Abstract
Background: Drying is an important step for the thermochemical conversion of solid fuels, but it is energy-intensive for treating highly moist materials. Methods: To inform the thermal treatment of faecal sludge (FS), this study investigated the drying characteristics and kinetics of various faecal wastes using thermogravimetric analysis and isothermal heating conditions. Results: The findings show that FS from anaerobic baffled reactor (ABR) and ventilated improved pit (VIP) latrines exhibit similar drying characteristics, with maximum drying rates at 0.04 mg/min during a constant rate period that is followed by a distinct falling rate period. On the contrary, fresh human faeces (HF) and FS from urine-diverting dry toilets (UDDT) exhibited a falling rate period regime with no prior or intermittent constant rate periods. The absence of constant rate period in these samples suggested limited amounts of unbound water that can be removed by dewatering and vice versa for VIP and ABR faecal sludges. The activation energies and effective moisture diffusivity for the sludges varied from 20 to 30 kJ/mol and 3∙10-7 to 1∙10-5 m2/s at 55°C and sludge thickness of 3mm. The Page model was consistent in modelling the different sludges across all temperatures. Conclusions: These results presented in this study can inform the design and development of innovative drying methods for FS treatment.
Highlights
More than one-third of the world’s population are without access to modern sanitation, a situation that disproportionately affects low-income countries, rural dwellers[1]
Isothermal drying behaviour The drying profiles of the anaerobic baffled reactor (ABR), human faeces (HF), urine-diverting dry toilets (UDDT) and ventilated improved pit (VIP) at a drying temperature of 55°C are shown in Figure 1 by means of a) moisture ratio (MR) vs time and b) drying rate vs moisture content
Drying characteristics and kinetics of various faecal sludges were examined using thermogravimetric analysis under isothermal conditions. The results from this investigation suggested a high level of boundedness of the moisture in the FS samples, for the HF and UDDT sludge, which would lead to a high energy consumption for drying
Summary
More than one-third of the world’s population are without access to modern sanitation, a situation that disproportionately affects low-income countries, rural dwellers[1]. On-site sanitation facilities are being developed in many parts of the world This includes the development of i) ecological toilets e.g. ventilated improved pit (VIP) latrines, composting toilets, and urine-diverting dry toilets to safely collect and convert human waste to an environmentally-friendly form (e.g. compost) and for recovery of useful products such as fuel and energy[2], and ii) advanced waste-to-energy technologies to convert human waste to fuel, heat and/or electricity, without putting undue pressure on natural resources[3,4]. An appropriate drying method is important for the safe removal of moisture In this regard, thermogravimetric techniques can provide insights into the drying characteristics of faecal sludges and kinetic processes governing internal mass transfer. Conclusions: These results presented in this study can inform the design and development of innovative drying methods for FS treatment
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