CIATION ACHERS
0
SOME PROBLEMS OF WATER TREATMENT HAROLD E. WATSON Newport Waterworks, Newport, Rhode Island
INTRODUCTION
There are, in the Unit,ed States, about 14,000 public vater works systems. I n three days this industry supplies a greater tonnage of water tban the tonnage produced by the steel industry in a year. The transportation of this tremendous quantity of water is seldom thought of unless the system fails. Although some systems can be supplied by gravity, in many the water has t o be pumped, and in a city of 35,000 about 15,000 tons of treated water mould have to be delivered to the consumers' taps daily. The indispensable nature of water requires continuity of supply. This implies the necessity for continuity of the treatment. The water goes through the treatment plant but once, so there is no opportunity to change the treatment; it must be right the first time. Changes in treatment must be anticipated to meet changes in the character of the water. In a broad sense there is no naturally pure water. All natural water contains both organic and inorganic impurities in various amounts, inmmpension and solution, and live organisms. There may ako be industrial wastes. Since these impurities make the water unsuitable for use, treatments are applied to remove or mitigate the unwanted characteristics. If we omit water softening, such special treatment as is used for boiler feed water, and special treatments for certain industries, we may say that, in general, public water supply treatment has four objectives: (1) t o produce a water that is safe touse a t all times; (2) to remove objectionable color, turbidity, and suspended matter; (3) to control taste and odor; (4) to control corrosive characteristics of the water. But before considering these four objectives, I would like to discuss filters, because some kind of filter is the basic feature of all treatment plants. FILTRATION
In order to filter effectivelywith a sand filter more is needed tban just the sand itself. There must be some medium to prevent the water from passing too freely between the sand grains.
In the slow sand filter the reduction of the size of the spaces between the sand graills is accomplished by permitting the raw water t o pass through the filter and run to waste until enough material screened from the water has accumulated to form a jelly-like mat on top of and in the upper part of the sand bed. After this mat has formed the water delivered by the filter is greatly improved in character. After a slow sand filter has been in use some time the friction in the sand increases to such an extent that the filter must be cleaned. In cleaning, the top sand is skimmed off with shovels, removed completely from the filter, and then raw water is again permitted to flow through the filter until a new mat has been developed on the new surface, and the filter is again put in operation. This process is repeated 'as required until so much of the sand has been removed from the filter that it becomes necessary t o clean all of the sand and replace it in the filter to its original depth. In rapid mechanical filters the mat is produced by the use of chemicals, and the process is called coagulation. Mechanical filters are cleaned by versing the direction of flow through them, applying treated water a t the bottom under pressure, and washing the accumulated material over the top and to waste. In connection with both types of filters there usually are sedimentation basins and, in the case of rapid mechanical filters, also coagulation basins, where the process of coagulation actually takes place. Coagulation is produced by the use of chemicals which form insoluble gelatinous precipitates. Aluminum sulfate is the most common coagulant but sulfate of iron and ferric chloride are also used. Where the natural alkalmity of the water is insufficient it may be necessary a t the time of coagulation to s u ~ ~additional lv alkali in the form of hvdrated lime or soya ash. ~f suficient alkali is not present, some of the aluminum sulfate may not be hydrolyzed but may remain in the water. So one of the first conditions for a proper flocculation is the proper relative amount of alkali and coagulant. rlnother important consideration is the hydrogen ion concentration of the water to
401
402
be coagulated. The optimum pH range is around 6.5. If the conditions are properly controued, the hydrolyzed aluminum sulfate appears as smaU grayish-white flecks often called floe or flakes, the whole process sometimes being referred to as flocculation. The floe should be reasonably heavy and tough for quick settling and not too feathery to prevent breaking up. When the floc is formed in the basins there are possibly three functions which it performs: 1. The entrapping of suspended matter by contact in the water and also by coalescence of the various particles of floe and by the gradual descent of the floc through the water in the basins. 