Millions from Waste

by Frederick Arthur Ambrose Talbot

The presence of fat in material quantities has always been responsible for agricultural hostility towards this waste as a fertilizer. The fat is due to soap used in the household, and which is thrown down the drains, as well as the grease resulting from other domestic operations. The great objection to grease is that it has the tendency to clog the soil.

In turn efforts were made to dispose of the nuisance as a fuel, the heavy proportion of oil present in the dried cake being the attractive feature prompting this application. This recommendation found scanty favour. Another brilliant mind conceived the idea of consuming the refuse in gas-producers, thus obtaining a low-grade gas for power purposes. This attempt failed to meet approbation. A third expedient was its conversion into an illuminating gas, but this likewise failed to overcome the obstacle. In so far as lighting is concerned, in many places the practice is followed of allowing the gas thrown off by the decomposing fæcal matter during its passage through the sewers, to be led to the burners of adjacent street lamps to mix with the ordinary town gas and thus be consumed. But this is merely a safety precaution; it is not followed from economical motives. Now that electricity is widely displacing gas for street illumination, even this quasi-utilitarian system is meeting with defeat.

Under the Grossmann system, as practised at Oldham, the sludge is subjected to a complete scientific treatment. The process is continuous and automatic throughout. Moreover, the plant is designed and built upon the unit principle, which allows the standardization of parts and ability to meet any desired demand by merely acquiring a sufficient number of units to comply with the sewage resulting from a given population. Each unit is capable of dealing with sludge arising from the purely domestic sewage of 20,000 inhabitants. Thus a town of 100,000 inhabitants would require 5 units, a city of one million souls 50 units, and so on in arithmetical progression. Furthermore, any number of units can be worked together, so that in those centres where the population fluctuates according to season or other conditions, a certain number of units can be shut down during the off period.

The sludge passes to a special tank and is permitted to settle down to approximately 20 per cent. solid matter. It is then scooped up by bucket elevators to be lifted and discharged into another tank at the top of the building. This acts as the storage tank or hopper, whence it is moved automatically by means of screw conveyors and distributed among six hoppers. Each of these hoppers feeds a drying machine. The driers, set out in pairs with their brickwork casings and flues, occupy the upper room. The machines themselves comprise iron cylinders set in the brickwork and coal-fired furnaces. They are fitted with a specially designed gearing and pulley mechanism which gradually moves the crude wet sludge from the inlet towards the opposite end or outlet. Being exposed to heat during this passage the sludge is naturally deprived of the water it contains, this being evaporated to be led to the furnace where any offensive gases and other matter associated therewith in suspension are consumed before passing to the chimney to escape into the outer air. By the time the sludge reaches the outlet it has been completely dried.

The arrangement of the feed from the hopper to the drier is such that only a measured quantity of sludge can be passed through in a given time, which ensures the condition of the sludge at the outlet being uniform. The provision of a similar measuring system at the outlet of the drier ensures only a measured quantity of sludge being discharged at that point. It will be observed that these protective devices guard against forcing the apparatus to the detriment of the delivered sludge which emerges from the drier in the form of a dry powder.

If desired this residue may be burned. Mixed with coke it forms an excellent fuel, and can be employed towards raising the requisite steam to conduct the treatment of further sewage. But, in view of the fact that this powder contains about 15 per cent. of fat, its disposal as a fuel would constitute about the most wasteful conceivable. Accordingly, the next stage is the extraction of the fatty content. As it comes from the drying apparatus the sludge is passed automatically into a distilling retort which is bricked-in and heated. Above this retort is a tank containing acid, a certain quantity of which is passed into the retort to be automatically mixed with the powdered sludge. Simultaneously superheated steam is driven through the mass in such a manner as to permeate the whole. The interior of the retort is fitted with gearing and pulleys similar to those provided to the drier and for a similar purpose—the steady gradual movement of the sludge from one end to the other. By the time it has reached the outlet from the machine the sludge, completely deprived of fat, is automatically discharged as a valuable manure and is ready for distribution upon the land.

The superheated steam charged with the grease is passed into a condenser, where water from a feed tank condenses the water and throws down the grease. The mixture of condensed steam and grease is passed into a recovery tank. The grease settling out on the top is removed for boiling up in a separate vessel, upon the completion of which treatment it is ready for packing and sale. The fatty matter consists largely of stearine and palmitine, which to-day meet with a prompt sale at lucrative prices.

But it is the solid residue in the dry powdered form which attracts the greatest measure of attention. Disposal of the grease from sewage has never occasioned so much difficulty as the utilization of the ultimate residue from reasons already explained. In this particular instance the great problem has been solved. The manure is in the form of a fine powder, containing nitrogen, phosphoric acid, and potash, as well as about 40 per cent. of organic material. It is very fine, brownish in colour, odourless, and what is more to the point, absolutely innocuous, having been completely sterilized. Consequently there is no risk of infection being disseminated by its use.

The circumstance that the process is absolutely automatic from the time the sludge is charged into the hopper to the finished article issuing from the distilling retort, is a distinct recommendation. Not only does it conduce to extremely economical operation, but it reduces the necessity to bring human labour into one of the most offensive of industries, inasmuch as the atmosphere of such an establishment is scarcely fragrant, as may well be imagined, although familiarity breeds strange contempts. The only labour essential is that required for heating up the drying machines and retorts.

There is one overwhelming advantage incidental to this process which cannot fail to arouse attention. Pressing in any form is eliminated. This not only signifies a very pronounced saving in capital expenditure in the first instance, but contributes to lower working charges, while there is an enhanced recovery of grease and an absolutely grease-free residue.