Methods of Fresh Water Supply

Having discussed our various water requirements, we now investigate how these are met. Any water-supply system involves taking water from its source, treating it, and distributing it to the consumer. How this is done varies from one area to another, so we start by examining the various methods open to us at present and the principles that they involve. Then we go on to discuss in detail the water supply in various parts of the world so as to cover the most common types of system.

The sources of water today are much the same as they were thousands of years ago: we still rely mainly on rivers, lakes, springs, and wells, but we now exploit them more extensively. After the invention of the steam engine in the early 18th century, and the subsequent development of power-driven drills and pumps, it became possible to drill deep boreholes and to lift water from depths of over 1000 feet. We have increased the storage of natural lakes by building dams, and have created new reservoirs by impounding water in river valleys. Where there is no natural fresh-water source, a few places, like Kuwait, desalinate sea water.

Choosing a source is not easy, and exploiting it is a very complex procedure. Many mistakes have been made in the past: wells have been dug into brine-fields, and dams built on shaky foundations. If possible, a source should fulfil three conditions: it should be as near as possible to the population in need ; the water should be available in sufficient quantity; and it should not contain high concentrations of objectionable salts, or be too polluted.

A common source is the large river. When clean and near to the area of supply, a river is especially economical because it avoids the great expense of building distant reservoirs, and aqueducts. Unfortunately, the large amounts of water that many rivers yield for much of the year so often dwindles to a trickle in the dry season, just when the demand is greatest. Rivers also carry silt, and are a convenient flushing system for municipal sewage and a convenient disposer of industrial effluent. Thus river water needs thorough treatment. But recent developments in water treatment have made it possible to use polluted river water that would not have been possible before.

Natural lakes, such as the Great Lakes of America, often provide a good local, reliable supply to their nearby cities. But lakes, like rivers, are liable to be heavily polluted by the very people they are intended to serve. This experience is not met with so much in isolated lakes and reservoirs far from the area of supply. In these relatively unpopulated areas of runoff, called catchment areasy streams carry suspended matter into the lake or reservoir, where it slowly falls to the bottom, most of the bacteria die, and the water becomes relatively clean. Where such a source is high up in the hills, its water can flow to the cities under gravity, thus saving pumping costs. Unlike rivers, lakes and reservoirs are more able to store floodwaters, which may be slowly drawn upon during dry periods. But like all sources, lakes and reservoirs have their drawbacks. Their upper layers may be contaminated by algae, sometimes even by water weeds, and their lower layers may become polluted by the decomposition of living matter. Costly aqueducts also have to be built to convey the water to the treatment works.

More effective use can be made of both rivers and reservoirs by integrating the two sources—a method known as river regulation. For the same capital cost, river regulation produces a much higher output than using either source separately. When the river flow is adequate, water is taken from the river; reservoirs are only used as a flood retention basin. When the river level declines, water is added to the river from that accumulated in the reservoir during the previous wet season. River regulation is especially efficient when the river conveys this extra water part or all of the way to the supply area, for it saves the cost of an aqueduct. The benefit, however, is not all one way: the quality of the water inevitably declines with use and re-use in its passage down the river.

Water in rivers, lakes, and reservoirs is called surface water. Our last remaining source is ground water, which is used in much smaller quantities than surface sources ; rarely does a city rely solely on ground water. Occasionally an area obtains its ground water from artesian wells ; equally unusual and as fortunate are those areas that obtain water from springs. But in most places ground water has to be pumped from wells and boreholes, although with modern pumps this may be quite cheap. The most important considerations are how far away ground water lies from the areas of supply, how deep it lies, its quality, and how much water can be abstracted without permanently lowering the water table . A good-quality underground source near to a city is an invaluable asset; often the water is so pure that it needs only to be sterilized before reaching the tap.

The next stage in water supply is its transmission from the source to the treatment works. Sometimes the water is treated at source, at other times this is done at the area of supply, the choice depending partly on the risks of pollution en route. Water is taken from rivers, lakes, and reservoirs through an intake, well screened to keep out fish and debris.

