Recent developments in the components of the Nano Membrane Toilet

The team have been hard at work developing the components of the Nano Membrane Toilet and have produced three papers recently.

To separate clean water form urine, membrane distillation is used.   Good quality water was able to be produced, despite the high concertation of the urine.   Fouling was reversible using physical cleaning, without needing any additional pressure.   When faecal contamination was added to urine, it was still possible to reduce Chemical Oxygen Demand, but it would be better to have a pre-treatment stage to achieve the best water quality.

F. Kamranvand, C. Davey, H. Sakara,, O. Autin,E. Mercer, M. Collins, L. Williams, A. Kolios, A. Parker, S. Tyrrel, E.Cartmell, E.J. McAdam (2018) Impact of fouling, cleaning and faecalcontamination on the separation of water from urine using thermally drivenmembrane separation. Separation Science and Technology

The final stage of faeces processing in the Nano Membrane Toilet is a micro-combustor.   This paper describes the initial successful tests of this component on real human faeces.

Jurado, N., Somorin, T., Kolios, A.J.Email Author, Wagland, S.,Patchigolla, K., Fidalgo, B., Parker, A., McAdam, E., Williams, L., Tyrrel, S.,(2018) Design and commissioning of a multi-mode prototype for thermochemicalconversion of human faeces, Energy Conversion and Management 163, 507-524

 

To recover energy from this combustion we have modelled the performance of a Sterling Engine which they have shown could produce a daily power output of 27 Wh/h.

Sowale, A., Kolios, A.J.Email Author, Fidalgo, B., Somorin, T.,Parker, A., Williams, L., Collins, M., McAdam, E., Tyrrel, S. (2018)Thermodynamic analysis of a gamma type Stirling engine in an energy recoverysystem, Energy Conversion and Management, 165, 528-540

 

ICPET 2018: Best Presentation Award

Congratualtions to team member Ayodeji Sowale who was awarded the Best Presentation Award at the 20th International Conference on Power and Energy Technology last week for his paper on the "Numerical Investigation of the Integration of a Micro-Combustor with a Free Piston Stirling Engine in an Energy Recovery System".   The micro-combustor is a key component of the Nano Membrane Toilet.

Deji (far left) with the other presenters in his session.

Nano Membrane team member, Tosin Somorin, commended

We are really pleased that Nano Membrane Toilet team member Tosin Somorin has been highly commended at the Women Leaders Milton Keynes Awards.   Tosin's work on the combustor element is vital to the development of the toilet.   You can see Tosin speaking about her work in this video (from 4:35) and read her latest paper on the toilet here.

Lab stories video

The Ellen MacArthur Foundation has just made a video of interviews with the Nano Membrane Toilet team which helps explain some of our current thinking and gives an insight into the lab work behind the toilet development:

Fuel blending

Fuel blending is being considered to upgrade faeces prior to combustion in the gasifier.   This paper describes the results of experimental work to prove the concept.   It is free to download.

Onabanjo Somorin, T., Kolios. A.J., Parker, A., McAdam, E., Williams, L., Tyrrel, S. Faecal-wood biomass co-combustion and ash composition analysis, Fuel 2013, 781-791

Fuel blending is a widely used approach in biomass combustion, particularly for feedstocks with low calorific value and high moisture content. In on-site sanitation technologies, fuel blending is proposed as a pre-treatment requirement to reduce moisture levels and improve the physiochemical properties of raw faeces prior to drying. This study investigates the co-combustion performance of wood dust: raw human faeces blends at varying air-to-fuel ratios in a bench-scale combustor test rig. It concludes with ash composition analyses and discusses their potential application and related problems. The study shows that a 50:50 wood dust (WD): raw human faeces (FC) can reduce moisture levels in raw human faeces by ∼40% prior to drying. The minimum acceptable blend for treating moist faeces without prior drying at a combustion air flow rate of 14–18 L/min is 30:70 WD: FC. For self-sustained ignition and flame propagation, the minimum combustion temperature required for conversion of the fuel to ash is ∼400 °C. The most abundant elements in faecal ash are potassium and calcium, while elements such as nickel, aluminium and iron are in trace quantities. This suggests the potential use of faecal ash as a soil conditioner, but increases the tendency for fly ash formation and sintering problems.

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

Abstract:
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.

Understanding membrane processes

The membrane module is a core component of the toilet, allowing us to get clean water from faecally contaminated urine.   Experimental work has defined the tube-side mass transfer coefficient derived in hollow fibre membrane contactors of different characteristic length scales (equivalent diameter and fibre length) under the slow flow conditions that are expected in the toilet.

The work is described in detail in a paper in the Journal of Membrane Science, which is free to download:



Yang, C.Y., E. Mercer, F. Kamranvand, L. Williams, A. Kolios, A. Parker, S. Tyrrel, E. Cartmell, E.J. McAdam Tube-side mass transfer forhollow fibre membrane contactors operated in the low Graetz range,  Journal of Membrane Science 523, 235–246

Transformation of the tube-side mass transfer coefficient derived in hollow fibre membrane contactors (HFMC) of different characteristic length scales (equivalent diameter and fibre length) has been studied when operated in the low Graetz range (Gz<10). Within the low Gz range, mass transfer is generally described by the Graetz problem (Sh=3.67) which assumes that the concentration profile comprises a constant shape over the fibre radius. In this study, it is experimentally evidenced that this assumption over predicts mass transfer within the low Graetz range. Furthermore, within the low Gz range (below 2), a proportional relationship between the experimentally determined mass transfer coefficient (K_ov) and the Graetz number has been identified. For Gz numbers below 2, the experimental Sh number approached unity, which suggests that mass transfer is strongly dependent upon diffusion. However, within this diffusion controlled region of mass transfer, tube-side fluid velocity remained important. For Gz numbers above 2, Sh could be satisfactorily described by extension to the Lévêque solution, which can be ascribed to the constrained growth of the concentration boundary layer adjacent to the fibre wall. Importantly this study demonstrates that whilst mass transfer in the low Graetz range does not explicitly conform to either the Graetz problem or classical Lévêque solution, it is possible to transform the experimentally derived overall mass transfer coefficient (K_ov) between characteristic length scales (d_h and L). T_his was corroborated by comparison of the empirical relationship determined in this study (Sh=0.36Gz) with previously published studies operated in the low Gz range. This analysis provides important insight for process design when slow tube-side flows, or low Schmidt numbers (coincident with gases) constrain operation of hollow fibre membrane contactors to the low Gz range.