Industrial Water Conditioning Processes Part I WENDELL W . CERNA, Hall
E
Laboratories, Pittsburgh, Pennsylvania
Recognition of the needs of people and industry in IGHTY thousand tons of synthetic rubber will be produced in one year by one of the large new obtaining clear, safe water is illustrated by the type of plants now almost ready for production. Fifty times municipal water-conditioning plants constructed within as much or four million tons annually of river water recent years. The photograph in Figure 1 is a good will be chemically treatedfor usein steam power genera- illustration of a modem municipal water plant. tion and process steam requirements for the successful In the main, the North American continent is foroperation of this one plant. One hundred seventy-five tunate in having a plentiful supply of water stored in million tons more water, or twenty-two hundred times rivers, lakes, wells, etc., equal to any demand which as much water as synthetic rubber produced, will be bas been required or which can be anticipated for the used annually for service and cooling water and other necessary functions of sustaining life and manufacturuses in this plant. It is interesting to note that this ing processes. However, for most purposes, whether amount of water is approximately the same as supplied for drinking water or manufacturing process, this water to the municipal service lines of a city of 1,000,000in- must be treated to be satisfactory. The treatment to be used will vary with the purpose for which the water habitants. This relationship of water to plant product is typical is required, and also with the extent and nature of the not only of this one new synthetic rubber plant, but of impurities or contamination in the raw water supply many others in the field of synthetics as well as in other a t the point of use. Fortunately, the water-conditioning chemist has industries producing numerous necessities such as steel, non-ferrous metals, high-octane gasoline, plastics, available a background of more than 100 years of chemicals, ordnance, munitions, and hosts of other developments, which have gone forward like many materials now more important to our welfare and future other technological advances, a t an accelerated pace in the last few years. mode of life than ever before. The purpose of this article is to outline and trace the The successful operation of vast new and expanding plants, as well as older established plants, is dependent fundamental chemical processes and developments in upon the proper selection and operation of a water- use for conditioning water and bring to the attention conditioning system. Unsatisfactory water quality of the chemist and engineer some of the recent changes can be a real bottleneck in production. In former years in equipment designed to supply the vast quantities too little attention was often given to the matter of of properly conditioned water required for essential water conditioning, and when breakdowns and inter- industries with a minimum use of critical materials. ference occurred steps would be taken to alleviate these conditions by a belated consideration of methods VARIATIONS I N ANALYSES OF RAW WATER SUPPLIES There are considerable differences in the composition and tinally by applying some means of improvement. Frequently, with a plant already constructed and in and concentrations of dissolved and suspended solids operation, installation of a really suitable waterconditioning system was no longer possible because of space requirements and existing plant layout. In recent years, engineers entrusted with the planning of plants have given more thought to water requirements and called in the water chemists and chemical engineers for advice and recommendations. Thus, considerable progress has followed in the field of water conditioning by chemical means because of the intensified studies made as a result of the demands of industry. Today we are in the midst of an unprecedented program of plant construction and operation, the output requirements of which are such that no gamble can be taken on the production schedule. Too much is a t stake. These new plants are planned for practically continuous FIGURE1.-WATER PURIFICATION PLANTor MAHOKINO maximum output, without unscheduled outages which : VALLEYSANITARYDISTRICT, SUPPLYING YOUNGSTOWN AND can be caused by lack of sufficient quantity of propel NILES,OHIO. INCLUDES CLARIFIERS, SEDIMENTATION BASINS, FILTERS, AND CHLORINATION EQUIPMENT water supply. 107
in various raw water supplies, requiring differentmethods of correction. Consequently, there are a number of methods of treatment, and selection of the proper method or combination of methods is a real responsibility of the water-conditioning chemist or engineer. For proper selection to be made, knowledge must be had of the raw water composition with possible variations throughout the year, requirements of water characteristics for the use to which the water is to be applied, and knowledge of the methods of treatment with the limitations of each. Examples of the variety in the analyses of water resulting from differences in the composition of the earth's strata are illustrated in Table 1. TABLE 1 R m s S M Crn~ldlULLANALYSBB F ~ NI NDRAW WIT&. SOPPLBS R+~s Espressrd in Pam par Miiiion
Total solids Total soap hardness (-1eulsted as CaCOd Total acidity as HBOa Free acidity as Has01
26 3
'Surface water from small I& at Liviveroaol. . . Nova Seotia. b Lake Ontario water Sf Toronlo. Ontario. Mooonmhela River w a t u (filtered) a t Pittsburah. . Pa. d Well water near Houston, Texas. * Misiiwippi River water 30 miles above New Orleans, La. I wen watcr in western Pcnns~lvania.
