Water Technology Questions and Answers

Answer: A

Alternating cation and anion permeable membranes let the applied field concentrate the salts in alternate compartments and leave the others diluted. Its energy consumption rises with the salinity, which limits it to brackish water.

Answer: B

Alum or ferric salts hydrolyse to a gelatinous hydroxide that neutralises the charge on the colloidal clay and entraps particles too fine to settle on their own. Disinfection and softening are entirely separate steps carried out elsewhere in the plant.

Answer: C

Correct answer: a gelatinous aluminium hydroxide floc

Alum hydrolyses in water to give aluminium hydroxide as a voluminous gelatinous precipitate. That floc has an enormous surface area and a positive charge, so it neutralises the negative charge on the colloidal particles and then entraps them as it settles. The clarity of the treated water comes from the settling of the floc, not from any dissolution.

Answer: D

Chlorine first destroys the reducing matter and forms chloramines, whose residual passes through a maximum and then falls to a minimum; beyond that point further chlorine remains free. Dosing beyond it also removes taste and odour.

Answer: A

Hypochlorous acid kills far more rapidly and at a lower concentration than the chloramines do, which is why the break point is aimed for. Chloramine's one advantage is that it persists longer in a distribution system and so guards against recontamination.

Answer: B

Correct answer: calcium hypochlorite with calcium chloride

Bleaching powder is a mixed product of chlorine and slaked lime, containing calcium hypochlorite together with calcium chloride. The hypochlorite is the active disinfectant, releasing hypochlorous acid in water. It is convenient because it is a solid, but it loses strength in storage and leaves a lime residue, which is why liquid hypochlorite or chlorine gas is preferred at scale.

Answer: C

Correct answer: no taste, odour or chlorinated by-product

Ozone oxidises without adding chlorine, so it produces none of the chlorophenolic tastes and none of the trihalomethanes that chlorination of organic-bearing water can generate. Its weakness is the other side of the same fact: it decomposes quickly and leaves no residual, so a chlorine dose is usually added afterwards to protect the distribution system.

Answer: D

Correct answer: leaves no residual and needs clear water

Ultraviolet inactivates organisms only while they are passing the lamp, so nothing protects the water afterwards and any recontamination downstream goes unchecked. It also requires the water to be clear, since turbidity shields organisms from the light. It is therefore best suited to point-of-use treatment or as a final step on already clarified water.

Answer: A

The standard determination incubates the sample for five days at 20 degrees and measures the dissolved oxygen lost. It reflects only the biodegradable fraction, which is why it is always lower than the chemical demand.

Answer: B

Correct answer: matter that micro-organisms cannot degrade

The biochemical demand measures only what micro-organisms will oxidise in five days; the chemical demand uses a strong oxidising agent in acid and attacks nearly all the organic matter, biodegradable or not. The ratio of the two is therefore useful in itself, indicating how amenable an effluent is to biological treatment.