Experiences with Iron in Public Water Supplies
How Problem Was Solved by Engineers in Case of Shelby Water Company—Description of Methods of Aeration, Sedimentation and Filtration Employed
IT is a well recognized fact that the presence of a considerable quantity of iron in a public water supply is objectionable but unfortunately there are comparatively few instances where such public water supplies have been developed and improved by purihcation, so that it is believed that the experiences of the Shelby Water Company, at Shelby, Ohio, in the development of a ground water supply containing considerable quantities of iron will he of general interest, as read before the convention of the Central States Section of the American Water Works’ Association.
The water works at Shelby were installed by the local water company in 1897, and the supply was obtained without treat m e n t from drilled wells 40 to 60 feet deep, contained in a 10-acre tract bordering on Black Fork Creek and Situated about one mile above the center of the town. Preliminary tests were made of the quality of the supply, and the report of the analyst under date of October, 1896, contained the following:
“The’water is very high in free ammnnia, rather high in albuminoid ammonia, prelty well up on oxygen required. The water is a suspicious water. I should suspect it of containing animal sewage well fermented, but not exposed to oxidation. The practical abscence of nitric acid may mean, however, that the free ammonia is largely due to nitrates reduced to ammonia by working through soil and rock containing reducing agents, possibly sulphide. This is theory, however, and a full investigation of these features will be necessary before this water could be considered safe, as the analysis, on the face of it, condemns it This latter theory would be supported by the large amounts of sulphates—permanent hardness.”
The analyst’s statement indicating that the water was “suspicious” is of interest, and indicates the general lack of information relative to the significance of organic matter in ground water supplies. Bacterial analyses have always indicated that the water supply at Shelby is practically sterile, so that the high organic matter contained in the water is of no significance as regards its sanitary qualities. It merely indicates that the water takes up organic matter from the alluvial deposits, which are swampy and contain considerable deposits of a decayed vegetation, probably of pre-historic times.
Use of the water supply at Shelby soon indicated that it was generally very unsatisfactory for domestic purposes on account of its high iron content, and some eight years later, in 1905, the company installed devices to remove the iron. These included aeration through perforated, horizontal, steel baffles in the top of the standpipe, sedimentation in the standpipe and finally filtration through two horizontal, steel pressure filters, 8 feet in diameter by 24 feet long. The filters contained eighteen inches of crushed quartz, having an effective size of 0.75 mm. The water was improved by these devices, and the iron reduced from approximately seven parts per million to about 0.6 to 0.8 parts per million.
While associated with the Ohio State Department of Health, the writer made an examination of the plant of the company in 1909 and numerous chemical and bacterial tests of the raw and treated waters were made at the plant as shown in the tables on the following page.
At that time the total consumption of water was from 250,000 to 300,000 gallons daily. The iron was reduced to a little less than one part per million, and the consumers were generally satisfied with the quality of the supply. Subsequently the water consumption at Shelby has increased until, during the past year, the consumption has been about 700,000 gallons daily, or double that during the year 1909, with the result that the removal of iron by the purification devices available was not at all satisfactory. The writer was retained by the company to design a new iron removal plant, and subsequently his firm entered into a contract to con struct this plant from the design furnished.
Description of Iron Removal Plant
The general features or principles adopted in the new plant were similar to those in the old and comprise aeration, sedimentation and filtration. The supply is the same as that originally acquired, but the old drilled wells have been abandoned by the company and have been replaced by two dug wells, approximately forty-live feet deep. The new plant was designed for a normal capacity of 1,000,000 gallons daily. The aerator tower was based upon a normal capacity of seventeen gallons per square foot per minute. The aerated water is retained in the sedimentation or reaction basin for a period of one hour. The filters have a normal capacity of 100,000,000 gallons per acre daily. The clear well holds 250,000 gallons of treated water, equivalent to six hours’ storage. The filters are covered with a substantial brick superstructure. All of the basins and filters are of reinforced concrete.
During operation of the new plant, water is pumped bv electrically operated centrifugal pumps from either of the wells, and is discharged at the top of the aerator through which it flows by gravity into the reaction tank, thence on to and through the filters into the clear well from which it is pumped by electrically operated centrifugal pumps under about 60 pounds pressure, into the distribution system or standpipe. The latter is 16 feet in diameter, 130 feet high and holds 196,000 gallons.
Aerator—The aerator comprises three horizontal trays containing crushed coke. The trays are 12 inches deep with 12-inch openings between trays. The bottoms are covered by wire screens containing 3 1/2 meshes per inch composed of iron wire 0.105 inch in diameter. The coke layers are 12 inches thick, and are comprised of particles ranging in size from 1/2-inch to 3 inches diameter. The trays are each 4.5 feet by 9 feet in section with a total area of 40.5 square feet. The nominal rate of flow through the coke is 17.3 gallons per square foot per minute. The aerator is located over the inlet end of the basin, and water is distributed over the top tray by two lines of perforated 8-inch cast iron pipe. The centers of these pipes are 8 feet 8 inches above high water level in the basin.
Reaction Tank—The basin or reaction tank is uncovered and is 14 feet by 43 feet by 10.5 feet deep. It holds 47,300 gallons, so that the period of retention in the tank is 66 minutes.
Filters—The filters are two in number, and are open gravity sand filters, each containing 240 square feet. The total filter area is 0.011 acre.
The strainer systems comprise 10-inch cast iron main collectors and 1 1/2-inch extra heavy galvanized iron laterals 6 inches apart. The laterals are perforated with 1/4-inch round holes spaced in two rows, six holes per lineal foot of pipe.
