Intensity of Radiation and Rate of Sludge Drying

Field experience has indicated that drying time required by the conventionally designed and operated sludge drying beds is comparable to that required by beds modified to provide only limited drainage and cleaned with mechanical equipment. This investigation provides quantitative information that concerns the dewatering characteristics of digested sludge and is useful for the design of drying beds with limited drainage. The rate of water evaporated from a sludge surface and a free-water surface, as a function of the radiant intensity incident on the sludge surface, was found to be essentially equal when evaporation was the sole mechanism of dewatering. For an intensity of 1.00 cal per sq cm per min, the evaporation rate was 0.9 × 10^-3 gm per sq cm² per min. The evaporation rate from the sludge surface was depressed by 22% when both evaporation and drainage contributed to the dewatering. One-half of the energy incident on the sludge surface was found to be associated with the latent heat of vaporization. The emissivity was found to be dependent on the wave-length of radiation. The critical moisture for the sludge used ranged from 66% to 84%.