1.2.1 Transport and distribution
In general, a water supply system comprises the following processes (Figure 1.6):
1 raw water extraction and transport, 2 water treatment and storage, 3 clear water transport and distribution.
Transport and distribution are technically the same processes in which the water is conveyed through a network of pipes, stored intermittently and pumped where necessary, in order to meet the demands and pres-sures in the system; the difference between the two is in their objectives, which influence the choice of system configuration.
Water transport systems Water transport systems comprise main transmission lines of high and fairly constant capacities. Except for drinking water, these systems may be constructed for the conveyance of raw or partly treated water. As a part of the drinking water system, the transport lines do not directly serve consumers. They usually connect the clear water reservoir of a treatment
Table 1.2. World urban population growth 1975–2015 (UN, 2001).
Areas Population (millions)/% of total
1975 2000 2015
Urban, above 5 million inhabitants 195/5 418/7 623/9 Urban, 1 to 5 million inhabitants 327/8 704/12 1006/14 Urban, below 1 million inhabitants 1022/25 1723/28 2189/31
Rural 2530/62 3210/53 3337/46
Total 4074/100 6055/100 7154/100
plant with some central storage in the distribution area. Interim storage or booster pumping stations may be required in the case of long distances, specific topography or branches in the system.
Branched water transport systems provide water for more than one distribution area forming a regional water supply system. Probably the most remarkable examples of such systems exist in South Korea.
The largest of 16 regional systems supplies 15 million inhabitants of the capital Seoul and its satellite cities. The 358 km long system of concrete pipes and tunnels in diameters ranging between 2.8 and 4.3 metres had an average capacity of 7.6 million cubic metres per day (m3/d) in 2003.
However, the largest in the world is the famous ‘Great Man-made River’ transport system in Libya, which is still under construction. Its first two phases were completed in 1994 and 2000 respectively. The approximately 3500 km long system, which was made of concrete pipes of 4 metres in diameter, supplies about three million m3/d of water. This is mainly used for irrigation and also partly for water supply of the cities in the coastal area of the country. After all the three remaining phases of construction have been completed, the total capacity provided will be approximately 5.7 million m3/d. Figure 1.7 gives an impression of the size of the system by laying the territory of Libya (the grey area) over the map of Western Europe.
Source – water extraction
Production – water treatment
Distribution Transport
raw water
Transport clear water
Figure 1.6. Water supply system processes.
Water distribution systems Water distribution systems consist of a network of smaller pipes with numerous connections that supply water directly to the users. The flow variations in such systems are much wider than in cases of water trans-port systems. In order to achieve optimal operation, different types of reservoirs, pumping stations, water towers, as well as various appurte-nances (valves, hydrants, measuring equipment, etc.) can be installed in the system.
The example of a medium-size distribution system in Figure 1.8 shows the looped network of Zanzibar in Tanzania, a town of approxi-mately 230,000 inhabitants. The average supply capacity is approxiapproxi-mately 27,000 m3/d (Hemed, 1996). Dotted lines in the figure indicate pipe routes planned for future extensions; the network layout originates from a computer model that consisted of some 200 pipes and was effectively used in describing the hydraulic performance of the network.
The main objectives of water transport and distribution systems are common:
– supply of adequate water quantities,
– maintaining the water quality achieved by the water treatment process.
Phase one
UK
Germany
France
Libya NL
Phase two Phase three Phase four Phase five
Km 0 100 200
Figure 1.7. The ‘Great Man-made River’ transport system in Libya (The Management and Implementation Authority of the GMR project, 1989).
Each of these objectives should be satisfied for all consumers at any moment and, bearing in mind the massive scale of such systems, at an acceptable cost. This presumes a capacity of water supply for basic domestic purposes, commercial, industrial and other types of use and, where possible and economically justified, for fire protection.
Speaking in hydraulic terms, sufficient quantity and quality of water can be maintained by adequate pressure and velocity. Keeping pipes always under pressure drastically reduces the risks of external contami-nation. In addition, conveying the water at an acceptable velocity helps to reduce the retention times, which prevents the deterioration in quality resulting from low chlorine residuals, the appearance of sediments, the
Figure 1.8. Water distribution system in Zanzibar, Tanzania (Hemed, 1996).
growth of micro organisms, etc. Hence, potable water in transport and distribution systems must always be kept under a certain minimum pressure and for hygienic reasons should not be left stagnant in pipes.
Considering the engineering aspects, the quantity and quality require-ments are met by making proper choices in the selection of components and materials. System components used for water transport and distribu-tion should be constructed i.e. manufactured from durable materials that are resistant to mechanical and chemical attacks, and at the same time not harmful for human health. Also importantly, their dimensions should comply with established standards.
Finally, in satisfying the quantity and quality objectives special atten-tion should be paid to the level of workmanship during the construcatten-tion phase as well as later on, when carrying out the system operation and maintenance. Lack of consistency in any of these indicated steps may result in the pump malfunctioning, leakages, bursts, etc. with the possible consequence of contaminated water.
