CWS System Sizing & Design 3

Question 1

A reverse osmosis unit has a feed pressure of 60 psi, a permeate pressure of 0 psi, and an osmotic pressure of 15 psi. What is the net driving pressure (NDP)?

Accepted Answer

45 psi

Answer

NDP = feed pressure − permeate pressure − osmotic pressure = 60 − 0 − 15 = 45 psi.

Question 2

When selecting tank dimensions for a residential softener, a tall narrow tank versus a short wide tank of equal volume will produce what design difference?

Accepted Answer

Better service flow rates in the tall narrow tank due to longer contact time and lower upflow velocity

Answer

A taller, narrower tank provides longer contact time and lower linear velocity during service flow, improving ion exchange efficiency.

Question 3

Which condition would require INCREASING the size of a planned water treatment system beyond the initial design calculation?

Accepted Answer

Future plans for additional bathrooms or irrigation expansion

Answer

Anticipated future demand increases, such as adding bathrooms or irrigation, must be factored into design to avoid premature capacity exhaustion.

Question 4

For a UV disinfection system, the 'dose' delivered to the water is a function of which two parameters?

Accepted Answer

UV intensity and exposure time (flow rate)

Answer

UV dose (mJ/cm²) = UV intensity (mW/cm²) × exposure time (seconds), where exposure time is determined by flow rate through the chamber.

Question 5

A well pump delivers water at 10 gpm. A pressure tank is being sized to minimize pump short-cycling. If the minimum drawdown volume required is 5 gallons per cycle, what tank size is most appropriate using a 20/40 psi pressure switch?

Accepted Answer

20-gallon tank

Answer

At 20/40 psi with an air pre-charge at 18 psi, a 20-gallon tank provides approximately 5-7 gallons of drawdown, meeting the minimum cycle requirement.

(CWS) Certified Water Specialist Practice Test

(CWS) Certified Water Specialist Practice Test

CWS System Sizing & Design 3