Water and wastewater CFD and validation: are we losing the balance?

A literature screening on computational fluid dynamics (CFD) modelling in water treatment applications showed a vast range of validation ranging from no validation at all, over residence time distribution (RTD) and tracer testing, to velocity field, species concentration and, finally, turbulence properties measurements. The validation level also differs depending on process scale (laboratory, pilot, full) and type of system (rheology, single phase vs. multiphase). Given the fact that CFD is in more widespread use, a discussion on the extent and need of validation needs to be initiated. This paper serves as a discussion starter on the topic.

CFD-based process optimization of a dissolved air flotation system for drinking water production.

Dissolved air flotation (DAF) has received more attention recently as a separation technique in both drinking water as well as wastewater treatment. However, the process as well as the preceding flocculation step is complex and not completely understood. Given the multiphase nature of the process, fluid dynamics studies are important to understand and optimize the DAF system in terms of operation and design. The present study is intended towards a comprehensive computational analysis for design optimization of the treatment plant in Kluizen, Belgium. Setting up the modelling framework involving the multiphase flow problem is briefly discussed. 3D numerical simulations on a scaled down model of the DAF design were analysed. The flow features give better confidence, but the flocs escape through the outlet still prevails which is averse to the system performance. In order to improve the performance and ease of maintenance, design modifications have been proposed by using a perforated tube for water extraction and are found to be satisfactory. The discussion is further reinforced through validating the numerical model against the experimental findings for stratified flow conditions.

Effect of sensor location on controller performance in a wastewater treatment plant.

Complete mixing is hard to achieve in large bioreactors in wastewater treatment plants. This often leads to a non-uniform distribution of components such as dissolved oxygen and, hence, the process rates depend on them. Furthermore, when these components are used as input for a controller, the location of the sensor can potentially affect the control action. In this contribution, the effect of sensor location and the choice of setpoint on the controller performance were examined for a non-homogeneously mixed pilot bioreactor described by a compartmental model. The impacts on effluent quality and aeration cost were evaluated. It was shown that a dissolved oxygen controller with a fixed setpoint performs differently as a function of the location of the sensor. When placed in a poorly mixed location, the controller increases the aeration intensity to its maximum capacity leading to higher aeration costs. When placed just above the aerated zone, the controller decreases the aeration rate resulting in lower dissolved oxygen concentrations in the remainder of the system, compromising effluent quality. In addition to the location of the sensor, the selection of an appropriate setpoint also impacts controller behavior. This suggests that mixing behavior of bioreactors should be better quantified for proper sensor location and controller design.

Testing the Holtzworth-Munroe and Stuart (1994) batterer typology.

A. Holtzworth-Munroe and G. L. Stuart (1994) proposed that 3 subtypes (family only [FO], borderline-dysphoric [BD], and generally violent-antisocial [GVA]) would be identified using 3 descriptive dimensions (i.e., severity of marital violence, generality of violence, psychopathology) and would differ on distal and proximal correlates of violence. Maritally violent men (n = 102) and their wives were recruited from the community, as were 2 comparison groups of nonviolent couples (i.e., maritally distressed and nondistressed). Four clusters of violent men were identified. Three resembled the predicted subtypes and generally differed in the manner predicted (e.g., FO men resembled nonviolent groups: BD men scored highest on measures of dependency and jealousy; GVA men had the most involvement with delinquent peers, substance abuse, and criminal behavior; and both BD and GVA men were impulsive, accepted violence, were hostile toward women, and lacked social skills). The 4th cluster (i.e., low-level antisocial) fell between the FO and GVA clusters on many measures.