Speaker: Santiago Septien Stringel
Thermal drying is the most efficient method to achieve low moisture content in faecal sludge, but it involves a high energy demand. The use of solar thermal energy for drying proposes could reduce drastically the energy consumption, leading to a significant reduction in costs. Under this context, two solar thermal drying technologies adapted to faecal sludge, namely a greenhouse-type and screw conveyer solar driers, were developed to offer a cost-effective solution to municipalities and sanitation stakeholders for the treatment of faecal waste that could be applied in-situ or in a faecal sludge treatment plant.
Thermal drying is the most efficient method to remove the moisture from faecal sludge to low contents and to achieve to satisfying levels of disinfection, but it usually involves a high energy demand for moisture evaporation. The use of solar thermal energy could be a cost-effective solution to supply the heat required for drying. An important number of solar drying technologies have been deployed in the food and agriculture sectors (Belessiotis and Delyannis, 2011). Solar drying has also been widely applied for sewage sludge treatment, particularly in greenhouse-type solar dryers (Bennamoun, 2012).
However, this option has not been enough exploited, probably because of the lack of awareness, knowledge and data. In order to tackle these drawbacks, a previous project was undertaken in order to explore faecal sludge solar thermal drying through a bench-scale rig (Septien et al., 2018). As a continuation of this work, the current project aims at developing a pilot-scale solar thermal drier from the knowledge and lessons learnt in the previous project.
The first step of this project consisted of a preliminary study to perform a detailed assessment of the solar resource in Durban. The solar assessment determined the average monthly solar irradiance during the last 5 years. Based on these values, the evaporation rate that would be obtained using solar thermal energy was calculated. The results showed that the evaporation rate would vary between 4.3 to 7.5 kg/d/m2 along the year, with an average value of 5.8 kg/d/m2. The period of May to July exhibited the lowest values (winter), whereas the highest values were found in December and January (summer).
Concerning the development of the solar drier in this project, two technologies of interest, namely a greenhouse-type solar dryer and a screw conveyor, were developed and their functionality was tested with successful (for the moment without faecal sludge). The former case consists of a batch system where the sludge will be placed as a bed of a given thickness and will be removed after reaching the desired dryness. This system incorporates a ventilation system, a solar collector to heat the air inside the dryer, a scraping system for sludge mixing and possibly ground heating. These measures enable to maximize the drying rate, to avoid crust formation and to lead to a homogenous drying. Instrumentation is placed at different locations of the system in order to monitor and record the key parameters from the process.
The screw conveyer solar dryer is composed of a transparent tube, inside which the sludge will be dried in continuous mode as the sludge moves forward from the inlet to the outlet. The system includes a ventilation system allowing for air circulation inside the tube, instrumentation to measure the key parameters, reflectors standing next to the conveyer in order to increase the amount of solar radiation incidence on the sludge, a solar collector for air pre-heating and a dehumidification unit to dehumidify the air before its introduction in the drying zone.
This study confirmed that solar drying in a thermal system is an an interesting cost-effect alternative for faecal sludge treatment. Based on the figures from the solar assessment, the theoretical annual capacity of a solar plant to reduce the moisture content of sludge from 80 to 20% would be 2.8 tonnes of sludge per m2 in the eThekwini Municipality (Durban, South Africa). This would allow treating the faecal sludge from ventilated improved pit (VIP) latrines generated in a year (estimated to 12000 tonnes) using the surface area of 4 Olympic pools (5000 m2).
In the developed solar driers, the drying of faecal sludge is expected to occur within a few days if the weather conditions are favourable, and with the correct operation of the ventilation and scrapping systems. The drying process could become significantly more difficult to carry on in the last stage of drying, as the moisture boundedness should strengthen as drying will proceed. If the moisture content is already low before the increase of the moisture boundedness, the process could be stopped at this point in order to lead to higher performance.
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2018. Solar drying of faecal sludge from pit latrines in a bench-scale device
in: 41st WEDC Conference. Nakuru