The greatest advance in water supplies has been made in the last 50 years, when water became available that was free from disease-producing organisms. This, as we have said before, is far more important than providing water in large quantities or free from taste. But in many parts of the world, we also expect water to be palatable, or pleasant to drink. This means that water must be reasonably oxygenated, and free from tastes and odours produced by algae, fungi, industrial waste, or sewage; it should not have any colour, which apart from being unsightly, stains fabrics and appliances. Finally, we prefer water that is not too soft, which corrodes pipes and containers, and not too hard, which wastes soap, produces scum, and furs hot water pipes and cooking vessels. This article describes how we obtain such so-called pure water.
We have to be careful about the use of the word pure because pure water, consisting solely of H20 molecules, does not exist; it cannot even be prepared in the laboratory. So when we talk about purifying water we really mean making it safe and palatable. The purest natural water comes from dew and rain in areas free from industrial, air-borne pollution; in industrial regions, as housewives know only too well, rain can quickly mark clothes on the line with dark streaks. All rain, though, collects small amounts of gases and dust as it descends through the air.
Ground water is often very clear, and because it has slowly filtered through the ground, it is generally free from silt, organic compounds, and micro-organisms. But as water travels through the ground it dissolves part of the soil and rock, and so is usually hard; it may even contain objectionable concentrations of salts, such as those of iron and manganese. Surface waters contain the most pollutants, for as water flows over the land it picks up silt, inorganic salts, and organic compounds from decayed plants and animals; surface water, unlike ground water, also invariably contains micro-organisms.
Really severe pollution, especially by harmful micro-organisms, started only when rapidly growing populations in the Industrial Revolution began to discharge untreated sewage into rivers and lakes. Human waste was formerly disposed of in earth closets and then spread on the land, where it was naturally decomposed by soil organisms. Earth closets, however, became impracticable for large towns for lack of spare open land, and there was really no choice but to use rivers to remove waste. In some parts of the world, including parts of America and Europe, raw sewage is still discharged into rivers, so that many are now no more than open sewers. With the Industrial Revolution, however, came a new source of pollution in the form of effluents from industry, and today industry is responsible for more than twice as much pollution as domestic sewage. A single sugar-beet factory, for example, produces pollutants equivalent to the sewage of a city of half a million; the food, textile, paper, and petroleum industries are other prime offenders. Some industrial effluents are actually more serious than household waste, for as well as containing organic material, they may include inorganic compounds like sulphuric acid, arsenic, and cyanides, which are difficult to remove and highly poisonous.
With thousands of tons of municipal and industrial waste being produced each year, it is inevitable that ground water should occasionally become polluted. This is most likely to happen in populated chalk and limestone areas where there are large fissures through which water travels so quickly that there is insufficient time for the natural process of purification. In such areas, a fractured sewer, for instance, may quickly infect wells with pathogenic organisms. In general, most organic compounds and micro-organisms are destroyed before they reach wells, but this is not so for industrial effluents that contain substances that cannot be broken down by microorganisms. Detergents, for example, often enter wells in rural areas where household waste is disposed of in cesspools or by spreading on land that overlies aquifers.
A polluted aquifer is in many ways more of a problem than a polluted river or lake. It is very difficult or impossible to remedy, and an abandoned well is so much wasted capital. However, it is not always possible to avoid the contamination of aquifers, or to foresee whether a borehole is likely to penetrate an already contaminated aquifer. There is the case of a firm in Norwich, England, that drilled a 36-inch-diameter borehole and later had to abandon it because tarry substances and phenols suddenly appeared in the water. The pollutants appeared to have come from a gasworks, which was puzzling because the nearest one was some distance away. Eventually it was discovered that a gasworks built in 1815 had formerly occupied the actual site of the borehole and this had produced gas from the distillation of whale-oil. The tar had persisted in the ground for over 120 years!
