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What is meant by the biochemical oxygen demand (BOD) of a water sample? The BOD of a raw sewage sample was 350 ppm and the BOD of the same sample after treatment was about 25 ppm - Leaving Cert Chemistry - Question a - 2011
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What is meant by the biochemical oxygen demand (BOD) of a water sample? The BOD of a raw sewage sample was 350 ppm and the BOD of the same sample after treatment wa... show full transcript
Worked Solution & Example Answer:What is meant by the biochemical oxygen demand (BOD) of a water sample? The BOD of a raw sewage sample was 350 ppm and the BOD of the same sample after treatment was about 25 ppm - Leaving Cert Chemistry - Question a - 2011
Step 1
What is meant by the biochemical oxygen demand (BOD) of a water sample?
Answer
The biochemical oxygen demand (BOD) of a water sample refers to the amount of oxygen consumed by microorganisms during the decomposition of organic matter when the sample is kept in the dark for five days at a temperature of 20°C (293 K). It is measured in parts per million (ppm) and indicates the level of organic pollution in the water.
Step 2
Describe how the BOD was reduced by about 30% in primary sewage treatment.
Answer
In primary sewage treatment, the BOD is reduced primarily through the removal of solid debris and particles. This is achieved by:
- Screening: Large solids and debris are removed from the sewage through screens, which prevents clogging and reduces the load on subsequent treatment stages.
- Sedimentation: The sewage is allowed to settle in large sedimentation tanks, where heavier particles settle at the bottom, forming sludge. This process also helps in removing grit and other heavier materials, thereby decreasing the overall BOD.
Step 3
Explain the processes by which the BOD was further reduced in secondary sewage treatment.
Answer
In secondary sewage treatment, the reduction of BOD occurs primarily through biological processes. The key processes include:
- Digestion: This involves the breakdown, decomposition, and oxidation of organic matter by microorganisms.
- Aerobic and Anaerobic Processes: The treatment can take place in aerobic conditions (where air is mixed to provide oxygen) or anaerobic conditions (without air). The presence of microorganisms (especially bacteria) plays a crucial role in oxidizing the organic material.
- Activated Sludge: This method involves maintaining a population of microorganisms in suspension in the wastewater, promoting efficient breakdown of organic material, thus further reducing the BOD.