Comparative study of hot air drying and microwave drying for dewatered sludge

The seafood processing industries produce large amounts of wastewater sludge in Thailand. Most of sludge is dewatered and then sent directly to landfills without utilization nor resource recovery. The dried sludge, having a gross calorific value (GCV) higher than 21.9 MJ/kg, might be worthwhile to utilize as refuse-derived fuel. However, the critical obstacle is its high moisture content of 87.4 wt%. This study investigated two techniques for drying sludge: hot air and microwave drying. The effects of hot air temperatures (100 °C–170 °C) and microwave power levels (100–800 W) were studied for their specific energy consumption (SEC), dried product characteristics, and drying kinetics. The results showed that the drying times decreased with the increase in hot air temperatures and microwave power. Under the tested conditions in this study, the application of microwaves reduced the drying time by at least 37.5% compared to using hot air. The reduction of drying times led to decreased energy consumption. Approximately 72.5–80.3 wt% of dried sludge was the volatile matter (VM). As this component was devolatilized, the GCV of the dried sludge decreased. The sludge dried by the microwaves had a GCV of 20.8–21.4 MJ/kg, while by the hot air had a GCV of 19.4–21.8 MJ/kg. The results from product analysis suggested that the microwave technique could be an alternative drying method when one requires a shorter drying time than the conventional hot air technique.