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.

Tuesday, 20 December 2016

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 rangeJournal 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 (Kov) 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 (Kov) between characteristic length scales (dh and L). This 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.

Tuesday, 22 November 2016

New funding and new video announced!

We have just recevied news that we have successfully been awarded a new phase of funding;  we are also launchign a new video explaining our latest vision for the toilet.   Our press release is copied below:

Cranfield University's Nano Membrane Toilet project has landed a major funding boost to secure the next phase of development of a novel and sustainable sanitation solution for the benefit of the huge number of people around the world who currently have no hope of being able to access a clean and affordable toilet in their home.

Dr Alison Parker, from the Cranfield Water Science Institute, said; "This is a great moment; the new funding from the Bill & Melinda Gates Foundation will support our research teams in water, energy and design to tackle the considerable challenge of turning the laboratory prototype Nano Membrane Toilet into a product for the marketplace."

The Nano Membrane Toilet uses a waterless flush; a unique rotating mechanism that drops the waste into a holding tank whilst simultaneously blocking odour and the user's view of the waste. The solids then settle to the bottom of the tank, while the liquids float on the top. The solids are transported out of the tank by mechanical screw into a combustor where they are burnt and transformed into ash. The heat generated can be converted into electricity which is used to power toilet operations, and any residual energy is used for charging mobile phones or other low voltage items. The liquids pass over a weir in the holding chamber and into the membranes bundle. The unique nanostructure membrane allows clean water to be extracted from the waste which can subsequently be used in the household for washing or watering plants.

The toilet is designed for single-household use (up to ten people) and accepts urine and faeces as a mixture. Developed with the aspirations and needs of the user in mind, it is small and easy to transport to locations where there is no access to a water supply and sewer. In comparison to the public toilets relied on by urban communities around the world, a household toilet offers convenience, dignity and security especially for vulnerable groups like women, the disabled and the elderly.

A new video has also been released highlighting some of the recent innovations and improvements to the toilet. With World Toilet Day (19th November) helping to raise awareness and inspire action to tackle the global sanitation crisis, Cranfield's toilet is attracting interest from around the world, and was recently showcased at the Toilet Investment Summit in Mumbai, India.

Wednesday, 9 November 2016

Nano Membrane Toilet in India

A 25% scale model of The Nano Membrane Toilet will be on display next week at two venues in India.   Firstly at the Toilet Investment Summit in Mumbai (15-17 Nov), and secondly at the British Counil Education Fair in Delhi on 19th Nov.  

Thursday, 6 October 2016

Modelling water and energy recovery

The last in the current series of three papers from our Energy team.   This one describes the potential water and energy recovery from the Nano Membrane Toilet and how the recovery of these two resources can be balanced.   An exciting conclusion is that the toilet should have a net power output equivalent to a USB port peak power, enough for charging a mobile phone.

Hanak, D., Kolios, A., Fidalgo, B., McAdam, E., Parker, A., Williams, L., Tyrrel, S., Cartmell, E., (2016) Conceptual energy and water recovery system for self-sustainednano-membrane toilet, Energy Conversion and Management 126, 352-361

With about 2.4 billion people worldwide without access to improved sanitation facilities, there is a strong incentive for development of novel sanitation systems to improve the quality of life and reduce mortality. The Nano Membrane Toilet is expected to provide a unique household-scale system that would produce electricity and recover water from human excrement and urine. This study was undertaken to evaluate the performance of the conceptual energy and water recovery system for the Nano Membrane Toilet designed for a household of ten people and to assess its self-sustainability. A process model of the entire system, including the thermochemical conversion island, a Stirling engine and a water recovery system was developed in Aspen Plus®. The energy and water recovery system for the Nano Membrane Toilet was characterised with the specific net power output of 23.1 Wh/kgsettledsolids and water recovery rate of 13.4 dm3/day in the nominal operating mode. Additionally, if no supernatant was processed, the specific net power output was increased to 69.2 Wh/kgsettledsolids. Such household-scale system would deliver the net power output (1.9–5.8 W). This was found to be enough to charge mobile phones or power clock radios, or provide light for the household using low-voltage LED bulbs.

Wednesday, 21 September 2016

Latest prototypes on display at New Scientist Live

We have developed a new prototype based on our latest system configuration.  This will be on display to the public at New Scientist Live in the Excel Centre from 22-25 Sept.   We are on stand 841!

Update, here are some photos from the event:

Thursday, 18 August 2016

Experiments to combust human faeces

The next paper in the series from our energy team describes a series of experiments to combust human faeces, which is informing the design of the gasifer.   The paper is free to download from the journal "Fuel".

Onabanjo, T., Kolios, A.J., Patchigolla, K., Wagland, S., Fidalgo, B. Jurado, N., Hanak, D.P.,  Manovic, V., Parker, A., McAdam, E., Williams, L., Tyrrel, S. (2016) Cartmell, E., An experimental investigation of the combustion performance of human faeces, Fuel 184, 780–791

Poor sanitation is one of the major hindrances to the global sustainable development goals. The Reinvent the Toilet Challenge of the Bill and Melinda Gates Foundation is set to develop affordable, next-generation sanitary systems that can ensure safe treatment and wide accessibility without compromise on sustainable use of natural resources and the environment. Energy recovery from human excreta is likely to be a cornerstone of future sustainable sanitary systems. Faeces combustion was investigated using a bench-scale downdraft combustor test rig, alongside with wood biomass and simulant faeces. Parameters such as air flow rate, fuel pellet size, bed height, and fuel ignition mode were varied to establish the combustion operating range of the test rig and the optimum conditions for converting the faecal biomass to energy. The experimental results show that the dry human faeces had a higher energy content (∼25 MJ/kg) than wood biomass. At equivalence ratio between 0.86 and 1.12, the combustion temperature and fuel burn rate ranged from 431 to 558 °C and 1.53 to 2.30 g/min respectively. Preliminary results for the simulant faeces show that a minimum combustion bed temperature of 600 ± 10 °C can handle faeces up to 60 wt.% moisture at optimum air-to-fuel ratio. Further investigation is required to establish the appropriate trade-off limits for drying and energy recovery, considering different stool types, moisture content and drying characteristics. This is important for the design and further development of a self-sustained energy conversion and recovery systems for the NMT and similar sanitary solutions.