The device along which water flows from the source to the treatment works is called an aqueduct. As we mentioned before, in some cases it is best to use an existing river for part of the way, but few rivers run the way desired by the water engineer. Canals are the oldest type of aqueduct, and some magnificent examples were built by the Romans. Canals are still used today, but they have several disadvantages, the chief one being that as they are open they can only travel downhill. This is why the Romans built giant stone bridges across valleys to support their channels in a gentle and continuous downhill gradient. It is also impossible to increase the flow in canals to any extent, which can be a critical factor in modern water supply. A further disadvantage is evaporation from the water surface in hot, dry weather, which is particularly serious in hot climates. Considerable losses also occur by seepage through the unlined beds of canals in many parts of the world.

Some of these disadvantages are avoided in channels that are both lined and covered, often called ‘cut-and-cover’ channels. These are less liable to pollution and clogging with water weeds, but since water flows along them under atmospheric pressure, some polluted water may seep in. As with canals, water cannot flow along cut-and-cover channels under pressure, otherwise leakage would occur.

When high ground obstructs the route, a tunnel is sometimes built through the barrier. With modern methods of tunnel excavation, this is often cheaper than going round the barrier with, say, cut-and-cover channels. Some tunnels act like cut-and-cover channels in that the water is under atmospheric pressure; others act like pipes in that they carry high-pressure water. But both atmospheric and pressure tunnels are very difficult to repair if there is a blockage some way in. The best type of aqueduct is the pipe: the flow can be increased at will, there is less loss from leakage, and it is not liable to contamination. A pipeline can also carry water across valleys, and is more easily repaired.

After leaving the aqueduct, water from rivers, lakes, and reservoirs often enters a storage reservoir , whose principal function is to store a reserve of water near the city. From the storage reservoir, water passes to the treatment works, where the system of purification depends mainly on the quality of the incoming water. Treatment often consists of two stages. Primary treatment removes the large particles in the water by one of three common methods. The first passes water quickly through rapid sand filters, the second through fine mesh screens, and the third through a chemical blanket. Then, secondary treatment completes the clarification of water and further reduces the bacterial content, usually by using sand filter beds. At one or more stages of the treatment, chemicals may be added to correct acidity, hardness or both. At some stage of treatment the water is sterilized

Any bacteria that remain are destroyed by chlorine in a contact tank, which allows adequate time for the oxidizing agent to act. The tank sometimes also acts as a balancing tank, containing an emergency reserve that lasts several hours so that the supply continues when the treatment works is under repair.

Water now passes on to the distribution system, consisting of mains, service reservoirs, and booster pumps. A difficult and costly process in any water undertaking is the laying and maintenance of mains, since the perfect pipe has yet to be found. A 30-inch-diameter pipe one mile long set 2 feet 6 inches in rock may well cost £38,000, while the capital cost of a distribution network is about £7 per head in cities and £40 in rural areas. Once the mains are laid, they also need constant attention. In a city, one fracture a year occurs on average in every three miles of mains—twice as often as in rural areas. All types of pipes may fracture during the careless digging of holes for inspecting and laying gas pipes, electricity and post office cables, foundations, and sewers. Vibration and soil movement caused by traffic, together with natural land subsidence, may add to the damage. Also, in spite of valves and other devices to keep internal water pressures at the right level, occasional surges of high pressure may burst pipes that are already weakened by other forces. But the key factor in the life of a main is corrosion of the inside by aggressive water and on the outside by aggressive soils. Due to the greater concentration on waste prevention during the last century, leakage from pipe-lines has fallen by over 60 per cent, from 21 to 8 ghd in Britain, despite an almost hundredfold increase in the number of fittings and longer pipes per head of population, and a threefold increase in pressure. Leakage is normally investigated during the night, the consumption of an area being recorded on charts. As the streets are closed in turn, a step appears on the chart when an unknown leak is shut off. Then follows a more intensive and very laborious search for the leak, using sounding sticks or acoustic electronic equipment.

