Unit-1

Fresh water

Q1)Predict the population for the years 1981, 1991, 1994, and 2001 from the following census figures of a town by arithmetical progression method.

A1)

+=increase; - = decrease

Arithmetical Progression Method:

Pn = P + ni

Average increases per decade = i = 8.57

Population for the years,

1981= population 1971 + ni, here n=1 decade

        = 120 + 8.57 = 128.57

1991= population 1971 + ni, here n=2 decade

        = 120 + 2 x 8.57 = 137.14

2001= population 1971 + ni, here n=3 decade

        = 120 + 3 x 8.57 = 145.71

1994= population 1991 + (population 2001 - 1991) x 3/10

        = 137.14 + (8.57) x 3/10 = 139.71

Q2) What are the various factors that affect the consumption of water?

A2)

Rainfall, temperature and evaporation rates – Precipitation and temperature varies widely across the state. Areas with high temperature and low rainfall need to use more water to maintain outdoor landscaping. Even within the same hydrologic region or the same water supply district these factors can vary considerably, having a significant effect on the amount of water needed to maintain landscapes.

Population growth – As communities grow, new residential dwellings are constructed with more efficient plumbing fixtures, which causes interior water use to decline per person as compared to water use in older communities. Population growth also increases overall demand.

Population density – highly urbanized areas with high population densities use less water per person than do more rural or suburban areas since high density dwellings tend to have shared outdoor spaces and there is less landscaped area per person that needs to be irrigated.

Socio-economic measures such as lot size and income – Areas with higher incomes generally use more water than areas with low incomes. Larger landscaped residential lots that require more water are often associated with more affluent communities. Additionally, higher income households may be less sensitive to the cost of water, since it represents a smaller portion of household income.

Water prices – Water prices can influence demand by providing a monetary incentive for customers to conserve water. Rate structures have been established in many districts to incentivize water conservation, but the effectiveness of these rate structures to deter excessive use and customer’s sensitivity to water prices vary.

Q3) Describe the factors to be considered in fixing the design period for water supply components.

A3)

Factor governing the design period

a)     Useful life of component structure and the chances of their becoming old and obsolete.

b)    Is and difficulty in expansion, if undertaken at future dates. Difficult extension then high design period.

c)     Amount and availability of additional investment likely to be incurred for additional provisions.

d)    Rate of interest on borrowings and the additional money invested. Less interest rate than high design period.

e)     Anticipated rate of population growth including possible shifts in communities, industries and commercial establishments.

Q4) What is gravity conduit? And what are its various types?

A4)

The Hydraulic gradient line (HGL) will coincide with the surface of water and will be parallel to the bed of conduit because the water is all along at atmospheric pressure.

These are of three types (canals, flumes, and aqueducts).

a)  Canals: - These are open channels which are constructed by cutting high grounds and constructing banks on low grounds, maybe line or unlined. Generally, it is constructed in balanced cut and fill and are cheap to build in suitable soils.

b) Flumes: - Open channels supported above the ground are called flumes. They are used to convey water across valleys and minor depressions. May be masonry, RCC, metal or wood and are circular or rectangular in cross-section.

c)  Aqueducts: - These are closed rectangular or circular horse shoe sections, built of masonry or RCC. Although they are covered, or closed yet water does not flow under pressure as they are not allowed to run full (1/2 or 3/4th). They wind their way through landscape, with gradual slopes like roads, highways etc.

Note: - From hydraulic point of view, a circular section provides the maximum hydraulic mean depth and is therefore more efficient. But they cannot be supported on ground level and used less. A rectangular section is hydraulically inefficient but is more stable to support on ground. Horseshoe shape is a compressive between the two shapes.

Q5) What are the different types of pipe materials used in the water transmission?

A5)

The following are the different types of pipes

1. Mild Steel Pipes

2. Galvanized Iron (GI) Pipes

3. Poly Vinyl Chloride (PVC unplasticised) Pipes.

4. HDPE

5. Ductile Iron Pipes

Q6) What are pipe appurtenances? Explain in brief.

A6)

The structures, which are constructed at suitable intervals along the sewerage system to help its efficient operation and maintenance, are called as sewer appurtenances. These include:

(1) Manholes: - The manhole is masonry or R.C.C. chamber constructed at suitable intervals along the sewer lines, for providing access into them. Thus, the manhole helps in inspection, cleaning and maintenance of sewer. These are provided at every bend, junction, change of gradient or change of diameter of the sewer. The sewer line between the two manholes is laid straight with even gradient.

(2) Drop manholes: - The depth of these manholes is more than 1.5 m. The section of such manhole is not uniform throughout (Figure 8.3). The size in upper portion is reduced by providing an offset. Steps are provided in such manholes for descending into the manhole. These are provided with heavy cover at its top to support the traffic load.

 (4) Clean-outs: - It is a pipe which is connected to the underground sewer. The other end of the clean-out pipe is brought up to ground level and a cover is placed at ground level. A clean-out is generally provided at the upper end of lateral sewers in place of manholes. During blockage of pipe, the cover is taken out and water is forced through the clean-out pipe to lateral sewers to remove obstacles in the sewer line.

(5) Street inlets called Gullies: - Storm water inlets are provided to admit the surface runoff to the sewers. These are classified in three major groups viz. curb inlets, gutter inlets, and combined inlets. They are provided either depressed or flush with respect to the elevation of the pavement surface.

