Water Technology Questions and Answers

Answer: C

Correct answer: the assertion is false although the reason offered is a true statement in itself

The reason is true - zeolite softening does reduce hardness to a very low figure - but the assertion does not follow. The process exchanges rather than removes, so the total dissolved solids are unchanged and the water leaves carrying sodium salts, including bicarbonate, which decomposes in the boiler to give caustic alkalinity. A high pressure boiler needs demineralised water.

Answer: D

Correct answer: also removes organics and micro-organisms

Reverse osmosis rejects on the basis of size and charge, so it holds back not only dissolved ions but organic molecules, colloids, bacteria and viruses. Ion exchange acts on ions only and passes organics straight through. Ion exchange can achieve the lower dissolved solids of the two, so the choice is not simply which is better but which contaminants matter.

Answer: A

Correct answer: dosing hydrazine

Hydrazine is an oxygen scavenger, reacting with dissolved oxygen to give nitrogen and water and adding no dissolved solids. None of the others touches oxygen: sodium carbonate supplies carbonate for conditioning, calgon sequesters calcium, and a zeolite bed exchanges cations. Deaeration is a separate problem from softening and needs its own treatment.

Answer: B

The free chlorine liberates iodine from KI, and the iodine is titrated with N/50 hypo. The hypo used is 2.5 ml of 0.02 N, which is 0.05 milli-equivalents. One milli-equivalent of chlorine is 35.5 mg, so the chlorine in the 50 ml sample is

Per litre (1000 ml) this is 1.775 x 20 = 35.5 mg/L, that is 35.5 ppm. The options 0.0355 and 0.001 are factors of 1000 smaller, which would result from stating the answer in grams per litre or from mixing up the units, and 1 has no basis in the data.

Answer: A

When the zeolite bed (sodium zeolite, Na2Z) has exchanged all its sodium for the Ca and Mg ions of hard water, it is regenerated by passing a concentrated brine solution of NaCl (about 10 percent) over it. The large excess of sodium ions reverses the exchange and restores Na2Z. Dilute H2SO4 and dilute HCl are regenerants for the hydrogen (cation) exchange resins of a demineralisation plant, and dilute NaOH is used to regenerate the anion exchange resin, not the zeolite softener.