The larger diameter mains leading from the treatment works are called trunk mains. They divide, like the branches of a tree, into smaller district mains, which are usually laid under the streets. From these, service pipes lead to buildings. The first mains, dating from the 15th century, were made of cast iron, and many laid in the last century still provide good service. The great disadvantage of cast iron is its tendency to corrode, although today this is overcome by lining the pipes with concrete and sheathing them with bitumen. Cast iron has been largely superseded by spun iron and ductile iron pipes.

For large mains up to 60 inches in diameter, steel is usually the most economical material. Because of its great strength and flexibility, it can be relied on to provide a continuous supply in almost all conditions. Non-steel pipes, such as those made of cast iron and asbestos cement, collapse completely when they fracture, whereas steel pipes develop corrosion pit holes, which can be covered by watertight sleeves while the pipe is still in service. Steel pipes, like those of iron, need protecting from corrosion by bitumen and concrete.

In many cases, prestressed concrete pipes are better than those made of steel, for, as well as being able to withstand high internal pressures, they do not corrode. The pipe is made by moulding concrete around steel wires previously placed under tensile stress. When the concrete sets, the steel pulls the concrete together so that water pressures cannot cause cracks in the pipe. For a given water pressure, a prestressed concrete pipe is more economical than one made of steel because less steel need be incorporated. On the other hand, concrete pipes are very heavy, and if they have to be transported long distances it is cheaper to use steel pipes.

Another economical type of pipe in sizes up to 30 inches is made from asbestos cement, in which fibres of asbestos reinforce the concrete. Though brittle, this type of pipe is fairly strong, and it only corrodes in sulphate soils. Reinforced concrete pipes are suitable for low-pressure work, such as sewers and some aqueducts, and are used because they are cheaper than prestressed concrete or steel pipes, and can be made in large sizes. They are rarely used for water mains in Britain.

For pipes up to 10 inches in diameter there has recently been a very important development in the field of plastics. Polythene pipes are now in common use, being flexible, cheap, and resistant to corrosion. On the other hand, polythene is less robust than steel and, in the tropics, termites and baboons have been known to gnaw into these pipes. Also, coal gas is able to pass through its wall from the soil. PVC pipes are more resistant to gas; these are widely used on the continent and provide a good alternative to traditional materials where there is no danger of heavy impact in frosty weather.

Service pipes, which lead to houses, are up to about two inches in diameter. Lead is the traditional material for these, and is still used where water is hard; soft water in lead pipes can cause lead poisoning. Lead pipes first gave way to copper pipes in the 1940s, and then to the cheaper plastic ones. For internal pipes within houses and factories, copper and galvanized iron are now generally used, the former predominating in soft water areas.

In most areas today, water travels from the treatment works, via some of the mains, to service reservoirs, feeding some houses on the way. Service reservoirs are roofed over to prevent contamination and keep out light, which would induce the growth of algae. They are situated at such an elevation that gives enough head for water to flow to premises under gravity but not enough to produce an excessive flow or burst the mains. When no ground at the right elevation exists, the more costly water towers are used. The main purpose of service reservoirs, which fill up at night, is to supplement the flow from the treatment works during day-time peak periods. Service reservoirs also contain from one to two days’ supply of water so that the supply can continue when the mains fail or when they are under repair. When there are considerable variations in height over an area, water may have to be pumped through a succession of service reservoirs at different levels so that each provides enough pressure but not too much.

When water enters a building it automatically rises about four storeys under its own pressure. In parts of the United States and in Europe, all domestic fittings are under mains pressure, and therefore may leak badly. In South American and Mediterranean cities, when the demand for water is heavy, the mains pressure falls throughout the area so that water fails to reach the upper floors, except in the early hours of the morning. In London and a few other cities, all fittings, except for one drinking tap on each floor, are fed by water that flows by gravity from a storage cistern in the roof. The pressure is thus much lower than mains pressure, and fittings leak less. Moreover, if a main has to be repaired, storage cisterns provide a reserve of at least 50 gallons, which could, at a stretch, last a few days. In tall buildings, booster pumps supplement the mains pressure so that water can reach the upper floors.

