Whether a water supply is ‘hard’ or ‘soft’ depends upon the nature of its journey from raincloud to the Water Authority’s reservoir.
Water has the capacity to take into solution some part of practically any gas or solid matter with which it comes into contact. Even during its brief journey from the clouds to the earth it will take into solution measurable quantities of carbon dioxide and, where it falls over a town, sulphur dioxide and other pollutants of the atmosphere. If it falls onto mountains or moorland it will acquire acid characteristics as it flows, via streams and rivers, to the reservoir. Such water will be soft and will have a tendency towards plumbosolvency— the characteristic of dissolving lead from the surfaces of water pipes.
If, on the other hand, it falls on a chalky soil, or seeps through the various strata of the earth into natural underground storage reservoirs it will dissolve the bicarbonates, sulphates and chlorides of calcium and magnesium from the rock and soil surfaces with which it comes into contact. These are the chemicals that are responsible for hardness in water. Virtually all water supplies in southern England, the south Midlands and East Anglia are hard or very hard. Water supplies in Wales, Scotland and the north of England are predominantly soft though, even in these areas, pockets of hard water supply are to be found.
Hardness is usually expressed in terms of the equivalent of calcium carbonate in the water in parts per million though ‘degrees of hardness’ on Clark’s scale are sometimes used. One degree of hardness on this scale is equal to 14 parts per million or p.p.m. Water containing the equivalent of more than 100 p.p.m. of calcium carbonate is reckoned to be ‘moderately hard’ and water containing over 200 p.p.m. is ‘hard’. Some 65% of the homes in Great Britain are supplied with water that would be classified as hard or moderately hard.
‘Temporary hardness’ is hardness that can be removed by boiling and is the kind that is most serious from the point of view of the maintenance of water heating systems. It is caused by the bicarbonates of calcium and magnesium whereas ‘permanent hardness’ is caused by the dissolved sulphates and chlorides of these chemicals.
When water containing the bicarbonates of calcium and magnesium is heated to temperatures of 70 C and above, carbon dioxide is driven off and the dissolved bicarbonates are changed into insoluble carbonates which are precipitated out as boiler scale or kettle fur. A glance into the inside of any domestic kettle will establish whether or not temporary hardness is a problem in that particular area. If it is, the electric element and the base and sides of the kettle will be coated with creamy white scale. This scale will not have formed only on the inside of the kettle where it can at least be readily seen. It will also be present in the boiler and flow and return pipes of any ‘direct’ hot water system and adhering to the element of any electric immersion heater.
Scale acts as an insulator and prevents the heat of the boiler fire, or of the electric element, being transferred to the water in the boiler or cylinder. Loss of efficiency is often the first, and frequently unnoticed, indication of a scale build-up. Later, as scale continues to build up, hissing, bubbling and banging sounds will be heard as water is forced through ever-diminishing water channels. Unfortunately, scale does not only insulate the water from the heating effect of the fire or electric element. It also insulates the surface of the boiler, or of the element, from the cooling effect of the circulating water. Eventually the metal of the boiler will burn away until a leak develops or the electric immersion heater will overheat and fail.
Boiler scale, like the fur in the domestic kettle, can be removed by chemical means. The system is partially drained and a descaling chemical introduced into it via the cold water supply pipe to the hot water storage cylinder. After the introduction of the chemical the boiler is lit arid the water encouraged to circulate between boiler and cylinder to enable the chemical to act upon the scale accumulation. This treatment must, needless to say, be followed by very thorough flushing.
Permanent hardness does not affect hot water systems in this way but it has other harmful effects. Both forms of hardness prevent soap from dissolving and lathering properly in water. Insoluble, sticky deposits of lime soap or scum are formed that leave dirty ‘tide marks’ round baths and wash basins, matt woollens and damage other clothing washed in the water. Evaporation of hard water leaves the chemicals of both permanent and temporary hardness behind to produce marks below the taps of baths, jammed ball-valves and clogged up shower sprinklers.
Treatment of hardness and its effects will depend upon the particular aspect of the effect of hardness that is causing concern in individual circumstances. If, for instance, the main concern is the protection of boilers and immersion heaters from scale there are a number of mechanical measures that can be taken.
Check the setting of the thermostat of the immersion heater. In soft water areas it is usual to set the thermostat at 70°C. In hard water areas the setting should be 60°C. It is only when the temperature of water rises above 60 C, which is quite hot enough for domestic purposes, that scale begins to form.
Endeavour to control boiler temperatures so that the water does not rise above 60 C. This should be prac ticable where gas or oil are used though solid fuel boilers are less easily controllable. A dial thermometer clamped to the boiler flow pipe will permit the water temperature to be noted.
Never allow the water in a domestic ‘boiler’ to boil.
Convert a ‘direct’ hot water system to ‘indirect’ as suggested in a preceding post. The water in the primary circuit of an indirect is used over and over again, only the minute losses from evaporation being made up from the feed and expansion tank.
Any given volume of water contains only a given quantity of scale producing chemicals. The scale from these chemicals is deposited when the system is first heated and after that no more scale can form.