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METHODS OP CONDITIONING WATER
Sterilization. The chief purpose in controlling the properties and characteristics of water is the provision of safe, potable water supplies for domestic consumption. Successful chemical control has made possible the elimination of epidemics of water-borne diseases such as typhoid fever. One of the first means of eliminating water-borne germs was sterilization by heat, or boiling of the water. This method is still used as the most readily available when the quality of the water supply is uncertain or known to be unsafe. Such simple steriliiation by heat will prevent much illness and reduce the death rate among our military forces in isolated regions with coutaminated water supplies. Sterilization of water supplies by the use of certain chemicals is a cheaper and simpler method than heating in many cases. Chemical sterilization methods are used not only for safeguarding water for domestic consumption, but are also very important on a large scale in preventing organic growths in water used for cooling purposes where the temperature of the water is not raised much above 100°F. In this temperature range conditions are ideal for the rapid growth of organisms which interfere with water flow and heat transfer. An example of this use is that of condensers
used in steam power plants for condensing exhaust steam from turbogenerators. The exhaust steam is generally led to an enclosed metal chamber where it is condensed by means of cool water flowing through a multitude of small-diameter metal tubes. Such tubes can become completely plugged with these organic growths unless preventive measures are taken. The prevention of organic growths by steriliiation is usually accomplished by treatmeut of the supply with such chemicals as copper sulfate, chlorine, ammonia-chlorine (chloramiue), and, in case of cooling and circulating systems, sodium pentachlorpbenate. The treatment requires a means of control to get the proper amount of the chemical in solution as well as effective distribution and mixing. In some cases, intermittent dosage is sufficient and in others continuous chemical feed is required. This type of treatment frequently requires less than one part per million of effective chemical, and heavy overdosage must be avoided. Ewuporution and Condensation. Purification of water by evaporation and condensation produces water of the highest purity. This method is used on a lawe scale ranging from the requirements of the laboratory and the medical field to the furnishing of make-up water for boiler feed-water systems in high-pressure steam power plants. The quantities of distilled makeup water req&ed by boiler feed-water systems in many large central electric power generating stations is startling a t first to'chemists accustomed to the preparation of a few gallons of distilled water per day. Many of these stations will prepare in excess of one-half million pounds of distilled water daily for make-up water, not to mention the 50 million or more pounds of total distilled water produced daily by the condensation of steam generated in the boilers to drive turbogenerators. As this condensate is also returned to the feed-water system, i t will b e apparent that such a power plant is essentially a huge distilled water prodncing plant. Surprisingly enough, despite the immense quantities of distilled water produced in these plants, the quality of the distillate is frequently superior to that produced in the average laboratory still. Settling or Sedimentation. Sedimentation is frequently the normal result of solids gradually and steadily settling due to greater density than that of water. Thus many streams show considerable suspended solids a t the point of entry to a lake or reservoir, but the water delivered from such natural or artificial storage basins is practically clear, free from the undesirable suspended impurities. Surprisingly, such sedimentation may even reduce the bacteria coutent sufficiently to make water polluted a t the source safe for drinking. Sometimes, however, following storms or changes in temperature, turbid matter may be stirred up from the bottom and the sediment content changes so quickly that additional methods of controlling the quality must be available. While sedimentation is generally a natural process, i t is also purposely brought about by the use of dams
