Constituent Concentrations

Question

Hello 
 1- What is difference between &lsquo;first order&rsquo; and &lsquo;zero order&rsquo; in &lsquo;wall reaction rate&rsquo;? 
 2- In &lsquo;Analyzing Constituent Concentrations&rsquo; (help document in WaterGEMS v8i) it does not explain steps completely and just says to do this after that very briefly. How can I get some explaination about? For example about bulk reaction rate, wall reaction rate or diffusivity and their complex UNITs? 
 3- According the &lsquo;Help document&rsquo; for &lsquo;Analyzing Constituent Concentrations&rsquo; why we add concentrations for Junctions, pipes, Tank,Reservoir, Pumps and Valves? From which it injects and why we add concentration to anothers? 
 Sim

Akshaya Niraula · Accepted Answer

Zero Order: "A Zero-order reaction has a rate that is independent of the concentration of the reactant(s). Increasing the concentration of the reacting species will not speed up the rate of the reaction i.e. the amount of substance reacted is proportional to the time." The reaction rate is constant such as, 
 r = k 
 Example: Aging is of zero order rate. You are older one day, everyday. 
 First Order: "A first-order reaction depends on the concentration of only one reactant. Other reactants can be present, but each will be zero-order." The reaction rate can be represented by: r = k * concentration. 
 So, no matter what reaction rate you are interested in, the fundamental is the same. "Wall decay coefficients, however, are more difficult to measure and are frequently estimated using disinfectant concentration field measurements and water quality simulation results." - Advanced Water Distribution Modeling and Management Book References: http://en.wikipedia.org/wiki/Rate_equation http://en.wikipedia.org/wiki/Order_of_reaction http://chemwiki.ucdavis.edu/Physical_Chemistry/Kinetics/Reaction_Rates/Zero-Order_Reactions http://chemwiki.ucdavis.edu/Physical_Chemistry/Kinetics/Reaction_Rates/First-Order_Reactions

Akshaya Niraula · Answer

Q: How can get more information about Water Quality Modeling? 
 I encourage not to limit within the Help document for this broad and hot topic. External references might be the best to educate and stay up to date. 
 Some references: AWDM book: http://www.bentley.com/en-US/Training/Products/Resources/Books/AWDM.htm Modeling Water Quality, ISBN: 13: 9781583218167 Field Studies (EPA): http://nepis.epa.gov/Adobe/PDF/2000D2C2.pdf

Akshaya Niraula · Answer

Diffusivity: The way I understand in simple words is, it&rsquo;s a rate at which things travel over an area. &ldquo;things&rdquo; = concentration of substance, travel = flow = flux. It you look at the units it&rsquo;s Length squared over Time. Length squared is area and the time in denominator represents the rate. In other words it's a flux rate of a substance. Technically Diffusivity is a proportionality constant between flux and gradient of substance concentration. Diffusivity represents how quickly the substance reacts/disperse. 
 Wall Reaction Rate: A reaction rate constant between the boundary/surface/wall and the fluid. 
 Bulk Reaction Rate: Reaction rate constant between fluid and fluid itself.

Tom Walski · Answer

Our wall reaction model is based on that in EPANET. Here is some original documentation: 
 Wall Reactions 
 The rate of water quality reactions occurring at or near the pipe wall can be 
 considered to be dependent on the concentration in the bulk flow by using an 
 expression of the form 
 R = (A/V)K C 
 where Kw = a wall reaction rate coefficient and (A/V) = the surface area per unit 
 volume within a pipe (equal to 4 divided by the pipe diameter). The latter term 
 converts the mass reacting per unit of wall area to a per unit volume basis. EPANET 
 limits the choice of wall reaction order to either 0 or 1, so that the units of Kw are 
 either mass/area/time or length/time, respectively. As with Kb, Kw must be supplied to 
 the program by the modeler. First-order Kw values can range anywhere from 0 to as 
 much as 5 ft/day. 
 Kw should be adjusted to account for any mass transfer limitations in moving 
 reactants and products between the bulk flow and the wall. EPANET does this 
 automatically, basing the adjustment on the molecular diffusivity of the substance 
 being modeled and on the flow's Reynolds number. See Appendix D for details. 
 (Setting the molecular diffusivity to zero will cause mass transfer effects to be 
 ignored.) 
 The wall reaction coefficient can depend on temperature and can also be correlated to 
 pipe age and material. It is well known that as metal pipes age their roughness tends 
 to increase due to encrustation and tuburculation of corrosion products on the pipe 
 walls. This increase in roughness produces a lower Hazen-Williams C-factor or a 
 higher Darcy-Weisbach roughness coefficient, resulting in greater frictional head loss 
 in flow through the pipe. 
 There is some evidence to suggest that the same processes that increase a pipe's 
 roughness with age also tend to increase the reactivity of its wall with some chemical 
 species, particularly chlorine and other disinfectants. EPANET can make each pipe's 
 Kw be a function of the coefficient used to describe its roughness. A different function 
 applies depending on the formula used to compute headloss through the pipe: 
 Headloss Formula Wall Reaction Formula 
 Hazen-Williams Kw = F / C 
 Darcy-Weisbach Kw = -F / log(e/d) 
 Chezy-Manning Kw = F n 
 where C = Hazen-Williams C-factor, e = Darcy-Weisbach roughness, d = pipe 
 diameter, n = Manning roughness coefficient, and F = wall reaction - pipe roughness 
 coefficient The coefficient F must be developed from site-specific field 
 measurements and will have a different meaning depending on which head loss 
 equation is used. The advantage of using this approach is that it requires only a single 
 parameter, F, to allow wall reaction coefficients to vary throughout the network in a 
 physically meaningful way. 
 If you need more, see the paper 
 Rossman, L.A., Clark, R.M., and Grayman, W.M. (1994). “Modeling chlorine 
 residuals in drinking-water distribution systems”, Jour. Env. Eng., Vol. 120, No. 
 4, 803-820.

Akshaya Niraula · Answer

Sim, 
 For the third question, I assume you are referring to Initial concentration that's been added to the nodal elements? This is done under assumption that there will be some level of constituent concentration right at the beginning of the simulation. At any given time, there will be some constituents in the distribution system and to model that we add the initial concentration. 
 If you are not referring to this then please add some details. Thank you.

Akshaya Niraula · Answer

Hi Sim, 
 Yes, Initial Concentration = Concentration at the time zero = simulation start time. 
 The value that goes into the nodal points are generally some guesses, doesn't have to be exact. This is because, water quality scenarios are typically of a long duration so that the system could reach equilibrium (meaning, the pattern of the graph is nearly parallel to time axis, x-axis). You can simply enter some meaningful value and let the model run for a longer duration. 
 Regarding the Units, I encourage to understand the equations behind them and for that please look at some of the books I mentioned earlier in the thread. This help link will also show you some of the equation. http://docs.bentley.com/en/HMWaterCAD/Bentley_WaterGEMS_Help-19-32.html Thanks,

Tom Walski · Answer

In general, there will be some chlorine in the system at time 0. However, WaterGEMS doesn't know unless you tell it. If you don't, it assumes that the concentration is 0. What will happen is that as water from the chlorinated source reaches nodes, the correct concentration is calculated. This means that for several hours/days depending on system size, the chlorine concentration is initially unknown. You will need to make fairly long runs to reach a quasi-steady state condition. 
 If you set initial concentrations, especially at tanks, the model reaches this quasi=steady concentration much more quickly so that the run duration can be much shorter. 
 A good way to set initial condition is to make a long run, copy the concentration out to a spread sheet and then copy them into the initial conditions. Use those values as the starting point.