The Canadian Engineer, Volume 30. February 10, 1916. pg. 227
In The Canadian Engineer for December 2nd, 1915, there appears an abstract of an International Engineering Congress paper on “Disposal of Suspended Matter in Sewage,” contributed by Mr. Rudolf Hering, D.Sc, of New York. We are now able to present a very able discussion of this paper, submitted by Dr. Gilbert J. Fowler, of Manchester, who replies to some of Mr. Hering’s comments in an interesting way that will be appreciated by many of our readers who have been closely following our articles and references to the activated sludge process of sewage treatment.
Dr. Fowler states that experience at the Withington sewage works of the Manchester corporation bears out much of what Dr. Hering has stated in regard to the operation of the Imhoff tank. The necessity for periodically stirring the scum is a somewhat serious matter; if mechanical agitation is to be used it will introduce complication and cost. The statement that exposure to the air tends to increase putrefaction, appears a priori open to question, and he would like rigid scientific evidence on the point. His experience with the Imhoff tank has confirmed him in his belief that the final solution of the sewage problem is not to be found in processes involving anaerobic action but on the lines of aeration, putrefaction being avoided at every point.
The history of the development of what has come to be known as the “activated sludge process” is carefully given in the first paper by Messrs. Ardern and Lockett (Jour. Soc. Chem. Ind., No. 10, Vol. xxxiii., May 30th, 1914.)
The articles which have recently appeared in American technical journals, describing experimental work at various centres, are clear evidence that the work of the English investigators marks an advance on anything previously accomplished.
It is a matter for satisfaction that the interchange of scientific work on both sides of the Atlantic should eventuate in progress for the general good.
The question of priority where so many workers are involved is of small importance in itself. When, however, statements are made by Dr. Hering and others which obscure the scientific understanding of the process, it is important that they should be corrected.
It is quite true that he (Dr. Fowler) was much impressed by Mr. Clark’s work at Lawrence, and to the Massachusetts workers is due the idea of building up by prolonged aeration of successive quantities of sewage a growth which would rapidly purify sewage in the presence of air. But the question of expensive surfaces, difficult to construct and handle, still remained and because of this the possibilities of the process were not favorably considered by the Metropolitan Sewerage Commission with whose president the matter was carefully discussed. The writer, therefore, returned from New York considering the problem of how to bring about purification in open tanks with, at any rate, the least possible addition of costly chemical precipitants. The idea of adherent growths was therefore abandoned in favor of some process of bacterial or enzymic activity, a line of thought which had previously been present in the mind through a suggestion by Dr. Maclean Wilson (Jour. Soc. Chem. Ind., No. 23, Vol. XXX., p. 1348, 1911.) From this line of thought was developed what has come to be known as the “M7” process, which was described in a paper by the writer and E. M. Mumford at the Congress of the Royal Sanitary Institute at Exeter in July, 1913.
By this method bacterium was made use of, discovered in colliery waters, termed “M7,” which had the property of precipitating hydrated oxide of iron from solutions containing salts of iron, together with organic matter.
Sewage from which the grosser solids had been re-moved by sedimentation was treated with a small quantity of iron salt and inoculated with the organism referred to, and aerated for several hours. Perfect clarification took place, and a deposit containing a very high percentage of nitrogen (as much as 10 per cent.) was formed.
The effluent from this process could be nitrified at very high rates on percolating filters.
Inasmuch as preliminary settlement of the sewage was called for by this process, with production of ordinary sludge, and as the effluent still required final treatment on filters for complete oxidation, the method, although having many advantages, did not completely realize the object of the researches.
In the development of the field experiments in connection with this process, valuable practical experience on the economical application of air was, however, gained. Contemporaneously with this work, experiments were being carried on at Davyhulme, at the writer’s suggestion, on the continuous aeration of successive quantities of sewage, as in the Massachusetts work, and these experiments ultimated in the activated sludge process described in the various papers of Messrs. Ardern and Lockett.
It is now possible to correlate the various results which have been obtained and to get some steps nearer to a proper understanding of the nature of the process. The writer’s present idea is that it can be referred entirely to bacterial activity. It was distinctly stated, in the first paper by Messrs. Ardern and Lockett that their sludge did not contain any algal growths ; the process thus differs essentially from that which was in operation at Lawrence at the time of the author’s visit and which was subsequently described in the annual report of the Massachusetts State Board of Health for 1913, p. 289 and seq.
It would appear, therefore, that the activated sludge process consists broadly of three operations : a clotting or clarifying action, a rapid carbon oxidation process, and finally, nitrification. It is probable that the first process is, to some extent, the result of the activity similar in character to “M7” organisms which was definitely shown to depend on enzymic action whereby traces of iron appeared to start the flocculation of the whole sewage. The “M7” bacillus is probably fairly ubiquitous, as it has been found that sewage containing iron and a certain amount of partially activated sludge but in which clarification has not been effected, can be made to clarify almost at once by the addition of a small quantity of properly activated sludge. Simultaneously with clarification, the organic matters in solution follow the usual course of oxidation, which takes place rapidly owing to the enormously extended area of bacterial activity. In the writer’s opinion, the outstanding advantage of the process lies in the fact that the sewage is really clarified and the process of clarification results in the precipitation of the emulsified nitrogenous matter in the sewage. This has hitherto not been arrested in any process of tank treatment, with the possible exception of certain precipitation processes which involve the addition of large quantities of costly and inert chemicals. Experiment has shown that bacterially precipitated sludge is quite extraordinarily active as a manure and there seems every reason to believe that an important step has been taken in the ultimate aim of economic sewage disposal, viz., the return of nitrogen to the land.
A great deal of research remains to be done on the conditions of activity of the sludge, both as an agent in sewage purification and as a manure, but advance is only possible by patient and exact biochemical investigation, and it is of the utmost importance that unfounded assumptions and short-cuts of all kinds, which have been responsible for so much waste of public money on sewage treatment plants in the past, should be avoided.
It cannot be too strongly emphasized that the proper treatment of sewage is a matter, in the first place, for the scientific specialist ; when he has worked out the governing facts of the situation, it remains for the engineer economically to construct the plant which fulfils these conditions.
In the present case, the engineering problem is a comparatively simple one ; it is merely to keep the activated sludge uniformly mixed with the sewage in presence of the necessary air. A large amount of work has been done in this country and also by Mr. Chalkley Hatton at Milwaukee and Dr. Bartow at the University of Illinois in collaboration with the writer, and the experimental plants of various dimensions capable of dealing with quantities varying from 60,000 gallons to as much as 2,000,000 gallons per day are in course of operation or construction.
In all comparisons of cost between one process and another it is essential that result should be compared with result. Unfortunately, this rule is not always adhered to and a given process, e.g., is said to be cheaper when on examination it gives much less satisfactory results. Where strict comparison is made the advantages of the activated sludge process are, in the majority of cases, beyond question, and the writer considers that any further large expenditure on works of the conventional type is — in view of the results already obtained — seriously to be deprecated.
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