Analysis of biosolids equilibrium moisture and drying

Provided that moisture is well managed, biosolids, or digested wastewater sludge, can be converted from an expense to a resource of renewable energy. Toward this goal, the equilibrium moisture content (EMC) for biosolids was determined experimentally in the ranges of 10–84% relative humidity and 30–60°C. Several well‐known models were used to fit the EMC data, including a new one that considers mixing and swelling effects on water activity. The rate of drying was measured as a function of temperature and gas velocity in a 3‐inch fluidized bed. The drying rate increased with temperature and was independent of gas velocity. In all cases, the drying rate continuously decreased; no period of constant rate was observed. This indicates that the dominant resistance to drying was internal diffusion.