Georgetown has been drawing upon underground resources for its water supply. Underlaid as this region is with limestone formation, the ground water collects and flows through underground channels often of considerable size. Frequently these underground streams become visible on the surface only to disappear again in their underground channels. Underground streams are used almost exclusively in this region for water supplies. Their waters are often highly polluted, nearly all of them being in direct connection with surface water at many places, and frequently, also, with sources of contamination. To the use of such water untreated have been traced many of the typhoid epidemics in this region. Georgetown takes its water supply from one of these underground streams appearing at the surface as a spring of considerable size in the western portion of the city. This spring, known at Royal Spring, flows normally at the rate of several million gallons per day. Its rate, however, is not uniform, and varies from 1,000,000 to 5,000,000 gallons per day. The fluctuations respond very rapidly to the rainfall, indicating a nearby watershed. Its waters are more or less polluted. During and after rain storms, the rate of flow and the turbidity of the Royal Spring water increases, leading to the conclusion of its having direct connection with surface water at several, if not at many, points. A number of sink holes exist in what would appear to be the direction of the underground stream, and a few years ago the one nearest the spring was used as a dump for garbage. To the use of this spring water heretofore untreated has been ascribed the former high typhoid rate, and also the recent typhoid epidemic through which Georgetown passed a few years ago. In the fall of 1910 the writer was retained by the city of Georgetown to report upon the best method of treating the water. The water is quite hard, varying from 110 to 150 parts of calcium and magnesium per million. T he writer recommended chemical purification, consisting of water softening, with coagulation during turbid conditions, to be followed by sterilization with hypochlorite of lime at all times This plan was adopted by the city council, and the writer was directed to prepare plans and specifications in accordance with the preliminary report.

Description of Plant

The plant is designed upon the continuous flow plan. On account of the limited funds available, it was necessary to reduce the size of the various tanks required for treatment, without impairing the efficient operation of the plant. When operating at the full capacity of 750,000 gallons in twenty-four hours, the time of mixing is thirty minutes; the time of settling five hours In the dear-water well can be stored one and two-thirds hours’ supply. The results obtained from the plant since its operation in June show that the efficiency of the plant when operating at full capacity has not been materially affected by cutting down the period of treatment to a minimum. The dimensions of the purification plant on the operating or upper floor are 39 feet 8 inches by 88 feet 1 inch, housed over by a onestory flat-roofed concrete block building. The settling basins take up an area of 1,320 square feet The clear well takes up an area of 390 square feet, and is covered over by a concrete floor, which area, together with the remainder of the budding, is used for the storage and mixing of chemicals. The elevation of the operating floor is 10 feet above the ground level. The water is raised 14.7 feet from the spring into the purification plant into the weir box. Under that part of the operating floor not occupied by the settling basins and Clearwater basin have been placed the two high pressure pumps, two 750,003gallon, low-lift centrifugal pumps of the De Laval type, one direct connected to a De Laval stream turbine, and the other motor driven; four electric centrifugal pumps to circulate the chemical solution, and the hypochlorite, coagulant and soda and lime tanks, in every case the solution of the chemicals is assisted by mechanical agitation The introduction to the raw water of the proper amount of chemicals is regulated by a weir box, a float box and a scries of three regulating boxes The amount of chemical s du tion admitted is regulated by the sliding diverting valves in the regulating boxes. These sliding diverting valves are operated through a link motion by the rise and fall of the float in the float chamber. The water level in the float chamber is controlled by the depth of water over the weir in the weir box. The weir is of such shape that the rate of discharge is directly proportional to the depth of water over the weir. The structure is so designed and constructed that the capacity of the plant can be increased any reasonable amount by extending the plant in a direction perpendicular to the plans of the section. The raw water into which the proper amount of chemical solutions have been introduced enters the mixing tanks at one end and flows in series through the nine mixing tanks. Agitators located in these tanks, and revolving at the rate of 10 r.p.m., thoroughly mix the raw water and chemicals. The advantage of the thorough commingling of the water to be treated and the chemicals by means of properly devised mechanical agitating devices has not received in the past the attention from engineers that so important a function deserves. Concrete distributors located one on each side of the mixing tanks distribute the water collected from the mixing tanks into the settling basins. Eighteen 3-inch galvanized wrought iron down-takes, arranged as shown in Fig. —, admit the water to the settling basins. These down-takes deliver the water to the bottom of the settling tank and cause the water in passing through the settling tank to follow an outward and upward direction, producing the best condition for most effective sedimentation. Each settling basin is 38 feet 4 inches long, 17 feet 8 inches wide, and holds a depth of water of 15 feet. Concrete collectors are built on the side of the settling basin opposite the distributors. The water, before enterting these collectors, passes through a roughing filter of excelsior, the material of which is retained in position by two threesixteenth inch perforated steel plates curved to a radius of 5 feet. Before the clarified water is admitted to the clear water well it enters the hypochlorite sterilization tank located in one corner of the clear water well, where it is thoroughly sterilized with hypochlorite of lime. The clear water well is entirely covered and separated by a wooden partition from the uncovered settling basins and mixing tanks. The bottom of each settling hasin is perforated with a number of three-quarter inch holes spaced approximately 30 inches centers. These holes lead into a system of under-drains imbedded in the concrete. These under-drains are arranged in zones parallel to the side of the tank along which the water is admitted through the down-takes. The outlets from these zones are controlled by quick-opening valves. By opening any one of the eight quickopening valves connected to these systems of under-drains. any one zone of either settling compartment can be drained of the accumulated defxisits or sludge without interfering with the continuous process of settling. Furthermore, the length of time that the valves controlling the different zones can remain open is varied to meet the different depths of sedimentation in the bottom of the tank as we recede from the inlet. By removing the sludge periodically, the interval depending on the character of the raw water, the capacity of the basins for effective sedimentation is not interfered with to any material extent. As settling tanks are usually designed, the accumulation is greatest at that period when the greatest settling capacity is needed, and when it is inconvenient to spare a tank for the purpose of cleaning it out. In the design herein referred to, the full capacity of the tanks can always be maintained by varying the period between the operation of the outlet valves. The sludge is discharged into the city sewer located at a convenient distance from the plant. The plant has been so designed that either settling basin can be drained without interfering with the operation of the plant. Since the operation of the plant the average amount of water treated daily has been 500,000 gallons, and the amount of chemicals used per million gallons is: Soda ash, 500 pounds: lime, 1,552 pounds; hypochlorite of lime, 5 pounds. The cost of chemicals delivered at Georgetown is $20 a ton for soda ash. $8.10 a tone lor hydrated lime with 84 per cent. CaO. and $18 a ton for hypochlorite of lime. The hardness has been reduced from 132 parts per million to 39.5 parts per million. From these figures it will be seen that the chemicals alone cost $11.40 per million gallons, while the cost of labor for operation is approximately $5 per million gallons.