Let us examine the water supply systems of certain areas. We start with New York, a large city with a very dense population. New Yorkers are supplied by an upland system of large lakes and reservoirs from which chlorinated water flows by gravity to the city. New York also illustrates how even very large water resources can sometimes fail to provide an adequate supply when there is a combination of an inattentive administration, a very high leakage rate from the distribution system, and a high consumption.

Apart from a few boreholes on Long Island and Staten Island, New York’s water originates in the mountains to the north. These sources have a capacity of about 421,000 million gallons and also supply several other states, including communities along the Delaware River, of which Philadelphia is the largest. There are three separate reservoir systems feeding New York. The first, built in 1843-1911 in the Croton Mountains 45 miles from the city centre, consists of 12 reservoirs and 5 controlled lakes. Two cut-and-cover aqueducts carry water directly to service reservoirs in the city. Since the Croton Mountains are not very high, they supply water only to the low-lying areas of Manhattan and the Bronx, mostly by gravity.

Between 1905 and 1928, two large reservoirs, linked by a 12-mile tunnel, were constructed in the Catskill Mountains, 125 miles to the north, at a height of 900 feet. These trap the large quantities of spring runoff from rainfall and melting snow for use during the summer, when runoff is low due to the high absorption of water by vegetation and the soil, and the high rate of evapotranspiration. Leading from the reservoirs is the 92-mile Catskill aqueduct, consisting of cut-and-cover channels, steel-pipe inverted siphons, and pressure tunnels. After passing 1000 feet below the Hudson River, the aqueduct feeds the Kensico storage reservoir 30 miles from New York. This reservoir, with a capacity of 25,000 million gallons , is very useful when the Croton system is interrupted.

The Delaware system, begun in 1936, includes four reservoirs in the Catskill Mountains at a height of about 1400 feet. From these the 130-mile Delaware high-pressure tunnel pours water into the Kensico reservoir, whence it flows on through the Catskill and Delaware aqueducts to the Hillview service reservoir in Yonkers. From here, two city tunnels in hard rock, 18 and 20 miles long, 11 to 21 feet wide, and a few hundred feet below the ground, feed all parts of the city. Water from the tunnels flows under pressure up vertical shafts to the distribution system at the surface.

The large quantities of water supplied to New York are very wholesome, which is one of the advantages of building reservoirs in areas of sparse population. There is little or no need to filter the water, although much is aerated to remove iron and manganese and to improve its taste. Much of the water in both upland and service reservoirs is treated with copper sulphate to check algal growths. All water is chlorinated before passing into supply, sometimes before it enters the aqueduct from the upland reservoirs, and always again as it leaves the aqueduct for the city.

The distribution system contains over 6000 miles of mains up to 72 inches in diameter, mostly of cast iron, steel, and reinforced concrete—a densely packed network in which three quarters of a mile of mains supply 1000 people. The network is divided into contoured zones accocding to street elevations so that consumers in each zone have enough water pressure but not too much. Nearly all water enters the zones under gravity at a pressure of about 50 pounds per square inch, which is sufficient to lift water to the top floor of four-storey buildings; about five per cent has to be pumped up to the higher zones. Taller buildings pump water into a roof tank; skyscrapers pump water successively into tanks on each 10th floor, from which water feeds the floors below by gravity.

When a city like New York has such an abundant supply, it is worth examining the causes of the serious water shortage in the summer of 1965. The city’s water supply was said to be ‘drought-proof until the year 2000,’ and it was stated that ‘1550 mgd [U.S. gallons] could safely be depended on at all times.’ The shortage therefore shocked both consumer and administrator. People were urged to use less water, especially for the unnecessary flushing away in toilets of everything from cigarette ends to paper wrappings. The«use of air-conditioners was restricted; a fine of £6 was imposed on restaurateurs who supplied customers with a glass of water without request; no one was allowed to clean his car; Tiffany’s, in a melodramatic gesture, used gin in its fountains instead of water.

