the wooden shoes to the wooden bond beam. Internal, non-load-bearing walls are often made of a honeycomb of recycled cans joined by concrete; these are nicknamed tin can walls. These walls are usually thickly plastered with adobe, and resemble traditional adobe walls when finished. The roof is made using trusses, or wooden support beams called vigas, that rest on the wooden shoes or the tin can walls placed on the bond beams. The roof as well as the north, east and west facing walls are heavily insulated to reduce heat loss. The average cost in 2019 including labour and land is about $500,000. Water Earthships are designed to catch all the water they need from the local environment. Water used in an Earthship is harvested from rain, snow, and condensation. As water collects on the roof, it is channeled through a silt-catching device and into a cistern. The cisterns are positioned to gravity-feed a water organization module (WOM) that filters out bacteria and contaminants, making it suitable for drinking. The WOM consists of filters and a DC-pump. Water is then pushed into a conventional pressure tank to create common household water pressure. Water collected in this fashion is used for every household activity except flushing toilets. The toilets are flushed with greywater which has been used at least once already. Typically it is filtered waste-water from sinks and showers. Greywater, recycled water unsuitable for drinking, is used within the Earthship primarily for flushing toilets. Before the greywater can be reused, it is channeled through a grease and particle filter/digester and into a deep rubber-lined botanical cell, a miniature living machine, within the Earthship. Here the water is oxygenated and filtered using bacteria and plants to reduce the nutrient load. Water from the low end of the botanical cell is directed through a peat moss filter and collected in a reservoir or well. The reclaimed water is passed once more through a greywater board and used to flush conventional toilets. Black water is water that has been used in a toilet. Earthships utilize anaerobic digestion in their septic tanks, which naturally separate solid waste. The black water is used in concrete cells containing plants, separate from the grey water plants in the greenhouse; it may also be used in exterior planters. Studies on the safety of growing food plants in a black water system show low levels of E. coli bacteria.