1.2.2 Piping
Piping is a part of transport and distribution systems that demands major investments. The main components comprise pipes, joints, fittings, valves and service connections. According to the purpose they serve, the pipes can be classified as follows:
Trunk main Trunk main is a pipe for the transport of potable water from treatment plant to the distribution area. Depending on the maximum capacity i.e. demand of the distribution area, the common range of pipe sizes is very wide; trunk mains can have diameters of between a few 100 mil-limetres and a few meters, in extreme cases. Some branching of the pipes is possible but consumer connections are rare.
Secondary mains Secondary mains are pipes that form the basic skeleton of the distribu-tion system. This skeleton normally links the main components, sources, reservoirs and pumping stations, and should enable the smooth distribu-tion of bulk flows towards the areas of higher demand. It also supports the system operation under irregular conditions (fire, a major pipe burst or maintenance, etc.). A number of service connections can be provided from these pipes, especially for large consumers. Typical diameters are 150–400 mm.
Distribution mains Distribution mains convey water from the secondary mains towards various consumers. These pipes are laid alongside roads and streets with numerous service connections and valves connected to guarantee the required level of supply. In principle, common diameters are between 80–200 mm.
The schematic layout of a distribution network supplying some 350000 consumers is given in Figure 1.9. The sketch shows the end of the trunk main that connects the reservoir and pumping station with the well field. The water is pumped from the reservoir through the network of secondary mains of diameters D 300–600 mm and further distributed by the pipes D 100 and 200 mm.
Service pipes From the distribution mains, numerous service pipes bring the water directly to the consumers. In the case of domestic supplies, the service pipes are generally around 25 mm (1 inch) but other consumers may require a larger size.
The end of the service pipe is the end point of the distribution system.
From that point on, two options are possible:
Public connection Public connection; the service pipe terminates in one or more outlets and the water is consumed directly. This can be any type of public tap, fountain, etc.
Private connection Private connection; the service pipe terminates at a stopcock of a private installation within a dwelling. This is the point where the responsibility
100 Red Sea
200 300
400 500 600 Figure 1.9. Distribution system
in Hodaidah, Yemen (Trifunovi-and Blokl(Trifunovi-and, 1993).
of the water supply company usually stops. These can be different types of house or garden connections, as well as connections for non-domestic use.
One typical domestic service connection is shown in Figure 1.10.
1.2.3 Storage
Clear water storage facilities are a part of any sizable water supply system. They can be located at source (i.e. the treatment plant), at the end of the transport system or at any other favourable place in the distribu-tion system, usually at higher elevadistribu-tions. Reservoirs (or tanks) serve the following general purposes:
– meeting variable supply to the network with constant water production,
– meeting variable demand in the network with its constant supply, – providing a supply in emergency situations,
– maintaining stable pressure (if sufficiently elevated).
Except for very small systems, the costs of constructing and operating water storage facilities are comparable to the savings achieved in build-ing and operatbuild-ing other parts of the distribution system. Without the use of a storage reservoir at the end of the transport system, the flow in the trunk main would have to match the demand in the distribution area at any moment, resulting in higher design flows i.e. larger pipe diameters.
When operating in conjunction with the reservoir, this pipe only needs to be sufficient to convey the average flow, while the maximum peak flow is going to be supplied by drawing the additional requirement from the balancing volume.
Saddle
Distribution pipe Watertight seal
Pipe protection
Water meter Stop
cock
Figure 1.10. Schematic layout of a service connection.
Selection of an optimal site for a reservoir depends upon the type of supply scheme, topographical conditions, the pressure situation in the system, economical aspects, climatic conditions, security, etc. The required volume to meet the demand variations will depend on the daily demand pattern and the way the pumps are operated. Stable consumption over 24 hours normally results in smaller volume requirements than in cases where there is a big range between the minimum and maximum hourly demand. Finally, a proper assessment of needs for supply under irregular conditions can be a crucial decision factor.
Total storage volume in one distribution area commonly covers between 20–50% of the maximum daily consumption within any partic-ular design year. With additional safety requirements, this percentage can be even higher. See Chapter 4 for a further discussion of the design principles. Figure 1.11 shows the total reservoir volumes in some world cities.
The reservoirs can be constructed either:
– underground, – ground level or – elevated (water towers).
Underground reservoirs are usually constructed in areas where safety or aesthetical issues are in question. In tropical climates, preserving the water temperature i.e. water quality could also be considered when choosing such a construction.
Compared to the underground reservoirs, the ground level reservoirs are generally cheaper and offer easier accessibility for maintenance.
Both of these types have the same objectives: balancing demand and buffer reserve.
Water towers Elevated tanks, also called water towers, are typical for predominantly flat terrains in cases where required pressure levels could not have been
0 20 40 60 80 100
Percentage of the maximum consumption day 65
Figure 1.11. Total storage volume in some world cities (adapted from: Kujund∆i-, 1996).
reached by positioning the ground tank at some higher altitude. These tanks rarely serve as a buffer in irregular situations; large elevated vol-umes are generally unacceptable due to economical reasons. The role of elevated tanks is different compared to ordinary balancing or storage reservoirs. The volume here is primarily used for balancing of smaller and shorter demand variations and not for daily accumulation. Therefore, the water towers are often combined with pumping stations, preventing too frequent switching of the pumps and stabilising the pressure in the dis-tribution area at the same time. Two examples of water towers are shown in Figure 1.12.