2. The adsorption of color on the surface of the floc. Color can be removed practically completely by a proper adjustment of the coagulant dose. 3. Finally, as the water passes from the coagulation basins to the top of the filters, there will still be a sufficient amount of floe to form the filter mat or filtering medium on and in the sand of the filter. I n c o ~ e c t i o nwith coagulation there are certain special treatments that should be mentioned. First, the use of clay to weight the floc. This is useful in the newer type of plants referred to as "upflow plants" where the course of the water in the coagulation basins is up from the bottom to the top, rather than in a horizontal direction as in the more conventional -type. Frequently the upflow type of coagulation is accomplished in much smaller basins provided with slowly rotating paddle wheels which accelerate the coalescence of the floe and help produce a bed of coagulant which floats possibly two to four feet below the surface of the water, and through which all the water passes from the bottom to the top. In these cases, in order to keep the floe bed heavy enough to prevent its flowing over the top, clay is sometimes employed. In another special treatment sodium silicate is activated by an acid a t the time of introduction into the water. This method of producing a heavier and more idealgoc is, however, very delicate of control and, to date, requires too much control for practical use in the average treatment plant. There is still another method of coagulation referred to as "electronic coagulation." Aluminum hydroxide is produced from metauic aluminum plates placed in the water, spaced about one eighth of an inch apart, while a n electric current is sent through the alternate plates. The electronic method does not affect the alkalinity of t h e water. The equipment is patented, and a t present the greater operating expense prevents its use. The filtration process is controlled by observation of the color and turbidity removal and adjustment of the coagulant dosage to the point of o p t i i effect. Frequent determinations of the alkalinity should be made in order to enable proper adjustment by the use of additional alkali if indicated. Careful checking of the length of run of each filter between cleanings is a help in determining the proper dosage of coagulant. Finally, observation of the general appearance of the floe in the coagulation basin is extremely helpful to an experienced operator.
JOURNAL OF CHEMICAL EDUCATION ELIMINATION OF BACTERIA
Filters for clarifying water were introduced as early as 1829, with more or less success, and about 1857 it was demonstrated that their effectiveness in reducing water-borne disease was due to their abiity to reduce the number of bacteria. It was recognized, however, that filtering did not remove all the bacteria, and although some 80 per cent or lilore might be removed, the small number remaining might be harmful. So filtering alone is insufficient. It was not until 1908 that chlorinated lime was first used as a germicide in water treatment, its purpose being to kill any harmful organisms that may have passed through the filters. The chloride of lime was effective but its use presented difficulties. It was hard to handle in large quantities, and as soon as it was dissolved for feeding into the water i t gradually began to lose strength. It was difficult to feed accurately, and of course was extremely corrosive. Equipment capable of continuously measuring and feeding pure chlorine gas into the water appeared on the market about 1912, and in a few years liquid chlorine became the most common disinfectant in the water supply industry. In spite of the progress indicated there were stiU problems in making the water safe. First, the occurrence of chlorinous tastes which, while in themselves harmless to the people using the water, might in some instances encourage the use of a polluted water which did not have this taste. Then there were certain vagaries of action. The bacterial reduction was not always predictable, a%d under appaxently the same operating conditions and with the use of the same technique unexpected resultsmight be obtained. Furthermore there were certain organisms, mainly spores and cysts, that were able to resist this treatment sufficiently to be carried through into the treated water. So several improved techniques have been developed for the use of chlorine. In simple chlorination there are two factors to be considered, the dosage and the contact period. A check of these against the bacteriological analyses enable the operator to determine whether or not his dosage was correct a t any particular tiihe. Of course, one didficulty was the period of 48 to 72 hours between the taking of a sample and the report of the bacteriological analysis. So the operator had to build up an experience which would help him to modify tentatively the dose of chlorine until he could have the benefit of analyses showing the effect of the change. A n indispensable adjunct to chlorine control is the ortho-tolidine test for chlorine in water. Briefly, if water containing chlorine is treated with an acidified solution of ortho-tolidine, a greenish yellow color is produced, the depth of which is directly dependent upon the amount of chlorine present. Certain chlorinous tastes in treated water are due to compounds formed by the action of chlorine on various substances which may occur in the water. A typi-
JULY, 1948
