Page images
PDF
EPUB

Sampling point. The most convenient location which is sufficiently distant from the point of chlorine application to insure adequate and effective mixing of the chlorine and water.

Tap at sampling point. The size of the tap is of no importance-possibly the smaller the better. A long nipple extending well into the main for approximately one-third its diameter, will deliver a much more representative sample than will a tap just projecting through the wall of the main.

For very large mains it has proven advantageous to have a stuffing-gland arrangement through which an adjustable length tube may be changed at will to deliver from any desired point across the diameter of the conduit or main. It has been shown that in conduits and flumes the strongest concentration of chlorine tends to remain along the bottom unless thorough mixing is first provided.

Procedure of test. 1. Allow water to waste from sample tap to clear the line. Collect a pint or more of sample slowly in a bottle or jar and mix by whirling or gently shaking.

2. Hold the sample for ten minutes in the bottle if such period has not already been available between the point of application and the sample tap.

3. Fill the empty test bottle with water from the collecting bottle. For each 50 cc. of water 5 drops of orthotolidine should be applied.

4. After a one or two minutes contact only of reagent and water, compare the yellow color developed with the standards No. 1 and No. 2. The color indicating a correct dose should not be lighter than standard No. 1 nor deeper than No. 2. The range between No. 1 and No. 2 being appreciable it is not difficult to regulate the chlorine to keep within it. No color or one lighter than No. 1 calls for an increase of chlorine. A deeper color than No. 2 is usually unnecessary and the chlorine should be reduced.

Notes concerning the ortho-tolidine method

Fading of standards. Organic growths in the standards prepared from potassium dichromate and copper sulfate may be eliminated by adding 0.5 cc. of cold saturated mercuric bi-chloride solution to each 50 cc. of standard solution. This treatment reduces also the fading of the standards so that they hold sufficiently well for eight months. The standards should be kept in a dark place.

Correct time to add ortho-tolidine reagent to the sample. With certain waters it is essential to withhold addition of the ortho-tolidine until after the chlorine contact period with the water for the full ten minutes has been insured. After the ortho-tolidine is added the rate of chlorine absorption is materially retarded and the apparent residual is higher than that obtained in practice. Allowing the full ten minute period before applying the reagent is therefore essential for some waters. Waters comparatively free of organic matter do not show this characteristic.

False color. In very alkaline waters a blue or green color may be produced when only 5 drops of reagent are added. Increasing the amount to 10 drops per 50 cc. of sample will eliminate this trouble.

In rare instances ortho-tolidine added to the raw water will produce a yellow color indicating the presence of chlorine. Manganese will cause trouble when applying the test. It is a safe procedure, therefore, to test a sample of the water before chlorine has been applied. Organic content may sometimes cause a false color reaction after a 10 minute interval. To avoid this error, readings should be made not longer than one or two minutes after adding ortho-tolidine.

The efficacy of chlorination Wherever the supply warrants preliminary purification by filtration, one school of sanitarians claim the chlorination process should be considered purely as a last line of defence. The other group proposes that the filtration process should be operated so as to deliver an effluent practically free of turbidity, color and other foreign matter. They do not insist that the effluent shall have less than 2 B. coli per 100 cc., believing that a plant turning out a physically and chemically good filter effluent will in the course of events have removed the major portion of impurities, including dangerous bacteria. For removing the remaining coli chlorination may be relied upon as an integral part of the purification process taken as a whole.

It is conceivable that certain filter plants must handle raw waters of such heavy degree of pollution that the stand taken in demanding maximum filter efficiency at all times is a tenable one.

Since the perfection and simplification of the residual chlorine test make it available for all chlorination plants, maintenance of efficient dose control should remove the largest single element of chance in chlorination and should destroy the strongest argument which has existed against the dependability of the practice. Possible failure would then occur only through discontinuity of application.

The above argument has dealt only with a water physically and chemically fit for chlorination. When a water contains noticeable turbidity (approximately 100 parts) there is some question as to whether chlorination alone should be considered a satisfactory method of rendering the water safe. In such cases filtration is usually desirable.

Of particular significance in this connection is the Report of the Committee on "Limitations of Chlorination" at the State Sanitary Engineers Conference in 1922 (Bulletin 133, U.S. P. H. S., May, 1923). It should be stated that two of the members of this committee presented a minority report in disagreement "in toto" with the majority report. The section of the report quoted as follows may be helpful.

If chlorine is continuously applied in amounts determined upon by the residual chlorine method of control satisfactory waters result, from the standpoint of bacteria content. The details of insuring continuous application by providing duplicate apparatus and reserve supply of chlorine are problems of management and not of mechanism of the process.

The proposed protective effect of suspended material upon bacteria is, so far as we know, nothing more than an hypothesis probably given some support in the past by analytical results which may have had their origin as much in improper and discontinuous control of chlorine application as in “protective effect.'

The Committee prefers to suggest no limit upon the application of chlorine on the score that (a) in raw waters the limit is reached by aesthetic objection to a turbid or colored water long before chlorination fails; (b) in filtered water good management presupposes the most effective use of all treatment processes regardless of additional aids such as chlorination; and (c) in underground waters, the limits are set by sanitary survey and history of the water rather than by defects in the chemical process.

Most surface waters should not be used without filtration. Therefore, it is better to formulate a policy of filtration than one of maximum limits for chlorination.

It is hardly necessary to establish a maximum allowable bacterial limit for filter effluents prior tochlorination, if we establish as the working principle the theorem that each part of the filtration process should be operated at its maximum efficiency.

Paradoxical as it may seem, chlorination is both a factor of safety and an integral part of water treatment when used in conjunction with filtration.

It is difficult to see why we should place any more limitations on chlorine than on any other treatment processes, inasmuch as it is subject to no more idiosyncrasies than storage or filtration.

a

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

« PreviousContinue »