Back in the last 1800's, horses were used extensively for personal transport, as well as for hauling cargo and goods through the streets. These horses periodically had to relieve themselves, and used the streets to do so.
Apparently, There was literally enough poop to cover the street in inches of poop, and it was estimated that there would be a few feet of poop accumulation each day if the growth of the City kept up. This was enough to scare home builders into building front doors and stoops that were above street level to avoid the manure.
This video from the New York Historical Society explains about how much poop there actually was.
According to the 89th annual report of the NYC Board of Health, over 500 tons of horse manure were collected from the streets of New York everyday in the early 1890s produced by 62,000 horses in 1,300 stables. It was taken along with human waste to a place called "Barren Island," where it was reduced to fertilizer in a process called not "inoffensive" to the residents of the Long Island Sound.
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