CHAPTER VIII

AERATION

The advantages of aeration were investigated in this country as early as 1883 by Leeds in connection with the Philadelphia Water Supply (An. Reps., Water Dept., 1883-5). The subject was studied carefully by Dr. Drown in 1891 (An. Rep., Mass. Bd. Health, 1891), who made a very clear statement of underlying principles, needing little modification in light of later experiences. In connection with a report to the New York Board of Water Supply by Hazen and Fuller in 1907, Whipple made extensive laboratory experiments to determine the aeration effects as to oxygenation, removal of carbonic acid, hydrogen sulfide and aromatic oils. His results in condensed form are given in the 1913 Jour. N. E. W. W. A. and in the 1914 edition of "The Microscopy of Drinking Water." In spite of these investigations and subsequent discussion in the water works and engineering journals, there still exists a popular misconception as to the function and limitations of this process which is incorporated in a large number of water purification plants.

Surface water supplies ordinarily are not deficient in oxygen to a troublesome degree, and therefore are not helped by increasing the oxygen content. The benefits from aeration are generally due to the removal or "sweeping out" of certain gases and volatile organic substances which give the water unpleasant taste or odor, or which affect its "aggressiveness" toward iron pipe. The increase of dissolved oxygen in such cases is incidental, for it has been shown that inert gases can serve equally well in the "sweeping out" process. The removal of organic matter in water by a short intimate contact with atmospheric oxygen is entirely negligible.

Sometimes water drawn from sluggish streams or the deeper intakes of reservoirs is deficient in oxygen to the extent of having stagnation odors and high color due to soluble iron and organic matter. Aeration benefits by sweeping out the gases of decomposition, although oxidation of the iron is also accomplished. Similar conditions exist in polluted streams, ice-bound for protracted periods. With

surface water containing much organic matter and showing a deficiency of oxygen, it is advantageous to aerate, even though stagnation effect may not be noticeable, for the oxygen added serves to defer or postpone anaerobic conditions which would later take place, as in a basin of large retention period.

Aeration of surface water to eliminate tastes and odors incident to the growth or disintegration of microscopic organisms, or "algae," is provided much more frequently than for any other purpose. The aromatic substances responsible for such tastes and odors are difficult to remove, but experience has shown that aeration is very helpful for such conditions, success depending much upon the particular organisms involved. Besides reducing tastes and odors associated with plankton growths, violent aeration, as by spray nozzle, may actually destroy certain fragile organisms, though too much dependence should not be put on this effect as a practical means of plankton control.

Certain obnoxious trade wastes present difficult problems for aeration. The phenols from coal or wood distillations, for example, are particularly disagreeable to taste and smell, even in very high dilutions. Aeration is seldom effective to the extent of avoiding complaint, where these and many other industrial wastes are concerned. Aeration has been found helpful in connection with petroleum wastes.

Aeration of the raw water immediately after addition of coagulant is employed in several plants, with reported success in decreasing the carbon dioxide content and materially accelerating coagulation.

Aeration of the filtered water is not so generally practiced as aeration of the raw water, mainly on account of head requirements. The former has advantage in reducing the carbon dioxide and "aggressiveness" of the water to the minimum possible without lime or soda treatment. Some recent important purification works include both primary and secondary aeration as additional protection against corrosion or against taste and odors from algae. There is some evidence that the same degree of aeration is more effective with filtered water than with raw water containing turbidity and organic matter. Areation is capable of removing a substantial proportion of "residual chlorine" resulting from disinfection with this chemical.

In the case of ground water containing iron or manganese in solution, chemical oxidation is a necessary preliminary step to

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