Some scale may still form in the hot water storage cylinder. This is however of less importance as it will not impede circulation.
There is also a chemical means by which, without actually softening the water supply, scale formation can be prevented. Certain phosphates of sodium and calcium, when released in minute quantities into a hard water supply, stabilise the chemicals of temporary hardness so that they do not precipitate out as scale when the water is heated.
Micromet is prepared in crystalline form. The usual method of introduction into a hot water system is to suspend the crystals, in a purpose made plastic basket, inside the cold water storage cistern, as close as possible to the ball-valve inlet. The crystals will slowly dissolve and need to be renewed at six monthly intervals.
Micromet can also be used for the threshold treatment of water passing, direct from the main, to instantaneous gas or electric water heaters. A special container, filled with the crystals, is introduced into the pipe-line leading to the appliance. It is, of course, essential to provide a stop-cock on either side of the Micromet container to enable the crystals to be renewed when required. Micromet, it must be stressed, does not soften water. Its purpose is to stabilise the chemicals that cause hardness and to prevent them from precipitating out as boiler scale when the water is heated.
Any domestic water supply can be softened completely— reduced to zero hardness-by the installation of a base exchange or, as they tend to be called nowadays, an ion exchange water softener. Water softeners of this kind are usually plumbed into the rising main so that every drop of water flowing into the house is softened. Synthetic sodium compounds are used in modem base exchange water softeners but the principle was discovered by observing the way in which water could be softened by passage through a bed of natural zeolite sand.
The hard water chemicals ‘exchange bases’ with the water softening material. Calcium and magnesium are left in the softening material and sodium bicarbonate, which does not cause hardness, is to be found in small quantities in the water flowing out of the softener. After a period of time the water softening chemical becomes exhausted but can readily be reactivated by passing a solution of sodium chloride through it. Complaints about mains water softeners usually relate to corrosion from frequent contact with salt water and to the rather tedious weekly task of reactivating the appliance with salt. Modern softeners overcome both of these objections. The body of the appliance is made of corrosion free plastic and the softener reactivates itself automatically from a large salt container at the prompting of a clock control.
Mains water softeners are somewhat expensive appliances and take up what may be considered to be an unacceptable amount of space in a compact modern home. They may too, be considered to be somewhat wasteful since they soften every drop of water passing into the home including, for instance, that used for w.c. flushing where soft water offers no advantage. Nor is soft water wholly without disadvantages. Many people consider that tea made with soft water is less palatable than that made with hard. Then too, a statistical relationship has been established between soft water supplies and the incidence of cardiovascular disease.
No-one really knows the reason for this. It could be that the chemicals responsible for hardness form a part of the body’s natural defence mechanism against disease of this kind. It is more likely though that soft water’s capacity to take into solution iron, copper or lead from the pipes through which it passes may be responsible. The first two of these metals are harmless enough if taken into the body in the tiny quantities likely to be found in a water supply.
Lead, on the other hand, is a dangerous and cumulative poison. Amounts well in excess of those considered to be acceptable have been found to be dissolved in water passing through lead pipes, particularly where water has stood in the pipes overnight. This need not deter the householder from installing a mains water softener or the water engineer from advising installation. Lead pipes are rarely, if ever, used in modern plumbing work. Where a water softener is installed into the lead pipework of an existing plumbing system it is probable that the internal surfaces of the pipes will have acquired an eggshell coating of scale that will prevent the lead going into solution.
To be absolutely safe there is perhaps something to be said for installing the mains softener into the rising main after the branch to the kitchen sink cold tap has been teed off. This will mean that softened water will be available for washing purposes and for the hot water system but will not be used for cooking and drinking. If a house has a naturally soft water supply the householder might be well advised to run off, every morning, the few pints of water that have been standing in the pipes overnight. Hot water dissolves metals more readily than cold. One should never be tempted, particularly first thing in the morning, to fill a kettle to make tea from the hot tap.
As well ?c the large mains base exchange water softeners already mentioned there are available small, portable softeners which can be used to supply limited quantities of softened water for hair washing, laundry purposes and so on. These small softeners operate on the base exchange principle. They have a hose inlet with a connector suitable for pushing on to any domestic tap. A salt cap at the top of the appliance can be unscrewed to permit salt to be added for regeneration when required.
Small quantities of water can also be softened by the addition of chemicals to the water after it has been drawn off. Most of these chemicals, which are used as bath salts, are based on washing soda. They have the effect of causing the chemicals causing hardness to become insoluble, precipitating out and leaving softened water behind. Disadvantages of these chemicals include the fact that the precipitate may harm clothing and the alkaline nature of the softened water can cause some dyes to run and may irritate sensitive skins.
Anotherchemicalwater softener, sodium hexametaphosphate works in a rather different way. This is a fine white powder, used straight from the packet, which when added to a hard water, links up with the chemicals causing hardness and neutralises them, so that they do not prevent soap from dissolving.
Calgon does not form a precipitate. Nor does it make the water alkaline. It can be used with confidence for laundry purposes, hair washing, washing and shaving—in fact for softening, for any purpose, any relatively small quantity of hard water.