Question

Define \(B O D\) and \(C O D\), and explain why their values for the same water sample can differ slightly. Explain why natural waters can have a high BOD.

Short answer

BOD measures oxygen used by microorganisms to decompose organic matter, while COD measures total oxygen demand to oxidize all substances. Natural waters may have high BOD due to decaying organic matter and nutrient-rich conditions that promote microbial growth.

Step-by-step solution

Define BOD

BOD stands for Biological Oxygen Demand. It measures the amount of oxygen required by aerobic microorganisms to decompose organic matter in a water sample, usually over a period of 5 days at 20°C. It's expressed in milligrams of oxygen per liter of water (mg/L).

Define COD

COD stands for Chemical Oxygen Demand. It quantifies the total oxygen needed to chemically oxidize all organic and inorganic substances in a water sample. The test usually uses a strong chemical oxidizing agent. COD is also expressed in mg/L.

Compare BOD and COD

BOD measures only the biodegradable organic matter over a few days, reflecting biological activity. COD, on the other hand, captures all chemical reactions of oxidizable substances, including those non-biodegradable. Therefore, COD usually gives a higher oxygen demand value than BOD because it includes non-biodegradable substances.

Natural Waters and High BOD

Natural waters can have high BOD due to the presence of decaying organic matter from plants, animals, and microorganisms. High levels of nutrients, such as nitrogen and phosphorus, can also increase BOD by promoting the growth of bacteria and algae, which increase organic matter when they die and decompose.

Key concepts

Biological Oxygen Demand (BOD)

Biological Oxygen Demand, often abbreviated as BOD, is a crucial indicator of water quality. It measures the amount of dissolved oxygen required by aerobic microorganisms to break down organic matter present in water.

These microorganisms use oxygen to decompose organic materials over a period, typically 5 days, at a specific temperature of 20°C. The result is expressed in milligrams of oxygen per liter of water (mg/L).

The BOD test assesses only biodegradable organic matter, providing insight into the level of pollution in water due to biodegradable waste.

• A higher BOD indicates a greater amount of biodegradable material in the water, suggesting potential pollution.
• It can also imply that the water body is supporting a higher rate of biological activity.

Understanding BOD helps in managing the health of aquatic environments as high BOD levels can lead to oxygen depletion, negatively affecting aquatic life.

Chemical Oxygen Demand (COD)

Chemical Oxygen Demand, abbreviated as COD, is another vital parameter for evaluating water quality. Unlike BOD, it estimates the total amount of oxygen needed to chemically oxidize both organic and inorganic substances in water.

In a COD test, a strong chemical oxidizing agent, such as potassium dichromate, is used to ensure a comprehensive oxidation of all oxidizable matter. COD values are also reported in mg/L.

• Because COD accounts for all oxidizable pollutants, not just those that are biodegradable, it usually indicates higher oxygen demand than BOD.
• This aspect makes COD a more inclusive parameter for detecting pollution.

Understanding COD is critical for identifying the presence of non-biodegradable contaminants that can affect water quality and hinder treatment processes.

Organic Matter Decomposition

Organic matter decomposition is a natural process through which organic substances are broken down by microorganisms, leading to nutrient cycling in ecosystems. In aquatic environments, this process is essential but can significantly influence water quality.

Decomposing organic material, such as plant debris and animal waste, often introduces large amounts of nutrients into the water. These nutrients can stimulate the growth of bacteria and algae, increasing the BOD of the water body.

• Natural waters may exhibit high BOD due to abundant organic matter from fallen leaves, dead plants, and microbial biomass.
• Elevated levels of nutrients like nitrogen and phosphorus further enhance microbial growth, leading to more organic matter when these organisms decay.

While decomposition is a vital ecological process, excessive organic matter can lead to oxygen depletion, stressing aquatic habitats and potentially causing harm to fish and other aquatic life forms.