Steady-state analysis of activated sludge processes with a settler model including sludge compression

Steady-State Methodology for Activated Sludge Model 1 (ASM1) State Variable Calculation in MBR

The complexity of Activated Sludge Model No. 1 (ASM1) is one of the main obstacles slowing its widespread use, particularly among wastewater treatment plant (WWTP) professionals. In this paper, a simplification procedure based on steady-state mass balances is proposed for the conventional activated sludge process (ASP) configuration, consisting of an aerated bioreactor and a perfect settler (without particular compounds in the outlet). The results do, in fact, show perfect suitability to a membrane bioreactor process (MBR). Both organic carbon and nitrogen removal were investigated. The proposed approach was applied to ASM1, and simple analytical expressions of the state variables were obtained. These analytical expressions were then validated by comparison to simulations given by the original ASM1 (implemented in GPS-X software). A strong match (less than 4% of error overall) was obtained between both results in the steady-state; consequently, these analytical expressions may be useful as tools for quickly estimating the main state variables, feeding the filtration models, or identifying the interaction between operating parameters. Moreover, this enables a sensitivity analysis, covering relevant factors such as kinetics or operating parameters. For instance, the sludge retention time (SRT) effect is lower on XBH and XS at high SRT (˃20 days), while it is more pronounced on XP and XI as their variations with SRT are linear.

Development of an improved model for settling velocity and evaluation of the settleability characteristics

The characterization of sludge settling is important in determining the operating performance of the secondary clarifier and also evaluating the purification efficiency of wastewater treatment plants. Many models have been proposed to explain the activities of settling velocity and illustrating the relationship between settleability characteristics. The aim of this research was to present a model for the settling velocity as a function of the stirred specific volume index at 3.5 g/L (SSVI_3.5 ), based on the Vesilind function, the solids flux theory, and the experimental data. Using this model gives an important performance improvement after calibration of the different settleability parameters. PRACTITIONER POINTS: Batch settling tests are some of the most interesting source of information to determine the sedimentation velocities and their characteristics. The modeling as an optimum operation of wastewater treatment plants is widely used for design, optimization, and management of installations. The characterization of sludge settling is important in determining the operating performance of the secondary clarifier. The characterization of sludge settling is necessary to evaluate the purification efficiency of wastewater treatment plants.

Characterizing shallow secondary clarifier performance where conventional flux theory over-estimates allowable solids loading rate

The performance characteristics of relatively shallow (3.3 and 3.7 m sidewater depth in 30.5 m diameter) activated sludge secondary clarifiers were extensively evaluated during a 2-year testing program at the City of Akron Water Reclamation Facility (WRF), Ohio, USA. Testing included hydraulic and solids loading stress tests, and measurement of sludge characteristics (zone settling velocity (ZSV), dispersed and flocculated total suspended solids), and the results were used to calibrate computational fluid dynamic (CFD) models of the various clarifiers tested. The results demonstrated that good performance could be sustained at surface overflow rates in excess of 3 m/h, as long as the clarifier influent mixed liquor suspended solids (MLSS) concentration was controlled to below critical values. The limiting solids loading rate (SLR) was significantly lower than the value predicted by conventional solids flux analysis based on the measured ZSV/MLSS relationship. CFD analysis suggested that this resulted because mixed liquor entering the clarifier was being directed into the settled sludge blanket, diluting it and also creating a 'thin' concentration sludge blanket that overlays the thicker concentration sludge blanket typically expected. These results indicate the need to determine the allowable SLR for shallow clarifiers using approaches other than traditional solids flux analysis. A combination of actual testing and CFD analyses are demonstrated here to be effective in doing so.