Before the Corporation of Oldham decided to install this system upon a practical scale searching experiments were conducted with the resultant manure, to determine its plant-feeding value. It was the promise of being able to find such a ready market for the ultimate residue which constituted one of the attractions of the process. Experiments were conducted at several farms with various produce, and these proved that the manure gives remarkably good results and is more effective than any other plant-feeder containing the same proportion of nitrogen, potash, and phosphates. Finally it contains an ingredient which is absolutely missing from every chemical fertilizer. The latter is certainly a plant food, but it is imperative that the ground should be treated with a certain quantity of organic matter to assure the physical and mechanical working of the soil. Decaying organic matter fulfils this end admirably, as one would suppose, being a natural process, but during the past five years the bestowal of sufficient quantities of necessary humus has been impossible, owing to the shortage in supplies of farmyard manure.
For this reason every farmer regards a grease-free manure carrying substance of a humus-like nature for the improvement of his soil with a particularly friendly eye, and he is prepared to pay a good price for such an article. The sewage sludge fertilizer prepared under the Grossmann process offers the agriculturist just what he desires in this connection, inasmuch as it carries about 30 per cent. of the humus-like substance. Then, again, the active manurial ingredients are distributed over the mass in such a fine state of division as cannot possibly be attained by resort to mechanical grinding. Finally, it is excellently balanced, and the farmer keenly appreciates a well-balanced fertilizer. Here he gets it because the essential operation has been conducted by Nature, whose process cannot be rivalled. Applied to gardens this manure is found to prevent the growth of yellow leaves, while the green of the foliage is particularly rich and dark. In some quarters there has been a certain degree of hesitation to utilize the fertilizer merely because it is derived from sewage, owing to the prevalence of many fallacious notions. Its origin is regarded with revulsion, and its utilization with a certain degree of dread, but there need be no apprehensions whatever concerning its use. The fact that in the course of the treatment the material is raised to a temperature approaching 600 degrees Fahrenheit—where the superheated steam comes into contact with the waste to expel the fat—effectively disposes of all germ life inimical to the health of both animals and human beings, while it is also clean to handle and odourless, it being impossible, from mere cursory examination of the fertilizer, for the lay mind to determine its origin. Finally, it may be stored for any length of time without creating a nuisance, or deteriorating.
The whole of the output from the Oldham sewage works, which, owing to the process of concentration, is really limited, notwithstanding the volume of crude sewage handled, is readily absorbed by farmers. Disposal was entrusted to a firm to act as the selling agents for the Corporation. Owing to the number of repeat orders received, year after year, this house declares that it could easily place 20,000 tons of the fertilizer, were it forthcoming, without increasing its present staff of travellers.
While the outbreak of hostilities militated against the expansion of the process, although many other Corporations have expressed their readiness to introduce the process into their respective sewage works, the past five years have not been allowed to represent dead time. Improvement upon improvement has been incorporated with the object of securing still higher efficiency. In this direction the inventor has made many distinct progressive strides. The one objection levied against the process was the heavy expense incurred in regard to fuel charges for drying the sludge, and these costs naturally have become accentuated by the 200 to 300 per cent. rise in the price of coal. But in this direction it is now possible to record noticeable reductions.
As a result of experiment the inventor has evolved a new method for settling the sludge. He found that, by adding a very slight amount of sulphuric acid—about 1 part to 1,000—to the sludge coming from the settling tanks, the usual settling process is completely reversed. Instead of the sludge settling to the bottom, the addition of the acid causes it to rise to the surface, and in a much more concentrated form. The water settles to the bottom in a clearer condition and can be drawn off. By further settling and draining this top layer—virtually a thick scum—a sludge can be obtained carrying about 30 per cent. solid matter, and therefore as a less volume of water needs to be evaporated a considerable saving in the consumption of fuel and cost of drying is achieved.
Moreover, it is suggested that in laying down new installations, it will be possible and profitable to install a destructor upon the sewage works. In such cases it would be feasible to draw upon the waste heat from the destructor to conduct the drying and other operations demanding the application of heat. In combining the destructor with the sewage plant the question of transport of the refuse from the collecting ground to the destructor would demand very careful consideration when horse haulage is employed, but with mechanical traction the question of an extra mile or two in distance hauled is not of such moment, especially as it would be off-set by the saving of fuel which would attend the diversion of the waste heat to this useful application. In fact, in cases where new lay-outs are being contemplated it is a matter for serious reflection as to whether it would not be found profitable to centralize destructor, electric generating station and Grossmann sewage treatment plant in one centralized spot, interlocking them together, and taking full advantage of such inter-connection. The destructor would furnish the necessary steam from the combustion of cinders and other refuse which it does not pay at present to exploit, or preferably other low-grade fuel to drive the electric plant, the waste steam being carried to the sewage works for the drying and other machines together with the desired proportion of live steam, while the electric station would furnish the requisite power for operating the automatic mechanical appliances.
The grease recovered from the sewage, which is of a domestic character, is essentially that from soap, cooking and washing operations. It is totally free from all objectionable smell. It can be purified very easily and is of distinct value. In its crude condition the fat contains about 70 per cent. of stearic acid.
During the past few years the dry powdered residue has not only been utilized in a direct form, but has also been exploited in the production of compounded fertilizers. Mixed with phosphates, sulphate of ammonia, and other nitrogenous products it has yielded a manure which has given most excellent results in farming. Considerable improvements with regard to greater efficiency and cheaper production have been made in the manufacture of compounded fertilizers from this residue, and there is every indication that still greater developments are possible in this direction.
Were all the sewage of this country treated along these lines British agriculture would derive distinct benefit, while industry would also be presented with a new source of supply of essential raw material. It would go a long way to enable us to use our greases over and over again, because the drain is the most popular avenue for the escape of this material. Sewage represents the greatest waste incidental to this country. Dr. Grossmann estimates its value at approximately £22,000,000—$110,000,000—per annum, of which but only an infinitesimal fraction is at present recovered. The value of the fat alone thrown down our drains, and reclaimed in a marketable form would realize from £500,000 to £1,000,000—$2,500,000 to $5,000,000—a year. The value of the manurial product, of which at least 1,000,000 tons are recoverable during the twelve months, may be set down at least at £2,000,000—$10,000,000—the contents thereof being equivalent to 50,000 tons of phosphates, 50,000 tons of potash salts, with nitrogen equal to that forthcoming from 100,000 tons of sulphate of ammonia. This manure would suffice for the fertilization of at least 3,000,000 acres of land from which we might safely anticipate gathering, at a modest estimate, additional crops worth £5,000,000—$25,000,000.
There is one other fact which deserves mention. Sewage is eminently adapted to the feeding of sandy soils and other land which, at the moment, is considered too poor for agricultural purposes. If this manure were reserved for such land many thousand additional acres might be brought under cultivation in these islands. At the present moment these acres are being allowed to run to seed, constituting what we erroneously term waste land, but only waste because we are not sufficiently enterprising and energetic to reclaim it.