The almost total lack of pollution control during the past century has resulted in rivers and lakes throughout the world that are polluted and almost useless for water supplies. Until recently, the attitude has often been to dump waste into a river and to leave it to be dealt with by the next consumer downstream. Sometimes legal action by downstream consumers forced those upstream to mend their ways, while occasionally all the river users co-operated to prevent pollution. Such co-operation, however, was not possible for rivers and lakes bordered by several states or countries with different laws and degrees of ‘pollution consciousness.’ For example, Lake Erie, one of the Great Lakes of America, is bordered by five states and by 1965 was seriously polluted; profuse algal growths thrived on the nutrients from untreated city sewage, while numerous cities along its edge dumped raw acid, oil, iron, car tyres, and other rubbish into its shallow waters. Detroit alone poured in about 670 tons of waste each day. The fishing industry was destroyed, and more than £35 million are now needed to clean up the lake and prevent it from becoming an offensive swamp.
Probably the most polluted large river in the world is the Rhine, which, with its tributaries, passes through Switzerland, Liechtenstein, Austria, Germany, France, Luxembourg, Belgium, and Holland. Numerous industries in several of these countries have dumped waste, regardless of the next country downstream. A large amount of untreated sewage also entered the river from the 40 million inhabitants of the Rhine river basin; West Germany, for example, poured 73 per cent of its sewage into the Rhine and its tributaries, of which only 11 per cent was fully treated. Sometimes there were high levels of sodium chloride in the river from untreated sewage and industrial effluents, which, as we know, cannot be removed economically. High concentrations of detergents were also common, and caused tastes and foaming of tap water. In 1960, 10 per cent of the population of Essen had non-bacterial gastro-enteritis, which was attributed to the irritant effect of the high salt and detergent concentrations.
Pollution is now so serious that several countries have enforced the treatment of all wastes. Agreements are also being gradually drawn up to prevent undue pollution of international lakes and rivers, like Lake Erie and the Rhine. For the small firm, the cost of treating its effluents may be prohibitive, but this is still no excuse for polluting water used by other consumers. Under pressure, most firms incorporate effluent treatment in their budget without too much trouble, and fortunately, it is not too late to clean up our polluted lakes and rivers. For example, around Lake Michigan is the largest concentration of steel industries and oil refineries in the world. These have been forced to treat their effluents, and the lake is slowly recovering, which proves that pollution is not an inevitable outcome of large populations and heavy industry.
In practice, it is not necessary to remove all impurities from water before discharge into rivers and lakes. Such a course would be very expensive, especially as a city of 500,000 produces about 100 tons of solid sewage each day. Nature has kindly made water self-purifying to a large extent: suspended solids settle out, particularly in lakes and reservoirs; inorganic substances, such as iron, are oxidized by the dissolved oxygen in the water; organic compounds from sewage and industry are also oxidized into harmless carbon dioxide, water, sulphates, phosphates, and nitrates. This natural conversion is done by bacteria and other micro-organisms, which find in sewage a rich source of food and energy. During break-down, oxygen is gradually removed from the water, and is replenished by absorption from the air at the surface. If too much waste is present, oxygen is removed faster than it can be replenished. Decomposition is then taken .over by anaerobic bacteria, which do not require oxygen, and which produce obnoxious or toxic compounds like methane and hydrogen sulphide. The water is then both a public nuisance and unfit for use.
To estimate how much sewage can safely be emptied into a river or lake, it is necessary to know the purifying capacity of its water. This depends on how much oxygen is dissolved in the water, and this in turn depends on the surface area, since oxygen dissolves only in the top three quarters of an inch. Turbulent rivers have a greater surface area than smooth-surfaced rivers and are thus more self-purifying; slow rivers, like the Thames, are too smooth to cope with large amounts of sewage. Equally important is knowing the quality of the waste, that is, how much oxygen it requires for decomposition; one type of waste may well require ten times more oxygen than another. The strength of a waste is measured by its biochemical oxygen demand , which is the amount of oxygen it requires for decomposition.
