THE PURIFICATION OF WATER BY OZONE
Ozone is the ideal agent for purifying, since it leaves behind it only ordinary oxygen and nothing foreign to the water. It was first tried for this purpose by Fröhlich, and Ohmüller proved that it energetically attacked bacteria in water from which any excess of inert organic matter had been previously removed. These experiments followed the construction of large industrial ozonizers by Siemens and llalske at Berlin, who in 1898 erected an experimental plant at Martinikenfelde, and afterwards larger installations for the towns of Wiesbaden and Paderborn. Air ozonized to 2½ to 3 grams per cubic metre passed upwards through a tower filled with flints, and met a descending current of roughly filtered water. The cost was about 2)4d. per 1,000 gal. of water treated, and a very impure water was sterilized down to 2 to 9 organisms per cc. Earlier, in 1893, Tindal started his apparatus at Oudshoorn, Holland, and in 1893 worked it experimentally at Paris, also at Brussels and Blankenberghe, and it was adopted for limited supplies at several other places. The estimated cost was 0.43d. per 1,000 gal. The bacterial reports of Van Ermengcm, Marmier and Roux were satisfactory. Tindal’s apparatus obtained intimate admixture and duration of contact by passing the water and ozonized air either in the same or an opposite direction through towers divided by perforated diaphragms “or other equivalent dispositions”; another form had pulverizers or spray jets; and the partially exhausted air at the exit could be dried, re-ozonized, and returned. Subsequently, in 1897, appeared the Marmier-Abraham and Otto patents; the former had a mixing tower filled with flints or bricks, the latter injected the water and ozonized air together by means of a pulverizer called an “emulsor,” and later (1905) added a column of flints through which a second current of air was ascending. These patents are now amalgamated, and are at work on the town supplies of Nice and Dinard, and (experimentally) at St. Maur, Paris. Tindal’s previous patent, after his death, was acquired by De Prise, who has improved the apparatus and process, and has introduced into his plant an ozonizer identical with the Siemens-Halske form, and an ozone-recuperating circuit.
In September and October, 1908, I examined in detail the working of De Fuse’s installation for the sterilization of water by ozone at the St. Maur municipal waterworks of the city of Paris. These municipal waterworks furnish a large portion of the water supply of Paris, drawn from the river Marne, a stream which receives a considerable amount of sewage pollution above the intake. At the time of my visit the river was full, with a sluggish flow; the water was greenish-brown, slightly turbid, had a faint marshy odor, and contained large numbers of aquatic organisms.
At the works, for the ordinary supply, the water is pumped into sedimentation reservoirs, then drawn off and filtered through gravel and sand. I found that the filtrate had no odor, was clear, and in small quantites was practically colorless; in bulk it had a varying tint of greenish-brown. On the surface of the water in the reservoirs, and in the channels, gelatinous green masses containing water algae were frequent. The straining beds of gravel and sand did not completely remove these organisms, and the water which passed through them, although generally clear, was by no means pure or safe, as shown by my bacterial and chemical investigations, the results of which are recorded in the appended tables. It is obvious that further precaution is imperative, that the present reliance on filtration alone implies constant danger from pathogenic bacteria, and that sterilization, as far as it can be practically effected, is an urgent need.
For this purpose ozone has always seemed a most suitable and natural agent for two chief reasons: (1) It is only a more active form of ordinary oxygen, already existing normally in small quantities in the atmosphere, and continually assisting in natural oxidations. (2) It adds no residue of any chemical to the water, and in doing its work of purification it is simply resolved into ordinary oxygen, which dissolves in the water and adds slightly to its aeration.
Experience and researches have proved that it is more economical and practical to supply the ozone in a state of large dilution with air, avoiding contamination with nitrous compounds or other impurities, and in a series of careful tests I observed that the Siemens-De Frise ozonizers now in use at these works yield the gaseous mixture in a very pure state, as will be seen by my subsequent record of chemical results.
The plant consists of an efficient arrangement for the production of ozonized air and for its intimate admixture with the water. The curent was generated by means of a 45-horsepower steam engine and Mordey alternator of 110 volts, and changed by a transformer to 4,000 volts.