and artificially constructed reservoirs. Such reservoirs must have sufficient capacity to permit quiet settlmg for a period sufficient to furnish satisfactory water for the intended purpose, and this may vary from a m i n i m of several hours to several days, depending upon the nature of the suspended matter in the water and the use for which the water is intended. Coagulation and Clarification. In some water supplies, the suspended matter is of such low density, or is so finely divided, that satisfactory removal by simple settling is not feasible. Water containing considerable organic coloring matter extracted from dead vegetation is an example of this class. Such water can be clarified by the use of proper chemicals for coagulation. The coagulating chemicals may act by one or both of the following methods: 1. Changing or neutralizing the electrical charge on the colloidally suspended particles, permitting agglomeration and settling. 2. Formation of a gelatinous, spongy mass which absorbs and entangles the suspended matter. The first action may occur from the simple addition 9f-fln electrolyte to water. An example of this is found m the formation of the Mississippi delta, resulting from the rapid precipitation of solids from the muddy Mississippi River as the fresh water joins the salt water of the Gulf of Mexico. In artificial coagulation, a chemical which will also produce the second action is always used for greater speed in settling and clarification. Such chemicals are known as coagulants and consist of metallic salts which hydrolyze in the water forming gelatinous hydroxy precipitates. The chemicals most frequently used for coagulation are aluminum sulfate, Mz(S0n)r 18H20 (6lter alum), and ferrous sulfate, FeS0,.7H~0 (copperas). As both of these chemicals hydrolyze in water to form acids, neutralization is required to permit proper floe formation. With some waters the necessary alkalinity is naturally present in the water in the form of bicarbonate. A typical reaction with aluminum sulfate in this case is represented by the following:
dissolved oxygen of the water and the reactions also illustrate the oxidizing nature of the usual water supplies. In addition to coagulating colloidal solids by changing their charges due to the coagulating chemicals added, colloids are also removed by adsorption on the surfaces of the precipitated floc. For good results in clarification by coagulation, intimate mixing is necessary. In modern equipment this is obtained by mechanical stirring, first a t high speed to obtain thorough mixing, followed by slow agitation to accelerate growth of particle size of the precipitated floc. This is followed by quiet settling, requiring from thirty minutes to four hours for satisfactory clarification. One type of rapid-acting clarifier, with sludge blanket and recirculation of sludge provided, developed in recent years, is shown in diagrammatic form in Figure 2.
J
Caurlrrr Injdco, Inr.. Chkoro, Illinois FIGURG 2.-MODERN COMBINATION COAGULATION AND CLARIREDUCTION FIG%TION UNIT. CANALSO BE USED FOR HARDNESS BY
PRECIPITATION METHODS
Precipitation. The total equivalent calcium bicarbonate and calcium sulfate delivered daily in natural water supplies would be sufficient to build whole city blocks of limestone structures with alabaster and plaster-of-paris trim and ornaments. Unfortunately, If sufficient alkaline salts are not naturally present hard stone-like deposits are not prized in heating or in the water to form the floc necessary for coagulation, cooling coils, boilers, industrial products, etc. On then alkalies must he added in proper amount. For- the contrary, such deposits are definitely harmful and tunately two of the cheapest chemicals, lime and soda- an expensive nuisance, and may cause shutdowns if ash, are most suitable for the purpose. The reactions the accumulations are not prevented or removed involved in producing the flocculent iron hydroxide on promptly. mixing ferrous sulfate and one of these chemicals in The red om in ant scale-form in^ soav-consumina ions the water to he clarified are illustrated by the following: found in water are those of calcium and magnesium. Their presence is due to the vast quantities of calcium and magnesium-bearing rock formations in nature, and the relatively high solubilities of their bicarbonates, sulfates, and chlorides. The carbonate of calcium is It will be noted that oxygen also enters into the last com~arativelvinsoluble, as is the hydroxide of magtwo reactions to oxidize f&ns iron to form the desired nesium, and-early chemists quickly ;ought means of femc hydroxide floc. The oxygen is furnished by the precipitating these salts from h&d water, thereby
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A
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softening the water by precipitation and settling of the calcium carbonate and magnesium hydroxide. Among the numerous chemicals first used for softening water were lime and soda-ash. As early as 1767, Cavendish described the relationship of lime and "fixed air" (carbon dioxide) in spring water. One hundred years ago (1841), Thomas Clark proposed and obtained British patents on a forerunner of the modem types of precipitation and sedimentation softeners. He used lime for softening, which is effective for the removal of bicarbonate hardness, as indicated by the following basic reactions:
-
+
+
Ca(HCO& Ca(OH)* 2CaC01 2&0 2Ca(OH)1- 2CaC01 Mg(OH)2 Mg(HCO&
+
+
+ 2Hn0
Clark's process was a great step forward in that he developed and described mechanical equipment for use with the process. His proposal was unique in that he recognized the need for a method of chemical control, as results depend on the addition of proper quantities of chemicals, and excess lime would simply add to the hardness of the water. Clark devised the soap test for measuring the hardness of the water, and this test with modifications is still one of the most important tools in controlling softener operation. The function of heat in softening water containing bicarbonate hardness was also early recognized and Brooman received a British patent on this process in 1850. By simple heating carbon dioxide is driven off from the highly soluble bicarbonate to form the less soluble carbonate, as follows:
