Drying of faecal sludge from ventilated-improved pit latrines( VIP latrines) using solar thermal energy.

Abstract

Ventilated improved pit (VIP) latrines are a basic form of sanitation in South Africa. The main challenge facing the application of VIP latrines is the gradual fill up of the pit due to usage. A sustainable way of treating FS remains from sanitation systems is by using solar energy. This project is focused on the characterisation of solar drying for FS treatment. A solar thermobalance was designed and constructed to investigate the solar and open-air sun drying of FS. Mass variations, temperature, humidity and solar irradiance properties were monitored during the drying process. The drying experiments for solar drying were conducted with the following variables: weather conditions; air temperature; airflow velocity; sample thickness and surface area. The experiments were carried out in October and November 2017. The drying kinetics were observed to be influenced by the weather conditions, air temperature and air flow rate, but the sample thickness and surface area of the sample did not have any significant effect on the drying rate, under the explored conditions. Solar drying recorded drying rates which averaged 0.892 kg/h.m2 across the investigated parameters and were comprised between 0.622 to 1.135 kg/h.m2, whereas open-air drying averaged a drying rate of 0.479 kg/h.m2, varying in the range 0.686 - 0.078 kg/h.m2. The faecal sludge was found to dry best under sunny conditions, 30 oC air temperature and 0.5 m/s air flowrate. Qualitative and quantitative analysis was also conducted to characterise the influence of solar and open-air sun drying on the physico-chemical characteristics of faecal sludge. Density, shrinkage, thermal conductivity and heat capacity showed great dependence on the final moisture content which depended on the employed drying conditions. Crusting and cracking of the samples were observed to occur, and their intensity depended on the investigated conditions and were suspected to affect the drying rate in specific experimental conditions. This is shown in the lower drying rates for 1.0 m/s air flow rate and 60 oC air temperature. Odour strength increased with lower final moisture content. In summary, the application of solar energy for drying proved to be a feasible option for the treatment of FS. Drying in a solar thermal system with controlled conditions of air properties will consist in an improvement on the current sludge dehydration practices, such as the drying beds (open-air drying).