act in this manner is shown by the fact that a filter is more effective after it has been in operation for some time.

There are two methods of filtration in common use: (1) the slow sand process, and (2) the mechanical or rapid sand process. In (1), the water is passed through a bed of sand and gravel 3 to 5 ft. thick to a system of underdrains at an average rate of 1.6 million gal. per acre per day. This process is seldom used for small installations such as would be used in connection with a building. In (2), mechanical filters operate at much higher rates, 100 to 200 million gal. per acre per day. The rapid filtration of water is distinguished from slow filtration, not alone by the difference in the rates of flow, but by several other features, such as the formation of the colloidal coating, which naturally differentiates the two general methods of purification.

Mechanical filters may be divided into gravity filters and pressure filters. Gravity filters as the name implies, signifies that the water flows through the filter by gravity.

Gravity filters are usually arranged in two rows, with a common water supply pipe, valves, wash-water pipes, rate of flow-controlling apparatus, gages, etc., in a pipe gallery between the rows. These filters are in round wooden or steel tanks (old type) or in square concrete basins (new type). The clear water basin is usually placed beneath the filters. The wash water required for mechanical filters is 5 to 7 gal. per min. per sq. ft. of filter area. The head required for washing is 30 ft. or more depending on the type of strainer used at base of filter. The amount of water used for washing per day is equal to a column of water 5 or 6 ft. high over the entire area of the filter. A gravity filter has an advantage over a pressure filter in that it is easier to observe every step in the filtering operation, and it is possible to examine the condition of the sand at any time without shutting down the filter.

Pressure filters are closed cylinders or tanks of steel in which is placed a bed of sand or crushed quartz and through which the water is forced under pressure (see Fig. 1). Other filtering materials for special purposes, such as charcoal, coke, or zeolite are sometimes used. The strainer system also acts as a distributor for the wash water during the cleaning of the sand bed. During the washing process compressed air is often used.

6

28.27

56

8

50 26

100

10

78.54

157

1

113.10

226

14

153.94

307

15

176.71

353

344 G∞

12,500 22,250

$1400.00

1150.00

5

6

6

8

TABLE 3.-DATA ON AMERICAN WATER SOFTENER Co.'s PRESSURE FILTERS

The above capacties are conservative.

Use minimumcapacities for muddy waters. Where maximum capacities are exceeded, resistance will increase, the filter will require washing more frequently, and the filtered water will not be of as good quality.

TABLE 4.-DATA ON INTERNATIONAL FILTER Co.'s PRESSURE FILTERS

Pressure filters, where no preliminary treatment is required, are used extensively for the reason that they are simpler and cheaper than gravity filters. Filters treating water for industrial purposes should always be provided with a by-pass so water can be supplied direct in case filter is cut out for repairs. The bacteria removing efficiency of mechanical filters is approximately 95%.

13. Rain Water Filters.-Besides its use in the laundry where it is desired on account of its softness, rain water, when properly aerated and filtered, makes a wholesome beverage. It should, however, be caught and stored so as to preserve its original purity. The present methods of collection and storage in most cases are bad. Every roof is foul with excrements of birds, dead insects, leaves, dust, etc., all of which are washed into the cistern. The first part of every rain should be turned off until the roof is washed clean. There are several automatic devices for this, but few are used. It will not in most cases be economical to attempt to filter the water as fast as it may come from a roof. It will be preferable and cheaper to first store the water in a tank or reservoir of wood, masonry, or slate, which should be located above the ground if possible. From such a reservoir the water should be led to the filter which should consist of sand and gravel, similar to the above described gravity filters.

The filter should be large enough to care for the average daily consumption, or at least large enough to filter the contents of the storage tank before another rain fills it. The rate of filtration should be between 50 and 150 gal. per sq. ft. per day; a layer of sand 2 ft. deep is sufficient. If desired, charcoal can be added to free the water of undesirable taste and smell. When the filter becomes clogged, it should be scraped to remove the sediment, but the sand and gravel below should not be disturbed except at long intervals, or when the filter ceases to work due to severe clogging.

The filtered water should be stored in a clean, tight, and well protected cistern or underground reservoir, from which the supply for domestic use is taken. Where the use of water may be largely for mechanical or industrial purposes, there could be two cisterns, one for filtered and one for unfiltered water. In this way the storage tank

and filter could be made smaller. Upward flow filters built in a part of a cistern are worse than sueless, as the pressure of the water will force a passage through in one or more spots and thus defeat the purpose of filtration. Also, galvanized-iron receptacles attached to down spouts and filled with charcoal, are only strainers and do not purify the water.

