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Pig slurry has a very high biochemical oxygen demand (BOD) and therefore effluent from pig farming must be very carefully managed - Leaving Cert Chemistry - Question b - 2021
Question b
Pig slurry has a very high biochemical oxygen demand (BOD) and therefore effluent from pig farming must be very carefully managed. Spreading it on the ground as a fe... show full transcript
Worked Solution & Example Answer:Pig slurry has a very high biochemical oxygen demand (BOD) and therefore effluent from pig farming must be very carefully managed - Leaving Cert Chemistry - Question b - 2021
Step 1
Explain the underlined term.
Answer
Biochemical Oxygen Demand (BOD) is a measure of the amount of dissolved oxygen consumed by microorganisms while decomposing organic matter in a water sample over a specific period, typically five days, at a temperature of 20°C. It is an important indicator of the organic pollution level in water.
Step 2
Why is the run-off from pig slurry fertilizer, or the addition of any other solution with a high BOD, into waterways undesirable?
Answer
Run-off can lead to algal blooms (eutrophication) which depletes oxygen in the water, negatively affecting aquatic life. It can also cause de-oxygenation and result in fish kills, making water unsafe for leisure activities and unfit for human consumption.
Step 3
How could a process like the secondary stage of sewage treatment reduce the quantities of organic matter in pig slurry effluent?
Answer
The secondary stage of sewage treatment involves biological processes such as oxidation and digestion where microorganisms (bacteria) break down organic matter. This process helps in removing excess nutrients and organic pollutants, thereby reducing the BOD of the effluent.
Step 4
Why was the raw slurry diluted and aerated before analysis?
Answer
Diluting and aerating the raw slurry ensures that the oxygen available for testing is adequately measured and avoids results influenced by high concentrations of organic matter. It also prevents immediate deoxygenation that might occur if the sample is too concentrated.
Step 5
Calculate the BOD of the raw pig slurry.
Answer
To find the BOD, use the dilution factor and the change in oxygen concentrations.
BOD = (Oxygen decrease × Volume of diluted sample) / (Volume of raw sample)
Using the data:
BOD = (2.7 p.p.m. × 5.0 L) / (0.050 L) = 270,000 p.p.m.
Therefore, the BOD of the raw pig slurry is , ext{p.p.m.}$.