CWS System Sizing & Design 2 — Questions and Answers
Question 1: A household has 3 bathrooms, a kitchen, a dishwasher, and a washing machine. Using a fixture unit approach, what is the PRIMARY purpose of calculating total fixture units?
- To determine pipe diameter for drain lines only
- To estimate peak simultaneous water demand for system sizing (Correct answer)
- To calculate the total daily water consumption
- To determine the number of treatment stages required
Correct answer: To estimate peak simultaneous water demand for system sizing
Fixture unit calculations estimate peak simultaneous demand so the treatment and distribution system can handle maximum flow without pressure loss.
Question 2: When sizing a water softener for a family of 4 using water with 20 gpg hardness and 70-gallon-per-person daily usage, what is the calculated daily hardness load?
- 280 grains
- 1,400 grains
- 5,600 grains (Correct answer)
- 2,800 grains
Correct answer: 5,600 grains
Daily load = 4 persons × 70 gpd × 20 gpg = 5,600 grains per day.
Question 3: A softener resin bed is rated at 30,000 grains capacity. If the daily hardness load is 5,600 grains, what is the maximum number of days between regenerations before capacity is exhausted?
- 3 days
- 4 days
- 5 days (Correct answer)
- 6 days
Correct answer: 5 days
30,000 ÷ 5,600 = 5.35 days, so regeneration should be set no longer than every 5 days to avoid hardness breakthrough.
Question 4: Which factor most directly determines the minimum freeboard space required above the resin bed in a softener tank?
- The salt dose per regeneration
- The backwash flow rate and bed expansion percentage (Correct answer)
- The hardness level of the source water
- The diameter of the distributor tube
Correct answer: The backwash flow rate and bed expansion percentage
Freeboard must accommodate bed expansion during backwash, which is determined by the backwash flow rate and the expansion characteristics of the resin.
Question 5: A carbon filter is being designed to reduce chlorine from municipal water at a service flow rate of 8 gpm. The empty bed contact time (EBCT) required is 5 minutes. What minimum volume of carbon is needed?
- 1.6 cubic feet (Correct answer)
- 40 cubic feet
- 0.6 cubic feet
- 4 cubic feet
Correct answer: 1.6 cubic feet
EBCT = bed volume (ft³) / flow rate (gpm); rearranged: volume = 8 gpm × 5 min = 40 gallons ≈ 5.35 ft³, but converting gpm to gal/min keeps it simple: 8 gpm × 5 min / 7.48 gal/ft³ ≈ 5.35 ft³.
Question 6: When designing a reverse osmosis system for a commercial application, the recovery rate refers to which of the following?
- Percentage of feed water converted to permeate (product water) (Correct answer)
- The rate at which membranes are replaced
- The ratio of permeate TDS to feed TDS
- The flow rate of concentrate returned to the feed
Correct answer: Percentage of feed water converted to permeate (product water)
Recovery rate is the percentage of feed water that passes through the membrane as product water, with the remainder discharged as concentrate.
Question 7: For a point-of-entry iron filter using oxidizing media, which design parameter most critically affects the system's ability to remove ferrous iron?
- Tank diameter
- Service flow rate relative to media bed depth (contact time) (Correct answer)
- Inlet pressure
- Backwash water temperature
Correct answer: Service flow rate relative to media bed depth (contact time)
Sufficient contact time between water and oxidizing media is critical for converting ferrous iron to ferric iron so it can be filtered.
CWS System Sizing & Design 2
A household has 3 bathrooms, a kitchen, a dishwasher, and a washing machine.
Using a fixture unit approach, what is the PRIMARY purpose of calculating total fixture units?