Biological Oxygen Demand: Concepts, Calculation & Importance

It's uncommon to consider that water bodies contain oxygen; however, there is some dissolved oxygen present in the water - a small amount, but vital for aquatic life. The term "biological oxygen demand" (BOD) refers to the quantity of oxygen required to decompose organic materials in water.

What is Biological Oxygen Demand?

The amount of oxygen utilized by bacteria and other microorganisms during the aerobic (oxygen-containing) breakdown of organic matter at a specific temperature is referred to as "biological oxygen demand" (BOD). One thing you cannot see in a lake's water when you look at it is oxygen.

We tend to think of water as the antithesis of air, although the typical lake or stream does contain trace levels of dissolved oxygen. The presence of a suitable concentration of dissolved oxygen is essential to preserving the aquatic life and aesthetic quality of streams and lakes. Dissolved oxygen is a tiny component of natural water bodies, amounting to up to ten molecules of oxygen per million of water.

The term "biochemical oxygen demand" also refers to the biological oxygen demand (BOD). To preserve the aquatic life and aesthetic appeal of streams and lakes, there must be a sufficient amount of dissolved oxygen present. Water-quality management depends on knowing how organic matter influences a stream's or lake's level of dissolved oxygen (DO). Biochemical oxygen demand measures the decomposition of organic materials in water. Biochemical oxygen demand is a method of calculating the number of oxidizable substances that can reduce the concentration of DO in a water sample.

The amount of dissolved oxygen in a water body can decrease due to environmental stresses (such as hot summer temperatures) and human-induced causes (such as the addition of excessive fertilisers to a water body), which can put stress on the local aquatic life. The measurement of biochemical oxygen demand is one water analysis used to comprehend better the impact of bacteria and other microorganisms on the quantity of oxygen they consume as they break down organic matter under aerobic (oxygen is present) conditions (BOD).

Sources of Biochemical Oxygen Demand

Leaf and woody debris, dead plants and animals, animal dung, effluents from pulp and paper mills, wastewater treatment facilities, feedlots, food-processing factories, malfunctioning septic systems, and urban stormwater runoff are all BOD sources.

Biochemical Oxygen Demand Measure

The BOD level can be determined using one of two ways. The two approaches are empirical tests.

Method I: In the dark, a sample of the water is held at a constant 20°C temperature. Five days later, the oxygen content is determined. The oxygen consumption during the measurement time reveals the oxygen demand in the water relative to the initial value.

Method II: The sample can be diluted at the start if a very high BOD is anticipated or if other harmful or inhibiting chemicals are present in the water. By doing this, it can be avoided that there won't be enough oxygen to break down the organic materials. The measurement result would be false as a result. The oxygen consumption during the measurement time is now determined, as with technique 1, by comparing the before and after readings.

The residual dissolved oxygen in the water sample is calculated after the fifth day. The BOD level can be estimated using this oxygen. After 5 days, the BOD in drinking water should be significantly lower than 1 mg/l. A BOD of about 20 mg/l is required for sewage treatment plant wastewater to be considered acceptable.

Biochemical Oxygen Demand in Wastewater

Water-quality management depends on knowing how organic matter influences a stream's or lake's level of dissolved oxygen. Living bacteria require oxygen to function and decompose the waste organic material to stabilise or render it harmless. BOD is frequently employed as a gauge of the level of organic contamination in water in wastewater treatment facilities.

Biochemical Oxygen Demand in River Water

Drinking water should have a BOD level considerably below 1 after five days. Untreated sewage typically has BOD levels between 200 and 600 ppm, while unpolluted rivers often have values below 1 ppm (or 1 mg/L).

Interesting Facts

• The BOD indicator does not provide absolute results. Although the indicator offers an excellent way to compare samples, it does not precisely assess the level of contamination in the water.

• Chemical Oxygen Demand (COD) is an alternate indication to BOD.

• Sulphur-reducing bacteria (SRB) are an example of anaerobic bacteria that don't require dissolved oxygen in the water to thrive. Since these microorganisms do not require oxygen to survive, their biochemical oxygen requirement cannot be measured.

Important Questions

1. What is COD?

Ans: The quantity of oxygen needed to oxidise organic material and cause Chemical Oxygen Demand (COD).

2. What happens if BOD is elevated?

Ans: The rate at which oxygen in the stream is reduced increases with BOD.

Key Features

• Oxygen is both produced and consumed by the stream system. It acquires oxygen through photosynthesis from both the atmosphere and plants.

• Running water dissolves more oxygen than still water, such that in a reservoir behind a dam, because of its churning. Oxygen is used in breathing by aquatic animals, decomposition, and many chemical processes.

• Dissolved oxygen levels drop, and some sensitive animals may flee, get feeble, or even perish if more oxygen is used than is created.

• Seasonally and throughout a 24-hour period, DO levels change. They change with height and water temperature. Water stores less oxygen at higher elevations and more oxygen when it is cold. Water is heated by thermal discharges, such as the water used to cool equipment in a factory or a power plant, and its oxygen concentration is reduced.

Early in the day on hot summer days, when stream flows are low, water temperatures are high, and aquatic plants haven't been creating oxygen since dusk, is when aquatic creatures are most vulnerable to decreasing DO levels.