Mechanical Filtration

Mechanical filtration can he secured at the lowest cost even in a small plant. It is important, therefore, to determine the advantage accruing to a city by having a soft water as well as a purified water. Whipple, several years ago, made investigations as to the value of reducing the hardness of water. From these investigations we find that it will require 27,800 pounds of soap to soften water having a hardness of 130 parts per million, and 10,900 pounds to soften water having a hardness of 40 parts per million. Therefore, in reducing the hardness of water from 130 parts to 40 parts there is effected a saving of 16,900 pounds of soap. On the assumption that 10 per cent, of the water actually delivered to the city is used for purposes which would be benefited by softening and assuming soap at 3 cents per pound, we find that there is a saving in Georgetown of $50.70 per million gallons, or $25.40 a day on the basis of 500,000 gallons present daily consumption.

From the following chemical and bacteriological analysis prepared by J. W. Ellms. chief engineer of the Cincinnati filtration plant, it will be seen that the treatment is satisfactory, both from a chemical and from a bacteriological standpoint:


The calcium and magnesium found in the treated water when expressed in terms of calcium carbonate amounted to 4.086 grains per United States gallon. The bacterial examination resulted as follows:

The following is an extract taken from a letter written by G. F. Olsen, chairman of the Water Committee of Georgetown, to L. M. Booth Company, the sub-contractor for the equipment:

“With the assistance of the State Board of Health we are running weekly tests on the efficiency of the purification plant, and we are more than gratified with the excellent showing that it is making in purifying the water. In fact, all four bacteriological tests that have been made have shown the water to he free of bacterial growth in the first twenty-four hours, with a maximum of ten in forty-eight and forty in seventy-two hours. We are bound to admit that this is pretty close to perfection.”

