Leon Williams

Energy recovery from human faeces via gasification: A thermodynamic equilibrium modelling approach.

Highlights • On dry basis, typical human faeces contain 83 wt.% organic fraction and 17 wt.% ash.• The LHV of dry human faeces ranged from 19 to 22 MJ/kg, values similar to wood biomass.• Syngas from dry human faeces had LHV of 15–17 MJ/kg at equivalence ratio of ∼0.31.• Energy is best recovered from moist human faeces at equivalence ratio above 0.6.• Recoverable exergy potential from moist human faeces can be up to 15 MJ/kg.

An experimental investigation of the combustion performance of human faeces

Highlights • Dry human faeces have a Higher Heating Value (HHV) of 24 MJ/kg.• Faeces combustion was investigated using a bench-scale downdraft combustor test rig.• Combustion temperature of 431–558 °C was achieved at air flow rate of 10–18 L/min.• Fuel burn rate of 1.5–2.3 g/min was achieved at air flow rate of 10–18 L/min.• Combustion temperature of up to 600 ± 10 °C can handle 60 wt.% moisture in faeces.

Conceptual energy and water recovery system for self-sustained nano membrane toilet

Highlights • Energy and water recovery system from human excreta is modelled in Aspen Plus.• The Nano Membrane Toilet is proven to be a self-sustained system.• Up to 87% of total amount of water fed to the system can be recovered.• Net power output of the entire system is similar to the USB port peak power (2–6 W).• The specific net power output varies between 23.1 and 69.2 Wh/kgsettledsolids.

Thermodynamic analysis of a gamma type Stirling engine in an energy recovery system

Highlights • The performance of Stirling engine integrated to a micro-combustor in the NMT system was investigated.• Energy recovery and power generation of 27 Wh/h from combustion of human faeces.• The integrated position of the Stirling engine to the micro-combustor is highly paramount.• Sensitivity of the performance of the Stirling engine to working gas temperature.• Requirements for optimum performance of the Stirling engine for integration with micro-combustor.

Tube-side mass transfer for hollow fibre membrane contactors operated in the low Graetz range.

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.

Faecal-wood biomass co-combustion and ash composition analysis

Highlights • Co-combustion analysis was investigated using a bench-scale combustor test rig.• Raw human faeces (FC) contained 73.9 ± 4.4 wt% moisture as received basis.• Blending with wood dust (WD) in a 50:50 ratio reduced moisture levels by ∼40%.• Minimum acceptable blend for combustion without prior drying is 30:70 WD:FC.• Fuel burn rates are 3.18–4.49 g/min for all the blends at air flow of 12–18 L/min.• Oxygen, potassium and calcium are the most abundant elements in faecal ash.

Design and commissioning of a multi-mode prototype for thermochemical conversion of human faeces

Highlights • Ignition, gasification and combustion of simulant and real faeces were studied.• Trials using fuel flowrates of 1.2 g/min and 7.5–8 L/min of air were carried out.• Mean temperatures of 440–670 °C allowed self-sustained combustion.• Maximum temperatures reached for real faeces were in the range of 1210–1240 °C.• Combustion trials lasted up to 160 min without external heat supply.

Probabilistic performance assessment of complex energy process systems – The case of a self-sustained sanitation system

Highlights • A probabilistic model is developed to assess the performance of an NMT.• Energy and environmental performance uncertainties of the system are qualified.• A realistic prediction of the energy and environmental performance of the system.• Probabilistic approach can be applied in other complex engineering systems.

Selection of screw characteristics and operational boundary conditions to facilitate post-flush urine and faeces separation within single household sanitation systems

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.

Non-isothermal thermogravimetric kinetic analysis of the thermochemical conversion of human faeces

The “Reinvent the Toilet Challenge” set by the Bill & Melinda Gates Foundation aims to bring access to adequate sanitary systems to billions of people. In response to this challenge, on-site sanitation systems are proposed and being developed globally. These systems require in-situ thermal treatment, processes that are not well understood for human faeces (HF). Thermogravimetric analysis has been used to investigate the pyrolysis, gasification and combustion of HF. The results are compared to the thermal behaviour of simulant faeces (SF) and woody biomass (WB), along with the blends of HF and WB. Kinetic analysis was conducted using non-isothermal kinetics model-free methods, and the thermogravimetric data obtained for the combustion of HF, SS and WB. The results show that the devolatilisation of HF requires higher temperatures and rates are slower those of WB. Minimum temperatures of 475 K are required for fuel ignition. HF and SF showed similar thermal behaviour under pyrolysis, but not under combustion conditions. The activation energy for HF is 157.4 kJ/mol, relatively higher than SS and WB. Reaction order for HF is lower (n = 0.4) to WB (n = 0.6). In-situ treatment of HF in on-site sanitary systems can be designed for slow progressive burn.