BOD (biological oxygen demand) - The water quality indicator

Summary

Biological Oxygen Demand (BOD) is a key measure of water quality, indicating how much oxygen is needed for microorganisms to break down organic matter. The BOD5 test is commonly used to measure this, with a sample incubated for five days. Higher BOD levels reflect increased pollution. While BOD measures biological oxygen consumption, Chemical Oxygen Demand (COD) measures all oxidizable material in water. Wastewater treatment improves BOD removal, often through methods like film media blocks, which provide a large surface area for microorganisms. Typical BOD values vary from 2-8 mg/l in moderately polluted rivers to 300-600 mg/l in untreated sewage.

Takeaways

• 😀 BOD (Biological Oxygen Demand) is a key assessment to determine water quality, measuring the oxygen needed by organisms to break down organic matter.
• 😀 COD (Chemical Oxygen Demand) measures everything that can be chemically oxidized, but it is less specific than BOD.
• 😀 The BOD removal process is described by the equation: Organic matter + O2 → CO2 + H2O.
• 😀 Higher water pollution levels result in higher BOD, as more organic matter requires more oxygen for oxidation.
• 😀 The most common BOD test is the BOD5 test, which measures oxygen consumption over five days.
• 😀 The BOD5 test uses a sample of wastewater, incubated for five days, where oxygen consumption is measured under light absence.
• 😀 After five days of incubation, most organic material has been degraded, allowing for an accurate BOD measurement.
• 😀 Typical BOD levels in moderately polluted rivers range from 2 to 8 mg/l.
• 😀 Untreated sewage in the US has a BOD level of about 300 mg/l, while European sewage averages around 600 mg/l.
• 😀 The lower BOD level in the US is due to a higher per capita water use.
• 😀 Wastewater treatment plants often increase biological organisms by providing more surface area for attached growth, using film media plastic blocks.

Q & A

What does BOD stand for and why is it important for water quality assessment?

BOD stands for Biological Oxygen Demand, and it is crucial for determining water quality because it measures the amount of oxygen required by aerobic organisms to break down organic matter in the water.

How does BOD differ from COD?

BOD refers specifically to the oxygen needed by aerobic organisms to break down organic material, whereas COD (Chemical Oxygen Demand) measures the total oxygen required to chemically oxidize all substances, not just the organic matter.

What biochemical process is involved in BOD removal?

The biochemical process of BOD removal involves organic matter reacting with oxygen (O2), converting into carbon dioxide (CO2) and water (H2O).

Why is a higher BOD level indicative of more pollution?

A higher BOD level indicates that there is more organic matter in the water, which requires more oxygen for oxidation. This suggests a higher degree of water pollution.

What is the BOD5 test and why is it used?

The BOD5 test is a common laboratory method to measure BOD. It involves incubating one liter of wastewater for five days and measuring the oxygen consumed. The five-day period is chosen because most organic matter is degraded during this time.

How is oxygen consumption measured in the BOD5 test?

Oxygen consumption in the BOD5 test is measured by monitoring the decline in pressure caused by oxygen being consumed by aerobic organisms, as the test sample is agitated in the absence of light.

What are typical BOD levels in moderately polluted rivers?

Typical BOD levels in moderately polluted rivers range from 2 to 8 milligrams per liter (mg/l).

How does the BOD level of untreated sewage in the US compare to that in Europe?

Untreated sewage in the US typically has a BOD level of about 300 mg/l, whereas European sewage has a higher average BOD level of around 600 mg/l. This difference is partly due to the higher water use per capita in the US.

What technologies are used to increase BOD removal in wastewater treatment plants?

Wastewater treatment plants use various technologies to enhance BOD removal, including increasing the population of biological organisms by providing more surface area for attached growth. One common method is the use of film media plastic blocks that improve water and air mixing.

How much surface area can one film media plastic block provide for biological growth?

One film media plastic block can provide up to 250 square meters of surface area for biological organisms to grow.