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Water process practice questions

22 questions from the advanced water and wastewater operator practice test.

Water process practice test

10 questions, graded as you go, with the reason for every answer. No sign-up.

What this covers

Sample questions

During a storm an operator raises the alum dose from 25 to 55 mg/L. Settled water turbidity gets worse instead of better, and the coagulated water pH has dropped to 5.4. What is the most likely problem?

  1. The alum dose is still too low for the amount of turbidity in the raw water
  2. The rapid mixer is running too fast for the higher coagulant dose
  3. The colder storm water has made the alum solution thicken in the feed line
  4. The alum has used up the alkalinity and driven the pH too low
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D. The alum has used up the alkalinity and driven the pH too low

Alum consumes alkalinity and lowers pH. In a poorly buffered water a large dose can drive the pH below the range where aluminum hydroxide floc forms well (roughly 5.5 to 7.5), so performance falls off. The fix is supplemental alkalinity such as lime, soda ash or caustic, not still more alum, which would lower the pH further.

Reference: AWWA Water Treatment Plant Operation, coagulation chemistry: alum, alkalinity and pH

A jar test series with a cationic polymer shows settled turbidity improving up to a dose of 3 mg/L, then getting steadily worse at 5, 7 and 9 mg/L while pH stays the same. What explains the poorer results at the higher doses?

  1. Excess positive charge has restabilized the particles
  2. The polymer has consumed all of the water's alkalinity
  3. The higher doses have made the floc too heavy to stay intact
  4. The polymer needs a longer rapid mix time at higher doses
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A. Excess positive charge has restabilized the particles

Coagulation by charge neutralization works best when the negative particle charge is just cancelled. Past that point the particles take on a positive charge and repel each other again, so turbidity removal drops. Polymers have little effect on alkalinity, which is why pH did not move.

Reference: Jar testing and charge neutralization; AWWA M37

How does very cold raw water usually affect coagulation and sedimentation?

  1. Floc forms faster but breaks up in the basin
  2. Floc forms normally but floats to the surface
  3. Floc forms more slowly and settles more slowly
  4. Floc forms faster and settles more quickly
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C. Floc forms more slowly and settles more slowly

Chemical reactions slow down in cold water and the water becomes more viscous, so floc is slower to form and slower to settle. Operators compensate with a coagulant aid, a different coagulant, or more gentle mixing time. Nothing about cold water speeds settling; the higher viscosity works against it.

Reference: AWWA Water Treatment Plant Operation, effect of temperature on coagulation and settling

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