The main reason for the water shortage was attributed to a 25 per cent drop in the average rainfall in the north-east of the U.S.A. from 1961 to 1965; in other words, the system was more than a year’s supply deficient over the four years. Worse still, the actual runoff was 33 per cent below average, for when it did rain the earth was so dry that it absorbed more than usual. Nevertheless, the reservoirs were still 88 per cent full in June 1964, but as the summer and winter of 1964 continued dry, the reservoirs were only 55 per cent full by June 1965, and continued to drop throughout the summer. It was predicted that unless the rainfall reverted to normal in 1966, there would be an even worse shortage the following summer.

The administration claimed ‘no lack of foresight’ for this shortage, but how far is this statement justified? The average yearly rainfall in the catchment area for New York is a generous 47 inches. During the drought, this fell to inches—just below the average for the notoriously wet British Isles. Why then the shortage? The answer is that even 34 inches of rain were not enough for a city with a very high water consumption and leakage rate. New York uses about 930 mgd for a population of about 7 million, compared with 365 mgd supplied to a London population of 6£ million .

The high consumption is partly due to the low price of water, which encourages wastage in the home, and to the great use of water-consuming mechanical gadgets. An expert committee had already made known that commercial water meters were under-registering by about 33 mgd and suggested that the domestic consumer was wasting the same amount. Yet, up to the last moment New York continued to use unnecessarily large amounts of water, hoping for the return of normal weather conditions. As late as October 1964, the Water Commissioner said: ‘I do not at present see any danger of a water shortage.’

Just as important, however, was the fact that the mains and service reservoirs, many of which were old and inaccessible, leaked badly. The underground waste per head, in fact, equalled the daily consumption per head in Britain of about 50 ghd, which amounted to nearly 400 mgd for the whole city. Another reason for the high leakage was that the pipe network was not designed with a high enough factor of safety against leakage. When a pipe-line is newly laid it has to satisfy a specified maximum rate of leakage, and in America this is 24 times higher than in Britain. But it is important to realize the problems involved in waste detection. It is very difficult to locate leaks in a congested and noisy city using acoustic detectors, and it only takes 20 holes of half-an-inch diameter in the whole pipe network to cause a loss of a million gallons a day. Nevertheless, an expert committee suggested that at least 166 mgd could be saved by better waste inspection.

To a large extent the recent inability of New York’s water supply to cope with modern demands has been due to the apparent inadequacy of the administration, which believed until the last moment that water was available in sufficient quantity. In New York City there is the Department of Water Supply, Gas and

Electricity , created in 1898 on the German pattern, which operates and maintains the supply within the city. In 1905 the Board of Water was created for developing new sources of water. To this day these bodies remain separate with no common directive, a dangerous procedure where water is concerned. The metering of domestic supplies would have reduced consumption considerably, as has been demonstrated in many parts of the world. But in New York the Water Commissioner is appointed by the Mayor, who in turn is elected by popular vote. What Mayor would risk losing votes by advocating water metering and increased charges?

But we should not over-criticize the shortcomings of water administrations, which usually learn by hard experience. The sensible objective is not a perfect water supply, as many water undertakings suppose, but a supply just adequate to the needs. This means that a city must be prepared to suffer some hardship during really exceptional droughts, and in fact New York did reduce its consumption by 20 per cent during 1965. But a city must be prepared to take steps as best it can and in good time. Many authorities believe that New York should not have relied solely on upland sources, neglecting the large river that runs nearby. Today there is no valid reason why the polluted Hudson should not be used, especially with modern treatment methods; in fact a treatment works was built again in 1966 at Chelsea on the Hudson to deliver an extra 100 mgd. By this time a large new Delaware reservoir was also in action to supply 275 mgd.