We now describe how sewage is treated prior to discharging it into rivers and lakes, for this, in a sense, is the first stage in water treatment. Sewage consists of domestic waste together with industrial effluents that are emptied into sewers. In Britain and many other countries, industry is encouraged to use the sewers for disposal, rather than rivers and lakes. Some towns on the coast still discharge raw sewage into the sea without any treatment, although this is now widely discouraged in Britain. Inland cities, however, have no alternative but to discharge waste along sewers to sewage treatment works. The first stage in treatment depends on whether sewers also carry storm water, that is, rain that runs off pavements, roads, and roofs. Most cities and suburbs built in recent times have separate sewers and storm drains to relieve the load on the sewage works. But most cities built their sewers many years ago to transport both sewage and storm water, and it is now uneconomic to convert them into separate systems. Sewage works therefore usually have storm water tanks to store the large bulk of rain-swollen sewage; during dry weather, sewage from these tanks is fed to the works in amounts that can conveniently be dealt with.
The sewage first passes through screens to trap large pieces of wood, rags, wire, and so on. It then passes to grit tanks, where grit and sand settle out, thus avoiding blocked pipes and tanks, and preventing undue wear on pumps later on. Leaving the grit tanks, sewage passes to primary sedimentation tanks, where about 50 per _cent of the suspended solids settle out to form sludge; the BOD is also reduced by about half. Some works mix chemicals, such as alum or ferric sulphate, with the raw sewage to produce a thick mat that drags down solids; this method reduces the suspended solids by about 70 per cent and the BOD by about 85 per cent. Primary sedimentation tanks do not remove dissolved solids; nor do they remove colloids, which are minute insoluble particles that remain as a cloudy suspension.
What we have just described is called primary treatment: it produces sludge, and a liquid called settled sewage. Treatment may go no further than jthis, the settled sewage simply being discharged into a river. This is feasible, of course, only when the river is capable of purifying it. In most works, however, settled sewage undergoes some secondary treatment to reduce the remaining suspended solids, and to reduce colloids and dissolved organic matter. There are two main methods of secondary treatment— both of which rely on micro-organisms to decompose compounds in much the same way as takes place in the soil and in rivers; the process is merely speeded up by having controlled and well-oxygenated conditions. With trickling filters, rotating arms spray settled sewage slowly over beds of broken stones, on which a variety of micro-organisms soon find a home and oxidize the settled sewage to a high purity. The degree of purity is decided in England by the river authority concerned, one of whose jobs is to calculate how much sewage a rjver can take. The treated settled sewage, together with slime deposits that have sloughed off the stones, then passes to a sedimentation tank for clarification. Trickling filters are more suitable for rural areas because they take up a lot of space, and they are ideal where there is a shortage of labour because they need very little supervision.
Sewage works in cities normally use the activated sludge process, which takes up less space. Activated sludge tanks are more sensitive to change and to certain pollutants than trickling filters and so need constant, skilled supervision, which is more available in towns. In this process, settled sewage is mixed with sludge that contains micro-organisms, and the whole mixture is agitated and oxygenated by compressed air for about 10 hours to encourage rapid decomposition. The mixture then flows to sedimentation tanks, where the sludge settles out. The upper liquid part is discharged into lakes or rivers and a portion of the sludge is returned to the activated sludge tanks to start a biological decomposition of the new incoming settled sewage.
To produce a reasonably pure settled sewage is no great task, and once produced it can be discharged through pipes, if necessary by pumping, or made to flow along open channels, called culverts. By far the greatest problem in sewage treatment is disposing of the sludge, which can be neither pumped to, nor dumped into, lakes and rivers. Raw sludge, which forms in storm tanks and in all sedimentation tanks, is about 96 per cent water, and this is almost impossible to remove economically. Small communities often spread raw sludge on the land, or place it in open lagoons, but decomposition by either method may take several years. Urban areas use a much quicker method by placing raw sludge in sludge digestion tanks, where it is digested anaerobically by micro-organisms in carefully controlled conditions. In cold tanks, the process takes about four months, but if the tanks are maintained at about 90°F, digestion takes about a month. The methane gas produced during digestion is collected and used to heat the digestion tanks, and to drive pumps, air compressors, and generators; it may even be fed into the national gas grid.
