Multi-objective Optimisation
Published on: 17 Oct 2023
By: Andrew Slaughter
This #TechnicalTuesday, Gabriel Pronek explains the methodology, application and benefits of WaQCoM, WRc's proprietary water quality modelling tool.
WRc provides an in-house water quality and corrosion model, commonly referred to as WaQCoM. This tool provides a method for rapid assessment of the effects of treatment and blending on water quality. The tool also models scaling or corrosion behaviour of materials in contact with water. WaQCoM can provide a simple means of estimating chemical dosing requirements and exploring different treatment scenarios. This is useful for those involved in both operation and design of plants. The output is given in both tabulated and graphical form.
The process model bundles together a range of equations and parameters, which relate to water chemistry, blending, corrosion, and scaling potential in one easy-to-use package. This is what sets WaQCoM apart from other similar tools which do some of what WaQCoM does but don’t have the extensive blending functionality - what happens when different waters are mixed together.
The Specific and Detailed Outputs
The software outputs parameters relating to carbonate saturation status, including pH, alkalinity, carbon dioxide concentration, total inorganic carbon concentration, acidity, total buffer capacity, Langelier Saturation pH, Langelier Index, Ryznar Stability Index, saturation ratio, driving force index, momentary excess, Larson Ratio, aggressivity index, and calcium carbonate precipitation potential (CCPP).
The corrosion calculations include copper pitting propensity rating, Rosette corrosion potential, dezincification potential, zinc solubility, and estimated lead solubility. The software can give estimated compositions resulting from the blending of two different water sources over the full range of blending ratios, or more than two water sources in specified proportions. WaQCoM can also be used to simulate the effects of chemical treatment with regards to the above parameters.
The Chemistry Explained
The fundamental physical chemistry behind how the model works revolves around chemical equilibria. In an ideal solution, the equilibrium of a reversible reaction is related to the concentrations of the reactants. In reality, deviations from ideal behaviour occur because of interactions between ions; opposite charges in the solution attract one another whilst like charges repel one another. These deviations are accounted for by multiplying concentrations by activity coefficients.
These activity coefficients are dependent on factors such as the charge of an ion, the ionic strength of the solution, and temperature. In WaQCoM, activity coefficients are estimated using the extended Debye-Huckel equation, which is applicable for dilute solutions. This departure from ideality also applies to pH, the strict definition of which is in terms of hydrogen ion activity (concentration x activity coefficient) rather than concentration.
Of Interest To
Though WaQCoM is principally of interest to water companies, with several of them already using the current version, it is also a tool that applies well to the consulting sector.