CWS System Sizing & Design 3 — Questions and Answers
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)?
- 45 psi (Correct answer)
- 60 psi
- 75 psi
- 15 psi
Correct answer: 45 psi
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?
- Higher salt efficiency in the tall narrow tank due to better brine channeling
- Better service flow rates in the tall narrow tank due to longer contact time and lower upflow velocity (Correct answer)
- Lower backwash flow requirements in the short wide tank
- No meaningful operational difference between the two configurations
Correct answer: Better service flow rates in the tall narrow tank due to longer contact time and lower upflow velocity
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?
- Lower-than-average daily water usage by occupants
- Future plans for additional bathrooms or irrigation expansion (Correct answer)
- Source water with naturally low TDS
- A high-efficiency dishwasher replacing a standard model
Correct answer: Future plans for additional bathrooms or irrigation expansion
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?
- UV lamp wattage and water temperature
- UV intensity and exposure time (flow rate) (Correct answer)
- Flow rate and water hardness
- Lamp age and inlet pressure
Correct answer: UV intensity and exposure time (flow rate)
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?
- 2-gallon tank
- 6-gallon tank
- 20-gallon tank (Correct answer)
- 32-gallon tank
Correct answer: 20-gallon tank
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.
Question 6: An acid neutralizer using calcite media is sized for a 10 gpm service flow. To prevent calcite loss during backwash, what maximum backwash rate is typically specified?
- 2 gpm per square foot of bed area
- 5 gpm per square foot of bed area (Correct answer)
- 10 gpm per square foot of bed area
- 15 gpm per square foot of bed area
Correct answer: 5 gpm per square foot of bed area
Calcite backwash rates are typically kept at or below 5 gpm/ft² to achieve bed expansion without washing calcite particles out of the tank.
Question 7: When sizing a chemical feed pump for chlorination, the pump output must be matched to which primary variable?
- The chlorine demand of the water multiplied by the flow rate (Correct answer)
- The pipe diameter of the injection point
- The distance from the chemical tank to the injection point
- The inlet water temperature
Correct answer: The chlorine demand of the water multiplied by the flow rate
Chemical feed rate (gpd or ml/min) must equal the chlorine demand (mg/L) × flow rate to achieve the target residual concentration.
CWS System Sizing & Design 3
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)?