be placed in the most constant connection with the external air, by making openings at every point where they can be put with safety. In London there are numerous gratings which open directly into the streets, and this plan, simple and apparently rude as it is, can be adopted with advantage whenever the streets are not too narrow. But in narrow streets the sewer gratings often become so offensive that the inhabitants stop them up. In such cases there must be ventilating shafts of as large a diameter as can be afforded, and running up sufficiently high to safely discharge the sewer air.' In some of these cases it may be possible to connect the sewers with factory chimneys.' The sewer should never be connected with the chimneys of dwelling-houses.

In making openings in sewers it seems useless to follow any regular plan. The movement of the sewer air is too irregular to allow us to suppose it can ever be got to move in a single direction, though probably the most usual course of the air current is with the stream of water, if this be rapid. The openings should be placed wherever it can conveniently be done without creating a nuisance. Some of these openings will be inlets, others outlets, but in any case dilution of the sewage effluvia is sure to be obtained. Mr. Rawlinson considers that every main sewer should have one ventilator every 100 yards, or 18 to a mile, and this should be a large effective opening.'

But there may be cases when special appliances must be used. For example, in what are called "sewers of deposit," as when the outflow of the sewer water is checked for several hours daily by the tide or other causes, it may be necessary to provide special shafts, and the indication for this will be the evidence of constant escape of sewer air at particular points.

The use of charcoal trays has not answered the expectations that were formed of them.

Inspection of Sewers.

The inspection of sewers is in many towns a matter of great difficulty, on account of the means of access being insufficient, and also because the length of the sewers is so great. Still inspection is a necessity, especially in the old flat sewers, and should be systematically carried out, and a record kept of the depth of water, the amount of deposit, and of sewer slime on the side or roof.

Choking of and Deposits in Sewers-Causes.-Original bad construction; too little fall; sharp curves; sinking of floor; want of water; check of flow by tides, so that the heavy parts subside.

Well-made sewers with a good supply of water are sometimes selfcleansing, and quite free from deposit, but this is, unfortunately, not always the case.

Even in so-called self-cleansing sewers, it has been noticed by Mr. Rawlinson that the changing level of the water in the sewers leaves a deposit on the sides, which, being alternately wet and dry, soon putrefies.

1 In Liverpool there were small shafts with Archimedean screws at the top. From the observations of Sanderson and Parkes, it appears that these screws did act, but not to such an extent as to warrant the expense.

* It seems inadvisable to erect chimneys and use fires with an idea of ventilating the sewers on a general plan, the air would simply be drawn with great force through the nearest openings. But local ventilation by a factory chimney, when gratings cannot be used, is a different thing.

* Others have recommended 1 in 50 yards.

In foul sewers a quantity of slimy matter collects on the crown of the sewers; it is sometimes 2 to 4 inches in thickness, and is highly offensive. When obtained from a Liverpool sewer by Drs. Parkes and Burdon-Sanderson, it was found alkaline from ammonia and containing nitrates.' On microscopic examination, this Liverpool sewer slime contained an immense amount of fungoid growth and Bacteria, as well as some Confervæ. There were also Acari and remains of other animals and ova.

When deposits occur, they are either removed by the sewer-men, or they are carried away by flushing of water.

Flushing of Sewers.-This is sometimes done by simply carrying a hose from the nearest hydrant into the sewer, or reservoirs are provided at certain points which are suddenly emptied. The sewer water itself is also used for flushing, being dammed up at one point by a flushing gate, and when a sufficient quantity has collected the gate is opened. An automatic system is however preferable, such as is carried out by Field's annular siphon, before mentioned.

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Almost all engineers attach great importance to regular flushing, and almost the only advantage of allowing the rain to enter the sewers is the scouring effect of a heavy rainfall which is thus obtained. This, however, is so irregular that it is but a doubtful benefit.

DISPOSAL OF THE SEWER WATER.

The great engineering skill now available in all civilized countries can insure in the case of any new works that the construction of sewers shall be perfect. If an engineer can obtain good materials, good workmen, and a proper supply of water, there is no doubt that sewers can be so solidly constructed and so well ventilated that the danger of deposits in the sewers, or of sewer air entering and carrying disease into houses, is removed.

But the difficulty of the plan of removing excreta by water really commences at the outfall. How is the sewer water to be disposed of?

This difficulty is felt in the case of the foul water flowing from houses and factories without admixture of excreta almost as much as in sewer water with excreta. The exclusion of excreta from sewers, as far as it can be done, would not solve the problem-would, indeed, hardly lessen its difficulty. In seaboard towns the water may flow into the sea, but in inland towns it cannot be discharged into rivers, being now prohibited by law. Independent of the contamination of the drinking water, the sewer water often kills fish, creates a nuisance which is actionable, and in some cases silts up the bed of the stream. It requires in some way to be purified before discharge. At the present moment the disposal of the sewer water is the sanitary problem of the day, and it is impossible to be certain which of the many plans may be finally adopted. It will be convenient to briefly describe these plans.

