penetration and often manages overlying vegetation to avoid growth of plants with deep root systems. History 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 operated 20 sewage farms occupying about 10,000 hectares. In Wales, it was the common means of sewage treatment when cess-pits became unusable as the population grew in towns during the industrial revolution. The initial response to overloaded local disposal was often a trunk sewer conveying sewage to the nearest river but as populations increased further, sewage farms were established. 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 infectious pathogens and sometimes with industrial waste, sewage was not always suitable for use as a fertilizer. Therefore, sewage plants began to replace sewage farms. Modern sewage farms are usually combined with such plant, so that they irrigate the land with treated sewage (reclaimed water). Some types of untreated sewage can be used on a sewage farm, or filtered through a constructed wetland. Examples A small-scale example of a sewage farm exists at Chester Zoo, where water fouled with elephant droppings and urine is filtered through a reedbed. Western Treatment Plant in Melbourne. See also On-site sewage system Reuse of excreta References Sewerage infrastructure