September, 1923
INDUSTRIAL AIVD ENGJNEERIXG CHEMISTRY
915
Purifying Industrial Water By D. H.Killeffer 19 EAST2 4 T ~ ST., N E W YORK, N. Y
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ATER is our most important industrial raw material. No industry can operate without using water in some form a t some point in its processes. Fow general classes of uses may be recognized in which water owes its importance to its solvent action, its high latent heats j ,steam and ice production, its high heat capacity in heat transfer, and its mechanical properties as in the production of hydroelectric power, the operation of hydraulic presses, and other similar mechanical operations. I n the first two of these uses the composition of the impurities present in the water becomes important, and it is with these that the present discussion will deal. As to its source, water may be classified, in the order of its purity, as distilled, rain and snow, surface water from streams and shallow wells, deep well water, and sea water. Distilled water naturally contains no impurities except dissolved gases, unless carelessly prepared, and hence need not be subject to purifying treatments. Rain and snow waters contain minimum impurities and are the most sought for industrial waters. Unfortunately, supplies of either are too uncertain and too meager to meet the needs of industry. Water from streams and shallow wells generally is of sufficient purity for industrial use. The objections to surface waters are Based on the wide fluctuations of both quantity and quality u-hich occur during the course of €he year. Their quantity depends upon rain and snow, which are widely variable, and their quality depends upon the character of the soil over which they pass, as well as upon their velocities. In quality, surface waters vary from a close approach to the ideal of rain water to extremely hard, calcareous water in the limestone regions and to extremely alkaline waters in the arid regions of the West. Deep wells are most frequently calcareous in character, although they may vary through wide ranges depending on the character of the rock formations through which they pass. Deep wells and springs coming from great depths possess uniform clarity, with few, if any, exceptions, which property distinguishes them from surface waters that are frequently muddy from excessive r a i d a h Sea water, with which might also be classed some of the western alkali waters, contains principally salts of sodium and potassium with smaller amounts of alkaline earth metals. In industrial operations four principal points are raised as to the nature of the impurities in water: its freedom from suspended matter, its freedom from dissolved salts of calcium, magnesium, and the heavy metals, its freedom from excessive amounts of salts of the alkali metals and other dissolved matter, ttnd its freedom from permanent acidity-i. e., acids other thrtn carbonic. Upon these points depends the usefulness of water after the question of volume available has been settled.
ever, in the case of boilers working under variable loads on extremely pure water, the absence of traces of suspended matter may increase priming to an alarming extent. Priming is analogous to bumping in laboratory operations, and results in throwing water into the steam lines through the sudden formation of steam.
DISSOLVED SALTS
The effects of dissolved impurities are so widely different in different operations that they will be discussed in connection with the uses. PAPERINDUSTRY-The largest single use of water as a solvent is probably in the paper industry. Estimates placed on this use run into the billions of gallons per day on the basis of the fact that the manufacture of each ton of paper requires from 50 to 200 thousand gallons of water. In order that the paper be up to standard in all respects, it is necessary that the water used be free from the slightest turbidity and from any elements which are likely to produce insoluble compounds with any of the reagents used in the processes of manufacture. Water for paper manufacture must also be free from color and from salts of metals, such as iron, which might produce color in the product. The principal faults which develop in papers made by the use of impure waters are undesirable color, improper or lumpy size, and harshness in some of the finer papers. Aside from actual faults in the paper as finished, larger quantities of bleach and of alum (in applying the size) are consumed by impure water, thus increasing cost of manufacture. Impurities which seem inconsiderable, and would be so under other conditions, become important when the quantity of water here employed is taken into consideration. In this case the amount of sodium salts in the water is relatively unimportant so long as the objectionable metals are removed. The presence of organic matter in the water used by the paper mill is to be avoided in all cases, for no treatment short of distillation has been devised for removing this satisfactorily, and marked discoloration is caused in the paper produced by the use of water so contaminated. FABRIC FIXISHERS, BLEACHERS, AND LavNDRIES-Here it is also necessary to have as pure water as it is possible to obtain. In the degumming of silk, particularly, the soap solutions must be prepared from water free from metals which produce insoluble compounds with the fatty acids of the soaps, as these attach themselves firmly to the fiber and make the finished silk harsh to the touch, and seriously affect its subsequent dyeing. Wool scouring and all operations involving the use of soap must take place in water free from salts of calcium, magnesium, and the heavy metals, if best results are to be obtained. In bleaching operations, it is also necessary that the water be as free as possible from organic matter-such as is frequently found in shallow wellsSUSPENDED MATTER since such matter must of necessity be oxidized before the b p e i i d e d matter is objectionable wherever water is used, bleach can have the desired effect on the fiber. The boiling as it clogs pipes, erodes the vanes of turbines, in boilers it out of cotton is less likely to be affected by impurities in the becomes a part of any scale that may be formed, and in all water used, but even here it is necessary to have absolut(e1y w&g operations, particularly those involving fibrous clear water and highly desirable that it be free from hardmaterials, it becomes firmly enmeshed in the material washed ness on account of the effect dissolved salts may haye on its and preyents the completion of the washing operation. How- subsequent dyeing.
