HIGH IRON, HIGH CO2 GROUNDWATER- A DIFFICULT COMBINATION
Clara Laydon 1, Gary Hallsworth 2, Karl Woods
1. Hunter Water Australia, NSW
2. Aqwest, Bunbury Water Board, WA
ABSTRACT
2
holiday location of the South West. The location
and region of Bunbury is shown in Figure 1.
Aqwest is the water supply utility for the Bunbury
region in Western Australia. Aqwest source their
water from the Yarragadee aquifer.
The groundwater can have high levels of iron and
low pH ranges (6.2 – 6.5) due to high levels of
dissolved CO2. The levels of iron in Aqwest’s bores
are highly variable ranging from approximately 1
mg/L to 25 mg/L. Aqwest curently uses areation
and prechlorination to removed dissolved CO2 and
to oxidise the iron before filtration.
Aqwest engaged HWA to review pH correction and
water stability options, to raise the current lower
pH levels to the target of 7.6 and reduce CO2
levels to 15 mg/L.
Water quality modelling, jar testing and pilot testing
for lime dosing was undertaken, however it was
found that chemical addition alone was not able to
fully correct the pH to the target level, this was due
to the high levels of aggressive CO2 .
CO2 stripping investigations where undertaken and
it was found that the existing spray areators and
bench testing of traditional cascade or diffused
areation was not able to reduce the CO2 levels
without a considerable and extended residence
times. It was theorised that the high iron levels
where interfering with the CO2 stripping and a more
aggressive aeration technique would be required.
After futher investigations, including pilot testing, it
was found that aggressive aeration, such as by
using a surface areators could reduce both CO2
levels and increase the pH to target levels. The
aeration also had cost offsets, as the aeration also
oxidises the majority of the iron and therefore the
chlorine usage would drop significantly.
INTRODUCTION
Bunbury is located approximately 2 hours south of
Perth in Western Australia. The city has
approximately 31,500 people and is a well-known
Figure 1: Bunbury Location (Google earth)
Aqwest sources its water from the Yarragadee
aquifer. This groundwater source can have high
levels of iron and low pH ranges (6.2 – 6.5) due to
high levels of CO2. The levels of iron in Aqwest’s
bores
are
highly
variable ranging from
approximately 1 mg/L to 25 mg/L of iron.
This investigation focused on Aqwest’s Tech Water
Treatment Plant (WTP). Tech WTP is a 12.5 ML/d
plant which treats ground water through:
 A spray bed aerator which assists in stripping
disolved CO2 and for oxidising the iron.
 Chlorine is added post aeration to oxidise any
remaining soluble iron.
 The iron particulates are then filtered out
through Dynasand® (continuous up-flow) filters.
 The chlorine residual is maintained from the prefilatrion dose point for disinfection.
Tech WTP produces water which has a low pH
range (median pH 6.7) and is potentially corrosive
in nature. Given these factors, Aqwest engaged
HWA to review options for increasing the pH and
improving stability levels.
BACKGROUND THEORY
Theory - Aggressive CO2
The measured free CO2 is comprised of two
components, these are the aggressive and
equilibrium CO2 fractions.
There is a direct relationship between equilibrium
CO2, bicarbonate and pH within water. While the
pH is between 5 and 6.35, the majority of the
equilibrium CO2 is in a free state, and as pH
increases above 6.35 the equilibrium CO2 is
increasingly converted into bicarbonate which
remains in solution.
At a pH around 7.5 the majority of the equilibrium
CO2 has been converted into bicarbonate. This
explanation is simplified in Figure 2.
Figure 2: Relationship between Equilibrium CO2,
Bicarbonate and pH
As the equilibrium CO2 is in equilibrium with
bicarbonate, the fraction of equilibrium CO2 at
different pH values can be calculated by using the
alkalinity value.
Alkalinity is the measure of:
[ALK] = 2[CO32-] + [HCO3-] + [OH-] - [H+]
However the fractions of carbonate, hydroxide and
hydrogen are so low that alkalinity can be taken to
be directly equivalent to bicarbonate. However as
alkalinity is generally measured as CaCO3,
conversions need to be undertaken to convert
CaCO3 back to bicarbonate. Further calculations
then need to be undertaken to determine the molar
fractions, and the equivalent equilibrium CO2
amount. A formula has been created which
simplifies this process:
pH = log(2.2x106 x [ALK]/[CO2])
Therefore the equilibrium CO2 levels can be
calculated as long as the pH and alkalinity is also
known.
Natural water may contain a quantity of free CO2
which is greater than that necessary to keep the
equilibrium relationship between CO2 and HCO3- .