If the roof tributary to the filter plant has an area of 1000 sq. ft. and the ordinary maximum rainfall is 2 in. per storm, then the capacity of the storage tank should be 1250 gal. and the filtered-water cistern should have a capacity sufficient to tide over dry spells. A cistern of 10,000-gal. capacity (for dimensions see Table 40, page 1212) will supply 400 gal. per day for about one month.

14. Removal of Iron.-If iron occurs in water to the extent of 0.2 to 0.3 parts per million it will cause little or no annoyance. In some cases 0.5 parts per million have given no trouble. Where iron occurs in the ferrous condition as hydrate, bicarbonate, or sulphate, its removal is comparatively easy by aeration and filtration, but where it occurs as a chloride or nitrate or in the presence of manganese, organic matter, vegetable acids, or in a polluted water, its removal is difficult. Iron may be said to exist in waters of three classes: (1) Those which begin to precipitate on exposure to air, iron in hydrate form; (2) those which will hold iron in solution indefinitely even when aerated, iron usually combined with vegetable acid and appearing in a colloidal form; (3) waters which contain iron in both forms and therefore deposit a part, but not all, of the iron contents after aeration.

The principal methods of treatment for class (1) are: aeration, sedimentation, filtration, coke contact, sprinklers, and filtration through sand. For class (2): aeration, coke contact, sand filtration, chemical treatment, sedimentation, and filtration. For class (3): a hypochlorite with filtration through chemically treated sand beds, or filtration through aged beds of sand.

Iron-bearing waters are of the greatest annoyance in the laundry where they stain the clothes. Waters of the first class readily give up their iron when heated so that a simple pressure filter will remove the difficulty to a large extent if not overtaxed.

Iron also causes trouble by clogging mains and service pipes due to the presence of the iron organism, crenothrix, which has the ability of causing a deposit of iron to form on the inside of the pipes.

15. Removal of Manganese.-Waters containing manganese are more difficult to treat than those containing only iron. Where the same processes are effective, they react much more slowly. In a water containing both iron and manganese, the iron will be found deposited near the source while the manganese will be found on the outskirts of the system and in dead ends. Manganese is no doubt a much more troublesome element than is generally supposed. Manganese often gives to the water a milky appearance due to its colloidal form. When present with iron, organic matter, carbonic acid, and vegetable acids, it is a most difficult element to remove. Salts of sodium, especially a hypochlorite, seems to have the quickest and greatest effect on it when in this condition.

The higher oxides of manganese appear to have a very favorable effect on the removal of those of a lower order, so that if water containing manganese is applied to a filter in which higher oxides have been precipitated, a large percentage of the manganese in the water will be removed. Such a condition can be brought about by treating the filter with manganese sulphate, sodium hydroxide, and a hypochlorite, or by letting the filter become automatically coated as it will do in most cases if not disturbed. The rate of filtration for the removal of iron and manganese varies greatly with the character of the water, but would generally come between 1000 to 2000 gal. per sq. ft. of surface of filter per day.

16. Causes of Incrustation.-Incrustation of steam boilers, water heaters, furnaces, coils, etc., is caused by deposition of the following: suspended matter; deposed salts from concentration; carbonate of calcium and magnesium by boiling; sulphates and chlorides at temperature above 270 deg. F.; manganese at high temperature; and lime, iron soaps, etc., formed by saponification of grease.

17. Effects of Incrustation.-Incrustation reduces efficiency of steam boilers, water heaters, furnace coils, and siphon jet closets; causes boiler plates to become overheated and distorted, and in some cases, causes failures to occur. Hot-water heaters, especially of the coil type, and furnace "backs" or coils become so incrusted as to stop circulation. Hot water pipe systems become incrusted to such an extent as to greatly reduce their capacity, if not clog them entirely.

Means of preventing incrustations are: filtration; blowing off boiler; use of internal collecting apparatus, or devices for directing the circulation; heating feed water keeps scale caused by temporary hardness from entering

the boiler; use of boiler compounds; introduction of zinc into the boiler; water softening. Other remedies for these troubles will be found in Art. 19.

18. Hardness of Water.-Waters are said to be hard because of their action upon the skin of the body and because of their neutralizing effect on soap. Hardness is of two kinds: (1) that caused by the bicarbonates of calcium, magnesium, and iron, which is called temporary hardness and can largely be removed by boiling; and (2) that caused by the sulphates, chlorides, nitrates, and silicates of calcium and magnesium, which is called permanent hardness because boiling not only does not remove it, but, on the other hand, tends to increase it. The sulphates, chlorides, etc., form much harder scale than the bicarbonates. Hardness is measured by grains per gallons, parts per 100,000, and in parts per million in equivalent calcium carbonate. Waters have been classified as follows in parts per million:

0-50

50-100.

