The screw is a critical component in the toilet, allowing the settled solids to be removed from the bottom of the holding chamber and transported towards the gasifier/combustor. This separates them from the liquid. Traditional source separating designs require a change of user behaviour, whereas our aspiration is to make using the Nano Membrane Toilet a comparable experience to using a "conventional" water-flush toilet.
We have been doing excperiments on the screw to determine the best charactertistocs to transport the solids but minimise the transport of liquids. This has included changing the taper of the central shaft, makig the flights closer together near the top of the screw, and changing the rotation speed. the results are described in this recently published, free-to-access paper:
McAdam (2016) Selection of screw characteristics and operational boundary conditions to facilitate post-flush urine and faeces separation within single household sanitation systems, Environ. Sci.: Water Res. Technol., 2, 953-964
To ensure adequate access to sanitation in developing
economies, off-grid single household sanitation has been proposed which
obviates the need for significant infrastructure capital investment.
Whilst treatment at this scale is most efficient when coupled to source
separation (i.e. urine from faeces),
existing source separation solutions have proved difficult to implement
in this context. In this study, screw extrusion is therefore
investigated to provide ‘post-flush’ source separation. Both screw
characteristics and operational boundary conditions were evaluated.
Preferential screw characteristics included tapering of the shaft and
progressive pitch reduction, linked to a small extrusion aperture, the
combination of which enhanced solids extrusion efficiency and promoted
higher solids concentration in the extruded fraction. Whilst maximum
extrusion efficiency was observed at high rotational speeds (over 400
rpm), this also promoted free water transport. Operating below 300 rpm
instead introduced selectivity for transport of faecal sludge over
urine, enabling phase separation. Constraining the volumetric ratio of
urine to faeces also enhanced the extrusion rate of faecal sludge by
increasing feed viscosity sufficient to overcome backpressure imposed by
unmasticated food particles that would otherwise restrict separation.
Importantly, this study demonstrates the feasibility of screw extrusion
for ‘post flush’ separation of urine and faeces which constitutes a
significant advancement towards realising sanitation at a single