Part 12

Mr. Pitkethly's apprehensions as to the adequacy of the system of ventilation adopted have not been realized, in part perhaps because the houses, though generally of only one story, have such high ceilings that the tops of their vent pipes are generally higher than the ventilating columns at the heads of the branch sewers.

GEORGE ROBERT GRAHAM CONWAY, M. AM. SOC. C. E. (by letter).--The writer regrets that some features of the works described in this paper have failed to call forth the many useful criticisms which he expected, and his remarks, therefore, are limited to the few points which have been raised. He is particularly indebted to Messrs. Schuyler, Meyer, and Saucedo for adding supplementary information of value to the paper, but regrets that he cannot support Mr. Binckley in his claim that "the entire general design of the system, as well as the extensive hydrological studies and final selection of the sources of water supply, was completed in 1906," etc. On May 1st, 1907, when the writer assumed responsibility as Chief Engineer, he was unfortunately confronted with the fact that very little data and only a few preliminary and incomplete plans were available. His first duty was to report upon the final sources of supply, and the recommendations made in his report (dated July 12th), received Mr. (now Sir William) Mackenzie's approval during the same month. The final plans, upon which the approval of the State Government was definitely obtained, were prepared by the writer during the summer of 1907, were submitted to the Governor of the State, Gen. Bernard Reyes, on October 19th, and received his approval on December 12th, 1907. No works, with a long preliminary history, such as those at Monterrey, can rightly be said to be due to any one individual; many engineers contributed to the final result, and the writer willingly acknowledges his indebtedness to the able men, who, for ten years prior to the construction of the works, investigated the particular problems which were met--problems which were not only of an engineering and physical nature, but racial and financial. The responsibility of constructing the works in their present form, and leaving them practically complete, did, however, fall on the writer's shoulders.

Messrs. Pitkethly and Hammond have criticized the basis of the calculations upon which the sewer system was laid down. In considering this problem, it is necessary to remember that, in designing this system, there was practically no information upon which to base the calculations; and the writer believed that the wisest course was to anticipate a liberal growth, and provide a large margin of safety. In designing a sewer system in older and well-established communities, the engineer is generally able to compile considerable information regarding the probable sewage flow for which it is necessary to provide. In Monterrey this quantity was absolutely unknown. The writer's practice in other places has been to assume that about 8% of the total daily discharge of sewage will flow off in one hour; and, from many curves which he has plotted regarding sewage flow in British towns, this rate appears to him to be approximately correct. In Monterrey, however, the old Mexican traditions are rapidly changing, and the city is now becoming one of the most Americanized in Mexico; the old one-story houses will give way in time to buildings of several stories--a change, already noticeable, which has occurred during the past few years, particularly in the business portion of the city. Taking these facts into consideration, it is believed that it would be, not only bad engineering, but bad business, for a company whose concession lasts 99 years, to provide sewers as small as 6 in., as Mr. Hammond would recommend, and then be called upon, under the terms of the concession, to relay larger sewers at a future date, thus incurring further capital expenditure upon which no Government guaranty would apply, and no further revenue be obtained. In matters of this kind, not only the engineering, but the commercial, aspect of the question must be kept in view, and this point, the writer must frankly admit, he has always seriously considered.

The writer's experience with reference to the method of ventilating sewers by tall columns extends over many years, and he still maintains that no other system gives such satisfactory results. In this view he finds considerable support in a recent paper on "Salisbury Drainage," by Mr. W. J. E. Binnie,[11] written since the system at Monterrey was installed, in which the result of a series of experiments carried on during 1906-07 are given. At Salisbury, England, 68 ventilators, 6 in. in internal diameter, 30 ft. high, were connected to the main sewer by 6-in. stoneware pipes. They were placed about 540 ft. apart, and, from careful anemometer readings, the following conclusions were reached:

[11] _Minutes of Proceedings_, Inst. C. E., Vol. CLXXXI, p. 317.

"(1) That four ventilators all lying in the lower portion of the town acted sometimes as air-inlets and sometimes as air-outlets, and that the other sixty-four acted as air-outlets.

"(2) That the average velocity of the air escaping up these columns was 3.2 feet per second, representing the circulation of 3,600,000 cubic feet of air per diem, or sufficient to change the air in the sewers every 10 minutes.