The ozonizers were of the Siemens-Halske type, placed in a dark cool room. In my experiments from Monday, September 28, to Tuesday, October 6, there were six of these ozonizers running. On Wednesday, October 7, I caused the number of ozonizers at work to be increased to nine, in order to ascertain what effect would be produced in the treatment by an increase of the electrical power employed. I have given the electrical factors in table I. which also notes the amount of ozone in the air entering and in the water discharged. The ozonizers were working throughout in a very steady and quiet way and without sparking. Careful tests showed that the power was also kept almost regular. The electrical energy required for the production of the ozone during my experiments averaged 1.31 kilowatt hours per 104 cubic metres of water sterilized, equal to 57 B.T.U. per 1,000,000 gal. In addition, the energy required for compression, 1.675 kilowat hours per 100 cubic metres (or 70 B. T. U. per 1,000,000 gal.), must be added to the cost, giving a total of 133 B.T.U. per 1,000,000 gal.
T understand that M. Colmet Daage, the chief engineer to the Paris municipality, taking .11 fr. as the price per kilowatt hour (being that charged by the Societe de l’Est Parisien), estimates the average cost of treatment at .0184 fr. per cubic metre. He further estimates that for treatment of 300 cubic metres at an electrical cost of .055 fr. per kilowatt hour, the expense would only be .0072 fr. per cubic metre, say one-third of a penny per 1,000 gal., excluding interest, amortization, and repairs.
The ozonized air was introduced laterally into the stream of filtered water in a proportion which was kept constant throughout the run. the volume of filtered water being 104 cubic metres per hour, equal to 22,880 gal., and that of the ozonized air 42 cubic metres per hour (1,483 cu. ft.), so that the volume of ozonized air supplied averaged 40.4 per cent, of the volume of filtered water treated.
During the main portion of the trial, from September 28 to Ocotber 6, I found that the content of ozone in the air supplied was almost constant, the lowest amount in grammes of ozone per cubic metre of air entering being 1.45 on the morning of September 30, and the highest 1.85 on the morning of October 3. with an average of 1.63. This would be a proportion of ozone in the water of 0.658 grammes ner cubic metre in the proportion used at present of 100 volumes of water to 40.4 of air.
The water and. air are drawn together, by means of an injector, into the bottom of a sterilizing tower. This is a vertical cylinder of enamelled cast iron constructed in sections, and divided at regular intervals by horizontal diaphragms consisting of carefully levelled and finely perforated trays made of celluloid, with holes .7 mm. (.026 in.) diameter, whereby the ascending curent of gas and water is intimately mixed. In addition the height of the column is sufficient to make the air and water meet under an extra pressure of from 10 to 15 lb. per square in., which facilitates the absorption of the ozone. The columns are provided at intervals with small glass windows for inspection, and with cocks for collecting samples. T noticed that the process of solution of the air was indicated by the bubbles becoming smaller and less numerous as the tower was ascended, and in my chemical results it is shown that the oxygen dissolved in the filtered water entering is usually about 3 to 4 cubic centimetres per litre, whereas the escaping treated water is practically saturated with oxygen at the temperature, containing constantly about 6’/> cubic centimetres per litre.
I his fact in itself is a considerable advantage for the following reason: The aeration test, commonly accepted as a valuable index of the character of a natural water or an effluent, shows that such a liquid is not ordinarily in a healthy condition unless it contains nearly its full complement of dissolved oxygen. If this be deficient it is a general sign that organic matter is using it up by the agency of bacteria, which is an objectionable feature, as aeration alone is antagonistic to many pathogenic species. 1 have found that the amount of dissolved oxygen in the filtered water from the Marne is much below saturation, and my results are in agreement with those obtained by Drs. Ogier and Bonjean in 1904, hence the phenomenon appears to be constant, and points to stagnation and a want of aeration in the filter beds.