same period, among the most notable of which was the work of the Frenchmen, Boutron and Boudet (1854). The German chemist, Fresenius, was also attracted to the field of water softening and recommended the use of sodium carbonate (soda-ash) in 1853. While lime and heat could only be used to remove bicarbonate or so-called "temporary" hardness, in addition to magnesium hardness, soda-ash could be used for reducing the sulfate or "permanent" hardness. The fundamental type reactions which make soda-ash effective are the following: CaSO,
MgS04
--
+ Na2COa
+ Ca(OHh + N ~ * C O I
+
C ~ C O I Na,SO,
CaCOa
+ Mg(OHL + NanS04
Thus, by a proper combination of lime and soda-ash, with or without heat, water of low residual hardness can be produced to the limits of solubility of calcium carbonate and magnesium hydroxide under the conditions of temperature, pH, carbonate, and hydroxide balance maintained in the treating system. Basically, modern precipitation-type softeners consist of a mixing and sedimentation chamber, a method of controlling chemical feed and sometimes a means of heating the water and removing dissolved gases. Water from the settling chamber is drawn off and frequently the residual suspended solids are removed by filtration; this will be considered later. Cold-process softeners may operate by either the batch or continuous systems. In the former, the treating tank is filled with water while the treating chemicals are added, further mixed by stirring, and then permitted to settle. In the Ca(HCO& + Heat CaCO. + CO. + H30 continuous process, the treating chemicals and raw Similar investigations and proposals were brought water are added continuously to the mixing chamber out by a number of European investigators during the of the softener in proportion to the amount of raw water flowing to the softener. The hot-process softener always operates continuously and may be used where hot water is advantageous, such as for make-up water to boiler feedwater systems. Coagulants are seldom required in the hot-process softener, as the softening reactions and settling are accelerated by the temperature. In coldprocess softeners longer reaction and settling periods are required, and coagulants are generally necessaty to obtain satisfactory removal of the precipitated hardness. These coagulants are the same as previously mentioned, usually the sulfate of aluminum or iron. A special coagulant, sodium aluminate, is also frequently used in these softeners. Its main advantage, aside from not increasing the sulfate concentration of the treated water, is one of convenience to the user. Precipitation-type softeners lend themselves to a large variety of special treatments in addition to the common lime-soda softening. Among the special applications are softening with orthophosphate (hot process only), and silica reduction in high silica waters by the use of magnesium or iron salts. With some water supplies, silica removal can be combined with softening AND FEEDING TANK(LIME AND SODA-ASH) AT LEPTAND PRESSURE by the use of magnesium sulfate, magnesium oxide, or TYPE FILTERAT RIGHT
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dolomitic lime (calcium-magnesium oxide) along with the lime and soda-ash added to the precipitation chamber of the treating tank. Silica removal by means of iron or magnesium salts is largely an absorption process, although with the latter some precipitation of magnesium silicate also occurs. Waters high in sodium bicarbonate can also be treated for alkali reduction by use of lime and gypsum (calcium sulfate) in the treating tank. The main reactions involved in this treatment are as follows:
+
2NaHCOp NaaCOs
--
+ &SO4
+
+
NaCOs CaCOs 2HI0 Na*SO, CaCO,
+
The type of clarifier illustrated in Figure 2 is also used as a continuous operation, cold-process, precipitation-type softener, and can be used for silica removal as well. Cold-process precipitators may be constructed of concrete or wooden basins as well as steel, while hotprocess softeners are always made of steel. A standard type of the latter is illustrated in Figure 3. Filtration. Filtration of water is simply straining i t to remove suspended solids. The suspended solids may -ge particles such as sticks of wood, leaves, mud, etc., or very small particles such as bacteria and colloids. Naturally, the types of filters to accomplish the desired results will vary from relatively simple screens with large openings to graded beds of filter material requiring a top layer of jelly-like floc to remove the very finely divided solids from the water. Nature supplies an excellent earth filter for vast quantities of well and spring waters. However, some of these waters containing ferrous iron may become turbid on reaching the atmosphere, from oxidation of the iron by air, so that further filtration