SWEDISH WATER WORKS METHODS
Plants 85 Per Cent. Municipal—Town Group Supply-Many Manufacturer* Have Own System— Only 10 Per Gent, of Citie Coinpletcly Metered
THE following article gives a very dear and concise review of the water works situation in Sweden and will be found of interest in comparison with the systems in this country:
Sweden is a large country though relatively sparsely populated. Its superficial area is 448,460 sq. kilometres (173,150 sq. miles) or about 5 per cent, of the total area of Europe. On January 1, 1923, the population amounted to 5,987.520. and the density of population to 14 inhabitants per square kilometre (35 inhabitants per sq. mile) or about the same as in l’. S. A , hut only l-14th of the density of population in England. Of the people 1.813.177 live in the 111 cities and about 250,000 in small country towns and similar communities with more than I.0C0 inhabitants. Of the cities, three have more than 100,000 inhabitants, and thirty-six above 10.000.
Thus one-third of the population of Sweden live in communities in which, as a rule, it is advisable, on account of a relatively great density of population, to lay down one water supply for the whole community. This solution of the water problem has been applied extensively. Not less than 85 per cent, of the Swedish cities have municipal works. As only a few of the smaller cities are without water works, the cities provided with such water works embrace together not less than 98 per cent, of the total population of the cities. Within the various cities an average of 95 per cent, of the inhabitants are supplied with water from the municipal water works. Thus most of the people in the cities enjoy the advantages provided by water works.
In the following the natural resources, as far as water is concerned, will he set forth and an account given of the Swedish water works.
The Presence of Water
The rock foundation of Sweden consists largely of primary rocks. On account of its homogeneous structure, it cannot collect ground water to any noteworthy extent. The looser layers of earth consist almost entirely of the post-glacial formations, which arose in connection with the great glacial periods of northern Europe. As these formations to a large extent consist of clays and very fine sand, they do not contribute to forming any considerable ground water streams. Only when this material has been sorted by the melting water of the glaciers, have water-bearing sand and gravel appeared. It is especially worthy of note that the rivers running under the glaciers have worn the stones round and washed away the fine particles, leaving the remarkable rubble-stone ridges, which at the present time rise above the surrounding country and constitute interesting memorials of the glacial periods. On account of rock sand other irregularities projecting into the ridges there are often great difficulties in using the ground water.
Precipitation is relatively small, and only averages 515 mm. a year. It is fairly evenly distributed over the whole year, with somewhat less precipitation in the spring than in the autumn as a rule. It is estimated that at least one-third of it is carried off by the rivers, which fact is due to the low mean temperature and the resultant low evaporation. On their way to the sea the rivers pass through a great number of lakes, the total surface area of which amounts to not less than 8 per cent, of the whole area of the country. In Sweden is to be found, in point of size, the third great lake of Europe, Vanern, with a surface area of 5,568 sq. kilometres (2,150 sq. miles).
From this general survey it may be gathered that ground water occurs relatively sparingly, while there are no difficulties in furnishing the cities of the country with water from the numerous lakes and rivers.
The first water works distributing water with sufficient pressure to carry it to the upper stories of dwellings, were built in the ’sixties. The most complete collection of statistical data on the Swedish water works is to be found in the “Statistical Data” of the Swedish Association of Municipal Engineers (Svenska K.ommunal-Teknisha Foreningen. or S. K. T. F.), ami the data given below have been compiled from Matistics from this source for 1922.
(Continued on page 944)
Sweilish Viater Works Methods
(Continued from page 940)
The water consumption per capita is relatively small in Swedish cities, due partly to the fact that manufactories using gn at quantities of water often have water works of their own. li we deduct the water used for manufacturing purposes (which in the twenty larger cities averages 25 per cent, of the total consumption in these cities) the consumption of those cities, where the water is generally metered, reached a daily average of 80 litres (17.6 imperial gallons) per capita reckoning the average over a full year, and 120 litres (26.4 imperial gallons) in cities not metered.
Thus cities not metered to any considerable extent consume 50 per cent, more than cities with meters. In the case of ground-water plants the consumption is somewhat lower than in the case of surface-water plants. Water used for manufacturing purposes is generally controlled by meters, but only in 10 per cent, of the cities is the entire consumption controlled by meters. In the other cities the domestic consumption is largely not controlled. The meters used are generally rotary meters; to some extent volume meters occur, most often in the form of disc meters.
Water works engineers in Sweden have endeavoured in the to place to obtain ground water for the water requirements of the cities As mentioned above, the natural resources arc not so well adapted to serve this purpose, but in spite of this fact they have succeeded to a remarkable degree, due partly to the small size of the cities. Not less than about half the number of Swedish cities use exclusively ground water. The two largest cities. Stockholm and Gothenburg, fill only a minor part of their requirements with ground water, but the third city of Sweden, Malmo, with a population of 115,000, used exclusively ground water. Here it has been possible because of a preglacial water-bearing sand-stratum in the vicinity, which on account of a dislocation in the rock surface ha> withstood the action of the glaciers.
The condition of the ground water in its natural state is usually good, and thirty-eight cities use the water without any special treatment. When purification is necessary, it is generally for removing iron, present, however, in easilyoxidized compounds. The largest iron content amounts to 6 milligrams per litre (p.p.m.). The purification is commonly effected by aeration and in gravel and sand filters of the open type. Only in two places are pressure filters to be found. Where there are natural sand strata, but the quantity of ground water is small, plans for producing artificial ground water have been rather extensively built. In this case the surface water is first purified hv filtration and then conducted by open basins or pipe wells into the sand strata.