The water supply of London differs from New York in almost all respects. There are no upland reservoirs, though the idea was suggested a century ago. Instead, the main source is rivers that are heavily polluted, mainly with the sewage effluent of Oxford and Reading. The water must therefore be carefully filtered and sterilized. All water is pumped from the rivers, and then enters a large number of storage reservoirs. To a great extent, the form of London’s water system is dictated by its history. When in 1903 the Metropolitan Water Board was formed from eight existing water companies, they took over a system that had evolved with little planning since the early 17th century. Cautiously, the

Board interconnected, adapted, and expanded the complex pipe network, the storage reservoirs, and the old slow sand filter beds. Through the more efficient use of these time-honoured methods, and by a scrupulous bacteriological examination of all water, the Board now produces a cheap and reliable supply that continues during both drought and flood.

Sixty-eight per cent of London’s water comes from the River Thames, 16 per cent from the River Lee, and 16 per cent from wells and boreholes. The runoff from the catchment area is not high, but fortunately the Thames does not only depend on immediate runoff; much of the water entering the river comes from springs and ground-water seepage, with a contribution of effluent from sewage works. The Thames is thus a good source because its flow during the dry summer is to a large extent maintained by seepage of water that accumulated during the previous winter. However, infiltration is decreasing as new housing estates cover the porous ground with impermeable roads and houses.

London is also fortunate in obtaining good quality water from wells and boreholes, mainly in Kent and in the Lee valley. The city is built over a large chalk basin up to 700 feet thick, covered by a blanket of clay. The exposed porous chalk of the Chiltern Hills and North Downs provides a large gathering ground for water to sink downward and flow into the basin under London, where it accumulates. In the past the city derived much of its water from wells, many of which were artesian, but inevitably its excessive abstraction led to gradual depletion. But as we shall see, ground water will play a vital role in the future of London’s water supply.

Water is taken from the Thames above its tidal limit at Teddington Weir, where the flow averages 1560 mgd. Normally the Board is not allowed to reduce the flow below 170 mgd. Water passes through several intakes, each well screened to catch debris, before it is pumped into 37 storage reservoirs. The total capacity of these reservoirs, about 40 days’ peak supply, is not large compared with the 200 days of many upland reservoirs; they are drawn down only on the few occasions when the river flow is seriously diminished. An important function of the reservoirs is to reduce the number of bacteria and to allow suspended matter to settle out.

The reservoirs provide the usual algal outbursts, which may impede the flow through sand filters and give water a taste, but this has been alleviated by forced circulation in the reservoir, and dosing with chemicals.

From the storage reservoirs, water passes to the treatment works, where primary filtration involves passing water quickly through rapid sand filter beds or microscreens. These hold back coarse suspended matter, including algae, and prevents them reaching and blocking the more critical secondary treatment in the slow sand-beds. As water passes very slowly through slow sand-beds, a very large filtration area of 154 acres is needed. No chemical methods of filtration are used in London; nor are chemicals added during filtration to correct acidity or hardness. This is in marked contrast to modern practice in most other large waterworks.

All water finally receives a small dose of chlorine, after- which it passes into a large contact tank. Here water stays for a long time, and only a trace of chlorine remains when it enters the distribution system. Water from wells rarely needs filtering, but as a safety measure receives a heavy dose of chlorine followed by dechlorination with sulphur dioxide. This entire process is known as superchlorination.

A distribution network of some 9000 miles of underground mains, interconnected with 92 service reservoirs and water towers, supplies an area of 540 square miles. Water from the treatment works has to be pumped into the mains, the old ones of cast iron, the new ones of bitumen-lined steel pipes. For ready access beneath rivers, tunnels are used to carry pipes, and a 19-mile pressure tunnel has been driven to transfer Thames water to the Lee valley, using the natural surrounding clay as the impervious wrapper. After repeated branching, the trunk mains end up by supplying premises through a lead service pipe.

No more water can be taken either from the River Lee or from the present boreholes. The Thames could supply more water than it does already if more storage reservoirs were built, but they are very expensive to build and no more land is available. The Metropolitan Water Board therefore takes great care to conserve its water and to eliminate waste. Another reason for water conservation is that every drop has to be expensively pumped through several stages— from the river into the storage reservoirs, to the treatment works, to the mains, to the service reservoirs; booster pumps lift water to high ground and to tall buildings. In other words, 351 mgd have to be pumped at least three times for the consumers.