The aerated and ozonized water flows from the top of the cylinders into an open collecting tank of white enamelled cast iron. During its passage through the columns it parts with a considerable portion of its ozone in oxidizing the organic matter of the water, but it still retains a small excess of ozone dissolved, and in order to utilize this as much as possible in the purification (lie lank is divided into three chambers, so that the water flows through submerged orifices from the first compartment to the second, and then over the top of another partition into the third. Itt the discharge pipe just before entering into this reservoir a small exit, always running, and carefully protected by a glass bell jar, is placed for the periodical collection of samples. The physical features 1 observed constantly in the tank were: (I) The change in the appearance of the liquid was very striking, from the dull greenish brown of the filtrates to the bright, sparkling and colorless or faint bluish tint of the treated water. (2) T lie odor of residual ozone which was dis tinct in the first compartment had decreased in the second, and at the same time a large number of bubbles of gas were evolved; in the third the water hail become almost inodorous. I allude later to my further observations on the slow continued action of ozone after the first effect.
A feature of the method is that in order to economize the fraction of ozone that passes away with the residual air from the top of the cylinders the process is worked in a cycle, and the escaping gases arc strained, dried, cooled, re ozonized, and used over again. To replace the gas absorbed, an automatic valve just above the top of the cylinder opens at very frequent intervals to admit fresh air.
COLLECTION OK SAMPLES FOR ANALYSIS.
I have taken special care that the samples should be fair and representative. Those of the filtered water were gathered from the underground collecting tank into which the mixed water from the filter beds flowed. I found that this filtrate required twenty to thirty minutes to reach and traverse the ozon iing plant, therefore 1 took my samples of ozonized water from the sampling bell jar already mentioned, thirty minutes after collecting the filtered water.
The course of the inquiry involved a number of chemical and physical questions which I was able to satisfactorily elucidate.
1. As to how far the temperatures are affected in the process. The solution of the gases in water, the oxidation of the organic matter by ozone, and the compression, all nd to produce an increase of temperature, vvhiie evaporation during the transit would effect a decrease. Atmospheric conditions might also have some influence. I therefore took observations of the temperatures of the filtered water throughout, and of the atmospheric conditions from October 1 to 7.
The results prove: (a) That that water under treatment instead of being heated is cooled to a slight extent, and that it issues at a constant temperature independent of that of the filtered water, or of atmospheric conditions during the time, (b) That the cooling arrangements in the plant are working efficiently.
2. The question as to whether any corrosion of the apparatus by ozone occured which could occasion metallic contamination of the water. My tests in this direction were entirely negative.
3. The possibility that oxides of nitrogen, chlorine compounds, or peroxide of hydrogen might he formed in the process and remain in the water. That this is not so is established by my daily tests as recorded in table 1. page 37; neither the chlorine nor nitrate show any increase ove r the amounts naturally present in the filtered water: nitrates are usually absent, but are occasionally present in traces both in the filtered and in the ozonized liquids; a faint trace of hydrogen peroxide was discovered in only one case, on October 1. The hardness was not ailered. Saline ammonia in about an equally minute trace was sometimes found both in the filtered and treated waters.
4. The physical characters are greatly improved, and the organic matter, as shown by the oxygen-consumed figure, is considerably reduced. The average of 14 determinations showed a reduction from .125 parts per 100,000 to .072 parts, corresponding to a removal of Hi per cent, of the organic matter as judged by the oxygen-consumed standard. if this oxygen (.125—.072=053 parts) divided itself between the hydrogen and the carbon of the organic matter in the approximate diagram matie manner CH»-4-0*=M»0-|-C0i, it would produce a weight of COa very nearly equal to its own (48 to 44). or .0486 parts per 100,000 of ( (),. corresponding to 0.486 grammes, or about 25o cubic centimetres, of carbonic acid per cubic metre of water. As a matter of fact, I found by experiment that the current of air in ascending displaced not only this carbonic acid but also a portion of that naturally pres ent in the filtered water, so that the treated water contained a little less carbonic acid, and hence had a slightly lower acidity, but it was not therefore less aerated, as the loss of CO., was more than compensated by the increase in the dissolved oxygen.
