That sample contains from 55 to 60 per cent of water and is as it comes from the filter press, and it is called filter-press cake.
Mr. Gardner. That is not in commercial form?
Mr. Paterson. No; that is in the form, as it comes from the sewage works, and it is material that gives us much trouble to dispose of. The analysis of that material, not air dried, is approximately—by “approximately” I mean the general analysis of that class of material over the United Kingdom—as follows:
Analysis of press cake (not air dried).
| London. | Leeds. | Wimbledon. | |
| Per cent. | Per cent. | Per cent. | |
| Water | 50.00 | 58.05 | 56.15 |
| Organic matter | 15.40 | 16.69 | 11.36 |
| Silica | 6.40 | 8.08 | 7.10 |
| Carbonate of lime | 10.30 | 7.94 | 11.14 |
| Nitrogen in sewage sludge (dry) | 1.63 | ... | ... |
| To ammonia | 1.95 | ... | ... |
| To ammonia sulphate | [2]7.67 | ... | ... |
[2] Or per ton sludge, 171.8 pounds.
The ordinary air-dried sewage—we do not have very much of it in England—mostly comes from the small towns, in which the sludge is pumped up into lagoons and allowed to dry there, and the composition of that material is, generally speaking, as follows:
| Air dry. | London. | Leeds. |
| Per cent. | Per cent. | |
| Water | 11.86 | 16.40 |
| Organic matter | 24.61 | 27.92 |
| Phosphoric acid (P₂O₅) | 1.04 | .75 |
| Sulphuric acid | 1.12 | 1.02 |
| Carbonic acid | 10.98 | 13.11 |
| Lime | 14.33 | 17.51 |
| Magnesia | 2.34 | 7.67 |
| Oxide of iron | 3.02 | 2.32 |
| Alumina | 4.13 | 6.33 |
| Nitrogen | .86 | .70 |
These analyses will give a general idea of the composition of filter-pressed sewage cake, which we treat in Great Britain, and of which we have 1,800,000 tons per annum.
The object of the process under consideration is to dry the solids so that they may be available either for a fertilizer or, secondly, to be in a condition that by-products of commercial value can be recovered. The greatest difficulty with which we have had to contend was to provide a way of liberating the moisture, of which the pressed cake contained from 55 to 60 per cent, without volatilizing valuable material, and at the same time achieve the object in a short space of time and at low cost. One of the difficulties that has caused a great deal of time and trouble to solve is how to get rid of that moisture and dry the material at a moderate cost without losing any of the valuable constituents, because the valuable products are volatilized at a very low temperature. A very large amount of money and time has been spent in Great Britain in trying to dry sewage pressed cake, or sewage sludge, economically, and many ingenious mechanical appliances have been invented to try and solve the difficulty, but they have not been a success, as they have been costly to operate and required high temperature, with consequent loss of some of the valuable constituents of the sewage. This is due to the water in the sewage being in so many different forms, namely, hydroscopic water, water of combination, and water of crystallization; the first being easy to drive off and the two latter extremely difficult. So it is recognized now that the problem is one of chemistry and mechanics applied, and I can safely say that there is now a satisfactory solution of these difficulties on a practical scale.