Rendering

Historical Rendering

In the early to mid-1800s, rendering was often done in an iron vat with a tight lid. The vat was filled with water and the materials were boiled. The lid kept the steam inside the vat, but the pressure was generally not allowed to rise above a few pounds per square inch. After the grease was removed from the surface of the water, the larger bones were charred and used as a filter medium in sugar refining. The remaining solids, because they were rich in nitrogen and phosphorous, became a feedstock for fertilizer manufacture.

Rendering produced noxious odors, which frequently prompted complaints. Another drawback to boiling was that the water became a foul, smelly soup that was simply poured into the nearest body of water.

However, the desire to remove putrescible wastes from cities, the demand for industrial feedstocks, and the need to dispose of animal carcasses in the era before the internal combustion engine made rendering plants in or near cities a necessity. In July 1853 alone, 1,113 tons of butcher's offal and 425 dead horses were removed from Manhattan Island for reprocessing into grease and fertilizer.

The development of closed tanks for rendering allowed higher pressures and had the added benefit of allowing noxious gases to be captured. Higher pressures also caused bone to disintegrate. These tanks were heated by introducing steam through a pipe opening into the bottom of the tank. Pressures of 50–60 pounds per square inch were possible. By 1874, technologies were introduced that would direct any noxious gases created during the digestion process into the same fires that were used to create the steam. These technologies were far from perfect, and the odors from rendering plants continued to be a public nuisance.

Rendering plants were highly profitable. During the 1800s, the recovered grease was refined and used for manufacturing soap, candles, paints, and lubricants. At the start of the 20th century, New York City was able to finance its garbage collection entirely through the sale of recovered grease.

During the 1890s, rendering technology was used by large cities to process household and restaurant food wastes—a process known as “waste reduction.” The fertilizing value of the tankage was not as great as that produced exclusively from animal flesh, but there was still sufficient nitrogen, phosphorous, grease, and potash to make recovery profitable.

In addition to steam heating, at the start of the 20th century, there were two other commonly used rendering technologies. Crushed and dried [Page 744]food wastes were extracted with gasoline. Another method was to extract wet wastes with gasoline at a temperature equal to the boiling point of gasoline. The advantage of this approach was that both water and grease were removed in the same step. Whichever process was used, the tankage was usually pressed to recover the last of the grease.

...