"(3) That the average velocity of the current of air in the ventilating-column increases with the size of the sewer to which it is connected, averaging 2.4 feet per second with the 7-inch sewer, 3.6 feet per second with the 9-inch sewer, 3.7 feet per second with the 12-inch sewer, and 4.1 feet per second with the 15-inch sewer in these experiments.

"(4) That the draught in the column is very largely dependent on the wind, being at its minimum on a still day, and could therefore be readily increased by the use of a suitable cowl.

"(5) That the draught is very little affected by the sewer-gradients. It was expected that, in ventilating-columns placed in connection with the upper end of a sewer laid at a steep gradient, a strong draught would have been obtained. No direct connection, however, was traceable."

As the result of these experiments, Mr. Binnie rightly came to the conclusion that this system of ventilation was efficient.

Mr. Hammond anticipates that the house connection trap system at Monterrey will lead to bad results, but the writer has seen the system at work in many widely different cities with excellent results. He believes that it is in accord with the best practice of the most eminent sanitarians during the last 20 years, and has no apology to make for introducing that system in Monterrey.

Regarding Mr. Hammond's summary of the advantages of concrete pipes for sewer construction, the writer is in entire agreement, and would willingly have introduced them throughout the whole of the Monterrey system, but for the fact that it was an exceedingly difficult matter to obtain suitable sand for their manufacture during the early days of construction, and considerable delays would have arisen if a complete network of such pipes had been used. His later experience at Monterrey, when the sand difficulty had been solved, clearly showed that concrete pipe could be laid down at much less expense than fire clay.

Both Mr. Pitkethly and Mr. Hammond refer to the system of liquefying tanks used at Monterrey preparatory to turning the sewage on the irrigation lands, and both express doubts as to their efficiency. The writer is now separated from his library and notes by many thousands of miles, and cannot quote "chapter and verse" as accurately as he would like, in order to support his views that the system adopted was adequate for dealing with a system such as that at Monterrey. It must be pointed out that not only was it intended to prevent the sewage from becoming a nuisance, but that the sewage flow plus a large quantity of surplus water was intended to be used profitably for irrigation purposes. With that object, the Company--or rather its allied Company, the Monterrey Railway, Light, and Power Company--obtained the control of 2,246 acres of the very finest arable land, with almost perfect natural drainage conditions, so that this land could be utilized to create a profitable revenue from the use of the sewage. The outfall sewer was accordingly designed to carry sufficient water and sewage to irrigate about 2,500 acres of land, which area could be considerably extended if necessary at any future time.

Most authorities now agree that before turning sewage upon land, a preliminary treatment is required to remove as much as possible of the suspended matter, and then reduce the latter by subsidence in liquefying or septic tanks, so that the quantity remaining in the effluent is so small and finely divided that it may be readily decomposed and oxidized by bacterial action without risk of clogging the surface or interstices of the land upon which it may discharge.[12]

[12] See Raikes, "Sewage Disposal Works," pages 97-98.

Mr. Pitkethly quotes Messrs. Watson and O'Shaughnessy as saying, in their evidence before the Royal Commission on Sewage Disposal, that not more than 10% of the solids are digested in septic tanks, but it must be remembered that in many other places evidence was given before the same Commission showing that from 25 to 30% was actually obtained.

Mr. J. D. Watson, in his paper, "Birmingham Sewage-Disposal Works,"[13] read in March, 1910, points out that:

[13] _Minutes of Proceedings_, Inst. C. E., Vol. CLXXXI, p. 259.

"The much-maligned sewage-farm still may be allowed (where the conditions are favourable) to rank as one of the best methods of sewage-disposal. Diverse opinions may be held as to what are favourable conditions, particularly as conditions are sure to vary widely with locality; but it may be assumed that where there is 1 acre of suitable land per 100 persons, as in Berlin and several other important cities, the efficiently-worked sewage-farm, when judged solely by the standard of the effluent produced, is still in the front rank. Effluents from such a farm are remarkable for their paucity of micro-organisms, their low albuminoid ammonia, and their unvarying character."

Assuming that not more than 2,000 acres of the irrigated land at Monterrey were available for sewage purposes, this area would represent the sewage treatment of the present population of not more than 45 persons per acre, and on the basis of the design, that is, for a population of 200,000 persons, this represents not more than 100 persons per acre. In many sewage farms on the continent of Europe, the number treated per acre varies between 80 and 200 persons; for example, at Breslau it is 187, at Berlin 105, at Brunswick 88, and at Steglitz 185.