Report on the Sanitary State of Liverpool, by Drs. Parkes and Burdon-Sanderson, 1871. The amount of free ammonia was .025 parts per cent.; the albuminoid ammonia was .00462, and the nitric acid .2035 parts per cent. Photographs are given of the microscopic appearances of the slime in this report.

Baldwin Latham points out that there is a point of flow in all sewers when they discharge more than when running full. A good flushing power may be obtained at considerably less than the full discharge. Tables are given in his Sanitary Engineering.

1. STORAGE IN TANK, WITH OVERFLOW.

The sewer water runs into a cemented tank with an overflow-pipe, which sometimes leads into a second tank similarly arranged; the solids subside, and are removed from time to time; the liquid is allowed to run away. Instead of letting the liquid run into a ditch or stream, it has been suggested to take it in drain pipes, to 1 foot under ground, and so let it escape in this way into the subsoil, where it will be readily absorbed by the roots of grasses. The fat, grease, and coarser solids may be intercepted by a strainer, and daily removed and mixed with earth. The liquid portions may be discharged periodically by means of the automatic flush-tank.' In a light soil this could no doubt be readily done; and, if the drain pipes are well laid, a considerable extent of grass land could be supplied by this subterranean irrigation. The tank plan is, however, only adapted for a small scale, such as a single house or small village, and there should be ventilation between the tank and the house in all cases. This plan is applicable to the disposal of slop-waters in villages, even when the excreta are dealt with by dry methods.

2. DISCHARGE AT ONCE INTO RUNNING WATER.

All new works of this description are now prohibited, and the plan will probably ultimately cease in this country."

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3. DISCHARGE INTO THE SEA.

The outlet pipe must be carried to low water, and, if possible, should be always under water. A tide flap opening outward is usually provided. If not under water constantly, special care must be taken to prevent the wind blowing up the sewers. The tide will fill the outfall sewers (which are generally made large) to the level of high water, and to that extent will check the discharge, and in the sewers filled with the mixed sea water and sewage there will be deposit. To remove this special attention is necessary.

See Mr. Rogers Field's evidence, Annual Conference on the Progress of Public Health at the Society of Arts, 1880.

When the sewer water passes into a river it undergoes considerable purification by subsidence, by the influence of water plants, and in a lesser degree by oxidation. Although some oxidation of nitrogenous organic matters into nitrous and nitric acids and ammonia must take place, it appears from Franklin's experiments, that in the River Irwell, which receives the sewage of Manchester, after a run of 11 miles, and falling over six weirs, there is no formation of nitrites and nitrates, and there is even an increase in the organic nitrogen (?), though the suspended matters are less (from 2.8 to 1.44 parts per 10,000) than at first. Average London sewage diluted with 9 parts of water and siphoned from one vessel into another so as to represent a flow of 96 and 192 miles, gave a percentage reduction in the organic nitrogen of 28.4 and 33.3 respectively. The oxidation of sewage appears, then, from these experiments, to take place slowly. Dr. Letheby considers, however, that oxidation takes place more rapidly, and that if sewage is mixed with 20 times its bulk of water, and flows for 9 miles, it will be perfectly oxidized. Of course, it is clear that ova, and solid parts of the body, like epithelium, might be totally unchanged for long periods,' and we may conclude that oxidation of sewage in running water cannot be depended on for perfect safety.

Reports of the Commissioners appointed to inquire into the Pollution of Rivers, 1870, vols. i., ii., and iii.

• Reports of East London Water Bill Committee (1867), p. 430, questions 732-4.

As formerly mentioned, Dr. Parkes found unchanged epithelium in unfiltered Thames water after a transit in a barrel of 180 miles, and after keeping for five months. It was transparent and worn, but quite recognizable.

VOL. II.-3.

If the sewage cannot be got well out to sea, and if it issues in narrow channels, it may cause a nuisance, and may require to be purified before discharge. In the Rivers Pollutions Act (1876) power is given to prohibit discharge into the sea or tidal waters under certain circumstances."

4. PRECIPITATION.

Another plan is not to pour the whole sewage into rivers, but to precipitate the solid part, or the greater portion of it, and then to allow the liquid to pass into the stream or over the land.

This is sometimes done by simple subsidence, the sewage being received into settling reservoirs or trenches, with strainers to arrest the flow to some extent. When the solid matter has collected to a certain amount, the sewage is turned into another reservoir, and the thick part, being mixed with coal refuse or street sweepings, is sold as manure.

The thin water which runs off must be almost as dangerous as the sewage itself when poured into streams, and consequently the prohibition to discharge sewer water extends to it also.

In order to produce greater purification, the sewage in the subsiding tanks is now usually mixed with some chemical agents which may precipitate the suspended matters.

Numerous substances have been employed as precipitants."

Lime Salts.-Quicklime (proportion 8 to 12 grains per gallon), or 1 b of lime for 600 gallons of sewage (nearly); chloride of lime, which is added to quicklime in the proportion of about 1th part of chloride to 1 of lime ; calcic phosphate dissolved in sulphuric acid, or a mixture of mono- and dicalcic phosphate with a little lime (Whitthread's patent), are said to be good precipitants. Chloride of calcium has also been recommended.