Great store is placed on the detection of leaks in the mains and service reservoirs. The Board also depends heavily on the good nature of its consumers to report leakage and not to waste water. Since two thirds of the supply is domestic and unmetered, there is no way in which the board can detect if a consumer wastes water. But strangely, the Englishman seems to be born with an innate civic sense, which includes an antipathy to water waste. Londoners consume only about 58 ghd, which is very little compared with many other cities of Europe; tfrey even accept the responsibility of telling the Board when there is a leak, and when informed that the leak is their fault, of repairing it within a certain time on pain of prosecution.

As for the future of London’s water supply, there is little land left for more storage reservoirs. It is therefore proposed to drill numerous boreholes into the large chalk aquifers under the western end of the Thames valley, covering 900 square miles of Berkshire, Wiltshire, and Gloucestershire. These will supply an extra 270 mgd, which will be pumped into the tributaries of the Thames when the river is at its summer low; in winter the ground water will be allowed to replenish itself. The Thames will thus be used as a natural pipe-line, and although this means that clean ground water is added to polluted river water, it will be cheaper than building a long pipe-line; it will also help to dilute sewage effluents in the river.

Having described the supply of two cities, we now turn to a rural area—Mid Cheshire, England. Here a relatively small quantity of water is supplied to a large area of 500 square miles. Cheshire’s special feature is the integration of underground and surface supplies by a simple process to make the best use of both. Ground water is deliberately underdrawn in wet weather when the river level is high, and overdrawn in dry weather when the river is low. This prevents the water table from permanent decline and increases the availability of water in drought. In addition to this integration of sources for local use, the river and upland reservoirs are combined for river regulation over a wider area.

Mid Cheshire was also the first European authority to install a computer to type out incoming data from widely distant pumping stations. This type of computer eliminated the necessity of reading dials and resulted in a very efficient water-supply system. The running of all water undertakings is affected by human fallibility. It was found, for example, that 60 per cent of night plant failures were caused by mistakes made during the day; it was also discovered that man is less efficient and more expensive than an automatic alarm. This type of automation freed skilled staff from the laborious task of continually watching dials and keeping charts of information just in case something went wrong.

The computer digested the incoming data, discarding the unimportant, and gave the alarm only when something went wrong. Thus not only were errors reduced, but staff had time for more skilled work, heavy labour costs were saved, and productivity was doubled.

In Britain’s first river regulation scheme, Mid Cheshire, Wrexham, East Denbighshire, Wirral, and Liverpool formed a consortium with the river authority to control the flow of the River Dee in central Wales by two regulating reservoirs. Bala Lake is a natural lake in which sluice gates have been inserted in the natural barrier across its outlet. During the dry summer, the declining river flow is augmented by water from the reservoir; in anticipation of winter floods, the reservoir is kept low in order to accommodate floodwaters that would otherwise damage agricultural property in the valley below. The other regulating reservoir, called Llyn Tryweryn , is formed by an earth dam, and supplements water from Bala Lake.

For Mid Cheshire in particular, water is pumped from the River Dee into the 40-mile-long Llangollen canal, constructed in 1795-1805. The water gravitates leisurely to an old raw-water reservoir at the Hurleston treatment works, crossing difficult ground by attractive stone bridges, one of which is reminiscent of the aqueducts of ancient Rome. Canals, as we have pointed out, are not the ideal type of aqueduct, especially unlined ones, since they experience large losses by seepage and evaporation; in order to get 8 mgd at Hurleston during very dry weather, nearly twice as much has to be pumped from the Dee. The reasons for using the canal in Mid Cheshire are the same as those advanced in favour of rivers rather than pipelines: the canal was already there, underemployed and ran in the right direction; the cost of a more efficient pipe-line would have been much greater.

