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(2) The air supplied to intermittent filters may be more than five times the volume of the sewage treated, therefore more highly oxidised effluents are attainable.

(3) The filter does not deteriorate if properly made, the only plugging which takes place being on the surface.

It will be seen that, on the whole, the advantages rest with percolating filters; there are, however, circumstances in which contact beds may be preferred; this may be the case where the subsoil is so stiff a clay that the beds may be watertight without any retaining walls, and where there is little fall.

The advantages of the bacterial treatment of sewage is that the smallest populations can be as effectually treated as the largest towns, the area of ground required being small in comparison with land treatment.

In private houses the sewage varies greatly in strength and quantity ; sudden flushes of water from baths causes too quick a flow through the tank. This has to be provided against by ponding the sewage in the tank, allowing only a small flow to pass through an aperture, provision being made to pass any further increase or excessive volume by an overflow pipe.

Grease must be rigidly kept out of the tank. This is done by providing efficient grease traps to catch the slop water from the kitchen and pantry. The soapy water from laundries must also be treated in the same manner, and the hot water should be cooled off by having the traps of sufficient size to permit of this. These traps require to be cleaned of the grease frequently.

The great difficulty in all these small installations is to overcome the smell which emanates from the effluent of the Septic Tank when being distributed on the filter beds. In many cases it has been necessary to construct these installations close to the dwellings, sometimes within 40 feet, and, of course, if any smell, it is highly disagreeable to the occupiers. As it is impossible to get rid of the smell entirely, I find it best to enclose the filter bed in entirely on the top, and to have a fresh air inlet and an outlet, which can be carried up, say, 12ft., or fixed against some adjoining wall or building, when any smell will be carried up this vent pipe and distributed without any annoyance. This is only necessary when the installation has to be put down in a confined space.

Percolating filters are, I think, more suitable for small installations, as an effluent is obtained by one bed quite equal to that obtained from double contact, and the cost is less in construction.

The best distribution is obtained by Stoddart's trays. The liquid is brought into a gutter, overflows the margins, and, on reaching the under surface, meets with a series of drip points, from which it drops upon the filter. There are no mechanical parts about it, so it is impossible to get out of order from this cause, and the only attention required is the occasional brushing out of the troughs. Another good point is that it is completely unaffected by variations of flow, whether great or small.

The effluent from the filter bed can be used for watering the garden. A cubic yard of filter is required for 15 persons; where the drainage from stables is taken in, it is usual to allow one horse equal to five persons.

The bacterial system is very adaptable for native compounds, a great saving being effected in the annual cost, compared with the pail system.

The initial cost of an installation and drainage varies from 17/6 to 20/- per native ; this includes drainage, building of latrine, and purification works.

In the latrine troughs containing water are provided ; two troughs are allowed for a thousand boys, but six will be sufficient for four thousand. These latrines are each about 22ft. 6in. long, and contain 6in. of standing water. At regular intervals these are flushed out by a discharge of water by automatic cisterns of 100 gallons each, all the contents being washed into the drains. Urinals are also provided. The floors of the building are laid with granolithic pavement, which can be easily washed down, so that perfect cleanliness is obtained.

The drains conduct the sewage to the Septic Tank; after passing through the tanks the effluent is distributed on to the filter beds. The distribution is effected by revolving sprinklers, of which there are various types on the market. Fixed sprinklers are also used, which spray over the bed ; these are good when the flow is constant, and sufficient head of water is obtainable.

If the filtrate is required to be very clear, and the water returned to be used over again in the latrines, it is advisable to have straining beds. These are filled with sand, in which the liquid is filtered through at a constant head. Any flocculent matter is thus caught, and on the flow becoming too slow the bed must be drained off, and the film formed on the top of the sand raked off and thrown on one side. The filter will then be ready for work again.

The working cost of this system is very small, as after the first charge of water, which must be at least 15 gallons per head of population, only a small amount is required per diem for freshening purposes. This is often obtained from the washing out of the kitchens and that used in the cooking, so that the only direct charge is the pumping back of the water.

The purification effected by the filter in an intallation of this kind is shown by the following table : Oxygen absorbed 4 hours. Alb, ammonia. Nitric nitrogen.

In grains per gallon.

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For the information of those who are unfamiliar with analytical methods, it may be mentioned that the reduction of organic ammonia in the filter, compared with the tank effluent, affords a very good index of the decrease of the putrescent matter, and therefore of the efficiency of the filtration. Also the amount of nitric nitrogen in the filtered effluent indicates the extent of conversion of putrefactive nitrogenous matter in the tank effluent into non-putrefactive nitrogenous compounds, so that the higher this figure is the more active, as a general rule, is the purifying power of the filter.

The question of sewage disposal is a highly technical one, and only those who have had practical experience can imagine the difficulties which have to be overcome, and therefore Local Authorities and others who have this matter to deal with should be careful before committing themselves, unless advised by experienced engineers, as a method of disposal which may answer in one case may prove useless in another. It is important to remember that it is a chemical and bacteriological, as well as an engineering question, and the chemical composition of the sewage, which I have previously mentioned, varies greatly, and has to be taken into consideration in forming an opinion as to the method which is likely to prove most satisfactory.