(6) Catch basins: - Catch basins are provided to stop the entry of heavy debris present in the storm water into the sewers. However, their use is discouraged because of the nuisance due to mosquito breeding apart from posing substantial maintenance problems. At the bottom of the basin space is provided for the accumulation of impurities. Perforated cover is provided at the top of the basin to admit rain water into the basin.

(7) Lamp Holes: - It is an opening or hole constructed in a sewer for purpose of lowering a lamp inside it. It consists of stoneware or concrete pipe, which is connected to sewer line through a T-junction as shown in the Figure 8.12. The pipe is covered with concrete to make it stable. Manhole cover of sufficient strength is provided at ground level to take the load of traffic. An electric lamp is inserted in the lamp hole and the light of lamp is observed from manholes. If the sewer length is unobstructed, the light of lamp will be seen.

Q7) Explain the leakage test in brief.

A7)

A water distribution system should supply necessary amount of potable water at demand points, for domestic, commercial, industrial, and firefighting purposes; the system should be capable of meeting the demands almost any time at required pressures. However, sometimes considerable water leakages from the system may cause problems related to the pressures at the consumers tap and significant water losses. Leakages might occur from the main feeder, distribution pipes, service pipes, or storage tank; the sizes of the leakages might change from small cracks to large breaks.

After pressure test, this leakage test is done, at a pressure to be specified by the authority for duration of 2 hours. Leakage is the quantity f water that is required to be supplied for maintaining the specified leakage test pressure after the pipe has been filed with water and the air is expelled.

The allowable leakage during the maintenance stages of pipes should not exceed the value given by:

                                         qL =  

Where,                           qL = Allowable leakages in cm3/hr

                                                N = Number of joints in the length of pipe

                                                D = Diameter of pipe in mm

                                     P = the average test pressure during the leakage test in kg/cm2

Q8) Predict the population for the years 1981, 1991, 1994, and 2001 from the following census figures of a town by different incremental increase method.

A8)

+=increase; - = decrease

Incremental Increase Method:

Population for the years,

1981= population 1971 + average increase per decade + average incremental increase

        = 120 + 8.57 + 3.0 = 131.57

1991= population 1981 + 11.57

        = 131.57 + 11.57 = 143.14

2001= population 1991 + 11.57

        = 143.14 + 11.57 = 154.71

1994= population 1991 + 11.57 x 3/10

        = 143.14 + 3.47 = 146.61

Q9) Predict the population for the years 1981, 1991, 1994, and 2001 from the following census figures of a town by Geometric Progression method.

A9)

+=increase; - = decrease

Geometric Progression Method:

Average percentage increase per decade = 10.66

P n = P (1+i/100) n

Population for 1981 = Population 1971 x (1+i/100) n

= 120 x (1+10.66/100), i = 10.66, n = 1

= 120 x 110.66/100 = 132.8

Population for 1991 = Population 1971 x (1+i/100) n

= 120 x (1+10.66/100) 2, i = 10.66, n = 2

= 120 x 1.2245 = 146.95

Population for 2001 = Population 1971 x (1+i/100) n

= 120 x (1+10.66/100) 3, i = 10.66, n = 3

= 120 x 1.355 = 162.60

Population for 1994 = 146.95 + (15.84 x 3/10) = 151.70

Q10)What are the various types of valves according to their functions?

A10)

1. Sluice valves: - These are also known as gate-valves or stop valves. These valve control the flow of water through pipes. These valves are cheaper, offers less resistance to the flow of water than other valves. The entire distribution system is decided into blocks by providing these valves at appropriate places. They are provided in straight pipeline at 150-200m intervals. When two pipes lines interest, valves are fixed in both sides of intersection. When sluice valve is closed, it shuts off water in a pipeline to enable to undertake repairs in that particular block. The flow of water can be controlled by raising or lowering the handle or wheel.

2. Check valves or reflex valves: - These valves are also known as non-return valves. A reflux valve is an automatic device which allows water to go in one direction only. The swing type of reflux valve as shown in fig is widely used in practice. When the water moves in the direction of arrow, the valve swings or rotates around the pivot and it is kept in open position due to the pressure of water. When the flow of water in this direction ceases, the water tries to flow in a backward direction. But this valve prevents passage of water in the reverse direction. Reflux valve is invariably placed in water pipe, which obtain water directly from pump. When pump fails or stops, the water will not run back to the pump and thus pumping equipment’s will be saved from damage.

3. Air valves: - These are automatic valves and are of two types namely

1. Air inlet valves: - Air inlet valves these valves open automatically and allow air to enter into the pipeline so that the development of negative pressure can be avoided in the pipelines. The vacuum pressure created in the down streamside in pipelines due to sudden closure of sluice valves. This situation can be avoided by using the air inlet valves

2. Air relief valves: - Air relief valves Sometimes air is accumulated at the summit of pipelines and blocks the flow of water due to air lock. In such cases the accumulated air has to be removed from the pipe lines. This is done automatically by means of air relief valves. This valve consists of a chamber in which one or two floats are placed and is connected to the pipe line. When there is flow under pressure in the pipeline water occupies the float chamber and makes the float to close the outlet. But where there is accumulation of air in the pipeline, air enters the chamber, makes the float to come down, thus opening the outlet. The accumulated air is driven out through the outlet.

4. Drain valves or Blow off valves: - A valve used to drain off material that has separated from a fluid or gas stream, or one used to empty a process line, vessel, or storage tank.

5. Scour valve: - These are similar to blow off valves. They are ordinary valves operated by hand. They are located at the depressions and dead ends to remove the accumulated silt and sand. After the complete removal of silt; the value is to be closed.