Regarding the crops to be grown on the land, very satisfactory results were obtained from growing Indian corn, and two excellent crops per annum were taken from an area of 500 acres during the period in which the writer was responsible for the works. It was also his intention to grow alfalfa, and turn a part of the land into a pecan grove, and, although he does not share the apprehensions of danger of either Mr. Pitkethly or Mr. Hammond as to growing root crops, he believes the growth of alfalfa, Indian corn, oats, barley, and pecan and fruit trees is eminently suitable for the land, which is a deep rich loam, from 4 to 8 ft. deep, overlying the "sillar" formation referred to in the paper. The writer has seen many sewage farms during the last 18 years, upon which root crops of excellent quality have been grown, and not the least suspicion has ever been raised regarding their use.

In reference to the adoption of the monolithic form for constructing the South Reservoir, the writer is so convinced as to its economy that had he to build this reservoir again, he would adopt the same method. Mr. Binckley, in drawing attention to the method of construction, has overlooked the fact that the cost of forms for a reservoir 30 ft. deep was a very serious item, and warranted the adoption of this new method, not only on account of economy but because of rapidity of construction; while, in the case of the Obispado Reservoir, which is very much shallower, simpler forms could be and were adopted.

Mr. Saucedo's remarks regarding the repetition of the extraordinary floods of August, 1909, in September, 1910, are particularly interesting, and show how abnormal conditions are in so dry a section of Mexico as the State of Nuevo León. These two floods, the writer believes, are among the most instructive in North America, particularly when one remembers that prior to 1909 the average rainfall during a period of 15 years, was less than 22 in. per annum.

TABLE 18.--COMPARISON OF VOLUME OF FLOODS, ETC.

+------------------------------+-----------+----------+-------+--------+ | | | Maximum |Cu. ft.| Annual | | | Drainage | recorded | /sec. | amount | | River. | area, in | flow, in | per | of | | | square | cu. ft. |square | rain- | | | miles. | per sec. | mile. | fall. | +------------------------------+-----------+----------+-------+--------+ | Santa Catarina, Monterrey, | | | | | | August 27th, 1907 | 544 | 235,000 | 432 | 22 | | Estanzuela, near Monterrey, | | | | | | August 28th, 1909 | 3.5 | 2,900 | 825 | 25 | | Tansa, India | 52.5 | 35,000 | 666.7 | 101 | | Krishna, India | 345 | 118,000 | 342.6 | 258 | | Coquitlam River, Vancouver | 100 | 12,000 | 120 |147-189 | | Sweetwater, Cal. | 186 | 18,150 | 99 | ... | | Delaware, Lambertville, N. J.| 6,820 | 250,000 | 36.5 | 45 | | Colorado, Austin, Tex. | 37,000 | 123,000 | 3.3 | 24.5 | | Ohio, Cairo, Ill. | 214,000 | 700,000 | 3.3 | 54.9 | +------------------------------+-----------+----------+-------+--------+

Table 18, compiled by the writer, shows how very extreme the floods of 1909 were compared with those on other rivers, while those of 1910, referred to by Mr. Saucedo, although not so great, would appear to have reached a rate of flow of about 300 cu. ft. per sec. per sq. mile of the drainage area.

The writer agrees with Mr. Saucedo that in the semi-arid regions of Mexico and the Southern States, and also in India, the possibility of these abnormal floods is an important consideration in the design of hydraulic works.

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Changes To This Document

Transcriber's Note: The table of contents has been added. Blank pages have been deleted. Illustrations may have been moved. Discovered publisher's punctuation errors have been corrected. Some wide tables have been re-formatted to narrower equivalents with some words replaced with commonly known abbreviations and possibly a key. Some ditto marks have been replaced with the words represented. In addition, the following changes or corrections were made:

p. 501: but the tampers had had[del 2nd had] previous experience p. 508: shown on Plates VI to IX[VI, VII, VIII, IX[to accomodate links]] p. 516: at this place there is a considererable[considerable] area p. 538: based on the following rates and and[del 2nd and] percentages p. 579: by crossing the river, build-the[building the] reservoir p. 550: [For Table 14: added "Total materials cost"] p. 566: respectively (Fig. 5)[(Fig. 4)], together with lack of p. 584: [Table 17 renamed to Table 18 to avoid duplication.] p. 584: Table 17[18], compiled by the writer, shows how very extreme

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