Potable water treatment
Potable water treatment

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Potable water treatment

6.2 Service reservoirs

Transmission mains convey treated water from the water treatment works to the service reservoir throughout all, or most of, the day. However, as with the demand for electricity or gas, the demand for water varies with the time of day. The variations are greater in small networks. Typically, the water demand at night is about 20% less than the average daily demand, whereas the peak demand, occurring around midday, is about 40% greater than the average daily demand.

Figure 38(a) shows the cumulative volume of water entering and leaving a service reservoir during the day. The straight line of constant slope represents the constant rate of inflow (supply) to the reservoir, and the variable line running at first below and then above the supply line represents the changing demand from the reservoir. Figure 38(a) is an example of a mass diagram for a reservoir. The effect of variations between supply and demand on the level of water in the reservoir is shown in Figure 38(b).

Figure 38
Figure 38 Supply and demand in service reservoirs: (a) mass diagram; (b) reservoir levels

For example, starting at midnight, the water level in the reservoir is H. During the early hours of the morning, supply exceeds demand and the water level in the reservoir increases. By 0600 hours the level has reached a maximum value H + h1. This corresponds to the situation shown on the mass diagram where 1100 m3 has been supplied to the reservoir, whereas only 500 m3 has been withdrawn. This represents a surplus V1 of 600 m3.

After 0600 hours the rate of demand increases (as the nation awakes) and by 1500 hours the level in the reservoir has returned to its initial level H with supply matching demand exactly.

By early evening the situation has reversed. At 2000 hours, the mass diagram shows that the deficit V2 between the amount of water supplied and that withdrawn is 400 m3, at which time the water in the reservoir has reached its lowest level, H − h2.

Thereafter, water demand 'eases off' again with the water level in the reservoir rising back to its initial level at midnight.

The volume V1 + V2 is called the equalising storage, because it performs the function of equalising supply and demand. Without equalising storage the transmission mains would need to be large enough to cope with peak demand and would be underutilised most of the time. With a service reservoir the transmission mains need only carry the average daily demand (together with an extra 15–25% to account for leakage). Extra capacity may be incorporated in a service reservoir, in addition to equalising storage, for emergency and fire-fighting use. Such an extra capacity is shown in Figure 38(b).

Usually, service reservoirs (Figure 39) are constructed of concrete and frequently, for reasons of both economy and appearance, they are sunk wholly or partly below ground level. The reservoir needs to be positioned with sufficient elevation to provide an adequate flow to the distribution area and to raise the water to the top of buildings. In flat areas, where elevated sights for ground level tanks are not available, or where it is necessary to supply tall buildings, water towers (Figure 40) may be used. In exceptional cases, tall buildings may require their own system of pumps to raise water to the top.

The elevation of the water surface in a reservoir above a given baseline is a measure of the static head of water available (Figure 41). For example, the elevation of the reservoir level above Point A in Figure 41 is 50 m. So the static head at A is 50 m. In other words, the water pressure available to the house at A is equivalent to the pressure at the base of a 50 m high column of water. At point B in Figure 41 near the top of the high-rise building, the static head is only 10 m.

Figure 39
Figure 39 Prestressed concrete service reservoirs
Figure 40
Figure 40 A water tower
Figure 41
Figure 41 Static head in distribution mains

The water pressure in the distribution mains should usually be such as to be able to fill a storage tank at second-floor roof level, and enable the cold-water tap in a ground-floor kitchen to fill a 4.5-litre container in 30 seconds. This will be achieved if the static head at the boundary of the property is 10 m.

Self-assessment question

What should the flow rate at the kitchen tap be to satisfy the requirement that a 4.5-litre container be filled in 30 seconds?

Answer

If the 4.5-litre container has to be filled in 30 seconds, the flow rate has to be 9 litres a minute.

In a distribution main, the head should not normally be lower than 30 m for fire-fighting purposes and should not exceed 70 m. Many domestic fittings, including taps, ball valves, stopcocks and domestic washing machines are designed to operate at pressures between 30 m and 70 m (appropriate pressures). At higher pressures, wear of fittings becomes excessive, seals need frequent replacement and the system is noisy and more liable to 'knocking' and vibration. Furthermore, at high pressures the system is likely to leak at a greater rate than at low pressures.

Pressure-reducing valves are sometimes used if the water pressure is too high. These valves are designed to limit the pressure downstream of the valve to a predetermined value, irrespective of the pressure upstream.

Figure 42 shows a possible arrangement of service reservoirs, water towers, mains and buildings. We can see how this arrangement is used to create appropriate pressure zones.

Figure 42
Figure 42 Distribution of water from service reservoirs

It is not always possible to take advantage of flow under gravity in transmission systems. The necessary head for flow then has to be raised by pumping through part or all of the pipeline. Where pumping occurs, the pipeline is called a rising main.

In England and Wales, water companies are legally required to provide fire hydrants as requested by the fire service, but there is no guidance on the quantity of water to be made available or its pressure. Nevertheless, fire-fighting requirements govern the size of main that is considered to be desirable. When a pipe is fed from both ends (e.g. in a loop), a diameter of 75 mm is usually considered to be satisfactory. A 100-mm diameter pipe is preferred where only one end is fed. If the fire service considers that there would be insufficient water available for quenching a fire, it will ask for a larger main. Under these circumstances the extra cost is borne by the fire service, both for the larger main and for the provision and maintenance of the hydrant.

Self-assessment question

Figure 43 shows the hourly variation of demand from a service reservoir. Plot a mass diagram for the service reservoir and estimate the volume of equalising storage required.

Answer

The mass diagram is plotted in Figure 110. Maximum surplus in volume supplied V1 is 300 m3 at 6.00 a.m. Maximum deficit in volume extracted V2 is 200 m3 at 8.00 p.m.

So the equalising storage = V1 + V2 = 500 m3.

Mass diagram for Self-assessment question 29
Figure 43
Figure 43 Hourly variation in demand from service reservoir

Self-assessment question

Select the appropriate option(s) 1–4, for each of the components A, B and C.

A Storage tower 1 To meet fire-fighting requirements
B Spur water main of diameter 100 mm 2 To give a reasonable water pressure at the top of a tall building on flat land
C Pressure-reducing valve 3 To avoid a water head of less than 30 m
4 To avoid a water head of greater than 70 m

Answer

A 1, 2 and 3

B 1 and possibly 3

C 4

T210_1

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