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The design of the toilet is a critical aspect of ecological sanitation. Conventional toilets are designed to dispose of excreta by combining faeces and urine and then either flushing them away or burying them in deep pits. It is generally not feasible or cost effective to retrofit existing facilities, as new designs are needed to achieve the goals of ecological sanitation. The three primary goals of ecological sanitation being disease prevention, reduced water use & pollution, and the recovery of plant nutrients. As they aim to ‘close the loop’ in nutrient recycling they usually follow decentralised approaches. Each unit is free-standing and not connected to a centralised network, resulting in lower construction and maintenance costs.Ecological toilets have three basic components:a pedestal or squatting pan,a slab and a chamber,and sometimes a superstructure (if it is sited outdoors).The two principle types of ecological toilets are the urine-diverting units and the non-urine-diverting units (see diagram alongside).These toilets operate on the principle of keeping urine and faeces separate during treatment. They sanitise faces through one or more of the following processes: dessication (drying), increase in pH (more alkaline) or increase in temperature.Urinals can either be constructed separately, or as part of a modular unit (see photo alongside). Urine is collected and stored in a separate tank. After a couple of days it is completely sterile and may diluted with water in a ratio of between 1:10 and 1:5 and then applied to vegetation. During the storage period it is important that the nitrogen gas does not escape, as it converts to amonia and raises the pH to about 9, killing off any pathogens present. For this reason the storage chamber should have limited ventilation.To effect easy application of urine to fields the toilet should, if possible, be situated above the fields. If this is not possible it will have to be collected in buckets and applied manually.Faeces are collected in a chamber located directly below the pedestal unit. After each use lime, soil or ash is sprinkled on the faeces in the chamber (see photo alongside). This raises the pH and assists in the destruction of pathogens as well as preventing foul odours from escaping. Of the three main methods of killing pathogens – pH, reduced moisture & increased temperature – raising the pH is found to be the most effective. At a pH of 9 the faeces are rendered safe and odourless after six months.Most of these systems use double chambers. While one chamber is in use under the pedestal the second chamber is stored while the faeces inside are dessicated. Once dessicated the faces are odourless, light in colour and consist of large particles. They can be added directly to the soil or composted further to produce humus. It has been found that raising the temperature of the chamber to between 50 to 55 degrees celcius can halve the length of time it takes for faeces to dessicate. However, elevating temperatures to this level is not possible without a change in technology.These toilets are suitable for use within the home, and can fit seamlessly into a modern bathroom. Several have been installed in middle-class households in Mexico, Vietnam and Sweden. As they require no water and are not connected to a network they can also be cheaply and easily introduced in poorer households and rural areas.These toilets keep urine and faeces together and, through composting, convert them into humus, and are commonly referred to as improved pit-latrines. The faeces and urine collect in a ventilated pit, with lime or ash sprinkled on top after each use. As long as the humidity levels are kept low, by ensuring thorough ventilation, the excreta will form composted humus within three to four months. The diagram alongside shows the design of such a toilet produced in Australia. It has a built-in fan to ensure good ventilation, and produces compost in less than two months. Although technologically advanced, it is based on the same principles as the improved pit latrine. The only differences are that the latter relies on convection for ventilation; and the chamber is underground. Once the excreta have been converted into humus it can be applied to fields for the production of plants.There are some variations to the basic improved pit-latrine. In Zimbabwe the ArborLoo is commonly used in rural areas. This consists of a portable slab, pedestal and superstructure; and the chamber is a shallow (one metre in depth) pit dug into the ground. It is used in the same manner as the improved pit-latrine, with ash or soil added after each use to keep odours at bay. Once the pit is three-quarter full the slab, pedestal and superstructure are removed and the pit filled in with soil. A young tree is then planted over the contents of the pit and the toilet is erected in another place. Because the toilet is portable and moves on a never-ending journey, a sanitary orchard or wood lot appears over time. The trees can either provide fruit or construction and fuel wood. The advantage of this system is that there is no handling of excreta. The risk of groundwater contamination is reduced because of the shallowness of the pits. Due to the large area needed the concept is most popular in rural areas.The Fossa Alterna (alternating pit) toilet also has a portable slab, pedestal and superstructure. There are two permanent, semi-lined chambers, with one being the under the toilet while the other stores excreta while they compost. Once the humus has formed, a process which takes between three and four months, it is dug out and placed on fields. Then the toilet is sited over the empty chamber and the cycle is repeated.Another non-sewer approach to waste processing doubles as a source of energy. Since the 1970s, China has installed more than 5,000,000 anaerobic digesters. These consist of large chambers, mostly underground, that break down a rural family’s organic waste, including manure, human excreta, and crop residues, producing methane gas in the process. Toilets and pigsties drain directly into the digester, yielding enough biogas to meet 60% of a family’s energy needs, mostly for cooking and for fueling gas lamps. The unit produces an odourless dark slurry, used primarily for fertiliser.