Modelling biogeochemical cycles in wastewaters

[Rob Colston]

I'm trying to show the stability of struvite (NH4MgPO4.6H2O, which I've had to create in GWB) in wastewater, using the typical chemistry of wastewaters I've been able to find and calculated the ion species activities. I was then wondering if there's a way of showing bacterial impact for precipitating, i.e. thermodynamics overcome when the solution isn't saturated. Any help or advice would be appreciated, it's been a while since I've used GWB so any suggestions to get me up to speed would also be appreciated. 

Cheers!

[Jia Wang]

Hello Rob,

If you’re doing a simple calculation of the equilibrium state of your wastewater, or making an Eh-pH diagram or stability diagram of some sort, there’s nothing extra to incorporate.

You can certainly account for the effects of microbes in a process model, however. Please note that a mineral still has to reach saturation to precipitate. Microbes can catalyze redox reactions that are thermodynamically favored, but kinetically limited. I’m not especially familiar with the details of biological wastewater treatment, but perhaps the byproducts of microbial metabolism (e.g. NH4+ from dissimilatory NO3- reduction) help drive the saturation of struvite.

If you’re interested in simulating the effects of microbial metabolism and growth, you’ll probably want to use a kinetic rate law in React. A simple option is to use an enzyme-mediated reaction to predict the influence on species concentrations. In enzymatic catalysis, a substrate combines with an enzyme to form an activated complex, which can decay to give a catalytic product.  React can also consider more complex models of microbial metabolism and growth, including competing strains. Perhaps a good place to start is chapter 4.6.2 (Enzymes and biotransformation) and chapter 4.7(Microbial metabolism and growth) in the GWB Reaction Modeling Guide.  

Best regards,

Jia Wang