Development of a greenhouse and screw conveyor solar driers for the treatment of faecal sludge from on-site sanitation facilities

Abstract

Thermal drying is an important unit operation in faecal sludge treatment. However, this process requires a high energy input, leading to high running costs due to high electricity or fuel consumption. Solar thermal energy can be used to decrease the costs of the drying process. This work aims to develop solar thermal drying technologies that efficiently harness solar thermal energy and are adapted to the sludge characteristics. Two different types of technologies were selected for their development, i.e. greenhouse-type solar drier and screw conveyor solar drier. After construction, the technologies were tested to measure their performance, find the optimum operating conditions, and identify improvement points. During the tests in the solar driers, it was observed that temperatures inside the enclosure were higher than the ambient conditions (+10-15°C for the greenhouse and +15-25°C for the screw conveyor drier), leading to relative humidities lower than 30%. An evaporation rate in the greenhouse was measured in the order of 0.8 kg/h/m² when conducting the tests with water. The drying tests with wetted soil in the screw conveyor solar drier resulted in a good performance with most of the moisture being removed from the soil after less than 30 minutes of operation. Testing with synthetic sludge also showed positive results, with moisture removal in the region of 20% to 50% in 2 hours. The most optimal conditions were obtained when the ventilation was operated at a low air flow rate, as the maximum temperatures achieved in the system were higher (after the solar air heater), leading to a higher moisture removal from the synthetic sludge. The results from the tests were promising. After the testing phase for both prototypes, improvements have been identified for the next round of iteration. In particular, the sludge stickiness is a critical problem that must be resolved to achieve sustainable long operation times