5. The amount of ozone absorbed. When the ozonized air is kept regularly at 40.4 per cent, of the volume of filtered water treated, the ozone divides itself into three portions: (a) oxidizing the organic matter in the water, (b) dissolving in the water and passing out with it into the collecting tank, (c) escaping with the air curent at the top of the tower to be used again in the cycle. I have determined the ozone left in the escaping air with the fol lowing results:
On October 6. with six ozonizers as usual, the proportion of ozone in grammes per cubic metre was:
In the air entering at the bottom. 1.68
In the water entering (40.4% of 1.68=). .679
in the air 40.4′, of .440. . .157 Kscaping .185
in the water .048
I’setl in oxidizing the organic matter.494
The filtered water showed considerable fluctuations in the total number of bacteria (organisms growing at 20° C.). and during the period corresponding to the continued dry weather a steady improvement in this respect was observed. The frequency of coli organ isms during the period was about 1 per 100 cc. in the filtered water, out of twenty-two tests coli was found to be present eight times in to cc.
The ozonized water was found not to be sterile, but showed an average bacterial content of t.l per cc., these coresponding to innocuous spore-bearing organisms of the subtilitype.
Entirely negative results were recorded in all cases for coli organisms in the ozonized water, no growth whatever being obtained in the MacConkey medium. During the testing a total volume of 1.580 cc. of the ozonized water was submitted for the coli test, with a maximum amount of 200 cc. at one test. The frequency of organisms in the filtered water, not producing gas but showing acid growth in the MacConkey medium at blood heat, reach ing at time 1 per to cc. of water, is noticeable. This class of organism, while not having the significance of coli, is undesirable. They in variably accompany contamination to a greater extent ‘hart true coli, anil in my opinion their frequency in the filtered water is an unsatis factory sign.
In order to exclude any possible interfer ence due to the presence of ozone, the treated water was not examined until at least several minutes after collection. To demonstrate any retrogade action in the ozonized water by short storage, samples of the filtered and ozonized water were kept (I) twenty-four hours at 0° C., and (2) forty-eight hours at room temperature. The ozonized water showed coli absent in both cases from 100 cc. (the maximum amount tested), and the filtered water showed coli present in 40 cc. and 50 cc. respectively.
The results of the bacteriological examination prove, as a general conclusion, that the ozone treatment destroys all but the more resistant spore forms of bacteria present in the filtered water: coli and allied intestinal organisms are entirely eliminated, and subsequent contamination being excluded, there is no recrudescence of this class of organism. The ozonized water during the period of the tests showed a uniformly constant degree of sterility as against a very’ variable bacterial content of the unozonized filtered water.
The water as leaving the sand filters at the St. Maur works, and augmenting the Paris supply, is decidedly inferior to the London water supply. Taking the coli test with 100 cc. of water as a method of comparison, the former gives positive results approaching too per cent., while all London waters for the year ending March, 1908, showed 58.6 per cent, of positive results on this basis (presumptive coli bacili test). The results with ozonized water -how that it iof far greater bacterial purity than that obtained by any adopted method of sand filtration or storage: the ozone treatment produces water which is superior to the high class London waters obtained from the Kent deep wells, or that have been subjected to natural filtration.
Am ICATION OF THE METHOD.
