Drying of faecal sludge from VIP latrines through a medium infrared radiation process

Abstract

In order to treat faecal sludge from ventilated improved pit (VIP) latrines, eThekwini Municipality (Durban, South Africa) developed an infrared dryer, ‘LaDePa’ (Latrine Dehydration Pasteurization). Parameters that influence its operation were investigated using a laboratory-scale replica of the full-scale machine. For this, faecal sludge collected from VIP latrines was pelletized and dried under different operating conditions. Drying curves were obtained by plotting medium wave infrared intensity (MIR), height of emitters above the belt, air flowrate and pellet diameter against the residence time. These curves were then used to determine the drying rate and energy consumption. The results show that the drying rate increased while the energy consumption decreased by increasing the power of the MIR emitters and decreasing the size of the pellets. For example, the drying time to get a moisture content of 0.8 g water/g dry solid was shortened from 27 to 6 min while the energy consumption for this reduced from 1.5 to 0.8 kWh after increasing the MIR power from 1.5 to 3.3 kW. Similar drying curves were obtained by varying the distance between the pellets and MIR emitters, and adjusting intensity of the MIR radiation to obtain the same temperature in the drying zone. It was also observed that higher airflow rates enhanced mass transfer rates, but led to a cooling effect. No effect on the drying rate was observed after pre-drying the sludge or adding sawdust. The study shows that for the process to be efficient, the MIR intensity should be high enough for fast drying to occur (T ≥ 150°C), but without causing thermal degradation (T ≤ 220°C). The height of emitter above the belt and the pellet size should be as small as possible (8 mm); airflow rate should be optimised to maximize the mass transfer rate and minimize the cooling effect.