Tuesday, 3 January 2017

Designing the screw

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:

Mercer, E.,  P. Cruddas , L. Williams , A. Kolios , A. Parker , S. Tyrrel , E. Cartmell , M. Pidou and E. J. 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 household scale.