another major source, including preparation scraps and discarded meals that contain complex carbohydrates, fats, and oils. Fats, oils, and greases (FOG) are particularly problematic because they are difficult for microbes to break down and can accumulate in sewer systems. Household cleaning products also contribute to the organic load, as they contain surfactants and carbon-based molecules that are subject to microbial decomposition.
Industrial and Commercial Contributors
Certain commercial and industrial operations produce wastewater with BOD concentrations that far exceed those of typical municipal sewage. These industrial effluents are characterized by high volumes of specific organic compounds concentrated by manufacturing processes. For example, the food processing industry, including dairies, breweries, and slaughterhouses, generates wastewater high in complex sugars, proteins, and fats.
Brewery wastewater often contains high loads of complex carbohydrates and alcohol, while dairy operations discharge whey and milk solids that are rich in protein and lactose. Similarly, pulp and paper mills release lignin and cellulose fibers, which are complex organic polymers that require oxygen for biological breakdown. This high-strength industrial waste can have BOD values reaching into the thousands of milligrams per liter, placing stress on municipal treatment facilities if not pre-treated.
The Environmental Impact of High BOD
The release of high-BOD wastewater into natural aquatic environments initiates a chain reaction that alters the ecosystem. As the organic matter is introduced, the microbial population rapidly increases and consumes the dissolved oxygen in the water. This rapid oxygen depletion leads to a condition known as hypoxia.
When dissolved oxygen concentrations drop, aquatic organisms that depend on it, such as fish and invertebrates, cannot survive, resulting in widespread fish kills and a reduction in biodiversity. Furthermore, this heavy organic load places a burden on municipal wastewater treatment plants, which rely on controlled microbial processes to reduce BOD before discharge. If the treatment system is overloaded, it may fail to adequately process the organic material, leading to effluent that violates discharge regulations and continues to degrade receiving waters.