This extra free CO2 is classified as aggressive
CO2, and this is the CO2 that causes the majority
of corrosion in copper pipes. The aggressive CO2
amount can be found by subtracting the equilibrium
CO2 (found by using the above equations) from the
measured free CO2 levels (determined onsite
through titrations).
As aggressive CO2 is not part of the equilibrium
curve it cannot be converted into bicarbonate, the
only effective manner of removing aggressive CO2
is through aeration stripping.
Theory - Stability
The other important aspect of water stability is the
impact on cement lined structures and pipes. This
is related to pH, alkalinity and therefore calcium
carbonate concentration (CaCO3). Water which is
considered unstable can strip cement linings as
there is insufficient CaCO3 in the water. This also
has implications on the long term sustainability of
concrete assets, such as reservoirs and cement
lined pipes. The increase in pH throughout a
distribution system which has concrete assets.
Due to the increasing pH this can also reduce the
efficiency of chlorine disinfection, due to the
preferential conversion of chlorine to hypochlorite
at higher pH levels, which can lead to biofilms and
microbiologically induced corrosion. Alternatively if
there is too much CaCO3 in the water, precipitation
or scaling can occur. This can also impact on the
assets as scaling can form uneven deposits which
reduces pipe diameter, increases friction headloss
and can cause pit corrosion.
There are a number of indices that have been
developed to determine how stable the water is in
relation to the CaCO3. The Langelier Saturation
Index (LSI) and Calcium Carbonate Precipitation
Potential (CCPP) are two indices that are
commonly used to determine the stability of water.
The Langelier Saturation Index (LSI) is an index
that provides a measure of the stability of a water
with respect to its degree of CaCO3 saturation.
 If LSI is negative: No potential to scale, the
water will dissolve CaCO3 impact on concrete
assets and increase pH in the reticualtion
system.
 If LSI is positive: Scale can form and CaCO3
precipitation may occur
 If LSI is close to zero: Borderline scale
potential. Water quality or changes in
temperature, or evaporation could change the
index.
 In practice, water is considered to be in an
optimal range if the LSI is less than 0 and is
considered to be aggressive if less than -1 at
25 ºC.
The Calcium Carbonate Precipitation Potential
(CCPP) is often considered a more reliable water
stability index to use since this index provides a
quantitative measure of the calcium carbonate
deficit or excess of the water, giving a more
accurate guide as to the likely extent of CaCO3
precipitation.
 If CCPP is positive: Scale can form and CaCO3
precipitation may occur
 If CCPP is -5 to 0: The water is conisderd to be
passive
 If CCPP is -10 to -5: The water is considered to
be midly dissolving
 If CCPP is < -10: : The water is considered to
be dissolving
 In practice, the target CCPP range is
5<CCPP<-1 at 25 ºC.
METHODOLOGY/ PROCESS
Due to the low pH values and in particular, high
levels of free CO2, there was concern that the
water potentially corrosive. This was confirmed as
water stability calculations for the LSI and CCPP
stability indices showed that the Tech WTP (after
current treatment) was producing aggressive water
and was likely to be causing damage to concrete
pipes and reservoirs.
During this initial review it was also found that the
current spray bed aerator which was originally
sized for 350 - 400 kL/h, were overloaded as the
can plant operate at 455 kL/h at peak periods.
At this stage of the project HWA and Aqwest set
the optimal water quality targets that the
investigations would aim to achieve, with the pH at
7.6, and free CO2 at 15 mg/L.
Plant Optimisation
Given the possibility that spray beds where
overloaded, it was determined that the most cost
effective method for correcting CO2 levels was to
attempt to improve the aeration efficiency. New
nozzles where fitted, and a low cost lime treatment
option of running aerated water over lime chips
was trialled as shown in Figure 3.
Tech WTP Water Characteristics
Tech WTP has three main production bores, the
characteristics of the water are summarised below
in Table 1.
Table 1: Raw Water Characteristics
Typical
Draw
Typical
Typical
Iron
Rate
pH
Bore
CO2
Levels
(kL/hr)
(mg/L)
(mg/L)
Bussel
6
115
240
6.5
Bore
Tech 1
6.4
110
105
6.4
Bore
Tech 4
21
170
215
6.2
Bore
As can be seen in Table 1, Bussel Bore has the
lowest iron levels of the three (3) Bores. However,
as it is only hydraulically rated for 240 kL/hr. In
peak periods it requires supplementation by Tech 1
or Tech 4 Bores.
Currently the ‘normal’ bore configuration in peak
periods is Bussel and Tech 4 Bores as it has the
highest capacity.
As shown in Table 1, the Tech WTP bores have
CO2 range of approximately 110-170 mg/L and
after current aeration the CO2 levels are reduced
to 50-90 mg/L with a pH of 6.6.
