removal of these metals by contact beds or filtration. For this purpose only slight aeration is required and excessive aeration is often distinctly detrimental. However, ground water is usually high in carbonic acid and frequently contains objectional amounts of hydrogen sulfide. A high degree of removal of these gases requires efficient aeration. Ground waters, devoid of oxygen, furnish an excellent example of the "exchange" of gases by aeration, for the degree of CO2 removal is closely related to the extent of oxygen saturation produced.

Methods

For securing aeration two general methods are available, first, by pumping air into the water through perforated pipes, strainers or porous plates; and second, by exposing water to the atmosphere, in the form of thin sheets, small streams, drops or spray.

Aeration by applying air under pressure is not generally practiced because less efficient and less economical than by letting the water drop. The method has proven useful, however, in emergencies where the plant layout would not permit the use of the other method. It has obvious advantages in cold northern climates where cascade or spray devices would not be feasible.

The efficiency of aeration by agitation of the water in contact with air depends upon the head utilized, the time of contact and the fineness of subdivision. Often a drop of 1 or 2 feet will give desired results, though heads commonly utilized range around 5 feet. Sometimes drops of 10 feet and over are provided even if the water has to be pumped.

Of the numerous aerator designs in use the simplest and most common type is a vertical riser pipe on basin inlet. Multiple riser pipes give better aeration by subdividing the flow into thinner sheets. Special caps or spreading aprons are frequently added for the same purpose. Auxiliary pans, shelves or steps add to the efficiency by increasing the spattering effect and the time of contact with the air. The pans may be solid, perforated or slotted. Many aerators instead of getting distribution of water by riser pipes use distributing flumes with weir effects on one or both sides. The thin sheet may discharge directly into a basin, down a succession of steps, or into successive weir troughs. In one well-known type, the water from the distributing flume discharges down an inclined

plane studded with iron plates arranged in herring-bone fashion. There are a few installations where the velocity of the water is utilized to draw air by inspirator effect into the water.

Trays filled with coke or stone are used for aeration at a number of installations, chiefly in connection with iron removal, where aeration and contact action are combined, or where compactness is desirable.

Where head is no consideration, or where maximum efficiency is called for, it is doubtful if any aeration device can compete with the spray nozzle, which gives a much better dispersion or subdivision of the water than can be obtained by any other method. Any degree of fineness may be obtained by a spray nozzle varying from atomization up to coarse drops, but for practical reasons only the larger size nozzles having an orifice of one inch diameter and larger are used. Spray nozzles require a minimum head of about 11 feet, but much better results are gotten from higher operating heads. Possible damage to adjacent property from wind-carriage of water has to be considered in connection with spray aeration.

Principal data relating to typical spray nozzle installations are given in table 8. Where underground waters are pumped by air-lift, the incidental aeration is quite effective, both as to oxidizing soluble iron and in removing a substantial portion of the carbon dioxide and hydrogen sulfide gases. Often no further aeration is needed.

Performance

In the case of surface water supplies, it is difficult to place a definite value on the improvement brought about by aeration. Such installations are generally made for the specific purpose of removing or reducing taste and odor, which objectionable qualities are obviously not susceptible of satisfactory numerical appraisal. However, a certain degree of removal is nearly always evident from the distinct odors in the vicinity of such aerators. Chemical determinations are of little value in indicating the actual benefits. Analytical data available shows in general that (a) the organic content represented by the nitrogen and oxygen consumed figures undergoes no change; (b) carbon dioxide is reduced materially with consequent raising of pH value (decrease of hydrogen ion concentration); and (c) dissolved oxygen is increased, possibly to supersaturation.

It is well to remember in connection with the aeration of surface

water supplies that there are numerous purification works provided with optional aeration where the aeration feature has not been found of sufficient value to justify operation of the device. The experiences where aeration has been installed in connection with filtration would seem to indicate that filtration, generally speaking, is a more effective barrier against tastes and odors than is aeration alone, particularly where microscopic organisms are involved.

In the case of ground waters the results of aeration may be expressed quantitatively with some satisfaction, in terms of the dissolved gases displaced. Table 9 gives some results of aeration at a number of typical iron removal plants handling ground water.

The primary purpose of natural subsidence is to remove turbidity, particularly the coarser particles more economically removed thus than by coagulation. Subsidence devices are of two general types: (a) storage and impounding reservoirs having retention periods stated in weeks and months and (b) artificial basins having comparatively small retention, stated in minutes, hours or days.

Storage reservoirs are primarily water conservation devices intended to make available a larger proportion of stream runoff. However these storage basins or impounding reservoirs have a distinct value in clarifying, decolorizing and reducing the bacterial content of the water. Such purification in large reservoirs often gives a satisfactory quality of water without the aid of additional clarification processes, as in the supplies of the Metropolitan Water District (Mass.), of New York City and of Los Angeles.

The following discussion deals only with artificial subsidence basins installed as a distinct purification step for the purpose of reducing the load on subsequent processes, particularly filtration. Practically all of the larger slow sand filter installations include natural subsidence as preliminary to filtration.

With rapid sand filtration plants natural subsidence is not so generally used, because the basins furnished in connection with coagulation take care satisfactorily of relatively high turbidity.

Where the water carries high turbidity during a considerable part of the year plain subsidence is distinctly valuable ahead of coagulation in saving chemical and increasing the useful runs of subsequent basins. Thus practically all of the purification plants taking supply from the western tributaries of the Mississippi River include natural subsidence as a separate purification step.

Note: From published results.

* Plant abandoned in favor of filtered surface supply.

Retention period. The retention period for plain subsidence is governed by the amount and character of the suspended matter, the degree of clarification required and the nature of subsequent processes. The period varies from a few hours to several days. The retention period of small basins must take into account the depth occupied by the deposited mud between cleanings. Depths in excess of 20 feet are usually uneconomical for artificial basins, whereas basins shallower than 8 feet are likely to be inefficient due to scouring of material from the bottom, or disturbance by wind action.