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What is meant by the biochemical oxygen demand (BOD) of a water sample? Describe clearly the processes involved in the primary and secondary stages of urban sewage treatment - Leaving Cert Chemistry - Question d e - 2005
Question d e
What is meant by the biochemical oxygen demand (BOD) of a water sample? Describe clearly the processes involved in the primary and secondary stages of urban sewage... show full transcript
Worked Solution & Example Answer:What is meant by the biochemical oxygen demand (BOD) of a water sample? Describe clearly the processes involved in the primary and secondary stages of urban sewage treatment - Leaving Cert Chemistry - Question d e - 2005
Step 1
What is meant by the biochemical oxygen demand (BOD) of a water sample?
Answer
Biochemical Oxygen Demand (BOD) is a measure of the amount of oxygen that microorganisms will consume while decomposing organic matter in a water sample at a specified temperature, usually at 20°C, over a period of five days. It is typically expressed in parts per million (p.p.m.) or milligrams per liter (mg/l) of oxygen consumed. A higher BOD value indicates a greater amount of organic material in the water, suggesting poorer water quality.
Step 2
Describe clearly the processes involved in the primary stages of urban sewage treatment.
Answer
The primary stage of urban sewage treatment involves the removal of large solids and debris from the sewage. This is achieved through processes such as screening and sedimentation.
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Screening: This initial step involves passing the sewage through screens to filter out large particles such as twigs, plastics, and other debris.
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Sedimentation: After screening, the sewage is allowed to settle in a sedimentation tank, where gravity pulls the suspended solids to the bottom, forming sludge, while clearer liquid moves to the next treatment stage.
Step 3
Describe clearly the processes involved in the secondary stages of urban sewage treatment.
Answer
The secondary stage focuses on biological processes that further treat the sewage by breaking down organic matter.
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Aerobic Treatment: This process uses oxygen to break down the organic material. Microorganisms, including bacteria, consume the organic matter, which significantly reduces BOD levels.
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Anaerobic Treatment: In some treatments, without oxygen, microorganisms decompose organic materials, producing biogas and further reducing the sludge volume. This stage is crucial for enhancing the quality of the treated water before it is released into the environment.
Step 4
What substances are removed by tertiary treatment of sewage?
Answer
Tertiary treatment of sewage is designed to remove residual nutrients, particularly nitrogen and phosphorus compounds. The treatment focuses on the following substances:
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Nitrates (Nitrogen Compounds): These are often removed through biological processes or chemical treatments to prevent eutrophication in receiving water bodies.
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Phosphates (Phosphorus Compounds): The removal of phosphates can occur through chemical precipitation or biological uptake by algae and other organisms, improving the quality of the effluent.