Part 12 (1/2)
The water-works were designed to supply 40,000,000 liters (10,582,000 gal.) daily, which it was a.s.sumed would be sufficient for all future developments in Monterrey for a population of 200,000 at a per capita consumption of 200 liters (about 53 gal.) per day. The present population of the city is given as less than 90,000, there having been an increase of 22,000 in ten years (1891-1901), but it is evident that in the last ten years (1901-1911) this rate of increase has not continued. Taking into account all the data known to the writer, it does not seem that the city will attain a population of 200,000 in a great many years, if it ever does; but this is a matter of personal opinion, and is only stated as such.
The present requirements of the city's population, a.s.suming that each person uses 200 liters (53 gal.) per day, would be, at that rate, which is a very liberal one, only 18,000,000 liters (4,762,000 gal.) per day, or less than half the amount which may be provided.
If the water were not to be metered and the sewage discharge paid for by measure, it is possible that the free use of water might lead to the usual waste with which all are fairly familiar; but the use of meters, and the rates charged, will reduce the water consumption to a minimum. This end will especially result from Section 5 of the Tariffs which provides that:
”Groups can be formed of two or more small houses so as to obtain a joint service under the proportion shown in the tariff.”
This provision will keep down the per capita supply, among the majority of the people, to about 37-1/2 liters (10 gal.) per day. A similar provision led to abuse in Santiago de Cuba, as well as in other Cuban cities, where one householder, taking water, frequently delivers it to adjoining houses and tenements through rubber hose. As many as ten or twelve families are sometimes found to be supplied from one tap in this manner. Indeed, it may be stated as a rule, having but few exceptions, that where water is paid for by meter its use is always restricted.
The water mains and distribution system, however, are so well laid out, and the whole design is so good, that the writer would not antic.i.p.ate much difficulty because it is on rather too liberal lines for the present or probable future. It may, perhaps, be argued that it may cost more to keep the mains in such a system clean; but this extra cost will scarcely be of much moment, and will be offset by the greater lasting quality of the larger pipes. There is another feature of the problem, however, which is not affected favorably by a too liberal forecast of the per capita water supply, namely, the sewerage system.
If it is a.s.sumed that, using 200 liters per capita per day, the total water supply of the city for the present population will be 18,000,000 liters, and that this may double in fifty years, or even amount to 40,000,000 liters in that time, it would seem that a rather liberal provision has been made for the water supply, and that this will scarcely be exceeded by the sewage, for the latter must come from the water supply, there being little or no ground-water and no storm-water taken into the sewers. Designing the sewers to flow half full for all diameters less than 18 in., and seven-tenths full for all larger sizes, it would seem that this would give ample capacity for all time to come in such a city, and that good practice would not exceed these figures, it being more desirable that the sewers should not be too large to work well, than that they should be large enough in all places to meet every possible contingency.
If all the sewers of a system are too large, the condition is incurably bad; while, if a few miles prove to be too small, on account of growth and prosperity not antic.i.p.ated by the designer, it will be easy enough to relay such parts when this becomes necessary.
Mr. Conway states that:
”The sewers are designed on a very liberal basis, namely, on the a.s.sumption that when flowing half full the quant.i.ty to be dealt with will be 380 liters [100 gal.] per capita per day, with a maximum rate of flow of 200 per cent.”
If the writer understands this statement correctly, it means that the sewers, flowing half full, will carry 380 liters per capita in 12 hours, or are designed with 200% of the capacity required to take the a.s.sumed flow in 24 hours.
It was a.s.sumed that each house would be occupied by 7 persons and have a frontage of 12-1/2 m. (about 41 ft.), that is, about 700 gal. per day per house, the maximum flow rate being 200%, or at the rate of 700 gal. per house in 12 hours.
It is to be remembered that nearly all the houses are of one story, and that, as a rule in tropical and sub-tropical countries, the per capita use of water diminishes with some function of the increasing number of inhabitants in one house. Most of the water is used in the kitchen, and where there are 7 persons instead of 5, the quant.i.ty used by the smaller number will generally serve the larger.
The writer is unable to understand how this quant.i.ty of sewage will be produced, especially as the author states that, as far as the company is concerned, it is limited to the removal and disposal of the sewage, and is not required to provide for storm-water. He also states that:
”Apart from that fact, however, the best system for a city like Monterrey, where rainfall for many months at a time is very scarce, is the strictly 'separate system'.”
The minimum velocities in the sewers, when running full, vary between 0.91 and 1.5 m. (from 3 to 5 ft.) per sec., and will be the same flowing half full.
From the foregoing data it will be observed that:
(1) The water supply is the only source from which sewage flow is antic.i.p.ated;
(2) The water supply is very liberally estimated at 200 liters (53 gal.) per capita daily;
(3) For purposes of sewer design, the daily flow of sewage expected (all of which is derived from the water supply of 200 liters per capita) is estimated at 380 liters per capita, with a maximum rate of flow of 200% (or at the rate of 760 liters per capita), and with this quant.i.ty the sewers are designed to flow only half full;
(4) The gradients are such that a velocity of from 3 to 5 ft.
(0.91 to 1.5 m.) per sec. will be secured in the sewers flowing half full with the above quant.i.ty of flow per capita.
The writer does not agree with this method of computation, as he feels sure that it will give sewers which are too large, with grades too steep for the best obtainable results. His experience, extending over more than twenty years in sewer design and hydraulic work, convinces him that the method pursued is wrong in principle.
The principles involved in sewer design are first of all hydraulic. The quant.i.ty of flow, in the nature of things, cannot be forecasted accurately; success depends on getting the nearest possible approximation to average conditions. If 200 liters per capita per day is a liberal allowance, and 40,000,000 liters per day is a liberal expectation at this rate for double the present population, and the sewers are designed to flow half full only, why should this again be doubled?
The design of a sewer system for a city such as Monterrey is, in fact, a very difficult problem, especially as the quant.i.ty of sewage will be very limited, flush-water will have to be used in considerable quant.i.ties, and water in that part of the world is precious at all times and often scarce.
Under these circ.u.mstances, the size or shape of the pipes selected for the lateral sewers, should have been such as would more nearly agree with the requirements than does the 8-in. circular.
A. P. Folwell, M. Am. Soc. C. E., writing of the 8-in. circular size, states:[10]
[10] ”Sewerage,” by A. P. Folwell, M. Am, Soc. C. E.