is necessary before the water supply is fit for use. Artificial filters are made up of graded sizes of almost any relatively inert materials of greater density than water. Those of stone and silica sand are most common in the usual municipal and industrial plants. Other filtering materials used for special purposes range from sponges and activated charcoal to calcite and anthracite coal. The last of these is receiving particularly wide acceptance in filtering alkaline water after lime-soda softening for boiler feed-water use, because silica pick-up from the filter material is thus prevented. Furthermore, because of the relatively low density of the coal, less backwash water is required for cleaning the filter. Some filtration systems are open or gravity-flow, usually having the filter bed supported in concrete, or wooden basins; some operate under pressure in closed iron or steel tanks. Small pressure filters may also be constructed of wood. Both types are fitted with a means of backwashing with filtered water to lift and clean the filter bed, as it becomes clogged with the suspended material removed from the water. In some plants filtration is the sole treatment given the water. In many others, filters are used in conjunction with coagulation or softening systems to re-
move the residual finely divided solids not taken out by simple settling in the sedimentation basins or tanks. The use of filters after lime-soda softening reduces the residual hardness of the water and improves the clarity by removing the remaining suspended calcium carbonate and magnesium hydroxide. The value of filtration in increased efficiency of lime-soda softener operation was recognized about thirty years after the introduction of the chemical sedimentation type softener and a British patent on the system complete with the filtration feature was issued to A. C. Henderson in 1872. Cation and Anion Exchangers. These softeners are commonly known as "zeolite" softeners, as their operation depends upon the reactivity of certain natural and artificially prepared materials, among which natural zeolitic minerals were 6rst used. Americans have some claim to first directing attention to the possibility of using zeolite softeners, because soft water was noted in the course of sinking a coal shaft in zeolitic earth a t Urbana, Illinois. in 1884. However, the German, Robert Gans, first described and obtained patents on this process of softening water in 1906 and 1907. A little earlier, in 1905, Gaus also obtained the first of several patents on the artificial preparation of hydrated complex alumino-silicates, or artificial zeolite. The zeolite or base-exchange minerals first used for softening were composed of sodium-aluminum-silicates. When water containing hardness (dissolved calcium, magnesium, iron, etc.) is passed through a layer of the zeolite, an exchange takes place whereby the sodium in the mineral is given up and replaced by the calcium, magnesium, or dissolved iron. If we let the zeolite mineral be represented by NazZ, the typical reactions which occur in passing hard water through a zeolite bed are represented by the following:
The sodium salts remaining in the water as a result of zeolitic action are termed soft, due to their high solubility in cold or hot water, and non-reactivity with soaps, the latter already being sodium or potassium salts of certain organic fatty acids. As the zeolite bed changes from sodium zeolite to calcium or magnesium zeolite, its softening action is naturally reduced. The real industrial usefulness of the zeolite in water softening is that the base-exchange reactions are reversible. When the zeolite bed approaches exhaustion, i t is regenerated by treating with a strong solution of sodium chloride, the reaction being represented as follows: ZNaCl
+ CaZ
2NaCI
+ MgZ
-
NaZ Na2Z
+ CaClr + MgC4
The soluble calcium and magnesium chlorides formed are washed out of the zeolite bed by flushing with fresh water, after which the unit is again ready to supply
softened water. A pressure flow type sodium zeolite softener is illustrated in Figure 4. The typical reactions given show that there is no reduction in dissolved solids of the water when softened by the sodium zeolite system, as compared to the decrease in solids of a bicarbonate hardness water when treated in lime or lime-soda sedimentation-type softeners. For some uses, such as in laundry work, the lack of reduction in total dissolved solids is not important. With waters h?gh in bicarbonate hardness,
as that of the zeolitic mineral, the only advantage being that undesirable silica could not be dissolved from the organic material, which is a disadvantage of the siliceous natural or artificial zeolites for some uses. In 1936 patents were issued in England to the United Water Softeners Limited, and in Germany to Karl Jeager, which described base exchange materials made by treating peat, lignite, bituminous coal, anthracite, and wood with strong acids, such as fuming sulfuric acid, a t temperatures up to 160°C. These
Courlrsy Parmulil Cornpony. New York, New York
Frcune ZEOLITE BAse E X C H A NSOFTENER ~E (SODIUM CYCLE)