Special Features of the Design of Tanks

Wherever it was considered necessary the common walls of all tanks have been designed to withstand the full hydrostatic pressure on either side. There are several features in the design and construction to which attention should be called. One of these features is the junction of the sides of the tank to its bottom. The hydrostatic pressure against the side walls supported hv the buttresses on the outside creates considerable tension along the plane of junction between the side and the bottom. The development of on either side, was placed in the center. This location of the reinforcement coincides very nearly with the neutral axis of the section. However, this was not considered objectionable, as there rs very little, if any, beam action developed in these wall slabs, because of the short span, viz. 3 feet 10 inches, and the comparatively thin slab (10 inches). These slabs withstand a maximum load of 1,000 pounds per square foot over this span, principally by virtue of the arch action developed. The tension developed in the rein forcing rods serves to take up the unbalanced arch thrust at either end. The cost or this type of construction exceeds but little, if any. the other methods in use to attain the same object. The rigid steel frame obtained by this method is of considerable value in carrying out the construction in the field. Also, a positive anchorage is obtained for the reinforcing rods, which is to a considerable extent independent of the quality of the concrete work. In spite of the fact that the plant was constructed during the winter of 1010-1911, and the concrete work was carried on intermittently during the winter months, the structure developed no defects other than those which could be attributed to the severe winter weather. As it was necessary to construct the plant during the cold season, the writer expected a more or less porous concrete and called for waterproofing the inside of the tanks. The contractor applied a one-sixteenth inch coat of insulite mastic furnished by Girvan-Nachod Company, which rendered the tanks practically watertight. This material, however. while giving good results should not be applied in a covered tank without proper provision having been made for exhausting the heavy fumes rising during its application. Up to the present time no cracks whatsoever, or other defects objectionable in a construction of this kind, arc noticeable. this tension to its fullest extent is necessary for the stability of the structure, and proper provision should be made in the design for its full development. The passive forces which are called into action to develop this tension is the weigh! of the water on the bottom of the tank, and, in a rectangular tank, the restraining action at the corners. This restraining action at the corners is often of sufficient magnitude to insure the stability of otherwise unstable side walls. The percentage of additional stability thus obtained decreases with the size of the tank and reservoir, and this alone may account for some of the failures of tanks of considerable magnitude built with standard sections of side walls and but tressed and similar to those of existing but smaller structures which have offered every evideuce of strength and stability. To fully develop the forces necessary to prevent the overturning of the sides of the tank, sufficient strength must be given to the construction to permit of the full development of the passive forces. In the construction illustrated the weight of the water resting upon the bottom adjacent to the side walls is transferred by cantilever action to the sides to develop the tension necessary for the stability of the wall. To do this all of the vertical bars in the walls of the tank are bent to a sharp curve at right angles to the side, and are extended from 3 1/2 to 4 1/2 feet into the bottom slab, where they serve as cantilever reinforcement. An 1-inch round bar is placed in the sharp bend, and the unbalanced resultant tension is taken up by steel bands bent around the bar. These bands are 1/4 x 1 in cross-section, 40 inches long, and are spaced 6 inches center to center. The criticism may be advanced that unless such straps are in absolute contact with the round steel rod they are useless and cannot perform their function until sufficient yielding has taken place. If this were only partially the case the writer would most strenuously object to the use of such straps to take up the unbalanced tension. Some time ago a test was called to the writer’s attention in which two chains were embedded in concrete so that their ends protruded. One was placed in the concrete and allowed to set under a slight tension to insure that the links were in perfect contact with each other while the concrete was settling. The second was placed into the mold and concreted in such a way that there was a very large amount of slack in the chain and there were very few links in actual contact. Both of these chains, when tested, showed the same identical behavior and led to the conclusion that the mortar in the concrete sufficiently filled the voids between adjacent links to prevent any noticeable slip. It is impossible to judge from the behavior under test which of the chains had its links in absolute contact. This test corroborates the writer’s practise that the use of straps to take up the unbalanced tension is good, sound practise. There is nearly always a considerable increase in strength in the various structural elements when combined in a structure over and above that obtained when these structural elements are tested separately: first, because of the rigid connection resulting in reinforced concrete construction which distributes excessive local stresses over a larger area, and second because of the ability of reinforced concrete to relieve itself to some extent, wherever possible, of excessive stresses and to partially readjust itself to the loading. Unless the designer lias considerable experience, and can judge with some Ie grec of accuracy the increase in strength of the various structural elements when combined in the finished structure, he should not consider this increase in strength in designing the structure except as an additional safety factor. The writer also wishes to call attention to the extensive use of round rods with threaded ends and their to be imbedded in the concrete. This method of reinforcement was used in constructing the nine mixing tanks, and also for tying in the side vails at the end of the mixing tanks where the buttresses were omitted. On account of the com paratively thin sections used, it was deemed advisable that a more positive anchorage for the reinforcement should be provided for than could be obtained from the adhesion between the steel and the concrete in the short distance that it was possible to imbed the ends of the reinforcing rods Clamping the reinforcing rods to the steel plates by nuts at either side of the plate, as shown, prevents absolutely any slip between the ro 1 and the plat Shown in diagram, the reinforcement in the 10-inch side walls of the mixing tanks, which have to withstand the full hydrostatic pressure

No posts to display