Water Technology Questions and Answers

Answer: A

Correct answer: 4.8 mg/L

The biochemical oxygen demand is simply the oxygen consumed during the incubation: \[ 8.2 - 3.4 = 4.8\ \mathrm{mg/L}. \] The five days at 20 °C are the standard conditions, chosen by convention rather than because the oxidation is complete - the test measures a defined fraction of the total demand, which is why the conditions must always be quoted with the result.

Answer: B

Correct answer: 1-Q, 2-P, 3-R

Three distinct boiler troubles with three distinct causes. Caustic embrittlement is intergranular cracking where free alkali concentrates in the crevices at riveted or stressed joints. Priming is the carrying over of water with the steam. And scale is the hard deposit left where a salt such as calcium sulphate becomes less soluble at the hot surface and crystallises on to it.

Answer: C

Correct answer: 1-Q, 2-P, 3-R

Each process is named by the ion the bed gives up. Zeolite softening works in the sodium form and releases sodium ions in exchange for calcium and magnesium. In demineralisation the cation exchanger is in the hydrogen form and the anion exchanger in the hydroxyl form, so the two released ions combine to make water and the salts disappear entirely.

Answer: D

Screens take the coarse matter, coagulation and settling the colloidal, the sand filter the remainder, and chlorine is added last so that its residual survives into the distribution system. Disinfecting before filtration wastes most of the dose.

Answer: A

The first and third are correct. Zeolite exchanges cations only, so the sulphates and chlorides remain in the water paired with sodium and the total dissolved solids are unchanged. Removing the anions as well requires a hydroxyl-form anion exchanger in series.

Answer: B

Sodium salts are freely soluble and form no precipitate with soap or on a boiler tube, however concentrated they become. The other three are all hardness-producing salts of calcium or magnesium, one temporary and two permanent.

Answer: C

Lime-soda is an external treatment applied in its own plant before the water ever enters the boiler. The other three are chemicals dosed into the boiler itself to deal with whatever hardness arrives there despite the external treatment.

Answer: D

Correct answer: a deposit insulating that part of the tube

A bulge means the metal reached a temperature at which it lost its strength, and with normal firing the only way that happens is if heat cannot pass into the water. A local deposit - scale or a settled sludge - does exactly that. Soft feed water does not preclude it, since the deposit may be old, or may have come from a carry-over or a leak rather than from hardness.

Answer: A

Correct answer: the analysis is in error

Hydroxide and bicarbonate cannot coexist in the same solution: hydroxide immediately converts bicarbonate to carbonate and water. A sample may contain hydroxide with carbonate, or carbonate with bicarbonate, but never the first and the last together, so a result reporting both indicates a mistake in the titration or the calculation.

Answer: B

Correct answer: both the assertion and the reason are true and the reason explains the assertion

Both are true and the reason is exactly the mechanism. Because calcium sulphate becomes less soluble as temperature rises, saturation is reached first at the hottest surface in the boiler, which is the tube wall, so the salt crystallises directly on to the metal as an adherent scale. A salt behaving the other way would precipitate in the cooler bulk water and settle out as sludge.