Figure 3: Onsite Lime Chip Trial
This trial was successful as the pH targets where
reached, however the lime chips fouled with the
high iron levels after approximately 2 weeks of
operation, as shown in Figure 4.
Figure 4: Clean and Fouled Lime Chip
The next stage of the investigation was to
undertake a lime pilot plant investigation.
To provide some baseline aeration tests a number
of bench trials were undertaken using simple
equipment such as an aquarium fountain aerator
(Rig 1) and a diffused aeration stone (Rig 2) as
shown in Figure 5.
Lime Dosing Investigation
After the lime chip trial it was thought that the
majority of CO2 was thought to be part of the
bicarbonate equilibrium. Therefore it was thought
by raising the pH that this would effectively reduce
the CO2 levels through converting it into the
bicarbonate species.
Initial water quality modelling indicated that to
increase the pH to 7-7.5 and to reduce CO2, the
required lime dose would be approximately 30
mg/L.
To confirm that this dose rate would increase the
pH and reduce CO2 to the required targets, a small
pilot plant was established on site. Pilot testing was
considered to be the most practical to test different
bore configurations and to test water quality
parameters such as CO2 which needed to be
tested quickly onsite to gain meaningful results.
After running the pilot plant under a number of
different bore configurations it was found that the
pH could be corrected, however dependent on the
bore configuration higher doses than that modelled
would be required. The required lime dose rate
ranged between 23 – 60 mg/L. Further it was found
that at the corrected pH the CO2 levels where still
high ranging from 50- 90 mg/L dependent on the
bore configurations.
The stability indices where determined for the
corrected water (post aeration and after lime
dosing) and it was found that the LSI was within the
optimal range in both ambient and warmer water,
however the CCPP is moderately aggressive in
ambient conditions (-20.8 to -23.9 mg/L CaCO3)
and slightly aggressive (-6.1 to -12.6 mg/L CaCO3)
in warmer water.
After reviewing the results it was clear that the high
CO2 levels in the water were due to the additional
aggressive CO2. Therefore, while lime dosing alone
would correct the pH to meet the pH target value,
the high dose would not adequately addressing
stability issues associated with the high level of
CO2 in raw water.
Aeration Bench Investigation
From these results it was determined that lime
dosing with the existing aeration system was
insufficient, and an improvement to the aeration
system would be required.
Figure 5: Basic Bench Aeration Trails – Rig 1 (left)
with fountain aerator; Rig 2 (right) with air stone
These tests demonstrated that Tech 1 and Tech 4
bore water require a significant time period (3.5
hours) to overcome the high iron content and CO2
refer to Figure 6.
Figure 6: Bench Aeration - Rig 1 and Rig 2
As the time required to strip CO2 was in excess of
what would be practically possible, an aggressive
high velocity aeration system was trialled using a
kitchen stick mixer, as shown in Figure 7.
Figure 7: Basic Bench Aeration Trails – Enhanced
Aeration Rig (with kitchen mixer)
therefore, the greater the difference between CA,i
and CA, the greater the CO2 removal rate.
Figure
8
demonstrated
that
this
aggressive/enhanced aeration technique could
reduce CO2 levels in a shorter timeframe of 4
minutes. However even with the aggressive
aeration the 4 minute timeframe was still excessive
compared to CO2 stripping results from other water
treatment. The cause of this delay was unknown,
and a preliminary review of stripping and iron
removal theory was undertaken.
However the gas-liquid surface area in the surface
aeration trials is present both in the form of the
bubbles distributed throughout the aeration tank
and the surface of the water itself, as shown in
Figure 10.
Figure 8: Bench Aeration - Enhanced Aeration
Review of Aeration Theory for CO2 and Iron
Removal
Reviewing the theory of gas transfer and CO2
stripping it was theorised that both the transfer of
oxygen into water for iron oxidation and the
removal of CO2 from water was dependent on:
 The surface (interfacial) area available for
transfer of gas into liquid and from liquid into
gas
 The relative concentrations in both the liquid
and gas phase
 The rate of surface renewal at the gas-liquid
interface
 The pH of the water
The typical concentration gradient across a gasliquid interface (the “interface”) where a gas is
being dissolved in a liquid is shown in Figure 9.
Figure 10:
interface
Aeration
influence
on gas-liquid
When oxygen is first introduced into a liquid with
limited or no oxygen (such as water saturated with
CO2), the large difference between the oxygen
concentration at the interface and the water drives
a rapid transfer of CO2 into the air bubbles, and
oxygen from the bubble surface into the water.
Additionally, when aeration commences, the high
concentration of CO2 within the liquid drives a more
rapid transfer.
