Sewage farms comprise agricultural land irrigated and fertilised with sewage. The product being "farmed" at such a works is actually the combination of microbes and bacteria used to break down the sewage.
As a predecessor to modern sewage treatment systems, household sewage was collected from towns and cities and transported to nearby farm lands. During the Middle Ages this was accomplished with hand-carried buckets, but as local populations grew, during the Industrial Revolution sanitary sewer systems were built. These used a network of pipes and pumps to transport sewage beyond the city boundaries to large rented grasslands, into which the sewage trickled down. Berlin once rented 20 sewage farms occupying about 10,000 hectares.
Some of these farms remained in use until the end of the 20th century, at which point it became apparent that as it was usually contaminated with industrial waste, sewage was unsuitable for use as a fertiliser. Therefore, sewage plants began to replace sewage farms. Modern sewage farms are usually combined with such plant, so that they irrigate the land with reclaimed water. Some types of untreated sewage can be used on a sewage farm, or filtered through a constructed wetland.