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I S D USTRIAL A N D Eh'GIh'EERIXG CHEMISTRY
DYEIS-G-111 the dyehouse the faults of earlier mistreatment given the fabric are developed. Streaky dyeing is often the result of the presence of insoluble salts in the fabric or of the influence (either in resisting or mordanting the dye) of mineral salts left in the fabric from previous treatment. The water of the dye bath must be free from elements which would precipitate the dye. This condition is only to be satisfied in all cases by perfectly neutral water free from calcium, magnesium, or the heavy metals. CHEMICALS-In strictly chemical operations, where water is used as a solvent irom which to crystallize other substances, the purity of the product is essentially dependent on the purity of the water used. For this reason it is almost universal practice in the preparation of pure reagents and chemicals to use distilled water. In some cases it is possible to use natural waters which are practically free from dissolved solids, and in others softened water may be used, but in general the presence of even minute amounts of impurities is sufficient to disqualify a water for this use. STEAMPRoDucmow-Steam production becomes actually dangerous where very hard waters or those containing large amounts of dissolved salts are used. The use of hard waters in boilers is not only an economic crime, but an actual danger to operators of steam plants. The formation of scale on the heating surfaces of boilers causes immediate reduction in the heat conductivity of those surfaces, resulting in unnecessary losses of fuel, and, if the scale becomes thick enough and cracks, may cause dangerous explosions. The presence of large amounts of dissolved solids, particularly the soda alkalies and organic matter, in boiler feed water is no less a source of d:!nger from the fact that foaming and priming in the boiler are likely to result in exposing crown sheets and thus cause explosions. This danger is also less easy to overcome, and while many remedies have been proposed, few short of distillation have been found which will cure any but the mildest cases. Scale formation is less important in locomotive boilers than in stationary plants, since other causes, which do not affect stationary boilers, make necessary the complete cleaning out of locomotive boilers a t short intervals. However, the extremely variable load on locomotives, which may change by several hundred per cent of itself in successive minutes, makes them particularly liable to foaming and priming. Boilers in ocean-going steamships are less liable to the difficulties of other boilers, because it is necessary to operate them in connection with efficient condensing systems which reduce the amount of raw water needed to a low minimum, and even the make-up water is distilled. Vessels on fresh water do not lend themselves to the use of water-treating plants, and consequently boiler compounds are often used in them where necessity may require it. CONDENSERS AND ICEMAKING-co01ing water used for condensers need possess no other essential quality than low temperature, although excessively muddy water is to be avoided. Where water is frozen to ice, the quantity of dissolved solids becomes important. Aside from the lowering of the freezing point which comes from dissolved substances, the presence of excessive amounts of soluble impurities causes cloudy ice by being precipitated or crystallized from solution as the water freezes. Until quite recently it has been practice to distil all water used for this purpose, but the increasing commercial use of artificial ice, as distinct from its domestic use, and the development of efficient water-softening apparatus have made possible the use of treated water for this purpose. The absence of salts of calcium, magnesium, and the heavy metals is imperative and a reduction of the other dissolved substances to a minimum is necess'ary before undistilled water can be successfully used for ice manufacture.
