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GATE 2025 CE (CE1) – Question 45

Water and Waste Water Quality and Treatment · Primary, secondary and tertiary treatment, effluent standards · 2 marks · Multiple select

A facultative pond system is employed for wastewater treatment. Which of the following statements is/are true:

  1. The dissolved oxygen concentration will be high during daytime compared to night-time
  2. The pH will be high during daytime compared to night-time
  3. The dissolved oxygen concentration will be low during daytime compared to night-time
  4. The pH will be low during daytime compared to night-time

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Show answer and explanation

Correct answer: (A) The dissolved oxygen concentration will be high during daytime compared to night-time; (B) The pH will be high during daytime compared to night-time

Explanation

In the upper layer of a facultative pond algae photosynthesise in daylight, releasing oxygen and using up carbon dioxide. So the dissolved oxygen is high by day, and removing carbon dioxide also raises the pH. At night photosynthesis stops, respiration continues, and the oxygen and the pH fall. A and B are true.