CPO Pool Maintenance and Troubleshooting 2 — Questions and Answers
Question 1: Pool water appears cloudy despite free chlorine and pH being within normal range. What is the most likely cause?
- Insufficient total alkalinity below 80 ppm
- Poor filtration or inadequate water circulation (Correct answer)
- Excessive calcium hardness above 400 ppm
- Cyanuric acid levels above 50 ppm
Correct answer: Poor filtration or inadequate water circulation
When chlorine and pH are normal, cloudy water typically results from poor filtration efficiency or insufficient circulation rather than chemical imbalance.
Question 2: A pool has developed a widespread green algae bloom. What is the recommended initial treatment?
- Add copper-based algaecide and resume normal filtration
- Raise pH to 8.0 to create a hostile environment for algae
- Shock the pool with chlorine at approximately 10 times the normal dose (Correct answer)
- Drain the pool by 50% and refill with fresh water
Correct answer: Shock the pool with chlorine at approximately 10 times the normal dose
Shocking with a heavy chlorine dose is the first-line treatment to kill green algae, followed by brushing surfaces and filtering out dead algae.
Question 3: Mustard (yellow) algae is particularly difficult to eliminate because it:
- Only grows in pools with pH above 8.0
- Is resistant to normal chlorine levels and clings tightly to pool walls (Correct answer)
- Cannot be treated with any chlorine-based product
- Reproduces only in heated pools above 85°F
Correct answer: Is resistant to normal chlorine levels and clings tightly to pool walls
Mustard algae is chlorine-resistant at normal concentrations and adheres stubbornly to surfaces, requiring aggressive shock treatment and thorough brushing.
Question 4: Black algae is the most difficult pool algae to eliminate primarily because:
- Standard chlorine products have absolutely no chemical effect on it
- It has a protective outer layer and deep roots that penetrate porous surfaces (Correct answer)
- It only grows in areas with zero UV light exposure
- Pool filters are unable to remove its reproductive spores
Correct answer: It has a protective outer layer and deep roots that penetrate porous surfaces
Black algae's waxy protective coating and deep roots in plaster require aggressive physical brushing and very high chlorine concentrations to penetrate and kill.
Question 5: A sand filter's clean operating pressure is 10 psi. At what pressure reading should backwashing be initiated?
- 15–18 psi (5–8 psi above clean starting pressure) (Correct answer)
- When pressure drops below 5 psi
- Every 24 hours regardless of the gauge reading
- When pressure doubles to exactly 20 psi
Correct answer: 15–18 psi (5–8 psi above clean starting pressure)
Backwashing should occur when pressure rises 5–8 psi above the clean baseline, indicating sufficient dirt accumulation to reduce filtration efficiency.
Question 6: After backwashing a sand filter, what step should be performed before returning to normal filtration mode?
- Immediately switch the multiport valve back to filter mode
- Run on the 'rinse' setting for 30–60 seconds to re-seat the sand bed (Correct answer)
- Add fresh filter sand to replace any lost during backwashing
- Shock the pool to compensate for any chlorine dilution
Correct answer: Run on the 'rinse' setting for 30–60 seconds to re-seat the sand bed
The rinse cycle re-settles the disturbed sand bed and clears residual dirty water from the filter, preventing a turbid water surge back into the pool.
Question 7: At a pool pH of 7.5, approximately what percentage of free chlorine exists as the active hypochlorous acid (HOCl) form?
- About 25%
- About 75%
- About 10%
- About 50% (Correct answer)
Correct answer: About 50%
At pH 7.5, which is very close to HOCl's pKa of 7.54, the free chlorine is split roughly 50/50 between the active HOCl and the less effective hypochlorite ion (OCl-).
CPO Pool Maintenance and Troubleshooting 2
Pool water appears cloudy despite free chlorine and pH being within normal range.
What is the most likely cause?