In some cases, tanks can be installed at the consumer’s premises if:
– those consumers would otherwise cause large fluctuations of water demand,
– the fire hazard is too high,
– back-flow contamination of the distribution system (by the user) has to be prevented,
– an intermittent water supply is unavoidable.
In cases of restricted supply, individual storage facilities are inevitable.
Very often, the construction of such facilities is out of proper control and the risk of contamination is relatively high. Nevertheless, in the absence of other viable alternatives, these are widely applied in arid areas of the world, such as in the Middle East, Southeast Asia or South America.
A typical example from Sana’a, the Republic of Yemen, in Figure 1.13 shows a ground level tank with a volume of 1–2 m3, connected to the distribution network. This reservoir receives the water in periods when the pressure in the distribution system is sufficient. The
Figure 1.12. Water towers in Amsterdam (still in use) and Delft (no longer in use).
pressure in the house installation is maintained from the roof tank that is filled by a small pump. Both reservoirs have float valves installed in order to prevent overflow. In more advanced applications, the pump may operate automatically depending on the water level in both tanks. In areas of the town with more favourable pressure, the roof tanks will be directly connected to the network (Figure 1.14).
In theory, this kind of supply allows for lower investment in distribu-tion pipes as the individual balancing of demand reduces the peak flows in the system. In addition, generally lower pressures associated with the supply from the roof tanks affect leakages in a positive way. In practice however, the roof tanks are more often a consequence of a poor service level rather than a water demand management tool.
Pressure
Pump
Figure 1.13. Individual storage in water scarce areas (Trifunovi-, 1994).
Figure 1.14. Roof tanks in Ramallah, Palestine (Trifunovi-, Abu-Madi, 1999).
In Europe, roof tanks can be seen in arid areas of the Mediterranean belt. Furthermore, they are traditionally built in homes in the UK. The practice there dates from the nineteenth century when water supplied to homes from the municipal water companies was intermittent, which is the same reason as in many developing countries nowadays. Such tanks, usually of a few 100 litres, are typically installed under the roof of a family house and are carefully protected from external pollution. Their present role is now less for emergencies and more as small balancing tanks. Furthermore, the roof tanks in the developed world are frequently encountered in large multi-storey buildings, for provision of pressure and for fire fighting on the higher floors.
1.2.4 Pumping
Pumps add energy to water. Very often, the pumping operation is closely related to the functioning of the balancing reservoirs. Highly-elevated reservoirs will usually be located at the pressure (i.e. downstream) side of the pumping station in order to be refilled during the periods of low demand. The low-level reservoirs, on the other hand, will be positioned at the suction (i.e. upstream) side of the pumping station that provides supply to the consumers located at higher elevations. Apart from that, pumps can be located anywhere in the network where additional pressure is required (booster stations).
Centrifugal flow pumps Centrifugal flow pumps are commonly used in water distribution. They can be installed in a horizontal or vertical set-up if available space is a matter of concern (see Figure 1.15). The main advantages of centrifugal pumps are low maintenance costs, high reliability, a long lifetime and simple construction, which all ensure that the water pumped is hygienically pure.
The pump unit is commonly driven by an electrical motor or a diesel engine, the latter being an alternative in case of electricity failures or in remote areas not connected at all to the electricity network. Two groups of pumps can be distinguished with respect to the motor operation:
1 fixed speed pumps, 2 variable speed pumps.
Frequency converter In the first case, the pump is driven by a motor with a fixed number of revolutions. In the second case, an additional installed device, called the frequency converter, controls the impeller rotation enabling a more flexible pump operation.
Variable speed pumps Variable speed pumps can achieve the same hydraulic effect as fixed speed pumps in combination with a water tower, rendering water towers unnecessary. By changing the speed, those pumps are able to follow the demand pattern within certain limits whilst at the same time keeping
almost constant pressure. Consequently, the same range of flows can be covered with a smaller number of units. However, this technology has some restrictions; it cannot cover a large demand variation. Moreover, it involves rather sophisticated and expensive equipment, which is probably the reason why it is predominantly applied in the developed countries. With obvious cost-saving effects, variable speed pumps are widely used in The Netherlands where the vast majority of over 200 water towers built throughout the nineteenth and twentieth centuries has been disconnected from operation in recent years, being considered uneconomical.
Proper selection of the type and number of pump units is of crucial importance for the design of pumping stations. Connecting pumps in a parallel arrangement enables a wider range of flows to be covered by the pumping schedule while with pumps connected in serial arrangement the water can be brought to extremely high elevations. A good choice in both cases guarantees that excessive pumping heads will be minimised, pumping efficiency increased, energy consumption reduced, working hours of the pumps better distributed and their lifetime extended.