Your Answer : (Not Answered)

Correct Answer : C

The geometrical increase method gives \(P_{n} = P(1+ (i/100))^{n}\).

This method assumes the population grows by a constant percentage each decade, like compound interest. With present population P and average percentage growth rate i per decade, the population after n decades is:

\[ P_n = P\left(1 + \frac{i}{100}\right)^{n} \]

It suits young, rapidly growing towns. \(P + n \times i\) is the arithmetical increase method (constant absolute growth), and \(P + n(I_a + I_c)\) resembles the incremental increase method, which combines the average increase with the average incremental increase. The last option is not a recognised method.

Your Answer : (Not Answered)

Correct Answer : D

The process is Filtration.

After coagulation and sedimentation, very fine suspended particles, colloidal matter and many bacteria still remain. Passing the water through a bed of sand in slow or rapid sand filters removes them by straining, sedimentation in the pores, adsorption and biological action.

Screening removes only large floating matter before treatment, sedimentation is the stage these particles have escaped, and disinfection kills pathogens rather than removing fine particles.

Your Answer : (Not Answered)

Correct Answer : C

The incorrect statement is Screening is to remove objectionable tastes, odours and to remove dissolved gases from the water.

Screening is the first treatment step: bar screens remove large floating and suspended material such as leaves, sticks and debris. Removing tastes, odours and dissolved gases such as carbon dioxide and hydrogen sulphide is the job of aeration.

The other three statements are correct: raw water contains suspended, colloidal and dissolved impurities; disinfection, especially the residual chlorine it leaves, protects water against contamination on its way to consumers; and waterborne diseases are caused by pathogenic organisms.

Your Answer : (Not Answered)

Correct Answer : C

Alkalinity is due to excess of \(\text{HCO}_{3}^{-}\) or \(\text{CO}_{3}^{--}\).

Alkalinity is the capacity of water to neutralise acid. In natural waters it is caused mainly by bicarbonate (HCO₃^-) and carbonate (CO₃^2-) ions, and at high pH by hydroxide (OH^-), usually of calcium, magnesium and sodium.

Nitric acid and sulphuric acid are acids, which would reduce alkalinity, not cause it. Calcium carbonate is only sparingly soluble; the alkalinity it contributes is through the carbonate and bicarbonate ions it releases, which is what the correct option names.

Your Answer : (Not Answered)

Correct Answer : A

The carbonate and non-carbonate hardness are 150 and 200 mg/L.

Carbonate hardness is the part of total hardness balanced by alkalinity (bicarbonate and carbonate). When alkalinity is less than total hardness, carbonate hardness equals the alkalinity, 150 mg/L, and the rest is non-carbonate hardness: 350 - 150 = 200 mg/L.

The options pairing 150 or 200 with 350 treat total hardness as one of the two parts, but the two parts must add up to the total of 350 mg/L, and 100 does not equal the alkalinity.

Your Answer : (Not Answered)

Correct Answer : C

Steel pipes are best for laying in ground that is liable to subsidence, since they are ductile and flexible and have long lengths with welded joints, so they can undergo some settlement and movement without breaking or leaking. Cast iron pipes are brittle, asbestos cement pipes are also brittle and break under the uneven settlement, and PVC pipes are not suited to heavy loads and soil movement in large water supply schemes.

Your Answer : (Not Answered)

Correct Answer : D

The statement that submersible intake structures are normally used for lake-based water supply is the true one, as the key shows. A submerged intake sits below the water surface in the lake and draws water from there, so it works at every level of the lake. The other statements do not describe the usual practice: an intake tower is placed upstream of a dam (in the reservoir), not downstream, an underground chamber in the river bed is not the usual river intake, and a siphonic intake at the middle of the canal is not the way a canal supply is augmented.

Your Answer : (Not Answered)

Correct Answer : B

For a town of 50,000 population, IS 9668 recommends a fire demand of 1800 litres per minute for the scale of water to be kept available. The required quantity increases with the population served, so that bigger towns need larger fire flows. The values 1500, 2200 and 2500 litres per minute are not the figure that the code gives for this population, and they would apply to smaller or larger towns than the one named.

Your Answer : (Not Answered)

Correct Answer : D

The common methods of desalinating water are distillation (the water is evaporated and condensed, leaving the salts behind), reverse osmosis (water is forced through a semi-permeable membrane at a pressure above the osmotic pressure) and electrodialysis (ions are drawn out of the water through ion-selective membranes in an electric field). Filtration, by itself, removes suspended solids but not dissolved salts, so the options that list it are wrong.

Your Answer : (Not Answered)

Correct Answer : B

S1 is false and S2 is true. Infiltration galleries (horizontal, porous conduits laid in water-bearing ground near a river or lake) collect water from the unconfined aquifer or by seepage from a surface source. They are not used for confined aquifers, which are tapped by wells. S2 is true: water that has filtered through the soil is usually clear and of low turbidity and bacterial count, so groundwater from tube wells needs less treatment than the water of a river, though it may need softening or removal of iron.

Your Answer : (Not Answered)

Correct Answer : B

Water drawn directly from natural lakes, along with rivers, streams, ponds and reservoirs, is a surface water source. Rainwater is collected from the roofs or the ground as precipitation, groundwater is taken from wells and tube wells below the ground, and desalinated water is produced by treating sea or brackish water. None of those is taken by a direct intake from a lake.