(735ba) A Population Balance Based Model to Optimize Coagulation-Flocculation

Given the decreasing availability of drinking water, the growing needs of the global population, climate change, and desertification, drinking water treatment is increasingly necessary. The conventional drinking water treatment train comprises four steps: coagulation-flocculation, sedimentation, filtration, and disinfection. During the first step of the process, colloidal particles bind to each other and form flocs, whose physical properties, such as size and shape, determine their settleability and filterability. The success of subsequent steps will be mainly driven by these characteristics. Therefore, coagulation-flocculation can be deemed the essential component of the conventional drinking water treatment scheme, as it affects the next steps of the treatment and its efficiency strongly impacts the performance of the overall treatment (Tzoupanos et al., 2008).

In view of the above, the optimization of coagulation-flocculation is fundamental to optimizing drinking water treatment. Coagulant dosage is considered to be the most significant of the several variables that are known to influence this process, because it is largely related to ensuring an efficient and cost-effective treatment. Given its importance, the main focus of optimization strategies is thus the determination of the optimal coagulant dosage. Currently, jar tests, which are lab-scale simulations of the coagulation-flocculation and sedimentation stages, and/or machine learning models are used to identify the optimal coagulant dosage (Ratnaweera & Fettig, 2015). However, both methods have limitations that hamper their ability to successfully optimize coagulation-flocculation. The former is time-consuming and cannot cope with rapid changes in process conditions in real time, while the latter struggles with unforeseen events and can be difficult to interpret.

It is hypothesized that a potential way forward to improving the prevalent optimization techniques is the incorporation of floc properties (Khedher et al., 2023). To explicitly consider them in the optimization strategy, the Population Balance Modelling (PBM) framework is the ideal tool. This type of model describes the evolution of particulate systems suspended in a continuous phase by tracking the dynamics of key properties, such as size distribution and shape. The successful development of a PBM model could potentially lead to improvements in both state-of-the-art optimization strategies. This approach could be used to reduce the number of jar tests needed to identify the optimal coagulant dosage or even to replace them. Moreover, the PBM could be used to integrate mechanistic-based knowledge into the machine learning based approaches to improve interpretability and their capability of dealing with unforeseen events.

In this work, we present the first population balance based model capable of determining the optimal coagulant dosage. The model was calibrated and validated on an extensive data set obtained by the authors. The experiments performed describe the dynamic evolution of flocs’ particle size distribution and mass fractal dimension throughout a jar test for multiple coagulant dosages. The goal of this modelling exercise is to show the importance of considering floc properties in the optimization process, as well as deepen the current understanding of how these properties evolve and impact the particle separation stages of the treatment. Finally, this work seeks to develop a novel optimization strategy that could potentially be used as a complement or an alternative to the prevailing optimization methods.

References

Khedher, M., Awad, J., Donner, E., Drigo, B., Fabris, R., Harris, M., Braun, K., & Chow, C. W. K. (2023). Using the Flocculation Index to optimise coagulant dosing during drinking water treatment. Journal of Water Process Engineering, 51. https://doi.org/10.1016/j.jwpe.2022.103394

Ratnaweera, H., & Fettig, J. (2015). State of the art of online monitoring and control of the coagulation process. In Water (Switzerland) (Vol. 7, Issue 11, pp. 6574–6597). MDPI AG. https://doi.org/10.3390/w7116574

Tzoupanos, N., Zouboulis, A., Tzoupanos, N. D., & Zouboulis, A. I. (2008). Coagulation-flocculation processes in water/wastewater treatment: the application of new generation of chemical reagents Coagulation-Flocculation, 6^th IASME/WSEAS International Conference on Heat Transfer, Thermal Engineering and Environment (HTE’08), Rhodes, Greece, August 20-22, 2008