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PubMed | Biomath and Laval University
Type: Journal Article | Journal: Water science and technology : a journal of the International Association on Water Pollution Research | Year: 2015

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.


Burger R.,University of Concepción | Diehl S.,Lund University | Faras S.,Lund University | Nopens I.,BIOMATH | Torfs E.,BIOMATH
Water Science and Technology | Year: 2013

The consistent modelling methodology for secondary settling tanks (SSTs) leads to a partial differential equation (PDE) of nonlinear convection-diffusion type as a one-dimensional model for the solids concentration as a function of depth and time. This PDE includes a flux that depends discontinuously on spatial position modelling hindered settling and bulk flows, a singular source term describing the feed mechanism, a degenerating term accounting for sediment compressibility, and a dispersion term for turbulence. In addition, the solution itself is discontinuous. A consistent, reliable and robust numerical method that properly handles these difficulties is presented. Many constitutive relations for hindered settling, compression and dispersion can be used within the model, allowing the user to switch on and off effects of interest depending on the modelling goal as well as investigate the suitability of certain constitutive expressions. Simulations show the effect of the dispersion term on effluent suspended solids and total sludge mass in the SST. The focus is on correct implementation whereas calibration and validation are not pursued. © IWA Publishing 2013.


Torfs E.,BIOMATH | Bellandi G.,BIOMATH | Nopens I.,BIOMATH
Water Science and Technology | Year: 2012

Experimental data of Ca-induced activated sludge flocculation under different conditions of temperature and dissolved oxygen are investigated in order to model the influence of changing physical and chemical factors. However, current kernel structures for collision frequency and efficiency are unable to describe activated sludge flocculation data. Therefore, an earlier developed methodology based on an inverse problem is applied, yielding empirical models, to find out how flocculation is affected by these different environmental conditions. This contribution shows the useful application of inverse problems to improve the understanding of complex aggregation mechanisms. © IWA Publishing 2012.


Van Eerdenbrugh K.,Laboratory of Hydrology and Water ManagementGhent UniversityGhent Belgium | Van Hoey S.,BIOMATH | Verhoest N.E.C.,Laboratory of Hydrology and Water ManagementGhent UniversityGhent Belgium
Water Resources Research | Year: 2016

In this paper, a methodology is developed to identify consistency of rating curve data based on a quality analysis of model results. This methodology, called Bidirectional Reach (BReach), evaluates results of a rating curve model with randomly sampled parameter sets in each observation. The combination of a parameter set and an observation is classified as nonacceptable if the deviation between the accompanying model result and the measurement exceeds observational uncertainty. Based on this classification, conditions for satisfactory behavior of a model in a sequence of observations are defined. Subsequently, a parameter set is evaluated in a data point by assessing the span for which it behaves satisfactory in the direction of the previous (or following) chronologically sorted observations. This is repeated for all sampled parameter sets and results are aggregated by indicating the endpoint of the largest span, called the maximum left (right) reach. This temporal reach should not be confused with a spatial reach (indicating a part of a river). The same procedure is followed for each data point and for different definitions of satisfactory behavior. Results of this analysis enable the detection of changes in data consistency. The methodology is validated with observed data and various synthetic stage-discharge data sets and proves to be a robust technique to investigate temporal consistency of rating curve data. It provides satisfying results despite of low data availability, errors in the estimated observational uncertainty, and a rating curve model that is known to cover only a limited part of the observations. © 2016. American Geophysical Union.


Arnaldos M.,BIOMATH | Arnaldos M.,Acciona | Amerlinck Y.,BIOMATH | Rehman U.,BIOMATH | And 4 more authors.
Water Research | Year: 2015

The "affinity constant" (KS) concept is applied in wastewater treatment models to incorporate the effect of substrate limitation on process performance. As an increasing number of wastewater treatment processes rely on low substrate concentrations, a proper understanding of these so-called constants is critical in order to soundly model and evaluate emerging treatment systems. In this paper, an in-depth analysis of the KS concept has been carried out, focusing on the different physical and biological phenomena that affect its observed value. By structuring the factors influencing half-saturation indices (newly proposed nomenclature) into advectional, diffusional and biological, light has been shed onto some of the apparent inconsistencies present in the literature. Particularly, the importance of non-ideal mixing as a source of variability between observed KS values in different systems has been illustrated. Additionally, discussion on the differences existent between substrates that affect half-saturation indices has been carried out; it has been shown that the observed KS for some substrates will reflect transport or biological limitations more than others. Finally, potential modeling strategies that could alleviate the shortcomings of the KS concept have been provided. These could be of special importance when considering the evaluation and design of emerging wastewater treatment processes. © 2014 Elsevier Ltd.


PubMed | BIOMATH
Type: Journal Article | Journal: Water science and technology : a journal of the International Association on Water Pollution Research | Year: 2012

Experimental data of Ca-induced activated sludge flocculation under different conditions of temperature and dissolved oxygen are investigated in order to model the influence of changing physical and chemical factors. However, current kernel structures for collision frequency and efficiency are unable to describe activated sludge flocculation data. Therefore, an earlier developed methodology based on an inverse problem is applied, yielding empirical models, to find out how flocculation is affected by these different environmental conditions. This contribution shows the useful application of inverse problems to improve the understanding of complex aggregation mechanisms.

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