Hi,
I am using SWMM 5.0 in order to simulate TSS in stormwater.
I know that water quality routing within conduit assumes that the conduit behaves as a Continuously Stirred Tank Reactor (see CSTR.jpg file).
In reference to the substance involved in the reaction, a perfect and ideal mixing is assumed, and the universal law of material balance is applied (see transport formula.jpg file), where the coefficient K is the rate constant.
I would like to know if it is possible to modify directly K in the above mentioned formula. If yes, how?
I look forward to hearing from you soon.
Thank you,
Angela
Hi Angela,
I feel the issue is not the value for 'K', but the fact that the software you're using is not up to the job. A conduit is NOT a stirred tank. Of course you can try to calibrate your model against measurements, but you'll most probably end up with a poor fit and unrealistic values for the modelparameters. It depends on the application of the results whether or not you can be satisfied with the outcomes.
best regards
Francois Clemens
Hi Angela,
Under "Pollutants" double-click "TSS" (if this is the name of the pollutant) and in the Pollutant Editor there is a property "Decay Coeff.".
But as Francois said: CSTR is possibly in most cases not a good model for pollutant transport in sewers (especially for TSS where you have build-up and wash-off). Do you use a build-up / wash-off model (Land Uses in SWMM5)?
Maybe you could divide your conduits into short pieces to get better results.
But ask your question in the SWMM user forum [email protected].
Kind regards
Rolf
Thanks for this interesting question that may open a very useful discussion, Dear Angela. I am here for learning, as always. I took a look to the 13th edition from 2010 of the User’s guide to SWMM 5, by James, W.; Rossman, L.E.; James, W.R.E. that explains the approximation considered for the “water quality routing” in page 88 and in chapter 21 “Introduction to Background Theory of SWMM Transport” and how to use the pollutant Editor and Decay coefficient in page 141. (Link to the pdf bellow)
The equation as it is presented is not explicitly homogeneous giving ft3 mg/L, in both sides (instead of mg as could/should be)? For TSS (mg/L), this is probably also a question of units, given that although the concentration may vary between the inlet and outlet of any element, it does not look that there is (at least in practical terms) a decay of mass total in the element due to chemical and biochemical transformations. The increase of mass in an element is the difference between the mass that enters in the element and the mass that leaves the element in a given interval of time. Thus I suspect that for TSS, K should be null in the equation presented, at least approximately, even with the question of unities? Alternatively there is a visible transference of mass between phases?
With respect to the model considered, the art of engineering is also obtaining the maximum approximation possible in the simplest way. Did you formulate and develop any numerical model giving a better solution than the models presented by finite differences in the SWMM manual for solving this problem, Dear Clemens? I concur with Rolf. It is possible to ask also directly to USEPA, at least for user details. I made it once and they have been extraordinarily kind.
The importance of the biochemical and chemical transformations along the sewerage systems (that may have some relation with this decay coefficient K) is an area of concern for me since a long time ago. See the RG question bellow, for example.
http://www.chiwater.com/Files/UsersGuideToSWMM5Edn13.pdf
https://www.researchgate.net/post/What_is_the_real_relevance_in_the_wastewater_treatment_of_the_biochemical_and_chemical_transformations_that_occur_in_sanitary_sewer_networks
Hi Antonio,
actually there are better model descriptions for these processes, see for example the work of J. Huisman: http://e-collection.library.ethz.ch/view/eth:24110?q=recommendationof
best regards
Francois
Thanks, dear Francois for your prompt response and for the reference to the PhD Thesis entitled “Transport and transformation processes in combined sewers” even if it looks a little out of date, given that it is relative to combined systems that are since a long time ago being converted to separate systems. I would like to know what the information exactly is and the corresponding pages that you consider relevant in this Thesis for this discussion in storm water flows (supposing naturally that you read the thesis, and are aware of its content).
Thanks to all of you for your very intresting and useful suggestions.
They will help me to improve my model by understanding its weak points.
Hi Angela, Antonio,
with respect to the outdatedness of Huismans thesis: actually it is in my opinion the last word on understanding the transport processes in a sewer conduit, basically you should read the whole thesis as it addresses a quite comprehensive number of relevant interlinked processes. @Angela: working with models is always tricky: especially when you are confined to use a given software product.
best
Francois
Hi all,
What I found in the SWMM5 User Manual:
"3.4.7 Water Quality Routing Water quality routing within conduit links assumes that the conduit behaves as a continuously stirred tank reactor (CSTR). Although a plug flow reactor assumption might be more realistic, the differences will be small if the travel time through the conduit is on the same order as the routing time step. The concentration of a constituent exiting the conduit at the end of a time step is found by integrating the conservation of mass equation, using average values for quantities that might change over the time step such as flow rate and conduit volume."
But I don't know why they removed the plug flow option in SWMM5.
I think that transform processes are not important in case of TSS and stormwater. Concerning transformation processes in sewers I think that Jacco's work and the thesis of Maria Do Céu Almeida are the most important I know.
Kind regards, Rolf
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