It will now be useful to inquire under what conditions the ozone treatment of water for drinking purposes is indicated, and in this connection to make a comparison between the relative circumstances existing in Paris and in London. Both towns derive their drinking supplies partly from rivers and partly from wells, the latter giving water which has hitherto been regarded as of sufficient purity to require no artificial purification, while the riverwaters have for many years past been subjected to sand filtration. The amount of purification which a sand filter is capable of ensuring depends upon the original purity of the river water to be treated: and as in both cases the rivers have been getting more and more contaminated with the growth of the upper riparian populations, it has been found that this purification does not now result in a water which is equal to the standard of deep wellsupplies, and consequently there is no method of ensuring a given purity without largely aug menting the number of existing filter beds and at the same time maintaining a very efficient bacterial control. Thus in London at the present time the water board have 160 filter beds, the greater number of which are in constant operation, and despite the very great number of samples examined day by day at the laboratories, water is constantly passing into public supply’ the quality of which has not been bncterially determined. In Paris, awe have seen, it has been proposed to safeguard the purity of the water by an ozone treatment after the existing sand filtration, while in London during the last few yearthe water hoard has proposed to adopt storage of the crude river water for a length of time sufficient to ensure the destruction of all or nearly all the typhoid bacilli, and so to prepare a water by this “safety change” that the existing filter bedwill continue to do satisfactory work. So much is this the case that the advisers of the water board of London are of the opinion that a well stored rapidly filtered water is likely to be safer than an unstored slowly tihered water. From the work already done in this direction, it would appear that in London the average impurity of the present raw ri ‘T waters is of such a character that no less than two months of storage would be necessary in order to ensure the death of all typhoid bacilli, and. as a compromise, it isuggested that provision should be made in London for a four weeks’ storage before filtration. It is obvious that the length of time required to improve the quality of the water up to this safety’ standard is greater than the number of days which are necessary to ensure a sufficiency’ of quantity, and Dr. Houston, in his report, specially points om that after having provided a storage capacity of four weeks a slow rate of filtration should still be maintained at the existing filter works, so that no economy in the cost of filtration as ordinarily practiced is to be expected, at any’ rate until after the new regime has had sufficient trials. But stored water may behave very differently to the unstored xvaters hitherto treated. The time taken to form the “Schmutzdecke” will probably be longer, as there is less sediment in the stored water, and hence the time during which the filter delivers water at the proper rate will be less than under present conditions. It would seem, therefore, if the conclusions of Dr. Houston are correct, that the proposal to increase the storage of the river water to four weeks must he accompanied by aji increase of the number and area of the filter beds, if the rate of filtration is still to he maintained as at present. In order to ensure the storage of all the w’ater .supplied to London it is necessary to lecollect that this amount can only be taken from the rivers Thames and Lea in the absence of both high flood and of drought, and consequently the existing storage reservoirs, at present holding nearly forty’ times the daily average supoly. will not only require rearrangement, but also suitable additional works before they’ can fulfil the conditions laid down. Hence the subject resolves itself into an engineering problem, and an inquiry whether the outlay on these additional works will secure the same quality’ as an ozone treatment of the existing water as it leaves the filters.
I understand the Paris municipality have decided to ozonize the whole of their Marne supply at St. Maur, equal to 90,000 cubic metres per day. The water to be treated will pass through the e xisting filters at twice the present rate, 16 ft. instead of 8 ft. per twenty-four hours. An ozone installation capable of treating 10.000,000 gal. per day (half the above quantity) by the system described in this paper will be erected at St. Maur, involving no further outlay for storage or filters. The whole of the water passing through the filters, whatever its coli content, will be ozonized at an expenditure of 135 B.T.U. per 1,000,000 gal.
We have seen that under the ozone treatment an immediate chemical test gives the safety factor which is required for successful working in such a xvay that no doubt can be raised as to the purity of the water: but on the other hand by the present dependence on storage and filtration, although the safety change may be assumed to take place in the proposed reservoirs, the uncertainty of the quality of the water on leaving the filter beds still remains.
All towns wuth a river water supply have at present conditions similar to those of Paris and London, and the reasons for an ozone treatment seem, therefore, applicable. Many other places whose conditions of supply involve drawing on wells which may be subject to occasional or periodic pollution could by this treatment ensure a guaranteed purity.
My general conclusions as the result of this investigation are that:
- The ozone treatment of a filtered river water as carried out at St. Maur, Paris, works by the De Frise process is a satisfactory method of ensuring a standard of purification for a municipal water supply equal to modern requirements.
- The De Frise process differs, in my opinion. from all previous attempts at water sterilization by means of ozone in combining the De F’rise sterilizer and ozone recuperation system with the Siemens-De Frise ozonizer. which combination avoids the difficulties attending the use of the emulsifiers and sprayers which have been hitherto used for this purpose, and is attended with a low cost for renewals owing to breakages in the dielectric.