Sludge CO2 stripping and calcite precipitation

[Peppermint]

Hi all,

I am struggling with the modelisation of a process in which digested sludge (high pCO2) is stripped of its CO2 by being spit out by a pumping system in a poorly aerated chamber.

The incoming sludge is characterized by a pH (around 7,5-7,8) and alkalinity (around 7000 mg/L CaCO3) , and I am trying to see which pH should be reached by CO2 degassing to optimize the calcite precipitation potential, as well as how much calcite can precipitate. Also I know that kinetics must be of importance here as we observe on the field that the pH reaches a plateau at about 8, but have been failing to modelize this. I am also not sure what to charge balance on.

Would any of you have suggestions to help my case :)? Thank you so much in advance!

[dlparkhurst]

Two issues that I see are how much calcium is available, and how much CO2 degasses. Attached is a model that assumes 400 mg/L calcium (my arbitrary choice without any information) and calcite equilibrium as the log10 PCO2 is decreased from -0.75 (calculated from your initial conditions of pH and alkalinity) to -2.25.

In this simulation, virtually all of the calcium precipitates under all conditions, so the pH is mostly dependent on how thoroughly CO2 degases.

Water analyses before and after will give you information about whether calcite equilibrium is a valid assumption, CO2 dynamics, and kinetics.

If you have analyses of the major elements, and your solution is reasonably charge balanced, I generally do not try to charge balance the solution. If for some reason, you suspect errors in a particular ion, then you might charge balance on that element.

Code: [Select]

SOLUTION
-units mg/L
pH 7.6
Alkalinity 7000
Ca  400 # arbitrary guess
END
USER_GRAPH 1
    -axis_titles            "log10 PCO2" "pH" ""
  -start
10 GRAPH_X SI("CO2(g)")
20 GRAPH_Y -LA("H+")
  -end
END
USE solution 1
EQUILIBRIUM_PHASES
Calcite 0 0
CO2(g) -0.75 0
END
USE solution 1
EQUILIBRIUM_PHASES
Calcite 0 0
CO2(g) -1.5 0
END
USE solution 1
EQUILIBRIUM_PHASES
Calcite 0 0
CO2(g) -2.25
END


[Peppermint]

Hi,

Thank you very much for your answer :)! The issue is we don't have any analysis of the composition of the sludge except its pH and alkalinity...
Does phreeqC incorporates a way to look at the kinetics of calcite precipitation with an assumed initial Ca content?

Why did you pick a SI of -2.5 for CO2 and not ~-3.41, is this an arbitrary choice or is it based on the sample parameters?

Cheers!

[dlparkhurst]

PHREEQC has a kinetic rate expression for calcite based on Plummer and others in the phreeqc.dat, Amm.dat, and pitzer.dat databases. However, with so little analytical information, it seems a reach to use it. Analysis of Ca in the influent and effluent waters would be really useful to answer your questions.

The most relevant information you have is that the pH only reaches about 8.0. That indicates a relatively small loss of CO2 whether calcite precipitates or not. I used the range -0.75 to -2.25 because that more or less matched the pH that you indicated and because you said the receiving chamber was poorly aerated.

Thinking a little more about it, your sludge probably does not stop reacting, so you might have a continuing production of CO2, and I don't know what other reactions could be going on.