Estimation of sludge sedimentation parameters from single batch settling curves

In this paper a comparison is made between two means of obtaining the parameters for the settling velocity models that are at the core of the solid flux theory: (i) the traditional approach using zone settling velocity (VZS) data obtained from a dilution experiment and (ii) a new direct parameter estimation method relying on a single batch settling curve (SBSC). For four distinct sludges, settling curves were recorded at different sludge concentrations (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 SBSCʹs. Provided the dynamics of the sludge blanket descent were moderate a settler model incorporating these traditional Vesilind parameters could reasonably match the experimental batch settling curves. However, when the dynamics of the sludge blanket descent were fast, the Vesilind model failed. It was tried whether other settling velocity models could result in better fits to the SBSC. Here, the Cho model turned out to be the most effective one. When cross-validating the Cho model on the dilution experiment VZS-data, however, it was found to be less performing than the Vesilind model in describing the relationship between VZS and X. The fact that the Vesilind model is superior to the Cho model in describing such relationships, while the Cho model is better in describing complete settling curves, clearly points out that current settling models are still very empirical in nature. An important finding was that with all settling velocity models tested, practical identifiability problems appeared, indicating the need for better experimental designs. Finally, the flux curves associated with the SBSC-estimated Cho parameters were compared with the traditional Vesilind flux curves. Although both types of flux curves generally showed similar trends, the reliability of SBSC-based flux curve predictions is at present insufficient to warrant replacement of the traditional estimation of settling characteristics.