The plant for dealing with the sludge was moved from the old situation to new buildings specially erected for the purpose at Esholt, and the raw material is fed to the latter station through a special main. The sludge contains only 80 per cent. of water, the free water having been previously removed by settling. It is fed into the main by compressed air. Upon its arrival at the station the sludge is lifted, also by compressed air, into large vats, where it is heated by the waste steam from the engines of the power plant. In this heated condition the sludge passes into close-sealed vessels from which, still at a temperature approaching boiling point, it is forced by compressed air through the filter presses. Each of these presses, of which there are about 100 disposed in rows, contains 47 chambers, each 3 feet square.
As already stated, the sewage of Bradford is heavily charged with grease resulting from wool-washing and other industries, and it is this heavy proportion of grease which renders the process so attractive. Moreover, by keeping the sludge in a heated condition during the pressing process the expression of the fatty content is more readily effected. From 40 to 48 hours are required to fill a press with residuum, that is to say this period of time must elapse before the whole of the available space within the press is occupied by the dry cake from which the grease has been expressed, by which time from four to five tons of sludge have been passed through. Each cake is 3 feet square by 1¹⁄₂ inches thick and weighs about 30 cwt. The grease and water which is driven out of the sludge is carried away from the presses into tanks. Here the water and grease are separated, the water to be re-discharged into the sewage, while the grease is led to the purification tanks. Subsequently the fat is either drawn off into barrels or is pumped into tank wagons for dispatch to the works where it is worked up into articles of commerce, including soap. The oil is found to yield three valuable products—olein, stearine, and pitch. The two last named enter extensively into the dressing of leather, as well as the manufacture of candles and as an insulator for electric cables, respectively.
The installation yields from 12 to 15 tons of grease throughout the twenty-four hours, working, of course, being continuous. This product in the days before the war commanded from £8 to £10—$40 to $50—per ton, but the price is now higher. The sludge-cakes find favour as a fertilizer, mainly from the fact that they are free from lime and carry only from 28 per cent. to 30 per cent. of moisture. This residue fetched from 3s.—75 cents—upwards per ton at the works in pre-war days, when a healthy export was recorded, the product being shipped in appreciable quantities to France and even to the Southern States of America. The output of cake averages from 50 to 60 tons per day. In addition to proving useful as a fertilizer it has been found to furnish, when blended with coal-dust, a serviceable fuel.
The revenue derived from this example of sewage industry is certainly such as to attract widespread attention. In the early days of the process, when only two presses were maintained to establish its possibilities, the grease sales reached £222 10s. 6d.—$1,112.62—per annum. In 1911 the annual revenue had risen to a figure ranging between £20,000 and £30,000—$100,000 and $150,000—from the enlarged battery of presses. When the new works were opened it was anticipated that the Corporation would be deriving £50,000—$250,000—a year from the sale of the products derived from its sewage upon the attainment of the designed maximum output. Up to the year 1911 the total sales amounted to no less than £100,000—$500,000. From the recital of these figures it must be conceded that Bradford has a very profitable commercial enterprise in its sewage works.
Yet even the foregoing figures are undoubtedly capable of improvement owing to the advances made in the whole issue of the recovery of fats from wastes. The pressing system, even when conducted along the most modern lines with up-to-date plant, leaves much to be desired in point of yield. Under the most favourable pressing conditions at least 10 per cent. of the original volume of grease is left in the residue. The presence of this grease reacts against the value of the residue as a fertilizer, grease being the bugbear of the farmer. With the latest process for grease extraction this content can be reduced down to 1 per cent. Not only does this represent an increased yield of 9 per cent. of fat with its attendant enhanced financial return, but it gives a fertilizer which, being exceedingly low in fat, appeals more strongly to the farmer, and accordingly is able to command a higher price. This fact appears to have become appreciated by the Bradford authorities according to recent developments.
Because such a striking success has been recorded at Bradford, it is not to say that the self-same method would be equally profitable at other places, especially those handling what might be termed purely domestic sewage. The conditions existing at the Yorkshire city are peculiar, owing to the wool-washing trade. The process which is more likely to make the widest appeal, being the one adapted to meet the average conditions, is that which has been installed in the borough of Oldham. This is the invention of Mr. J. Grossmann, M.A., Ph.D., F.I.C., the well-known chemical engineer, who has made the exploitation of sewage his life-long study. The plant in question was laid down in 1912, being set in operation in October of that year, since which date it has been working without a break, giving the most satisfactory results. At the time the installation was carried out the population of the borough was 148,840, and both the water-carriage and sanitary-pan system were in vogue, although the latter was giving way to the former method at the rate of about one thousand per year. As the conversion system was carried into effect the quantity of sludge which the sewage works were called upon to handle increased, the quantity pressed in 1911 being nearly 8,000 tons a year as compared with 4,000 tons in 1899. This did not include the several hundred tons which were dealt with in lagoons without pressing. As the quantities of pressed sludge increased so did the difficulty of disposing thereof.
The outlook was somewhat disconcerting. The agricultural land in the vicinity could only absorb a portion of the available volume. The necessity to incur the expense of carrying the residue a considerable distance to dispose of it, which solution would have proved somewhat costly, appeared to be inevitable. Experiments innumerable were carried out, but to no purpose. Agriculture, which is regarded as the obvious outlet for such material, was adverse to the proposal to absorb the accumulation for the land, because it carried approximately 15 per cent. of grease. The only escape from the dilemma appeared to be the installation of further presses with the attendant expense for auxiliaries to yield a dry material, and then to pay for the cartage of this residue to some convenient tipping ground or carriage of the settled sludge to sea to be dumped. As a round 30,000 tons of sludge would have been involved, the sea-dumping expedient would have been extremely costly. Further consideration of the question established the possibility of converting the material into a marketable manure, but this would have required the utilization of a trade process and also would have incurred expense.
At this juncture the attention of the Corporation was attracted to Dr. Grossmann’s process. It was investigated and submitted to searching experiments spread over a period of three years at the sewage works. From the results obtained and the experience gathered, it gave promise of being completely successful when conducted upon a large scale. So it was adopted.
The Grossmann process may be said to represent the most logical exploitation of sewage yet attempted in accordance with the severe hygienic conditions imposed to-day. Curiously enough, when the disposal of sewage by water-carriage was first introduced, the critics of the principle did not hesitate to point out that it represented the most wasteful solution of the problem which had ever been accepted for practice. But against these contentions the advocates of the idea urged that the hygienic advantages to be gained were so overwhelming that the question should not be considered from the commercial view-point at all.
Other days, other manners. In this instance, however, not many years passed before the issue attracted such widespread attention as to demand searching investigation, the difficulty and cost attending the disposal of the sludge being responsible for a pronounced outcry against the method. The sludge problem was thoroughly probed by a Royal Commission, by which the opinion was expressed that the value of this waste, calculated upon the volume of dry substance contained therein, was no more than 10s.—$2.50—per ton at the very outside. But as the sludge is produced in a form showing a high percentage of water it was hopeless to expect farmers to absorb it, owing to the transport charges involved for such a comparatively low manurial return, unless their land happened to be situate close to the centres of production. To overcome the water difficulty attempts were made to dry the sludge, in the effort to reduce its bulk, but it was discovered that drying did not constitute a complete sterilization process, with the result that the material was liable to carry infection. But the greatest objection to drying is that this very process, while it achieves one end—the transport difficulty—provokes another disability. The sewage is worth less after drying than in the saturated form.