Re: CCPP calculation using the PHREEQC program

["David L Parkhurst" <dlpark@xxxxxxxx>]

> I tried to figure out how to get CCPP from the PHREEQC simulation based
upon
your example, but I have some difficulties understanding how to set up the
parameters and where to find the results  off the simulation. First of all,
we are using Phreeqci 2.4.2 from your web site. Let me know if you are
talking about a different program.

The latest version of PhreeqcI is 2.6.1. I recommend that you use it
although 2.4.2 should be ok.

> Then let me make sure I have the right set up for the calculation.
Currently
I have 4 blocks: SOLUTION, REACTION_TEMPERATURE, EQUILIBRIUM_PHASES, and
PRINT. Please tell me if I need more blocks.

The example I gave you is adequate. You do not normally need the PRINT data
block.

TITLE Example 1.--Add uranium and speciate seawater.
SOLUTION 1  SEAWATER FROM NORDSTROM ET AL. (1979)
        units   ppm
        pH      8.22
        pe      8.451
        density 1.023
        temp    25.0
        Ca              412.3
        Mg              1291.8
        Na              10768.0
        K               399.1
        Cl              19353.0
        Alkalinity      141.682 as HCO3
        S(6)            2712.0
END
USE solution 1
EQUILIBRIUM_PHASES 1
      calcite
END
REACTION_TEMPERATURE
      20.
USE solution 1
EQUILIBRIUM_PHASES 1
      calcite
      CO2(g) -3.5
END


> Now, let me list my difficulties and questions as follows:

> Questions related to your initial conditions:
> 1) I assume the first block in your example is the initial parameters
that
user provides.

Yes.

> Why is pe=8.451? The default is 4.0

pe is only important if you have total concentrations of redox active
species. The default is adequate in most cases for potable water. For
seawater, the pe has been measured by various methods, including platinum
electrode and various redox couples. Normally you do not have this
information. If the water is oxygenated, you can use that information to
set the pe, but for starters you can just use the default for your waters.


> 2) How do you set up the EQUILIBRIUM_PHASES parameters? Do you select
calcite and CO2 or just calcite?

It depends on your physical system. Will the waters be open to the
atmosphere by sitting in a reservoir, or are they pumped directly into the
distribution system, which does not have contact with the atmosphere? If
CO2 will not exchange with the atmosphere, do not include CO2.

> How do you choose the initial numbers (such as SI and amount)?

Choose 0 for the calcite SI, which means equilibrium with calcite. If CO2
is included, SI should be -3.5 for equilibration with the atmosphere. If
you just want to find out if calcite will precipitate, you may choose 0
moles of calcite initially present in the EQUILIBRIUM_PHASES definition. If
you want to find out how much calcite would dissolve or precipitate
(whichever will occur thermodynamically) then you should define an excess
of calcite that could react with the solution (1 mole is more than
sufficient). For CO2, I would define 1 or more moles to allow the solution
to ingas or outgas CO2 to equilibrate with the atmosphere.

> In the REACTION_TEMPERATURE block, do I choose linear or list? For
temperature, what should I use (25C)?

I don't know. You have to make some choices here about what you are
modeling. I assume you inject this water into the distribution system, so
you are concerned about precipitating calcite as the water is being pumped
through the system. In that case, the temperature in the distribution
system, which is likely to be the average annual temperature would be
appropriate. If you are worried about reservoirs or the treatment plant
itself, then choose an appropriate temperature.


> 3)You said "The final block equilibrates with calcite in a system open to
atmospheric PCO2 and a temperature of 20 C. In this case, 0.75 mmol/kgw of
calcite precipitate." Are you referring to the EQUILIBRIUM_PHASES block?

I am referring to the output file, After "Beginning of Batch-Reaction
Calculations", "Phase assemblage" printout, "delta" column, 2.4e-4 is in
moles per kilogram of water. Positive indicates an increase in the mass of
calcite, that is, precipitation from solution.

> Questions related to obtaining the results:
> 1) It is most confusing to find the results. Off what you said in your
example, all I could find in my simulation output is the Calcite SI, and it
is 0.76.

That is the saturation state of the initial water. It indicates that
calcite should precipitate from this water composition.

> However, I don't know where to find the "0.24 mmol/kg water of
calcite precipitate"

Farther down the output, it states "Beginning of Batch-Reaction
Calculations" twice. The first time gives the results for the closed system
(no CO2 in the Phase assemblage).

> and the "0.75 mmol/kgw of calcite precipitate." In
addition, how do I know which is closed and which is open to atmosphere?

Following the second instance are the results for the open system at 20 C
(CO2 is listed under Phase assemblage).

> 2) where is CCPP?

It is the amount of calcite that precipitates is it not?

> In my simulation using your parameters, I used 0.0 initially as SI for
both
calcite and CO2 in the EQUILIBRIUM_PHASES block (because I wouldn't know
what to put in there). After the first simulation, I found SI=0.76 and
-3.18
for calcite and CO2 respectively, then I plugged them in and started
another
simulation. I am attaching my second output as follows for your review. I
still can't find your 0.24 mmol/kg and 0.75 mmol/kgw.

You should not have made these changes, the results can be obtained by
running the original file.

David




David Parkhurst (dlpark@xxxxxxxx)
U.S. Geological Survey
Box 25046, MS 413
Denver Federal Center
Denver, CO 80225

Project web page: https://wwwbrr.cr.usgs.gov/projects/GWC_coupled