are some other reasons. Waters stored in a covered reservoir are more uniform in temperature than those stored in open reservoirs. In the winter little or no ice forms in them; in the summer the waters are not warmed by the sun. Thus in the winter there is less danger that service pipes, hydrants, and plumbing pipes may freeze, and in the summer the cooler water is more palatable. Experience has shown that, if ground waters are stored in open reservoirs, tanks or standpipes, microscopic organisms, especially diatoms, are likely to develop, imparting to the water their characteristic odors and making the water turbid. In order that these plant growths may occur it is necessary in the first place that the waters be seeded. Open reservoirs make seeding possible, because the spores of the organisms may be carried by the wind or on the feathers of aquatic birds. When, as in the case of the old water supply of Brooklyn, N. Y., surface waters are mixed with ground water, seeding is inevitable, because surface waters almost always contain, algae, protozoa, and other microscopic organisms. Algae contain chlorophyll, a substance which when exposed to sunlight has the power of converting carbonic acid into the sugars and starches of the plant cells. Exposure of water to the light provides the necessary energy and the fact that ground waters are clear and almost free from colloidal matter enables the sun's rays to penetrate further into the water than is the case with colored or turbid surface waters, although even in clear water the intensity of the sun's rays decreases rapidly below the surface. Ground waters usually contain a sufficient supply of dissolved carbonic acid as well as iron, silica, nitrates, and other mineral substances which diatoms need. Filtered waters exposed to the light in reservoirs are also susceptible to algae growths, but not quite to the same extent as ground waters, perhaps because colloidal substances are not entirely absent. from filtered waters and light penetration is less. Nevertheless, it is the best practice to provide covers for filtered water reservoirs. If algae, protozoa, and other microscopic organisms develop in an uncovered reservoir and enter the distribution system, they serve as food for fresh-water sponge and for several species of bryozoa which dwell attached to the inner walls of pipes and which are commonly known as "pipe moss." Such growths obstruct the flow of water and increase friction. They also break off and clog meters and water faucets. Providing a cover for a reservoir thus helps to keep the distribution pipes clean. Uncovered reservoirs in cities are exposed to atmospheric dust, leaves, pollen, bacteria, and other wind-blown substances. In the course of time, these make a considerable deposit on the bottom, foul the water, and promote growths of organisms. B. coli are often increased by exposure to dust and the droppings of birds. In the interest of good bacterial quality, covers are therefore desirable. Finally, reservoir covers are needed to protect the water from dirt and various substances carried in by animals or thrown in by human beings. Having gone to the expense of purifying water, it is good psychology to keep the filtered water under cover and its purity thus preserved. In Europe filtered-water reservoirs are all covered, practically without exception. Out of 80 reservoirs for the storage of filtered water of which a record is available, 30 were covered at the time they were built new. Of the remaining 50, four were covered after their original construction, on account of troubles with dust and odors due to the growth of microscopic life; two which gave trouble had been abandoned; and only 10 of the 50 show a record of such small growths of vegetable matter that they can be said to have caused no material difficulty. Of the remaining 34 reservoirs now storing filtered water without covers, the great majority show difficulties from time to time, although they vary in point of frequency, depending upon the size of the basins and the period of time that the filtered water remains in them. CHAPTER IX REMOVAL OF IRON AND MANGANESE Deferrization and demanganization Common manifestations of the presence of iron salts in a water supply are rusty or "red" water, staining of linens in laundry operations, staining of utensils or food in cooking, brownish films formed on the inside of glasses and water bottles and brownish stains on plumbing fixtures. In its less disagreeable form iron may be exhibited only as a slight turbidity in the water. In aggravated form, the rusty deposits may unfit the water for any satisfactory use. The presence of iron is sometimes manifested by a slight "styptic" or "metallic" taste. A not infrequent accompaniment of ferruginous waters from underground sources, is the appearance of "iron bacteria," e.g., crenothrix. These organisms established in the distribution system greatly aggravate the staining troubles named above, frequently cause clogging of service pipe and mains, and are responsible for disagreeable odors. In considering the effects noted, a clear distinction should be made as to whether the troubles are due to soluble