Estimation of sludge sedimentation parameters from single batch settling curves

In this paper a comparison is made between two means of obtaining the param eters for the settling velocity models that are at the core of the solid fl ux theory: (i) the traditional approach using zone settling velocity (V-ZS) data obtained from a dilution experiment and (ii) a new direct parameter e stimation method relying on a single batch settling curve (SBSC). For four distinct sludges, settling curves were recorded at different sludge concent rations (X) and the Vesilind parameters were calculated in the traditional way. The value of the resulting model was evaluated by cross-validating it on its ability to describe complete SBSC's. Provided the dynamics of the sl udge blanket descent were moderate a settler model incorporating these trad itional Vesilind parameters could reasonably match the experimental batch s ettling curves. However, when the dynamics of the sludge blanket descent we re fast, the Vesilind model failed. It was tried whether other settling vel ocity models could result in better fits to the SBSC. Here. the Cho model t urned out to be the most effective one. When cross-validating the Cho model on the dilution experiment Cis-data, however, it was found to be less perf orming than the Vesilind model in describing the relationship between ii, a nd X. The fact that the Vesilind model is superior to the Cho model in desc ribing such relationships, while the Cho model is better in describing comp lete settling curves, clearly points out that current settling models are s till very empirical in nature. An important finding was that with all settl ing velocity models tested, practical identifiability problems appeared, in dicating the need for better experimental designs. Finally, the flux curves associated with the SBSC-estimated Cho parameters were compared with the t raditional Vesilind flux curves. Although both types of flux curves general ly showed similar trends, the reliability of SBSC-based flux curve predicti ons is at present insufficient to warrant replacement of the traditional es timation of settling characteristics. (C) 1999 Elsevier Science Ltd. All ri ghts reserved.