Digested sludge is still about 96 per cent water, but at least it is inoffensive and it can be dried. The cheapest means of disposal for coastal works is to dump it by ship into the sea. Sometimes a proportion isused tofill in low-lying land, such as marshes and disused gravel pits. Another method of disposal has recently become common through advances in agriculture, which requires that the soil should be conditioned throughout the year. Digested sludge is applied to the land to provide humus and trace elements, and both farmer and sewage works manager are happy. Most digested sludge, however, has to be de-watered before disposal, so that it ends up with a water content of about 50 per cent and is therefore less bulky. This is usually done by laying sludge on sludge drying beds, made of sand and gravel, through which the water percolates and is collected by drains; the dried sludge is then often sold as compost. Raw sludge may also be dewatered by heating, or burnt with fuel, methods that are very expensive, -and more common in parts of America.
In this article we have discussed the treatment of sewage first because the first stage in water treatment is really the prevention of gross pollution. We have to face the fact that most sources of water are polluted and contain water that has been used at least once. As demand grows this use and re-use is bound to increase. We now describe how water is treated to make it suitable for domestic and industrial consumption. It is actually possible to make any water fit for use, no matter how polluted, but not necessarily at an economical price. There are several methods of treating water, the choice depending on the quantity of water required and the type and intensity of the pollution. The easiest water to treat is that from many springs and wells, which require only a precautionary sterilization with chlorine. Many upland lakes and reservoirs in unpopulated areas also provide very clean water, so long as it is not polluted on its way to the treatment works. In lakes and reservoirs, much of the suspended solids settles out. How much and how soon depends on the size and weight of the suspended particles; some very fine particles never settle out. In addition, all kinds of bacteria gradually die, so that 97 per cent have disappeared after about 30 days’ storage. Finally, many inorganic and organic compounds are oxidized, and both colour and hardness are reduced. These same advantages apply to the storage of very polluted river water in storage reservoirs, as in London.
Prolonged storage, though, does encourage the growth of algae, and outbursts are apt to occur just at the times of peak demand in the summer. In themselves, algae are rarely harmful to health, but they can produce unpleasant tastes and odours, and clog sand filter beds. Many water undertakings try to keep down algal outbursts by adding chemicals such as copper sulphate, potassium permanganate, and chlorine to lakes and reservoirs, but in practice this does not always work.
At the treatment works, water is usually filtered to remove or reduce suspended solids, colour, bacteria, algae, and various salts. The oldest method of filtration is by the use of slow sand filter beds. These became common in England after 1829, although various types of filter bed had been in use for thousands of years.
Slow sand filter beds consist of about two to three feet of sand resting on about two feet of gravel. As water slowly passes through the sand, a jelly-like film forms on the surface , which very efficiently strains out bacteria and particles in the water. Clean water is drawn off through perforated drains beneath the gravel. Eventually, the film becomes so clogged that the flow of water almost stops. How soon this happens depends on how polluted is the water, and especially on how many algae it contains. Slow sand beds therefore have to be cleaned frequently by taking them out of service and scraping off the dirty layer of sand, usually by hand. This sand is washed for re-use later.
The normal flow through slow sand beds of only 2 gallons per square foot per hour requires the beds to have a large surface area and so they take up a considerable amount of space. During algal outbursts, frequent pauses for cleaning reduce the output of water still further. Even so, some cities continue to use slow filter beds; London has 150 acres of them, built many years ago when land and labour were cheap. London has found it convenient to continue their use, but it no longer uses them alone: now water is first passed through rapid sand filter beds at the rate of about 100-150 gallons per square foot per hour to remove most of the larger solids, which lightens the burden on the slow filter beds. As with slow sand filters, water flows through these beds by gravity, but they take up less room and are easily cleaned by mechanical methods; the film is loosened by passing air and then water upward through the beds; the upper layer is then mechanically scraped. Some London works use. microstrainers instead of rapid filters; these consist of very fine-meshed screens constructed in the form of revolving drums through which water passes. Particles, including algae, are left on the screen and are continuously flushed away. Primary treatment by rapid sand filters or micro-strainers is now in common use as an, aid to slow sand filters.