iron present in the source of supply or to metal dissolved from the distribution system or from house plumbing, particularly hot water systems. Manganese in the water supply causes effects similar to those of iron, as to discoloration, staining, sediment, pipe clogging and assistance to organic growths. The manganese rust, however, has a darker appearance than the reddish iron rust and deposits of the oxide, if pure, are quite black. Manganese is frequently associated with iron in ground waters, but is usually present in smaller amounts than iron. There are notable exceptions, however, to this rule. Amounts objectionable Experience has shown that iron, if present in excess of 0.5 p.p.m. (expressed as Fe) will generally be objectionable to consumers. With pure ground waters 0.3 p.p.m. is the limiting amount beyond which complaints of deposition and staining are to be expected. In waters containing considerable organic matter, a content of 1 p.p.m. or over may be tolerated without complaint. It may be noted here that the tentative standard of the United States Treasury Department applicable to water on interstate carriers suggests a limit of 0.3 p.p.m. The amount of manganese required to produce complaint is not well established because this metal seldom is found unaccompanied by iron. The best information available is that the sum of these two metallic constituents may not exceed the limits stated above without complaint. Methods of removal No single method has been found applicable to all problems of iron and manganese removal because other impurities have a marked effect on behavior. These impurities in order of importance are organic matter, manganese, carbon dioxide and carbonates. The processes found applicable are as follows: Aeration, sedimentation, coarse contact beds, sand filters and chemical treatment. Aeration Oxidation of the iron and manganese salts present in the ground water is a necessary preliminary to any subsequent removal process. The amount of oxygen required is very small, being only 0.14 part for each part of iron present, while ground water at ordinary temperature, when fully saturated, contains about 10 p.p.m. of dissolved oxygen. It is often possible to supply the small amount of oxygen necessary for oxidation by adding chemicals such as permanganates, hypochlorites, chlorine, etc., but the addition of atmospheric oxygen is so simple and inexpensive as to leave no room for chemical treatment for this purpose. Oxidation therefore implies some form of aeration for bringing air in contact with water or water in contact with air. It is unnecessary to discuss here aerating devices in detail, but it is important to note, however, that some iron-bearing waters require a distinct limitation on the amount of aeration. At Lowell, Mass., for instance, it has been well established that, if aeration is carried beyond 50 per cent saturation of dissolved oxygen, the man ganese is not removed on top of the slow sand filters, but the action. is delayed and clogging of the lower part of the filter beds results. At Reading and Middleboro, Mass., complete aeration followed by filtration failed to remove iron satisfactorily from these supplies, but restricted aeration was successful. At Superior, Wis., the aerators were abandoned as distinctly harmful and the iron is now oxidized only by incidental handling of the water. Except where organic matter or manganese is present in sufficient amount to make necessary a limit on aeration, it is generally advisable in connection with ground water to provide an efficient means of aeration in order to reduce to the lowest amount the carbon dioxide and any hydrogen sulfide present as dissolved gases. Where ground water is pumped by air lift no other form of aeration may be needed, but carbon dioxide in excess of 50 to 60 p.p.m. generally requires additional aeration. In ground waters very low in alkalinity a secondary aeration may be necessary to secure satisfactory removal of carbon dioxide. In other words, the oxidized iron must be removed by contact action before a full release of carbon dioxide can be secured. Sedimentation The usefulness of plain sedimentation is limited to water containing considerable alkalinity, little organic matter and relatively high amounts of iron and manganese. Usually precipitation is too slow to make sedimentation economical, for satisfactory removal is seldom obtainable unless more than 24 hours retention is provided. Plain sedimentation is therefore less useful than other processes to be described. In connection with chemical precipitation referred to later, sedimentation has obvious usefulness. Sedimentation following coarse contact beds is useful in taking care of occasional "unloading" caused by rapid change in rate. Coarse contact beds Except in the presence of high organic matter, dissolved iron in ground water after adequate aeration will precipitate as red ferric oxide on any surface in contact with the water. This behavior is utilized in connection with artificial beds of various materials arranged to provide large contact surface. The kind of material is not important and may consist of broken stone, round boulders, |