In conclusion, I wish to say that the evidence given before the Royal Commission of Sewage Disposal has been of the greatest value to the Sanitary Engineer, but until the Commissioners' final report is published we are not in a position to say which of the bacteriological methods at present recommended is the best ; and no doubt, as I mentioned above, this to a large extent is governed by the cir·cumstances in each case ; neither can it be said that we have arrived at finality in our knowledge as to the best means of availing ourselves of Nature's processes, but that we are on the right lines is perfectly certain.

FERRO-CONCRETE

By H. KESTNER, C.E.

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There can be no question that the use of reinforced concrete

a building material has made steady progress in England, America, and on the Continent during the last few years, so that one is justified in asking, Why has this building system been so much neglected in South Africa ?

To arouse some interest and to draw the attention of engineers to this system of construction will be the object of this paper.

The practical use of ferro-concrete is of course known to every engineer, and it is only the theory which has not kept pace with the progress of this form of construction, about which there still exist very vague and erroneous views, especially with regard to the statical action of this method of construction. The reason that the theory has not kept pace is on account of the rapid development of ferro-concrete building.

A method of calculation conformable for the purpose must, as far as possible, correspond with the existing statical action of both materials forming the ferro-concrete, i.e., concrete and iron. Only under these conditions can safety and economy be gained. Very much depends on the kind of calculation adopted, and as to whether the formulæ are very long and intricate, or simple and easy of comprehension. It must at once be confessed that the various theories about ferro-concrete in the vain hope of obtaining complete accuracy, make the calculations so intricate that they are completely useless for the purpose they are intended. Every engineer who is a friend of this new building system, and is desirous of assisting its progress, should avoid these faults and only choose the simplest formulæ.

The most accurate theories which always consider the elastic qualities of both constituent materials, concrete and iron, in every point, have their fundamental rules always connected with certain presuppositions, and this disqualifies them of the term accurate, but on account of the care which has been applied in the solution of the question they are the most meritorious theories at present we possess.

In the present state of the theoretical investigations, it is in my opinion more justifiable to adhere to a theory connected with presuppositions instead of adopting a

fallacious accuracy.

It naturally depends entirely on the more or less careful investigation of the basis of this theory. This will be the object of my paper, and I will make use of the facts which experience has taught us.

CO-OPERATION OF CONCRETE AND IRON. In the theoretical investigation of ferro-concrete it is generally accepted that the concrete is in a position to follow the iron in its changes of form through tension, compression or other strains. we place instead of the modulus of elasticity of the iron the letter Ee, and of that of the concrete the letter Ec, we assume that the elastic strains of the reinforcement, and of the concrete,

a

which adjoins the reinforcement, are in the proportion of Ee to Ec. Most of the theorists have accepted this fundamental rule as basis of their formulæ, without investigating the correctness. It is, however, of great value to examine it more closely in order to prove whether this fundamental rule can be accepted or whether it is possible to disregard it completely.

Experience has not given us many facts to answer these questions. Harel de la Nae, a French Engineer, assumes that in a ferro-concrete body under strain the cross-section shows a point of contraction, but only then when the limit of elasticity of the concrete has been reached. According to this engineer's assertions, the slipping of the reinforcement within the body can either be caused by the elongation of the iron when it reaches the limit of elasticity, or by shearing of the concrete. These assumptions are completely proved by loading tests which were carried out to the breaking of test pieces. Should the breaking of such a test piece occur in the centre, the adhesion of the concrete to the iron ceases through the elongation of the iron. Should, however, the test piece break near the supports, the reinforcement is dislocated by the shearing forces. *This proves that the adhesion of the concrete to the iron is attacked by external forces which at a certain limit destroy the adhesion. In any case it is most interesting to know whether these forces give such an elastic change to the form of the body before reaching the limit, that both materials cannot be considered as co-operating.

From the purely theoretical point of view it is evidently inaccurate to assume that the fibres of the concrete and iron lying close together are in the same way pulled or compressed.

Even when shearing stresses resulting from the external forces are not appearing, the diversity of the material must necessitate a particular connection between concrete and iron at the point where both materials come into contact with one another. It cannot be disregarded that the theory of elasticity can really only be applied to bodies of the same material. The theory of the strength of materials which does not permit of the consideration of all irregularities in the shape of the homogeneous bodies, is more difficult to apply to the elastic change of form of those bodies which are built up of irregular voids, cracks and widely differing materials. Differently composed bodies do not therefore allow a purely scientific investigation. If we now consider the shearing strains which under the influence of the load come into action at the point of contact of the iron with the enveloping concrete, then we find a fresh reason for the probable disconnection of the concrete and the iron. At another page of my paper I will refer to the fact that the crosssection of a homogeneous body under a bending moment must be subjected to changes of form through shearing forces. In the ferroconcrete body the shearing force, which the iron exercises on the concrete, must have a reaction of the same kind. Experience does not give us any facts regarding the value of the modulus of shearing elasticity, and on the other hand such elastic changes of form are not taken into consideration in the calculation of the homogeneous

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