Sand filter beds alone are not effective for water that contains fine silt, and various tastes and colours. Coloured matter and tastes pass straight through the beds, while the silt quickly blocks them. This happens especially in peaty moorland areas; in parts of the tropics a stream may turn a coffee colour in minutes during heavy rain and carry 50 times more silt than ever occurs in England or America. For this reason, America departed in the early 1900s from the traditional European practice, and adopted a method of chemical treatment, which has now spread throughout the world. In this process, a coagulant such as alumina is mixed with the water entering the tanks. This clumps the solids together, including colloids and bacteria, into groups heavy enough to settle out. One type of chemical treatment tank is about 30 feet deep and shaped like an inverted pyramid; the coagulant forms a stationary sludge blanket about 10 feet thick through which the water passes. Water is pumped upward with a high initial velocity, which decreases near the surface and just prevents the blanket from falling. A highly turbid and coloured water passing through such a blanket is clarified and is led away at the surface. The sludge and trapped solids are periodically removed and dried. Various coagulants are available, and are supplemented by other substances, such as sodium alginate and activated silica, to strengthen the blanket. After treatment with coagulants, the water is always passed through rapid or slow sand filters.
Rapid sand filters are of two types: the open gravity filter, in which water percolates down by gravity, and the closed pressure filter, where water is forced through a bed of sand and gravel. The term pressure filter means that water flows through the bed at some pressure. As with gravity types, there is still a. loss of head, but the process is faster, yielding as much as 600 gallons per square foot per hour. Pressure filters are easier to install than gravity types, but are more liable to fail.
The treatment described above removes most of the bacteria, but some may get through— enough to cause very unpleasant epidemics. The final stage in water treatment is therefore sterilization, usually with chlorine. This was first used to sterilize water in 1897 , but it was not until the 1930s that its use became widespread; in developed countries today all domestic water is sterilized. Although chlorine is a very poisonous gas, small concentrations of dissolved chlorine have no known harmful effect on man, yet it is fatal to micro-organisms. Used at a concentration of 0.1 parts of chlorine in a million parts of water , it both kills bacteria and decolorizes the water. The actual concentration of chlorine and time of contact with water required depend on the degree of pollution, and should result in no Escherichia coli in 100 cc. of water. Although this human, intestinal bacterium is harmless, it is usually present in water, and the chances are that if this type is eliminated so will be all the harmful bacteria.
The only disadvantage with chlorine is that it imparts a taste to water if there is too much residual chlorine left. Sometimes it also produces unpleasant tastes by combining with certain types of decayed plant matter, particularly algae, and these tastes are almost impossible to remove economically. Tastes are sometimes said to be avoided by using the chloramination process, in which both ammonia and chlorine are added, but this is not altogether successful. One excellent method is superchlorination, in which a high concentration of chlorine thoroughly sterilizes the water and removes any residual colour and taste; the chlorine is then removed by adding sulphur dioxide to the water.
Chlorine is the best compromise between cheapness and effectiveness. Much better sterilizers exist, such as iodine, bromine, and ultraviolet rays, but such.is their cost that they are only used for small quantities of water in special situations. The only real rival to chlorine is ozone , which is used mainly in France. This is more expensive than chlorine and its power of sterilization is not quite so high, but it does not produce taste problems and it gives water a very pleasant sparkle. Once the reliability of ozone is well established, it will doubtless displace chlorine for those water undertakings who con- sider that a pleasant, sparkling water is worth the small extra cost.
At some stage of treatment, it is sometimes necessary to correct excessive hardness, softness, alkalinity, acidity, and persistent tastes. Temporary hardness is due mainly to the bicar-bonates of calcium and magnesium, which the domestic consumer can partially remove by boiling. Permanent hardness is due mainly to the sulphates of calcium and magnesium and is more difficult to remove. The degree of hardness is measured by the ability of water to destroy soap and produce scum. Hard water also produces scale in boilers, kettles, and hot water pipes, and is not good for cooking vegetables. There are two methods of softening water: precipitation with chemicals, and ion exchange. In the first method, lime, in the form of CaO or Ca 2, is added to the water, causing precipitation of the bicarbonates as insoluble compounds, which are then removed; sometimes soda-ash is also added to precipitate the sulphates. With ion exchange, water is passed through a bed of granules of zeolite, a substance that has the remarkable property of being able to exchange the calcium and magnesium in the water for sodium contained in the zeolite. The water then contains as much mineral matter as before but in the form of sodium bicarbonate and sulphate that are not hardness-forming. Zeolites, commonly used for domestic water softeners, are not used by water undertakings so much as lime-soda softening.
