Questions about reverse osmosis and total dissolved solids

Answers to these questions appear online at www.watertechonline.com/certification-action-line.

Certification Action Line features questions and answers typical of those appearing in Water Quality Association (WQA) certification examinations. Some answers may not satisfy everyone or every condition.

1. A stainless steel grade that is most resistant to corrosion, e.g. to the corrosivity of seawater or RO concentrate with several thousand milligrams per liter of chlorides, is:

a) 304

b) 304L

c) 316

d) 316L

2. The number of protons (or neutrons) whose weight totals 1 gram is:

a) a mole

b) 6.02 x 10²³

c) Avogadro’s number

d) all of the above

3. The attack and oxidation of resin and membranes by chlorine is (a) or (b) in the presence of iron, manganese or copper.

a) increased

b) decreased

4. Dissolved salts or total dissolved solids (TDS) in reverse osmosis (RO) feed water create a natural osmotic water pressure tending to pull low TDS product water back through the semipermeable membrane to dilute or equilibrate the TDS level in the feed and concentrate water. This natural osmotic pressure equals approximately:

          a) one psi for each milligram per liter (mg/L) of TDS differential across the membrane

b) 0.01 psi for each mg/L of TDS differential across the membrane

c) one psi for 100 mg/L of TDS differential across the membrane

d) b and c

5. RO permeate flow rate divided by RO feed flow rate equals:

          a) TDS rejection

b) permeate recovery

c) reject flow rate

d) gallons per day of product water

6. RO feed water TDS concentration divided by [1-recovery (as a fraction)] equals: TDS/(1-recovery) = ?

          a) TDS concentration of reject water

b) TDS concentration of product water

c) TDS rejection (as a fraction)

d) average TDS concentration of RO concentrate

7. The average natural osmotic pressure exerted by feed water/concentrate in an RO system operating at 30 percent recovery and with a feed water containing 1,200 mg/L of TDS (assuming negligible TDS in the permeate water) would be about:

          a) 12 psi

b) 17 psi

c) 15 psi

d) 10 psi

8. Which of the following salts in water will have the highest rejection by RO membranes?

          a) magnesium bicarbonate, Mg (HCO₃)₂

b) calcium sulfate, CaSO₄

c) sodium chloride, NaCl

d) sodium nitrate, NaNO₃

9 & 10. For most water supplies, RO permeate will have a (9) a, b or c pH and the reject water will have a (10) a, b or c pH than (as) the pH of feed water.

   a) higher

b) lower

c) the same