Of the cities using surface water, halt the number take their water from lakes and the others from rivers. On account of the relatively suitable condition of the looser la3’ers of soil, the numerous lakes and the low density of population, the water in lakes as well as rivers is generally a good raw material for the water works. The amount of suspended matter is small and the water usually but slightly coloured. Only a few cities have such a situation that they have to use surface water coloured by vegetable matter.
The number of bacteria in the raw water averages at the twelve largest surface-water plants, 875 per cubic centimetre. Thus surface water is well adapted for purification in ordinary slow sand filters, and the rate of filtration averages 100 mm. (about 4 in.) per hour. Through this treatment the bacteria are reduced from the above-mentioned 875 per cubic centimetre in the raw water to 36 in the purified water. Plants using water coloured by vegetable matter avail themselves of the same method of purification, although this is not sufficient for de-colourising the water. A couple of smaller plants use rapid sand filtration in combination with the addition of aluminium sulphate.
(Continued on page 948)
Swedish Water Works Methods
(Continued from page 944)
During the present year two cities, using water from rather densely populated drainage areas, have built rapid filters with disinfection by chlorine gas. Some smaller municipalities and private works use rapid filters in conjunction with disinfection by either chlorine or ozone. In this connection it may be mentioned that chlorine was discovered by the Swede Scheelc as early as 1774.
There are only five Swedish cities with their sources of supply situated so high as to render pumping superfluous. In the others the necessary pressure has to be obtained by means of pumps.
Among the latter works 30 per cent, are equipped with centrifugal pumps exclusively, and only 10 per cent, have piston pumps exclusively. The driving power is generally furnished by electric motors, taking their current, as a rule, from some hydro-electric power station; only 5 per cent, of the works do not employ such driving power. Of the latter the greater number use water power directly driving the pumps. The reserve machinery of the works is driven by oil motors, gas motors, steam engines or by electric motors with power from a different source from the other motors.
The water works distribute the water through mains and service pipes. The former consist almost without exception of cast iron pipes, the latter of cast iron as well as wrought iron pipes. Only to a small extent are there mains of wrought iron pipes, then usually covered with jute-cloth drenched with asphalt. The mains are planned for conducting water not only for normal consumption but also for fire-extinguishing purposes. Fire hydrants are placed at about every 100 metres (110 yds.), and almost without exception entirely below the ground surface.
For the manufacture of pipes as well as mountings there are regulations worked out by the S. K. T. F. On the premises the water is distributed by service pipes, generally of wrought iron with mountings of metal. The installations are covered by regulations and special instructions mainly worked out in close agreement with the standards of the S. K. T. F.
As pressure regulators open reservoirs are used almost exclusively. Half the total number of water works have their reservoirs situated on such a natural height that towers become unnecessary.
The reservoirs are generally made of reinforced concrete. The open reservoirs are intended to serve for equalising the variations in the daily water consumption, as well as for holding a reserve supply for fire-extinguishing purposes, the capacity of which must be at least 300 cubic metres (about 10,600 cubic ft.) when the reservoir is situated on a natural height, and 100 cubic metres (about 3,530 cubic ft.) when the reservoir is placed in a tower.
It has been emphasized above that the mains as well as the reservoirs are planned with a view of preparedness for fire extinction. Other parts of the works are also adapted to serve this purpose, and thus the water works fill the minimum requirements laid down by the fire insurance companies—viz., to be able to supply at arry time at least 1,600 litres (352 imperial gallons) per minute through at most six adjacent fire hydrants with a pressure in the main not below 30 metres (100 ft.). The observance of these regulations increases the erection cost of the plants, especially in smaller cities, but in return a safe protection against fire is obtained. Even within buildings there are fire hydrants in certain cases, and to a less extent sprinkler installations.
The water works in all Swedish cities, with one sole exception, are owned by the cities themselves. The water rates are generally calculated so as to make the revenue coyer the expenses for the running of the works as well as for interest and amorti?ation of the erection cost. Where there are no meter?, rate? are calculated on the assessed value of the estate, the number of rooms, etc. In 10 per cent, of the cities the water consumption for domestic use is still free. On an average for the twenty largest citie.-. the income from the water sold is 12 ore per cubic metre (about 8d. per 1,000 imperial gallon> of distributed water.
(Continued on page 951)
Swedish ’Rater Vi orks Methods
(Continued from page 948)
Even though natural resources have not been the most abundant in many instances, it has been possible for the cities of Sweden to obtain good water supply at a relatively low cost. However, the cities are growing, and the increase in water consumption is more rapid than the increase in population. Even if the consumption to a certain extent can be restricted by a more general use of meters, the consumption will probably tend ever to increase. As the demand grows, the water sources will become less satisfactory. The ground water will probablybe insufficient and the surface water polluted as places get more densely settled, and also pollution will occur from industrial wastes. It will be necessaryto plan in time to order to be able to meet the greater demand, and to reserve certain lakes for water-supply purposes, in order to make the proper methods of purification as simple as possible, so that good water maybe obtained at a reasonable cost even in the future.—Surveyor and Municipal County Engineer, London.