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REMOVAL OF SUSPENDED MATTER Turbidity is removed from natural water by ordinary processes of filtration. The most usual type of equipment used for this purpose is the ordinary sand filter. Where plenty of space is available, gravity filters are used which consist of large beds of quartz sand through which the water is passed. Municipal water-treating plants are probably the best examples of this type of apparatus. Here the filter consists of a large, comparatively shallow tank containing a sand layer several feet in thickness and provided with strainers a t the bottom through which the filtered water passes to storage. Gravity is depended upon to force the water through the filter. Bactericidal doses of chlorine are usually added to the water before filtration to reduce its bacterial content, and aluminium sulfate, with or without precipitating doses of alkali, is added to coagulate the suspended matter and increase the efficiency of the filtering operation. The sand filter beds become clogged in the course of time and are washed a t convenient intervals by forking the sand while water is passed through. Pressure sand filters are frequently employed where space is limited. These consist essentially of a sand bed in a tank so arranged as to permit pressure to be applied to increase the rate of flow. I n this type of filter the thickness of the layer of sand is proportionately greater than its area-just the reverse of the usual gravity filter. I n this case washing is effected by forcing a flow of water back through the filter. Coke, charcoal, and other types of sand sometimes replace quartz sand entirely or in part, but the preferred material in most cases is pure quartz sand. The use of limestone or marble chips is sometimes convenient where water high in acidity is to be treated.
REMOVAL OF DISSOLVED SALTS Two methods of removal of objectionable dissolved salts are in common use, the lime-soda ash precipitation method and zeolite interchangers. Both depend for their efficiency on the formation of an insoluble salt of the objectionable metal and its replacement by sodium. PRECIPITATION METHoD-The precipitation method is carried out either hot or cold as the circumstances may indicate, and consists essentially in adding to the water precipitants for the calcium and magnesium salts and filtering off the precipitate before use. Lime, either quick or hydrated, soda ash, and sometimes trisodium phosphate are the reagents most frequently employed. Lime neutralizes any acidity in the water, especially carbonic acid, and precipitates calcium from bicarbonate, which it converts to the insoluble carbonate. For this purpose it is preferred to soda ash or other neutralizing agents because it actually reduces the amount of dissolved solids in the water by being itself precipitated. Magnesium is also precipitated as hydroxide by the lime treatment. Soda ash is used to precipitate calcium from salts other than bicarbonate by converting it to the insoluble carbonate. Sometimes part of the soda ash is replaced by trisodium phosphate, where the water treated is high in magnesium, to insure its complete removal. Other reagents have been employed in special cases, especially barium salts to remove sulfates, but in general precipitation plants use only soda ash and lime, which suffice for the treatment of the great majority of hard waters. These precipitation reactions may be carried out either hot or cold. Ordinarily, the cold process is preferred in spite of the comparatively slow settling of precipitates. The hot process gains in the speed of precipitation, but the increased solubility of the precipitates in hot water is often a disadvantage. When preheaters are used, it is frequent practice to place these before the precipitating system in
September, 1923
IiYD USTRIAL AiYD ESGI,VEERING CHEMISTRY
order to reduce the calcium bicarbonate hardness of the water before treatment. The ease with which calcium bicarbonate breaks down in hot water to carbon dioxide and calcium carbonate is thus utilized. The equipment of a precipitation water softener consists of three essential pieces of apparatus, which may or may not be combined into a single unit: a reagent feeder, a reaction vessel in which the reagents are mixed with the raw water, and a means of separating the treated water from the precipitates. The difference between the hot and cold processes is that the former is carried out in a closed system and the latter in an open one. The reagents are dissolved in a sufficient quantity of water and, in the case of continuous softeners, added by mechanical means in a fixed proportion to the raw water as it goes into the reaction vessel. In discontinuous systems this is not necessary, the solution of reagents being run into the raw water in the reaction vessel from a dissolving tank. If sufficient time be allowed in the discontinuous system for the precipitate to settle, it is often possible to do away with filters. Any type of filter may be used, but sand is preferred. Continuous processes have the obvious advantage, however, of requiring less space, and are generally more efficient from an investment point of view. I n the continuous process, ineans are provided for feeding the reagents from pump or other device operated by the force of the raw mater as it runs into the reaction chamber. A small turbine or water wheel is placed in a by-pass from the supply line and is so regulated as to drive a pump which supplies quantities of reagent solution directly proportional to the amount of raw water. While separate units for precipitating and settling mag he used, it is common practice to build the reaction chamber as a vertical, open tube in the center of the settling tank. I n this case the water mixed with the reagents is fed into the top of the reaction chamber, through whichit flows downward to the bottom of the settling tank and upward in that to a filter built