good effluent, is estimated, it will be found that the crops have taken up a very small percentage of nitrogen indeed. Besides, it is doubtful whether all the nitrogen in the crops. is derived from the sewage. The researches of Nobbe and Hiltner on leguminous plants have shown that by the aid of certain bacilli small tubercles are formed on the roots of the leguminous plants which enable them to take up nitrogen from the air.

In many of the cases which have been quoted as instances where the total nitrogen has been greatly diminished in passing sewage through land, and where it is assumed that the nitrogen has been absorbed by the crops, it will be seen that the chlorine has also diminished, the fact being that the sewage has become diluted with subsoil water. The analyses published by the British Association of the sewage and effluent from Breton's farm, Romford, the Croydon farm, and Merthyr Tydvil, are all open to this criticism. It also applies to Sir Edward Frankland's experiments on the Barking farm, as reported in 1871. In this case 70 per cent. of the nitrogen came away in the effluent; but it is wrong to assume, as has been done, that the other 30 per cent. of the nitrogen was absorbed by the crops, for the chlorine was diminished 34 per cent., an amount which certainly could not be absorbed by vegetables, and a figure which clearly shows that the effluent was greatly diluted with subsoil water.

It can be shown that there is a certain manurial value in the ammonia in smoke, and when urging upon manufacturers the necessity for using smoke-preventing appliances, this fact has ingeniously been adopted as an argument why black smoke should be allowed to continue or even be encouraged. The argument is quite as sound as that of the money value of sewage.

When the English reports as to the money value of sewage went across the Atlantic, Forbes replied to them by asking, if a bottle of brandy were poured into a barrel of water, whether the mixture would be worth as much as the original brandy; while Professor Storer pointed out that Philadelphia stands upon a bed of clay which contains a pound of gold in every 1,224,000 pounds, from which data the money value of the gold in the clay within the limits of the city of Philadelphia is not less than 100,000,000 dollars, but no one would dream

of attempting to extract it. These American criticisms are strictly to the point. Agriculturists can get chemical fertilizers in a form in which they can be readily used for less money than the labour costs to apply the sewage in a manner in which it can be absorbed by the crops.

We must not lose sight of the fact that the nitrogen in the sewage has to be converted into nitrates, or nitrites, before it is assimilable by the crops, and to do this it must be applied with equal volumes of air, and the cost of the labour in so applying it must be taken into the estimate. Liebig's fantastic picture of Britain as a vampire on Europe, sucking out its life-blood and pouring it into the sea, was conceived without taking into consideration the fact of the enormous quantities of nitrogen which annually are imported into the country in the form of beef, mutton, corn, and nitrates, nor did it take into consideration the enormous quantities of fish brought from the sea to the land, and the large amounts of carbonate and sulphate of ammonia annually manufactured in the country at the various gas-works, which are now used as fertilizers.

It should also be remembered that, under the influence of electrical discharges, nitrates can be produced, if required, from the nitrogen and oxygen of the air.

When we bear these facts in mind, and also that leguminous plants have the power of absorbing nitrogen from the air, and that the sea requires its nitrogen to be replenished, on account of the fish taken from it, the fallacy of the manurialvalue argument becomes too ridiculous to entertain for one

moment.

CHAPTER VI.

PRECIPITATION, PRECIPITANTS, AND TANKS.

Limitations of the process-In practice solid matters only removed-Soluble matter removed by excess of precipitants-Comparative costs of precipitants Lime, alum, copperas-Various patent precipitants-Precipitation without tanks-Brewery sewage-Effects of excessive doses of lime -Absolute rest tanks-Continuous-flow tanks-Dortmund tank of Kine bühler-The Candy tank-Sludge, and methods of dealing with it.

By precipitation is meant the deposition of the insoluble matter in suspension in sewage, together with a certain proportion of the organic matter in solution, by the addition of some chemical which forms insoluble compounds with it, and at the same time deodorizes the sewage. For a considerable time it was claimed by one school of experts that by precipitation sewage could be sufficiently purified to be discharged into any river, while a rival school advocated the claims of land alone. To-day the limitations of each are recognized, and it is generally admitted that by precipitation practically only the solid matter in suspension is removed.

The chemicals which have been used for this purpose are almost innumerable; but only three need seriously be considered, viz. alum, copperas, and lime, or some combination of these. Most of the precipitants sold under fanciful trade names are composed of these substances in varying proportions. The general plan of the process of precipitation is for the chemical to be added to the sewage; the mixed chemical and sewage being then well mixed up by means of waterwheels or baffling plates. The mixture is afterwards made to flow into a large precipitation tank, where it is either allowed to rest for a few hours, or is passed continuously through the tank at such a slow rate of velocity that the suspended matters fall to the bottom of the tank, where they form a dark sludge, containing about 95 per cent. of water, the clarified effluent

overflowing from the tank. Details of different forms of precipitation tanks are given on subsequent pages.

The use of lime as a precipitant has been much prejudiced by the reports which have been published on the so-called "lime process" as it was carried out years ago, the effluent from the precipitating tank merely being passed into the river without any irrigation or filtration. It should be at once clearly stated that the effluent obtained by any process of precipitation is not fit to be turned into a stream without subsequent biological filtration. For practical purposes, it may be assumed that the precipitation process will merely remove the solid matters in suspension. It is true, as Mr. Dibdin has shown, that in the laboratory from 10 to 30 per cent., and even more, of the organic matter in solution can be removed; but it is a safe rule to assume that in the working of a sewage purification scheme by precipitation the suspended matters only are removed.

The following table gives the results obtained by Dibdin with various precipitants upon the organic matter in solution in the Metropolitan sewage. The results are expressed as a reduction per cent. in the oxidizable organic matter :

In addition to Mr. Dibdin's experiments, the Massachusetts. State Board of Health have made careful experiments on the relative value of various precipitants, and the proportion of

organic matter in solution removable by them. The following table gives in a condensed form some of the most practical of their conclusions, the quantities of the precipitants used being in each case of the same value, about 1s. 3d. per annum per head of the population dealt with :

Crude sewage yielding

...

0-40 part per 100,000 of albuminoid ammonia. 0.28 straining 0.26

after settling

copperas and 700 lbs. of lime per

million gallons

400 lbs. of ferric sulphate

...

100,000.

With regard to lime, what takes place when it is added. to sewage is that carbonate of lime is immediately formed; this acts as a weighting material, entangling the flocculent matters in suspension and carrying them down to the bottom of the tank with it. The lime also forms an insoluble compound with a certain amount of organic matter in solution, which also is carried down.

There is no doubt that the carbonic acid in the sewage holds a lot of carbonate of lime in solution, and if the tank effluent is allowed to pass into any stream with a short run the carbonic acid is given off, and the carbonate of lime is deposited as a white insoluble substance in the river. This is what is sometimes called the secondary decomposition of lime effluents, and was one of the objections to precipitation with lime without further treatment. If, however, the tank effluent obtained from precipitation with lime is irrigated over a large area, the carbonate of lime is deposited in the soil, where it is split up by the nitric acid formed by the nitrifying organisms, a stable nitrate of lime being formed in its place, and no secondary decomposition will result.

Another objection which there was to the use of lime as a precipitant, without subsequent intermittent filtration, arose from the danger of the sewage being overdosed, so that some free lime might escape into the rivers and thus kill the fish.