With the sorting toilet, faeces with toilet paper (if used and put in the bowl) is flushed with 4 - 6 l water to the tank where the solid and the liquid phase are separated and treated separately. There are many varieties of processes used in liquid-solid separation. They are usually based on two principal modes of separation: 1) filtration (gravity, vacuum, pressure and centrifugal), in which the solid-liquid mixture is directed towards a filter medium (screen, woven cloth, membrane etc.). The liquid phase flows through the filter medium while solids are retained, either on the surface or within the medium and 2) sedimentation or settling in a forces field (gravitational and centrifugal) whereby advantage is taken of differences in phase densities between the solid and the liquid. The solids are allowed to sink in the fluid under controlled conditions. In the reverse process of flotation, the particles rise through the liquid, by virtue of a natural or induced low solids densities.

In decentralised wastewater treatment systems, septic tanks, which are based on the principal of sedimentation are widely used for solid-liquid separation (Crites and Tchobanoglous, 1998). A new development is Aquatron (figure 24) which is manufactured and applied in Sweden for separating solid matters from toilet wastewater (Del Porto and Steinfeld, 1999). When the toilet is flushed, the flush water with faeces and toilet paper enters the top of the polyethylene module, which is constructed to initiate a whirlpool effect in the upper container. Here, by centrifugal force, part of the liquid, which moves to the outer wall and ultimately out of the system, is separated while the solids drop down in the middle into the composter beneath it.

Figure 24: The whirlpool, surface tension separator Aquatron and its function where the faecal water (FW) is separated into liquids and solids (Vinnerås, 2002, http://www.aquatron.se)

The Aquatron is a commercial separation system adopted for separation of faeces from faecal water. The efficiency of the system depends on correct installation. The disintegration of faecal particles should be minimised. An easy way to reduce disintegration is by shortening the length of the system, especially by decreasing the vertical drop and a smoother bend between vertical and horizontal transport of the faecal water. When using the system in a multi-storey building, one way of doing this is to install an Aquatron on each floor.