From the Hurleston reservoir , water passes to the treatment works, which uses very efficient chemical methods. Primary filtration consists of the upward flow of water through a blanket, mainly of aluminium hydroxide, which removes the large particles. At the same time chlorine is added to kill bacteria and control algal growths. The water is drawn off from the top and passed to rapid gravity sand filters. After this, lime is added to correct acidity, and water travels to the contact tank where it remains for six hours before entering the distribution system.

In this water undertaking, great pains are taken to ensure complete sterilization by using large amounts of chlorine and then dechlorinat-ing with sulphur dioxide. This is partly necessary because the bacterial count of the River Dee is higher than it is for London’s water after it has been stored. The process is a little more expensive than the marginal chlorination used in London and New York, but it does ensure that absolutely wholesome water always enters the distribution network.

From the contact tank, 8 mgd is pumped into the 1400-mile network of mains; these ‘are connected at the perimeter with those of neighbouring authorities to give help in times of drought. Some consumers receive water as it passes to service reservoirs; the majority receive water from the service reservoirs. For those living on high ground, such as Mow Cop , water passes through as many as seven reservoirs by consecutive pumping.

Mid Cheshire is fortunate in having a great expanse of thick underground sandstone, which supplies 6 mgd of excellent quality water— almost as much as is taken from the River Dee. The Delamere boreholes, 36 inches in diameter, penetrate 800 feet into the sandstone, although the water-level is only 100 feet below ground level; they thus expose a large vertical area of water-bearing rock. The cost of pumping 1000 gallons 100 feet to ground level and then a further 150 feet to the service reservoirs is only threepence halfpenny, compared with about Is. 6d. for river water treatment. The natural quality of the water is so high that only marginal chlorination is needed before pumping into supply.

We end this article by describing the supply of two cities in Germany, both of which use very interesting techniques. Much of Germany overlies a generous supply of cool, clear ground water, supplying as much as 75 per cent of the nation’s needs; most of it comes from shallow wells and boreholes. The ground water is usually not too hard, but it often contains objectionable amounts of carbon dioxide, iron, and manganese, in which case the water is filtered ; chemical treatment is rarely used. Carbon dioxide is removed with lime, the iron by aeration followed by sand filtration, and the manganese by filtration through zeolites on which the manganese is deposited.

The water supply of Essen is typical of several towns in the Ruhr and Rhine valleys, where very polluted river water is filtered through the alluvial sides of the river. Raw river water is diverted over a weir into a large sedimentation basin, and after some 12 hours’ retention, passes on to about 52 acres of filter beds. These are excavated out of the banks of the river by removing the natural silt until the gravel is exposed, which is then covered with sand. After a time, the top layer of sand becomes clogged and is then skimmed and replaced. Water percolates through the beds until it reaches perforated collector pipes 30 feet down and gravitates to wells on the other side of the river. No artificial filtration is necessary, and after adding chlorine, water passes into supply. This process, which makes full use of nature, amounts in fact to a conversion of polluted river water into clean ground water. An average consumption of 33 mgd is met by this system at the high cost of 4s. 6d. per 1000 gallons.

Munich is a large city with a population of over one million, which obtains all its water from underground. There are 7000 private wells in and around the city, mainly for industrial use. Tap water comes from more distant wells and from a very interesting system of infiltration galleries built in 1883. These are horizontal tunnels, or adits, about three to five feet in diameter, built in pervious rock and slotted to allow ground water to seep into them. They are situated 300 feet above the city, in the Mangfall valley of the Bavarian Alps, about 25 miles from Munich. Water flows from many of these galleries by gravity along aqueducts to two large, covered reservoirs near the city ; the rest is pumped from wells in the valley into the aqueducts. One reservoir supplies the higher zones of the city, the other the low zones, and their relative height is such that water flows by gravity to each zone at the right pressure. The high-quality water obtained needs only chlorination. The cost per 1000 gallons is £12—which is again a rather high figure if we consider that the generous natural supply does not need extensive treatm pent and also requires very little pumping.

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