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Plant wide chemical water stability modelling with PHREEQC for
drinking water treatment
A.W.C. van der Helm*/**, O.J.I. Kramer*, J.F.M. Hooft* and P.J. de Moel***/**
* Waternet, PO Box 94370, 1090 GJ, Amsterdam, the Netherlands
(E-mail: alex.van.der.helm@waternet.nl, Tel: +31 6 5248 0203)
** Delft University of Technology, Faculty of Civil Engineering and Geosciences, Department of Water
Management, PO Box 5048, 2600 GA, Delft, the Netherlands
*** Omnisys, Eiberlaan 23, 3871 TG, Hoevelaken, the Netherlands
Abstract
In practice, drinking water technologists use simplified calculation methods for aquatic chemistry
calculations. Recently, the database stimela.dat is developed especially for aquatic chemistry for
drinking water treatment processes. The database is used in PHREEQC, the standard in geohydrology
for calculating chemical equilibria in groundwater. The development of a graphical user interface for
PHREEQC in Microsoft Excel has made it possible to easily incorporate complicated chemical
calculations for use by technologists of drinking water treatment plants. By making use of PHREEQC
the calculations performed are more accurate because of inclusion of ionic strength, ion pairs and most
recent determined chemical equilibrium constants. Due to this development it is possible to for instance
validation of laboratory measurements and on-line sensors. The use of PHREEQC is demonstrated in a
simulation of the chemical water stability at drinking water treatment plant Weesperkarspel of
Waternet.
Keywords
Chemical equilibrium; drinking water treatment; modelling; phreeqc; stimela
INTRODUCTION
PHREEQC (USGS, 2014) has become the ‘de facto’ standard in geohydrology for
calculating chemical equilibria in groundwater. It is developed by the US Geological
Survey (USGS), starting in 1980 with regular updates and extensions to date. Major
aspects contributed to its success are:
scientific base, fully traceable;
adapted to newest scientific knowledge;
users can modify and extend the basics;
freely available;
availability of an MS COM module for communication with e.g. MS Excel.
Recently de Moel et al. (2014) developed the stimela.dat database for PHREEQC
which is specifically designed for water treatment. The stimela.dat database is based
on the phreeqc.dat database (de Moel et al., 2013) that is available in PHREEQC.
Adaptation of the phreeqc.dat database is necessary because, on the contrary to
geochemistry, in water treatment processes thermodynamic equilibrium of all redox
reactions cannot be assumed. Therefore, inert elements are defined for a number of
parameters such as:
non N(+5) oxidation states of nitrogen, with species NH4+, N2 and NO2-;
typical anaerobic groundwater species, such as Fe2+, Mn2+ and NH4+;
all anaerobic gases such as CH4, H2S, NH3 (already implemented in PHREEQC
version 3).
Waternet, the water cycle utility of Amsterdam and surrounding areas in the
Netherlands, is adopting the use of stimela.dat in PHREEQC in their operation
through the use of a Microsoft Excel user interface with PHREEQC. This paper
describes the use of stimela.dat through Excel for calculation of chemical stability in
drinking water treatment.
MATERIALS AND METHODS
Data is used from the Loenderveen-Weesperkarspel drinking water treatment, see
Figure 1. The pre-treatment Loenderveen consists of an intake of seepage water from
the Bethune polder and a consecutive treatment of coagulation and sedimentation,
self-purification in a lake-water reservoir and rapid sand filtration. The pre-treated
water is transported over 14 kilometers to the Weesperkarspel treatment plant without
chlorination. The first process at the treatment plant Weesperkarspel is ozonation for
disinfection purpose and oxidation of organic matter. Thereafter, pellet reactors are
used to reduce hardness (softening) and biological activated carbon filtration is
applied to remove natural organic matter and organic micro-pollutants. The last step
in the treatment is slow sand filtration. The drinking water is transported and
distributed without residual chlorine.
Figure 1 Process scheme of drinking water treatment plant Loenderveen-Weesperkarspel of Waternet
In the model, the water from the lake-water reservoir, with a detention time of about
100 days, is used as influent. The used laboratory measurements are temperature, pH,
calcium, magnesium, alkalinity, ammonium, natrium, kalium, ferric iron, chloride,
sulfate, nitrate, phosphae, oxygen, silica and total organic carbon. In Table 1 an
overview is given of the relevant chemical reactions within the treatment plants. All
these chemical reactions are defined within the PHREEQC/Stimela database, as
equilibrium reactions with all related equilibrium constants and compound
characteristics.
Table 1 Chemical reactions included in the PHREEQC/Stimela water treatment model
Process
Item
Reaction
pH control
HCl-dosing
HCO3- + H+ + Cl- -- > CO2 + H2O + Cl-
Filtration
NH4-oxidation
NH4+ + 2 O2 + 2 HCO3- -- > NO3- + 2 CO2 + 3 H2O
Ozone
O3-reduction
2 O3 -- > 3 O2
Softening
NaOH-dosing
CO2 + OH- + Na+ -- > HCO3- + Na+
NaOH-dosing
HCO3- + OH- + Na+ -- > CO32- + Na+
CaCO3-crystallization
Ca2+ + CO32- -- > CaCO3(s)
pH control
HCl-dosing
CO32- + H+ + Cl- -- > HCO3- + Cl-
AC filtration
DOC-oxidation
CH2O + O2 -- > CO2 + H2O
pH control
NaOH-dosing
CO2 + OH- + Na+ -- > HCO3- + Na+
O2 control
O2-dosing
O2 -- > O2
SS filtration
DOC-oxidation
CH2O + O2 -- > CO2 + H2O
The input file for the model only defines the quantity of dosed chemicals or the
quantity of the converted compounds. In this case the operation of the plant is
simulated for the caustic soda and hydrochloric acid dosages at Loenderveen and
Weesperkarspel based on the set-points used in the plants for the calcium carbonate
saturation index (SI) and total hardness (TH) after pellet softening. The calculated
dosages are obtained within the model by iterative calculations, by specific algorithms
or by ‘normal’ functionality of PHREEQC.