cal example is the combination of chlorine and certain phenolic products to form chlorophenols, compounds with strong chlorine, or medicinal, tastes and odors. To control these ammonia in very small quantities is added to water before chlorination. The chlorine reacts with the ammonia rather than with phenolic or other organic taste-and-odor-producing substances. The chloramine thus formed is tasteless and odorless, but its disinfecting value is less than that of chlorine, requiring larger doses of chlorine and longer contact periods. In another method of using chlorine, called superchlorination, the water is treated with much larger doses of chlorine than are used in conventional practice, and after a sufficient contact period the excess chlorine is removed by a dechlorinating agent such as sulfur dioxide or sodium thiosulfate. This technique is said to produce a better bacterial kill and better control of tastes and odors. The knowledge gained through the use of thk techniques described, particularly reactions between chlorine and ammonia in various amounts, has led to further research and the recent development of the "break point" chlorination method. In this method a sufficient amount of chlorine is added so that a t the end of the contact period there will still be present in the water a small amount of free, uncombined chlorine, in addition to the combined chlorine. At this dosage the tastes and odors will be a t the lowest. This method is the most exact yet developed and takes into consideration the amount of free ammonia present, the pH value, the color, and alkalinity. By its use the bacterial content is much reduced over that obtained by some of the former methods and the results are more uniform. The optimum final pH range for this procedure is between 6.5 and 8.5. By the use of these various techniques of chlorination, preceded by filtration, we can attain our first objective, a safe water. Thisisnicely shown by a comparison of typhoid fever death rates for the psFst H t y years. Fifty years ago about one person in two hundred died of typhoid; now the rate is about one in two hundred thousand. The second objective, control of color and turbidity, is not generally very difficult. Color and turbidity are controlled by coagulation and filtration, as described above. A proper dosage of coagulant and a controlled pH and alkalinity are the main requisites for color removal. Some color is also removed by chlorine when large doses are used.
403
the algae themselves in the raw, untreated water by the application of copper sulfate or chlorine. Copper is much more commonly used. If, after treating with copper, the water still has some objectionable odor i t probably may be removed by activated carbon. Activated carbon is also sometimes spread over the surface of the water in open resewoirs to produce a socalled "blackout," thus cutting - off the light - which the algae require. Chlorine dioxide has also been used recently for taete and odor control. When sodium chlorite became commercially available i t was possible to make chlorine dioxide by treating the sodium chlorite with chlorine gas a t the point of application. The chlorine dioxide has a very high oxidizing power and is very useful in removing tastes caused by chlorophenols, but probably not as useful for tastes and odors produced by algae. Its use does not eliminate the necessity for using chlorine. In spite of its high oxidizing power it does not appear to be very efficient as a disinfectant, chlorine still being necessary for that purpose. Ozone is also an effective disinfectant, leaves no chlorinous tastes, has high oxidizing ability, and produces good quality water. It is, however, very unstable and cannot be stored. This necessitates generating the ozone a t the point of application, which is costly. The cost has probably mitigated against its use to any great extent, although a t the present time there is a t least one municipal plant in construction and some smaller plants in operation. CORROSION
Finally we will consider our fourth objective-the control of corrosion, which is important not only to the individual consumer but also to the waterworks, because probably more than half of their total investment is in the underground piping, which is of course subject to corrosion. To date the most commonly empioyed methods of controllmg corrosion are the control of pH and the use of some protective coating on the surface of the pipe, both inside and out. This may be nothing more than a tar coating, or it may be the lining with bitumastic or with the very homogeneous and hard lining of cement. The pH of the water is carefully controlled and maintained a t the calcium equilibrium, so called, which i s that point where the water is so nearly saturated with l i e that more lime would not dissolve, but less lime would leave the water still slightly aggressive. Soda ash or calcium carbonate can be substituted for lime. Small amounts of sodium hexametaphosphate are also TASTE AND ODOR used continuously to produce a tough, thm coating on the inside of the pipe and thus protect i t from the acNow we may proceed to our third objective-taste and odor control-which has already been considered tion of the corrosive elements in the water. in connection with the use of chlorine. If we discount REFERENCES tastes or odors due to specific trade wastes, which would (1) STEIN, M. F., "Water Purification Plants and Their require special studies and treatments, then the odors Operation," John Wiley & Sons, New York, 1926. produced by essential oils from the green and blue(2) "Taste and Odor Control in Water Purification." Pubgreen algae present the principal problem. The first lished by the Industrial Chemical Sales Company. (3) '% New En& Watw Wwks Assoe., Dec., 1947. Pubstep in combatting algal odors and tastes is to destroy
404
lished by the New England Water Works Association. ( 4 ) Cast Iron Pipe News. January, 1948. Published by the
JOURNAL OF CHEMICAL EDUCATION Cast Iron Pipe Research Association. (5) The American City Magazine, Jsnuary. 1948.
