Water Technology Questions and Answers

Answer: C

Correct answer: hydroxyl form

Demineralisation uses two beds in series. The cation exchanger in the hydrogen form replaces every cation with a hydrogen ion, so the water leaving it is a dilute mixture of acids. The anion exchanger in the hydroxyl form then replaces every anion with hydroxide, which neutralises those hydrogen ions to water - so both the salts and the acidity disappear together.

Answer: D

Correct answer: dilute acid and dilute alkali

Regeneration drives the exchange in reverse, so each bed must be flooded with a large excess of the ion it is to be restored to. The cation bed is returned to the hydrogen form with dilute sulphuric or hydrochloric acid, and the anion bed to the hydroxyl form with dilute sodium hydroxide. Brine is for a zeolite bed, which works in the sodium form.

Answer: A

The resins exchange ions and pass micro-organisms through unharmed, and a bed standing idle may even harbour and multiply them. Softness, purity of dissolved solids and freedom from pathogens are three separate properties and each needs its own treatment.

Answer: B

Correct answer: treats the water after it has entered the boiler

Internal conditioning doses chemicals directly into the boiler so that any hardness getting through is converted into a harmless sludge that can be blown down. It is a supplement to external treatment rather than a substitute, since it deals with the residue that softening leaves rather than preventing the hardness from arriving.

Answer: C

Correct answer: soft sludge

Sodium phosphate precipitates calcium as calcium phosphate, which forms in the bulk of the water as a loose non-adherent sludge rather than crystallising on the hot metal. The sludge is removed by blowdown. This is the point of the treatment: it does not stop calcium precipitating but changes the form in which it does so.

Answer: D

Correct answer: forming a soluble complex with calcium ions

Calgon is sodium hexametaphosphate, and it works by sequestration: it binds calcium into a soluble complex ion so that the calcium is never available to react with sulphate at all. Nothing precipitates, so there is neither scale nor sludge to remove - which distinguishes it from phosphate conditioning, where a sludge is deliberately produced.

Answer: A

A film round each precipitated particle keeps it from bonding to the metal or to its neighbours, so it settles loosely and can be blown down. The agents are organic and would decompose at high temperature, so the method suits low pressure boilers only.

Answer: B

Correct answer: sodium carbonate in the boiler water

Maintaining an excess of carbonate ions ensures that calcium precipitates as the carbonate, which is a loose sludge, before it can reach saturation as the sulphate and crystallise on the tube as scale. It converts a scale-forming salt into a sludge-forming one, and the sludge is removed by blowdown.

Answer: C

Correct answer: water passes through the membrane and salt is retained

Osmosis drives water from dilute to concentrated across a semi-permeable membrane. Applying a pressure above the osmotic pressure on the concentrated side reverses the flow, so water is pushed through while the dissolved salts are held back. Nothing evaporates and nothing crystallises - the separation is entirely a matter of what the membrane will pass.

Answer: D

The membrane's rejection extends down to individual ions, which no conventional filter approaches, and it holds back bacteria and dissolved organic matter as well. Its main operating problems are fouling and scaling of the membrane surface.