| Aerobic digestion of waste activated sludge has been investigated largely as a continuous or batch process. The actual practice found in the field usually involves sludge thickening and decanting of supernatant; a semi-batch process. Two potential control and design parameters are the cycle time, the time period between additions of sludge and the suspended solids retention time, analogous to the mean cell residence time found in the activated sludge process.;The ANOVA involving coefficient of compressibility showed only the block effect as being a significant factor statistically. The magnitude of these variations, however, were slight and would probably not be considered important in actual practice. The ANOVA involving specific resistance showed the block effect, the block-suspended solids retention time interaction and the effect of individual waste activated sludge samples used to feed the semi-batch reaction as significant. A contrast comparing the undigested sludge and the digested sludge showed that digestion made the sludge more difficult to dewater. Batch reactors were operated to act as a comparison. Based on limited data, batch aerobic digestion was just as likely to improve dewatering as it was to make it more difficult.;The supernatant COD ANOVA also showed the block effect, the block-suspended solids retention time interaction and the sludge used to feed the reactors as being significant. The block caused about 80 percent of the significant variations. The block means varied from 80 to 408 mg/l. At these levels, the supernatant would usually require further treatment before discharge.;With the interface settling velocity, all the factors and the cycle time-suspended solids retention time interaction were found to be significant. The settling velocity also appears to be a function of the solids concentration, which in turn, varied with suspended solids retention time. The only satisfactory velocities, from a practical point of view, were found with a cycle time of 4-5 days with a suspended solids retention time of 12 days or less.;The investigation of these two potential control and design parameters proceeded in two phases. During the first phase, the response of supernatant COD, solid-liquid interface settling velocity and dewatering characteristics, as measured by specific resistance and coefficient of compressibility, were investigated as functions of cycle time and suspended solids retention time. A nested-factorial experiment was performed, with a "block effect" nested within the cycle time effect. Analysis of Variance (ANOVA) was used to evaluate the results.;The second phase was concerned with the fate of solids in the semi-batch aerobic digestion process. A first-order reaction, involving degradable suspended volatile solids, was used and a model in condensed form was developed for the semi-batch process. Nonlinear regression was used to obtain estimates of the paramaters involved. High correlation indices were obtained. The data do not indicate substantial mineralization of the volatile solids. Most of the change in form of solids, however, was conversion of suspended solids to dissolved solids. |