the lack of any reduction in bicarbonate is a serious drawback for many uses, as, for example, where the water is to be used for boiler feed water. This objection to zeolite softening has been overcome by the development in recent years of organic base-exchange mediums which can be used for converting the basic elements or cations in water to hydrogen, as well as to sodium. The forerunner of the organic base-exchange developments was the work of George Borrowman who obtained United States patents in 1931 and 1935 on methods of producing organic materials which could be used in place of zeolitic minerals, by treating peat, lignite, or brown coal with solutions of salts of alkali metals, such as sodium chloride. This procedure, however would have merely the same softening action
materials when properly made could be used in a baseexchange softener for either simple softening according to the reactions already given for sodium zeolite softeners, or they could be used to substitute hydrogen for the metallic cations, depending upon whether the organic base-exchange material was regenerated with salt brine or an acid solution such as sulfuric acid. With acid regeneration the type reactions which occur when the water is passed through the organic exchange material, represented by H Z , are as follows:
A study of these reactions is very interesting. The first reaction shows that bicarbonates, including sodium bicarbonate, can be entirely eliminated, because the carbonic acid formed can be largely removed by the simple expedient of liberating the carbon dioxide. This will be discussed later under the subject of gas removal. The second reaction shows that with sulfates in the water sulfuric acid is formed, and similarly chlorides are converted to hydrochloric acid. The concentrations of sulfates and chlorides in the principal sources of raw water are sufticiently low so that the actual acidity of such treated water is very slight. However, even such slight acidity would be undesirable for most commercial uses. By combining treatments of water containing bicarbonates, sulfates, and chlorides, passing part of the water supply through a sodium zeolite softener and the balance through a hydrogen zeolite softener, and combining the effluents, water of almost any desired degree of .alkalinity or acidity can be obtained (within the limits of the bicarbonate and sulfate plus chloride concentrations of the raw water). This results from the neutralization of the sodium bicarbonate, formed in /€he sodium cycle treatment, by the sulfuric and hydrochloric acids formed in the hydrogen cycle treatment. Water-conditioning chemists and engineers have been quick to recognize these advantages for many water supplies and uses, and a large number of industrial installations of this type have been made. A complete water-conditioning system of this type, including gas removal equipment, is illustrated in Figure 5 . In common with the discoveries that plastics could be made from a host of organic materials abundantly available from plant life, i t was soon found that many of the resins could be converted to base-exchange usage similar to the carbonaceous zeolites. Adams and
Holmes of England received patents on this process in 1935, and in a paper published in the same year described similar condensation products which possessed the property of adsorb'mg anions from d i i t e acids. Thus, by first passing raw water through a carbonaceous zeolite softener operating in the hydrogen cycle to convert salts to acids and then passing the water through an anion exchanger (or absorber) sulfate and chloride are removed, although carbonic acid remains in the efflnent. The carbon dioxide is then removed from the anion exchanger efflnent by passing the water through an aerator. By this series of treatments, water comparable in quality to distilled water can be economically obtained from raw water supplies which are not too high in silica and total dissolved solids. The reactions occurring in water passing through anion exchangers are indicated by the following (the anion exchanger is represented by Au(OH)~): HzSOI 2HU
-
+ An(OH), + A~(OH)Z
AnSO, AnCIz
+ 2H20 + 2H20
The spent anion exchanger is regenerated with sodium carbonate or bicarbonate solution, the reaction being represented as follows:
+ AnSO, + AnU,
2NaHCOa 2NaHCOs
-
+ +
An(OH)* NanSO, An(OH)% 2NaCI
+ 2C01 + 2C01
After regeneration, the salts and dissolved carbon dioxide are removed from the anion exchanger by rinsing with fresh water. Aeration, Odor, and Gas R e m w l . For some uses certain dissolved gases, such as oxygen in drinking water, are not objectionable and may be desirable, while others, such as hydrogen sulfide in drinking water or oxygen in boiler feed water, are detrimental to the quality of the water. In the case of cold-process lime-
courtesy Pnmulil cam*any, New York, New YO"*
FIGURE ~.-COMBWATION SODIUM AND HYDROGEN CATION EXCHANGERPLANTPOR REMOVAL OP HARDNESS, REDUCTION OP S o n m AND TOTALSOLIDS,AERATOR FOR CARBON DIOXIDEREMOVAL, AND SPRAYTYPE DEAERATOR POX OXYGEN REMOVAL FROM BOILER FEEDWATER
