Wastewater quality basics: Select the correct statements about pH and the typical pH of fresh domestic sewage.
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ApH value indicates acidity and alkalinity of sewage
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BIn acidic sewage, the pH value is less than 7
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CIn alkaline sewage, the pH value is more than 7
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DFresh sewage is generally alkaline
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EAll the above.
Answer
Correct Answer: All the above.
Explanation
Introduction / Context:
pH is a fundamental parameter in wastewater engineering. It influences biological activity, corrosion/scale tendencies in sewers, and the effectiveness of many treatment processes such as coagulation and disinfection.
Given Data / Assumptions:
- The statements describe what pH represents and typical pH ranges in fresh sewage.
- Fresh domestic sewage commonly shows mild alkalinity (pH slightly above 7).
- Anaerobic decomposition over time may reduce pH (becoming acidic), but that is not “fresh.”
Concept / Approach:
By definition, pH < 7 is acidic, pH = 7 is neutral, and pH > 7 is alkaline. Fresh municipal sewage typically has a pH around 7.2–8.0 due to bicarbonate alkalinity and the nature of household waste streams.
Step-by-Step Solution:
Confirm that pH expresses hydrogen ion activity and thus acidity/alkalinity.Relate acidic sewage to pH < 7 and alkaline sewage to pH > 7.Recognize fresh sewage is usually slightly alkaline; hence all statements are correct.Verification / Alternative check:
Design manuals for wastewater treatment list typical fresh sewage pH as mildly alkaline; long retention (septic conditions) often shifts pH downward.
Why Other Options Are Wrong:
- Each individual statement (a–d) is correct; therefore “All the above” is the most complete choice.
Common Pitfalls:
- Assuming all sewage is acidic; freshness and aeration conditions matter.
- Ignoring industrial inputs which can alter pH significantly.
Final Answer:
All the above.