Over the strainer systems is the filter gravel placed to total depth of 12 inches, and comprised of particles ranging from 1/12-inch to 2 inches in diameter.
Above the gravel is placed two feet of filter sand with an effective size of 0.49 mm. and uniformity coefficient of 1.75. Considerable thought was given to the selection of sand of the proper size, and experience indicates that the sand used is entirely satisfactory.
The filters are washed through a 10-inch cast iron pipe connection from the standpipe, and the rate of application of the wash water is from 20 inches to 24 inches rise per minute. The tops of the wash water troughs are 24 inches above the filter sand.
An unusual feature of the design is that high water in the dear well is three feet above the bottoms of the filters, so that the total effective depth available for filtration is three feet, when the clear well is full. No controllers are provided in the effluent pipes on the filters. It is obvious that as the clear well becomes full, water backs up on the filters, tending to reduce the head available for filtration and consequently to reduce the rate of filtration. This feature permitted considerable saving in the construction and operation of the plant. Moreover, the total head available for filtration has proven to be ample notwithstanding the fact that it is customary to keep the clear well full.
Operation of Plant
The new iron removal plant at Shelby was first placed in operation on March 19, 1920, and has continued in successful operation since that time. It is of interest to note that the elimination of iron from the supply has been entirely satisfactory from the beginning of operation. The actual results accomplished by the plant have been determined from time to time by analvses of samples collected by the superintendent of the company and shipped to the laboratory of the State Department of Health at Columbus. The results of analyses so obtained are shown in the two accompanying tables. The analyses are not entirely satisfactory or significant on account of the fact that considerable time elapsed between the collections and analyses, but they do serve to indicate in a general way the character of the raw and treated waters.
In the first table are shown complete analyses of five samples collected on June 12 and August 27. The second table shows only analyses of iron and free carbonic acid. Two of the samples in the first table were obtained from well No. 2, and show considerable variation in the iron content of the water which carries from 8.0 to 4.8 parts per million. Similar analyses in the second table show that the water in well No. 2 contained from six to eight parts per million and samples collected from well No. 1, on March 19, showed that this water contained sixteen parts per million. In general the iron content of the well water is high, ranging from about five to eight parts per million in well No. 2 and sixteen parts in well No. 1.
Analyses of the samples obtained from the supply to the filters, on August 27, indicate but slight change in the chemical and physical qualities of the water as affected by aeration. Later tests made by Mr. Roberts, assistant engineer of the State Department of Health, indicate that the dissolved oxygen content of the raw water was increased by aeration from 0 to 64 per cent, saturation, and that the free carbonic acid is decreased by aeration from 43 to 16 parts.
Analyses of the total iron content as shown in the two tables are of especial interest, and indicate that the filtered water at times contains only a trace of iron in the samples. The highest iron content shown at the plant is 0.3 part per million and from the mains in March was 0.4 part per million. The increase of iron in the main as compared with that in the filter effluents was caused by the former deposits in the mains. It will be of interest to see how long these deposits continue.
Operation of Plants Satisfactory
In general the operation of the plant has been entirely satisfactory, and the filtered water supply has been always clear and free from suspended iron. Experience indicates that the coke in the aerator tends to become coated with deposits of iron so that it is necessary to wash the coke every three or four weeks. This is done by washing the coke in place with a hose. But little sedimentation of iron occurs in the reaction tank, which is cleaned only once or twice yearly, and even at such times contains but a few inches of deposited precipitate. The filters are washed once daily at the rate of a little less than 24 inches rise per minute and generally for a period of four or five minutes. The total quantity of wash water used is only about 4 per cent, of the quantity of water filtered, so that in this respect the operation of the plant compares favorably with that of similar rapid sand filter plants provided to improve surface water supplies.
The contract under which the writer’s firm constructed the clear well and filters included all excavation and concrete structures, except the reaction tank which was built by the company; also all pipe lines except outside cast iron pipe lines. It did not include the brick superstructure nor back filling and grading around the clear well and filter house. The total contract price for the above was $16,500 Mr. Bricker, president of the company, has advised the writer that the superstructure cost about $1,500, and the aerator and reaction tank about $3,000 additional. It will probably cost about $800 to complete embankments and grading around the new plant, so that the total cost of the new plant was about $23,000.
In conclusion, it should be noted that the recent experience of the company in providing a new iron removal plant has been entirely satisfactory, and. in fact, has proven beneficial in a number of ways. The satisfaction to the officials of the company in providing a clear water supply free from objectionable features, and to the consumers who are enjoying the new supply, is very pronounced. Moreover, the new plant has resulted in a marked increase in quantities of water sold, thereby increasing the revenues of the company, due in part to the more abundant use of water by former consumers and in part to new consumers who have been attached to the property since the new plant is in operation.
Another feature of operation brought about by the new plant is that it is now very much easier to maintain service lines and meters than was formerly possible. With the use of raw water there were many instances where service lines became clogged with the deposits of iron, thereby resulting in a loss of considerable sale of water and a considerable increase in expense entailed for maintenance of service lines. Similarly the iron contained in the water supply formed deposits on the meters, eventually with the result that there was a general reduction in the quantity of water sold, not always apparent to the officers of the company. There is no question but what the sale of water and revenue received therefrom were materially reduced by the above features, which are now eliminated by the operation of the new iron removal plant. In general, the relations between the officials of the company and the consumers are much more satisfactory, a condition which has resulted in increased income and benefits to the company and increased enjoyment and use of the public water supply to the consumers. The benefits derived from the new plant have more than compensated the company for the expense entailed by its construction and operation.
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