The output of the model is fully in compliance with all related equilibria as defined
within the chemical database. The model calculates the content of all compounds,
after each treatment step, as well as typical calculated values for e.g. pH, SI and
electrical conductivity.
The model set-up allows for the incorporation of non-chemical compounds such as
turbidity and UV-extinction. In this particular model these non-chemical compounds
were not included, as the prime goal for this model was the prediction of acid and
base dosing levels.
The computer program PHREEQC version 3.1.4 (phreeqci-3.1.4-8929.msi) was
used to solve the mathematical equations which are generated from the chemical
database stimela.dat and an input file (.pqi), both adjustable by the user (Parkhurst and
Appelo, 2013). For use of PHREEQC through Excel the communication module
version 3.0.6 (IPhreeqcCOM-3.1.4-8929-win32.msi) was used.
RESULTS
In Figure 2 the TH and the SI of the influent and the set-points in the plants are
shown, as well as the calculations results for the acid and base dosages to meet the
set-points. These are the results of the dataset of one day as they are presented in MS
Excel and calculated with PHREEQC. In Figure 3 the calculation results for 4 years
are shown.
Winning Waterleidingplas Loenderveen Weesperkarspel Drinkwater
1 2 3 4 5 6 7 8 9
Tijdstip 2009 13-Jan 12:00 13:57 17:04 17:09 17:09 17:09 18:08 18:09 3:03
Flow m3/week 473.944 464.465 464.465 464.465 464.465 464.465 463.443 463.443 458.809
m3/h 2.821 2.765 2.765 2.765 2.765 2.765 2.759 2.759 -
Waterkwaliteit
Totale ha rdheid mmol/l 2,27 2,27 2,27 1,49 1,49 1,49 1,49 1,49 1,49
SI 0,39 0,14 0,14 0,79 0,79 -0,05 -0,24 0,35 0,31
Dosering
Werkzame stof HCl NaOH HCl NaOH
Streefwaarde (setpoints) SI 0,35 TH 1,49 SI -0,05 SI 0,35
Hoeveelheid mmol/kgw 0,000 0,14 1,049 1,49 0,189 -0,05 0,175 0,35 0,31
Verbruik
Online kwaliteitsmetingen mmol/ kgw 0,000 1,05 0,189 0,175
Debiet doseerpompen l/h 0,0 50,9 8,3 8,5
Verbruik kg/week (100%) 0 19.485 3.205 3.242
Beschikbare voorraad dagen -85325,6 2,5 8,7 2,5 Totaal
Verbruikskosten € 0,01 / m3 0,00 1,47 0,36 0,24 2,10
€ /week 0 6.820 1.690 1.135 9.645
-01
00
01
01
02
01
02
02
03
Run PHREEQC
Figure 2 Simulation results for chemical dosages at drinking water treatment plant Weesperkarspel for
January 13th 2009
0,0
0,2
0,4
0,6
0,8
1,0
1,2
1,4
Dosage (mmol/kgw)
Simulated acid-base dosages DWTP Weesperkarspel
HCl Lake NaOH Softening HCl Softening NaOH Activated carbon
Figure 3 Simulated chemical dosages based on Phreeqc modelling
CONCLUSIONS
The development of a graphical user interface in Excel has made it possible to
incorporate complicated chemical calculations in daily operation of drinking water
production for technologists. By using PHREEQC the calculations performed are
more accurate, because effects of ionic strength and ion pairs are included and most
recent determined chemical equilibrium constants are used. Furthermore the dosing of
chemicals with regard to the calcium carbonate saturation can be optimised. Important
for practice of technologists is the availability of a tool that can work with data arrays
instead of for instance average year values. This gives better insight in the processes
and leads to better decisions by process technologists.
REFERENCES
de Moel, P. J., van der Helm, A.W.C., van Rijn, M., van Dijk, J.C. and van der Meer, W.G.J. (2013).
Assessment of calculation methods for calcium carbonate saturation in drinking water for DIN
38404-10 compliance. Drink. Water Eng. Sci. Discuss., 6, 167–198.
de Moel, P. J., van Dijk, J. C., and van der Meer, W. G. J. (2014). Aquatic chemistry for engineers,
Volume 1, Starting with PHREEQC 3. Delft University of Technology, Delft, the Netherlands.
Parkhurst, D. L and Appelo, C. A. J. (2013). Description of input and examples for PHREEQC version
3—A computer program for speciation, batch-reaction, one-dimensional transport, and inverse
geochemical calculations, US Geological Survey, Denver, USA.
USGS: phreeqci-3.1.4-8929.msi, http://wwwbrr.cr.usgs.gov/projects/GWC_coupled/phreeqc/, last
access: 13 October 2014.