as an annular ring between the two a t the top. The flow in the settling chamber must be sufficiently slow to permit of the settling of the precipitates in spite of it. The filter a t the top is ordinarily operated by a slight head of water in the reaction chamber above the level of the filter overflow. The bottom of the tank (usually conical) is provided with means for removing precipitates periodically. The flow through the filter is also upward. It may be composed of any fibrous material which mats sufficiently to hold the precipitates, or it may even be sand. From the filter clear water passes to storage. The cubic capacity of continuous softeners should be sufficient to allow as complete settling as possible. Usually they are designed to hold four to six hours’ supply of water. ZEOLITE Ih.TERCHANGERS-ESSenti&lly, Zeolite SOftellerS consist of filters of granular sodium aluminium silicate of such a character that the sodium ion is easily replaced by an alkaline earth metal. The alkaline earth compounds are insoluble and remain in the filter, but they can be reconverted to the sodium compounds by passing in an excess of sodium chlorido solution. As installed, the equipment consists of one or more softening tanks in which the zeolite filters are placed and a supplementary tank for sodium chloride solution for the regeneration of the zeolite. The operation of these plants is extremely simple and requires little attention. The raw water is passed continuously through the bed of zeolite until the effluent begins to shorn hardness by the soap test-i. e., until the sodium of the zeolite has been replaced by other metals. At this point the zeolite is cut out of the system and regenerated by passing sodium chloride solution through it. After the regeneration has been completed the excess of salt solution and the waste products formed are washed out of the mass, which is then
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ready to return to the system. This operation requires an hour or less for completion froin the removal of the zeolite from the system until its return, since the whole thing is accomplished by means of the simple turning of valves controlling the flow of water and solution. Naturally, continuous operation can be had only by the use of two such softeners, but where relief periods are permissible, one will suffice. The two methods are identical in principle, the difference being in the solubility of the compounds formed. I n the precipitation treatment, complete removal of hardness is not possible on account of the slight solubility of the precipitated calcium carbonate in cold water. While its removal is virtually complete, it is impossible to reduce the hardness below one grain per gallon where calcium carbonate is in contact with the water, and in practice it is seldom possible to go below three grains per gallon consistently. In the case of zeolite softeners the limit reached in practice is so low that no hardness can be detected by soap solution. The precipitation method actually reduces the amount of dissolved salts, especially where a large part of the hardness is due to calcium bicarbonate, while the zeolite softeners simply exchange sodium for other ions without changing the ionic concentration. This latter may be a disadvantage where the mater treated is very hard and is used for a boiler where foaming and priming may be of importance. Variation in the hardness of the water treated causes a corresponding inverse variation in the capacity of the zeolite equipment, but must be equalized by variations in the quantity of reagent added to precipitators without changing their capacity. Frequently it is practice to precede zeolite softeners by lime precipitators to remove the calcium bicarbonate hardness where this factor would greatly reduce their capacity otherwise. Their usefulness is greatest in treating water of medium or low hardness to produce extreme softness in the treated water, as in laundry operations. Any considerable amount of sodium salts in the raw water nullifies the action of the zeolite, and iron and manganese in the raw water form compounds with the zeolite which are not regenerated by sodium chloride solution. Otherwise, there are no particular limitations on their use. Precipitation softeners possess the advantage of having their capacities unaffected by variations in the hardness of the water treated, they reduce the ionic concentration of dissolved matter, and they are equally efficient in treating all waters. Their chief disadvantage consists in the necessity of close control to compensate for variations in the raw water and to guard against the addition of excesses of reagents which may be as objectionable as the constituents they are designed to remove. TREATMEXT O F BOILER WATER-In the case Of boiler water a special type of treatment is frequently used which gives even better results in many instances than either of the foregoing methods and a t less expense. This consists in introducing treatment directly into the boiler itself. I n this way all equipment investments are eliminated and some of the treatments which can be applied in this way are more efficient than those used in other equipment. Treatment of electrolytic corrosion, which may result from the presence of appreciable quantities of soluble or hydrolyzable salts (magnesium chloride), by the addition of metallic zinc to the boiler itself has been very successful. Foaming can frequently be treated in this way which may be even aggravated by other treatments. This field, however, is a fertile one for quacks of various kinds, whose nostrums vary all the way from utterly innocuous mixtures to positively and dangerously harmful ones. Some few firms have given the subject careful thought, however, and have achieved remarkable results where other methods have failed. This subject is such a special case of water treatment that it cannot be given the full discussion it deserves here.
