LIMITING SOLIDS FLUX ANALYSIS: A STUDY CASE COMPARING 1-D AND 2-D MODELS

This paper presents a limiting solids flux analysis for a typical secondary settling tank (SST) of a trickling filter/solids contact (TF/SC) wastewater treatment plant (WWTP). For this clarifier, the permissible solids loading rate (SLR) was found using three different approaches: (1) applying a 1-D solids-flux analysis with the Vesilind equation for the zone settling properties, (2) applying a 1-D solids-flux analysis using a modified Vesilind equation accounting for zone settling and compression rate properties, and (3) using a calibrated-validated 2-D hydrodynamic model. The 2-D model used in the simulation was developed by the authors.The SLR was changed by keeping constant the surface overflow rate (SOR) and changing the mixed liquor suspended solids (MLSS), and also by keeping constant the MLSS and changing the SOR. The analyses illustrate the sensitivity of the 1-D flux theory (1DFT) to the settling properties. Similarly, the results indicate that the straight application of the Vesilind equation in the 1DFT, using only zone settling properties, may underestimate the limiting SLR. However, when the analysis is performed using zone settling and compression rate properties, the permissible SLR is over-predicted. The use of 2-D models, as the one presented herein, helps to reduce some of the uncertainties associated with 1-D models and to better asses the design and optimization of clarifiers.