Selective removal of water from mixed human waste
Introduction
Hollow fibre membrane
Hollow fibre membrane bundles are used in the nanomembrane toilet, immersed in raw human waste. During the toilet’s
operational cycle, vacuum and/or a dry sweep gas is applied to the inside of the fibres, which drives the transfer of water
from the waste as a vapour into the fibres. The removal of water dries the waste and reduces its volume to the point where
it is combustible. The vapour leaving the toilet can be condensed to liquid and is safe to use for cleaning around the home
or for discharge into the environment.
Two types of membrane material have been trialled: microporous and dense.
Microporous
Dense
Microporous membrane surface
Water and some volatiles pass
through the membrane pores at
high rates
Dry sweep gas or vacuum drives the transfer of water from
outside to inside the hollow fibres
Pathogens and solids are
too large to fit through the
pores, but can foul them
Dense membranes
have no pores, so do
not incur internal
fouling. The water must
dissolve into their
structure, resulting in
highly selective
Microporous
removal and rejection
membranes have pore
of volatile species
diameters in the region
such as skatole.
of 0.03 µm, too small
Dense membrane surface
for pathogens but large
enough for some other
compounds such as
ammonia. Also, the
pores can foul,
reducing performance.
Water vapour dissolves relatively
slowly into the dense membrane,
and volatiles are rejected
Dry sweep gas or vacuum drives the transfer of water from
outside to inside the hollow fibres
No pores means pathogens
and solids cannot pass, and
cannot foul the membrane
Results: which type of membrane to use?
Microporous
Dense
0.18
Microporous membranes allow a greater amount of water to pass
through them (up to 100% more), and use less energy to do so than
dense membranes. However, this comes at the expense of the
purity of the water passing into the hollow fibres.
Water flux
The more porous the membrane, the greater the amount of
other compounds are able to pass through – including ammonia
and volatile organic compounds such as skatole that will make
the recovered water smell unpleasant.
0.16
0.14
0.12
(L.m-2membrane
surface.h-1)
0.1
0.08
0.06
However, microporous units could be used where ammonia is recovered
for its nutrient value using downstream processes. Dense membranes
could be applied where the focus is on the purity of recovered water.
0.04
0.02
0
20
Results: commercial modules
0.2
30
35
40
Fluid temperature (ºC)
45
50
Next steps
Sweep
(LPM) and Vacuum (bar)
Dense membrane
using PDMS
Customised membrane modules have been
designed and are being tested for enhanced
water fluxes above those returned using
commercial membrane modules. The key
difference is in the looser packing of the fibres,
which allows greater contact with the waste.
Sweep
(LPM)membrane
and Vacuum (bar)
Microporous
using PP
0.15
25
Theoretical flux based on saturation
Water vapour 0.1
flux (l.m-2 h-1))
0.05
Conclusions
0
0
0.0001
0.0002
0.0003
0.0004
Sweep gas air flow rate, Qair (m3 s-1)
Whilst the driving force for water vapour transport through both
membrane types is ultimately the same, greater selectivity over
volatiles is provided by dense membranes.
0.0005
Both membrane types can be used for dewatering raw human waste,
and neither will allow the passage of pathogens. Both membranes can
produce water that has utility, and the application of either can greatly
reduce the volume of problematic (i.e. biologically hazardous) waste in
reinvented toilets.
Production of pathogen free water, reduction of waste volume