treated water, carbon dioxide is frequently introduced to stabilize the water to avoid after-precipitation and to lower the pH value of such water. Therefore, the water-conditioning system must be planned and installed with proper consideration for any necessary dissolved gas removal, or introduction, depending upon the particular use which is to be made of the water supply. Dissolved hydrogen sulfide, free carbon dioxide, and some organic gases and odors can be removed by thorough aeration. This method is used for many municipal water supplies and consists of passing the water into the air in a spray, letting it flow down a series of steps in thm sheets, over trays, or by various other methods of exposing considerable water surface to the air for gas liberation. Aeration also oxidizes ferrous iron in water, permitting precipitation of the ferric hydroxide which can then be removed by settling in a sedimentation basin or by filtration, as previously discussed. A special design of aerator, usually of wood or tile construction, is used for carbon dioxide removal when waters of fairly high bicarbonate concentration have been treated in carbonaceous zeolite softeners operating in the hydrogen cycle, or when sulfuric acid is added directly to a bicarbonate water to rednce the alkalinity. The treated water, usually the combined effluent from sodium zeolite and hydrogen zeolite units, or direct acid-treated water, is passed to the top of the aerator, also known as a decarbonator, and allowed to descend in thin streams over a series of baffles or trays, while air is blown up through the aerating tower by means of a fan (see Figure 5). This treatment will often reduce the residual free carbon dioxide to as low as 5 parts per million. Tastes and odors from gases not removed by aeration are also destroyed by adsorption on activated carbon, the carbon being removed by settling or final filtration along with solid impurities or coagulating chemicals. Tastes and odors developed from microorganisms or trade wastes, otherwise difficult to remove, can be destroyed by so-called superchlorination which consists of treating with considerable excess chlorine (one or more parts per million). The excess chlorine required is undesirable in itself but can be removed by treatment with activated carbon. One of the most important industrial applications of gas removal is the deaeration or removal of dissolved oxygen and other gases from boiler feed water. For industrial steam and power generating plants operating boilers and auxiliary equipment a t elevated temperatures, oxygen in the feed water is a serious cause of corrosion, due to the readiness with which iron oxidizes in the presence of moisture and oxygen. Therefore, very elaborate precautions are frequently taken to remove oxygen from the water to he used for boiler feed water. With proper provision for the escape of gases, dissolved gases which do not actually react with water can be removed by raising the temperature of the water to the boiling point. With liberal "venting" the
partial pressure of all gases except steam can then be reduced practically to zero, with corresponding reduction in the concentration of the dissolved gases in the water. Equipment for oxygen removal in boiler feed water is constrncted of iron or steel, frequently with special corrosion-resistant alloys in the area where the water is first brought to the boiling temperature and gases released, because of the severe corrosiveness of the watergas mixture a t this stage. Such equipment is usually designated as open-heaters, or deaerating heaters, with the former generally designed to reduce residual dissolved oxygen to 0.4 p.p.m. or less, while the more elaborately designed deaerators will reduce i t to less than 0.04 p.p.m. when properly operated and not overloaded. These gas-removing heaters are generally of either the tray or spray type. In the former, water is passed into the top of the units, intimately mixed with an ascending flow of steam and the heated water
Couriesy E l i i o l l Company, Jronnclle, Pennsylvnnio
FIGURE6.-TRAY-TYPEDBAERAT~NO HEATER
descends over a series of plate baffles or trays spreading trated in Figure 6. The system of treatment outlined the water out in a multitude of thin streams to permit in Figure 5 also illustrates tray-type carbon dioxide ready escape of entrapped gases. In the spray type removal equipment and a spray-type deaerating heater unit, the water is sprayed from a number of nozzles for oxygen removal. into a steam chest where it is heated by the steam to In the second and final article of this series suppleboiling temperature. The liberated gases and some mentary treatments of water will be discussed, such as steam are continuously removed by means of a vent a t boiler water-conditioning and the recently developed the top of the units, and the deaerated water is collected "threshold treatment." Applications of the methods in a storage chamber a t the bottom of the shell. of treatment described to actual industrial uses will A form of tray-type deaerating heater for removal of also be given with examples showing means of saving dissolved oxygen from the boiler feed water is illus- surprisingly large quantities of "critical" materials.
