42 BULLETIN 769, U. S. DEPARTMENT OF AGRICULTURE. 
fibrous substance to make it possible to press them. City garbage 
usually is cooked under pressure in large autoclaves, and the grease 
which separates from the disintegrated vegetable matter blown out 
with live steam through the strainers at the bottom of the tanks. 
The residue is often dried afterwards and extracted, to recover the 
remaining fats, and then sold as tankage. Fish scrap is sometimes 
boiled in open kettles, and the oil skimmed off. Better yields, how- 
ever, are obtained when it is treated by the same method as the 
garbage. A comparatively new process, the Cobwell System, 
which depends upon the removal of the water from the garbage by 
heating with a volatile solvent, is being used in New York City. 
In this plant the wet garbage is charged into a closed vessel known 
as a reducer which is equipped with a stirrer and flooded with a vol- 
atile solvent, usually gasoline. This is then distilled off, carrying 
with it some of the water. This 'process is repeated until the gar- 
bage is practically dry. It is then extracted several times with 
solvent which, however, is now drawn off instead of being distilled, 
and the fat it has dissolved recovered by the usual distillation 
methods. The degreased tankage remaining in the reducer is 
freed from solvent by blowing it with live steam, after which it is 
discharged ready for use as fertilizer. 
The refuse fats are, of course, suitable for industrial purposes 
only. They all contain from 7 to 10 per cent of glycerin, and many 
of them, especially garbage grease, which is high hi animal fat, con- 
stitute a satisfactory soap stock. Fish heads, tails, fins, etc., yield 
a fair grade of fish oil, if the material has not been allowed to decom- 
pose too badly. Grease recovered from wool scouring forms the 
crude lanolin, or wool grease, which when properly refined makes one 
of the most expensive of our greases, the lanolin of the pharmaceutical 
trade, and the basis of many ointments and salves. 
OILS FROM TRADE WASTES. 
With the growing demand for fats and the resulting increase in 
their price, manufacturers are paying more attention to the recovery 
of fats and oils from the wastes of their factories. This is particu- 
larly true where the disposal of the waste entails an expense, as in 
the case of canning factories located in or near large towns. Tomato 
seed oil is an example of an oil which may be recovered from such 
wastes. In Italy it has been extracted with volatile solvents from 
the tomato seeds thrown out by the tomato-pulp plants. The 
tomato seed oils made in the Bureau of Chemistry of this depart- 
ment by cold pressing tomato seeds from pulp and catsup factories 
in different parts of the United States are greenish yellow, and are 
easily refined and bleached. 
