This article has general information on getting ready to run a constituent analysis in WaterGEMS or WaterCAD. You can also find a Quick Start Lesson related to running water quality analyses, including constituent runs. In addition, Example1.wtg (located in the Samples folder within the product installation folder) has a scenario related to running a constituent analysis. This article will walk through some general steps on getting the necessary information to get started.
A constituent analysis is a type of water quality analysis that allows you to track the the concentration of a particular constituent at a given point in the network, over time. To compute a constituent analysis, you need to set the Calculation Type to "Constituent" in the calculation options, and configure the Constituent alternative.
A calibrated, extended period simulation (EPS) model is a good starting point for water quality modeling. An EPS run should be used because this will account for mixing and storage in tanks and reservoirs, which is a contributing factor to the degradation of water quality in a system. In addition, chemical reactions depend on pipe flows, pathways and the time that the water is in the system. A steady sate run will not account for these to the extent needed.
Note that as of August 2019, WaterCAD and WaterGEMS do not currently have a water quality calibration tool to calibrate coefficients such as the wall decay rate, based on measured constituent residuals. (related discussion here)
The information that you will include for a constituent include the diffusivity, the bulk reaction, and the wall reaction.
For the diffusivity, you should be able to find default values in various documentation. Example1.wtg includes a couple of constituents where you may be able to find the diffusivity, if you could not obtain this value from another source. However, the bulk and wall reaction is something that is best determined within your specific system. Getting these values will likely require field studies and lab analyses.
Bulk reaction data is typically determined through laboratory testing. Bulk reaction coefficients can be determined using an experimental procedure called a bottle test. A bottle test will find the bulk reactions by separating from other processes that may impact water quality and evaluated as a function of time. Determining the length of the bottle test and the frequency you take samples is critical to having accurate data.
Typically, the duration of the test should reflect the transport time that occurs in the system. For instance, if a water age analysis of the model indicates a range of 5-7 days, a 7-day bottle test would provide bulk reaction data for the entire range. The frequency of sampling is related to the rate of the reaction. In a typical case, sampling should be more frequent early in the test and can gradually be decreased later in the time range.
If the constituent in the bulk fluid has a first-order reaction, it will plot as a generally straight line on a semi-log graph and allow you to determine the bulk reaction rate.
Wall reaction data is typically found from field studies. They are more difficult to ascertain than bulk reaction coefficients. Wall reaction coefficients are similar to pipe roughness coefficients in that they can vary from pipe to pipe. wall reaction coefficients cannot be directly measured, so they must be found from results in the field. This can sometimes be difficult. In such cases, you can test different values of the wall reaction coefficient against field data using time-series data. If you see values in the model that do not match up will with the field test data, you can adjust the wall reaction coefficient until you see better results.
In addition to the bulk and wall reaction, you will need accurate initial information, including concentrations at the sources. Having accurate reading for concentrations, along with reasonable values for bulk reaction coefficients, wall reaction coefficients and diffusivity, will better assure accurate results in your modeling results.
For more in-depth information on water quality modeling, as well as additional information on the steps listed above, please see the Advanced Water Distribution Modeling and Management book. You can find Quick Start Lessons related to water quality runs, including constituent analyses, in the Help documentation.
Modeling DBP formation - Water Quality Analysis
What is the difference between the different Constituent Source Types used in a Constituent analysis?
Analyzing concentration of multiple constituents, or a multi-point trace