Natural soft water occurs when rain water flows over very insoluble rock, and it is very pleasant to use in the home. But it is more likely than hard water to dissolve dangerous substances, such as lead and copper from pipes. Soft water is also often acid, as in moorland areas where organic acids from decaying plants dissolve in the water. Acidity may also be caused by high concentrations of dissolved carbon dioxide. Acid water is very corrosive and is neutralized with lime.
Certain salts and tastes are not entirely removed by the methods of water treatment that we have described so far, and then costly techniques have to be employed. Common salt cannot be removed economically, and the only remedy is to dilute the water with non-salty water. Iron, manganese, and copper produce metallic tastes, while certain industrial wastes have very tenacious tastes and odours. Ozone deals with some of these troubles. Aeration, by a fountain or by cascading water over weirs, is also used to remove iron, manganese, carbon dioxide, and some tastes. The best taste-remover of all, however, is activated carbon. One cubic inch of this possesses a surface area of over 20,000 square yards, on which substances in the water are held, or adsorbed. Beds of granulated carbon are often used in large ships and have the special advantage of removing phenols, which are extremely offensive. When carbon is used in waterworks, it is usually in powder form. At one time, Los Angeles recharged reclaimed sewage into the ground, pumped it up, and then passed it through carbon; the water was said to taste better than from the ordinary supply.
There are many variations in water treatment. New York’s water, like that of Manchester, England, is not filtered at all, only sterilized, and occasionally aerated. At the opposite extreme, we have the Amsterdam waterworks, which has the misfortune to have to deal with the Rhine where it ends its journey in Holland. To produce a palatable water from this river, the following steps are performed in sequence: aeration, rapid filtration, aeration, chlorination at 6 ppm., aeration, artificial recharge underground through sand dunes for two to three months, aeration, activated carbon, rapid filtration, slow sand filtration, aeration, chlorination. Even after this all-out attack, bad tastes sometimes get through!
Finally, we deal with the treatment of industrial water—sometimes a more difficult proposition than preparing water for domestic consumption. Some industries abstract water directly from rivers and lakes, in which case the initial treatment follows the same lines as for domestic water. Water that is taken from the public mains sometimes has to undergo additional treatment. The domestic consumer does not object to moderately hard water, whereas industry often requires water of almost zero hardness. The salts that make water hard form scale in boilers, pipes, and heat exchangers, which decreases the conductivity of the heat-transferring surfaces, resulting in overheating and occasional damage. Hardness is also harmful in the laundering and textile trades, because scum damages and discolours fabrics.
The lime-soda method of softening has already been mentioned, and is often used by industry. The ordinary ion-exchange process using zeolites is even more efficient, producing water of almost zero hardness. But with both these methods, salts remain in the water. Some industries, ranging from power plants to the makers of transistors, need to remove all the salts from water, by a process called demineralization. This is an elaboration of the ion-exchange process, in which one zeolite removes the cations— mainly calcium, magnesium, iron, sodium, etc. —and another zeolite removes the anions—sulphate, bicarbonate, nitrate, chlorine, and so on.
As well as forming scale, water can also be very corrosive at high temperatures. The main cause of this corrosion is dissolved oxygen and carbon dioxide, so these gases have to be removed. Finally, some industries are also concerned with the organic matter and micro-organisms in water, but not usually for health reasons. Surface water generally contains small amounts of organic matter, even after treatment, and these damage the zeolites used in ion exchange. Organic matter may be removed by hydrogen peroxide and activated carbon. Then we have the slimes and encrustations that tend to form in pipes due to the presence of organic matter, and iron and sulphur bacteria. One way of eliminating these is by very thorough chlorination.