100-150.

150-200.

200-300..

Very soft
Fairly soft
Medium
Moderately hard
Hard

A water containing 0 to 70 parts per million of equivalent calcium carbonate is considered a very good boiler water; 70 to 150, good; and 150 to 250, fair.

19. Water Softeners-Gravity Type.-Waters may be treated with chemicals for reduction of hardness, when either hot or cold. There are two types of softeners, gravity and pressure, similar to filtration plants. The pressure type can be used only to remove temporary hardness after the water is heated, and when used with cold water must be of the zeolite type in which no chemical is applied directly to the water. Temporary hardness may be removed by the application of lime alone, but when sulphates, chlorides, and nitrates are present, sodium carbonate, sodium hydrate, or barium carbonate must be used. The entire hardness of any water cannot be removed by any of these processes, but they reduce it to a point where it is not objectionable. The Am. Ry. Engr's and M'n of Way Ass'n estimate the amount of chemicals required to remove 1 lb. of incrustating matter as follows:

The benefits derived from water softening lay chiefly in the reduction of the amount of soap required, and in the reduction or entire elimination of incrustations in boilers, heaters, and coils through which water passes to be used for bathing, laundry work, and culinary operations. Better products are made with soft water in such industries as paper making, tanning, dyeing, and bleaching.

Pressure Water Softeners-Zeolite Type.-Water which has been freed from suspended and organic matter, iron, and other interfering elements, or water which contains only small quantities of these elements, can be softened by passing it through a bed of natural or artificial zeolite 20 to 40 in. thick. The apparatus is similar to a pressure filter (see Fig. 1) When the softener has passed a stated quantity of water, depending on its hardness, it has to be shut down, drained out, and filled with a 10% brine solution. The period of contact is usually 12 to 16 hr., depending on local conditions. The amount of salt required to regenerate the zeolite, according to some authorities, is 3 to 6 lb. per 1000 gal. of water for each 100 parts per million of equivalent calcium carbonate. This process is the only one known that reduces the hardness to zero when properly cared for. The method, however, is not suitable for boiler water, as it leaves the equivalent of sodium carbonate which will cause foaming.

Some of the advantages of the zeolite process are:

1. It is the only practical process by which water of a zero hardness can be produced on a large scale. 2. Only one chemical is needed (common salt).

3. Variations in the hardness of the raw water are automatically taken care of.

4. There is no sludge to be removed.

Disadvantages:

1. Cost of operation is higher than with lime and soda ash.

2. Water to be softened must be perfectly clear, for if it contains turbidity, the pores of the zeolite become clogged.

3. The zeolite softened water contains residual sodium bicarbonate and if used in boilers may cause trouble by foaming.

TABLE 5.-REFINITE Co.'s ZEOLITE WATER SOFTENERS-VERTICAL PRESSURE TYPES

--

TABLE 6. REFINITE Co.'s ZEOLITE WATER SOFTENERS-HORIZONTAL PRESSURE TYPES

* On water of other hardness, capacity is inversely proportional to total hardness in grains per U. S. gal. The salt consumption is approximately 4 lb. per 1000 gal. per grain hardness.

20. Interpretation of Bacterial Count.-The fact that a water may contain a large number of bacteria means little unless some knowledge is had of their kind and characteristics. There are numerous varieties of water forms which are perfectly harmless if taken into the system. On the other hand, a very small number of typical sewage bacteria found in a majority of samples, especially in 1-cubic centimeter samples, should be looked upon with suspicion. Colon bacillus appearing in a few samples of 10 c.c. with no other characteristics of pollution should not be taken as indicative of an unsafe water. Water that is grossly polluted will usually show large numbers of bacteria growing on both gelatine and agar together with liquifiers, gas producers, and the presence of colon bacillus in 1-c.c. and 10-c.c. samples. Waters that are occasionally polluted will show some or all of these evidences a majority of the time. Therefore, the purity of a given water should not be determined upon a single analysis when that analysis reveals the fact that there is a possibility of contamination.

21. Disinfection and Sterilization.-A water supply is sometimes found where there is no objectionable organic matter present, but which may be subject to contact with pathogenic bacteria. In such cases no treatment such as filtration is necessary, but it is advisable to treat the water with a germicidal agent such as calcium, sodium hypochlorite, or liquid chlorine. These oxidizing agents may be applied in amounts varying from 0.2 to 0.3 parts per million of available chlorine without detriment to the water.

Where a water is pumped from a source, such as above described, direct to a distribution system, it is customary to apply the dose in the suction pipe, the rate of application being controlled by the rate of pumping.