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Aluminous Substances.-Aluminous earth mixed with sulphuric acid (Bird's process); impure sulphate of aluminum (Anderson's and Lenk's processes); refuse of alum works, either alone or mixed with lime or charcoal; clay mixed with lime (Scott's cement process); natural phosphate of aluminum dissolved by sulphuric acid and mixed with lime. In all these cases the amount of the substance added is from 50 to 80 grains per gallon of sewer water.

Magnesian Salts mixed with lime in the form of superphosphates (Blyth); impure chloride of magnesium.

Carbon in the shape of vegetable charcoal; peat; sea-weed charcoal; carbonized tan; lignite; Boghead coke.

The word "stream" (into which sewage is not to be passed) is defined by section 20 of the Act, thus:-"Stream includes the sea to such extent and tidal waters to such point, as may, after local inquiry and on sanitary grounds, be determined by the Local Government Board, by order published in the London Gazette. Save, as aforesaid, it includes rivers, streams, canals, lakes, watercourses, other than watercourses, at the passing of this Act, mainly used as sewers, and emptying directly into the sea or tidal waters, which have not been determined to be streams within the meaning of this Act by such order as aforesaid."

2 An interesting account of the precipitating process is given in a book called The Sewage Question, the author of which has had the advantage of Dr. Letheby's notes and analyses. A list of no less than 57 processes or proposals is given at page 38, from which it appears that the first precipitant was proposed by Deboissieu so long ago as 1762, and was a mixture of acetate of lead and proto-sulphate of iron.

This patent was found to give good results in removing suspended matters and organic nitrogen, and the Committee of the British Association considered the process deserved "further investigation." It appears, however, to have come at present to a standstill.

Iron in the shape of sulphate; perchloride (Ellerman's and Dale's liquid); the sulphate is sometimes mixed with lime and coal dust.

Manganese.-Condy's fluid.

Zinc sulphate and chloride.

The deposit obtained from any of these processes is collected and dried. It is usually dried on a hot floor, a stream of hot air being allowed also to pass over it. There is some little difficulty in drying it, but this is now being overcome. Of these various precipitants the best appear to be the aluminous preparations; the crude sulphate of aluminum prepared by Dr. Anderson, of Coventry; the solution patented by Mr. Lenk; the A B C process of Mr. Sillar, which consists of alum, blood, charcoal, and clay;' and Mr. Forbes's sulphuric acid solution of natural phosphate of aluminum. All produce rapid subsidence of the suspended matters, and clarify the liquid to a very great extent. The sulphuric acid also tends to prevent decomposition of the deposit. In using these substances the sewage water is received into a tank or well, and there, or on its way thither, receives the precipitating agent, which is generally mixed by means of a screw or turbine. After thorough mixing, the precipitate is allowed to subside, and the superabundant water is run off. The deposit is then dug out and dried. After drying the deposit appears to possess some agricultural value, and to be saleable at a price which, in some cases, leaves a small profit. The profit is never large, and in some instances there has been even a loss. The clear water from all these processes contains ammonia and oxidizable organic matters, as well as phosphoric acid (in most cases) and potash, and it would thus appear that a considerable part of the substances which give fertilizing power to sewage remain in the effluent water. The caustic lime process, when properly applied, appears also to be a powerful precipitant, but the deposit has no agricultural value.

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The metallic precipitants of various kinds (iron, zinc, manganese) are more expensive and less useful. Blyth's magnesian process was unfavorably reported on by Mr. Way.

When the sewer water is cleared by any of these plans, is it fit to be discharged into streams? In the opinion of some authorities, if the precipitate is a good one it may be so, and it appears certain that in many cases it is chemically a tolerably pure water, and it will no longer silt up the bed or cause a nuisance. But it still contains in all cases some organic matter, as well as ammonia, potash, and phosphoric acid." It has, therefore, fertilizing powers certainly, and possibly it has also injurious powers. No proof of this has been given, but also no disproof at present, and when we consider how small the agencies of the specific diseases probably are,

1 The proportions are stated to be, 6 grains of alum, grain of blood, 20 grains of clay, and 6 grains of charcoal, to 10,000 grains of sewer water. Sometimes a little lime is added.

Thus the pro

? This never exceeds one-third of the theoretical or chemical value. duct by Anderson's process at Coventry is estimated theoretically at 16s. 94d. per ton; the practical value is only 5s. 6d. to 8s. 4d. See Dr. Voelcker's Reports, in the Report of a Committee on Town Sewage (1875), p. lx. et seq.

* Many analyses are given in the First and Second Reports of the Rivers Pollution Commissioners, from which it appears that on an average the chemical processes remove 89.8 per cent. of the suspended matters, but only 36.6 per cent. of the organic nitrogen dissolved in the liquid. Mr. Crookes's analyses show that the A B C process, when well carried out, removes all the phosphoric acid. Voelcker's analysis of the effluent water treated by the acid phosphate of aluminum shows that it contains more ammonia than' the original sewer water, less organic nitrogen by one-half, and less phosphoric acid; it is pure enough to be discharged into streams.