Further the high iron content would also be
consuming O2 as it converts from the soluble Fe (II)
to the particulate Fe (III).
4 Fe
2+
+ O2 + 10 H2O  4 Fe(OH)3 + 8 OH-
The rate of iron oxidation is also pH dependant as
shown in Figure 11, with the rate increasing as the
pH increases.
Figure 9: Two-resistance concept
In the case of removing CO2 from water, the bulk
liquid concentration of CO2 (CA) is greater than the
concentration of CO2 at the interface (CA,i),
Figure 11: pH influence on iron oxygenation
Therefore as the oxygen is being introduced
through the aeration it is also being consumed
through the iron oxidation reaction. As the pH
begins to increase from the stripping process, the
iron oxygenation reaction will also increase in
speed.
The interaction between the two reactions, one a
mass transfer and the other a chemical oxidation
process has been proposed to be causing the
unusual difficulty in stripping CO2 from this water.
Although a number of possibilities for this delay in
stripping were suggested, resolving these
chemistry questions was not within the scope of
this project.
The focus of the next stage of the investigation
was to find a practical solution to reduce CO2 and
correct the pH.
Aeration Pilot Scale Investigation
Considering the high iron content of the water, the
iron sludge generated through the aeration process
would likely cause fouling issues with a number of
conventional aeration/stripping systems, such as
packed beds and diffusion systems. Also
considering the results from the bench test, a
surface type aeration system with a tank was
elected as the most practical solution to use for a
pilot plant trial.
A polyethylene aeration tank and an aquaculture
aspirator aerator were used for the pilot study, as
shown in Figure 11.
Figure 11: Aeration Pilot Trial
The aerator efficiency was established using
Standard Oxygen Transfer Rate (SOTR) testing at
the beginning and at the end of the trials.
A number of bore configurations were tested to
determine the required aeration requirement under
three aeration options:
1. Bussel Bore and Tech 4 Bore (This option is the
worst case scenario and would provide
suitable treatment for all bore configurations)
2. Tech 1 Bore and Bussel Bore
3. Tech 4 Bore (Bussel Bore would be aerated
through the existing spray aeration system)
To provide find the optimal aeration rates, the
volume within the tank was varied for each run.
Each run went for approximately 15 minutes and
the water was tested every 2 minutes.
After a series of tests the optimal water quality
targets where reached for CO2 and pH. It is also
worth noting that that the majority of the iron was
oxidised prior to removal of CO2 to the required
concentration. These results are summarised in
Figures 12 and Figures 13 at the end of the paper.
The stability results were also recalculated and it
was found that aeration alone significantly improve
the stability. However a minor dose of lime or soda
ash (3 – 10 mg/L) would bring the water into the
optimal ranges.
Establishing a Solution
The pilot plant was able to achieve the required
water quality targets which were established at the
beginning of the project. However it was important
to use these results to determine a feasible
solution that could be installed at the Tech WTP.
Using the SOTR results, tank volumes and the time
taken to remove CO2 a non-linear regression was
used to develop a power curve equation that
provided a direct relationship between power input
per unit volume, and CO2 removal time.
To calculate the required tank size and the
corresponding power demand, calculations where
undertaken on different contact times of 5,10,15,20
and 25 minutes. This was undertaken for each of
the three (3) aeration options. Capital costs were
then determined for each tank size and the
corresponding aerator size, the ongoing energy
costs where also estimated to run the aerator. It
should be noted that as the aeration also oxidised
the majority of the iron, the chlorine dose required
pre-filtration would drop signifcantly. Therefore the
reduction in chlorine was considered as a cost
benefit, and a NPV cost estimate was calculated
over 15 year period.
The most optimal cost based solution was
determined for each option, which is outlined in
Table 2.
Table 2: Aeration Solutions
Bore
BB & T4
Tech 1 &
BB
Tech 4
Bore
Aeration
Tank
Volume
(kL)
190
Aeration
Power (kW)
NPV
25
$110 K
115
20
- $ 80K
30
17
$110 K
Acknowledgments go to Aqwest for the oppurtunity
to produce this paper, and to the Aqwest
operations team that has worked on this project. In
particular acknowledgment goes to Karl Woods
who has been a key reasource for all of the onsite
trials.
REFERENCES
Gebbie. P, 2000. Water Stability - What Does It
Mean And How Do You Measure It ?
It was found that Option 2 was the most optimal as
it could provide a positive present value over a 15
year period for the required capital investment.
However it should also be noted that Option 2 does
not provide the same instantanous capacity as the
‘normal’ peak bore configuration of Tech 4 and
Bussel Bore. This would need to be considered
when determining the feasibility of the options.
OUTCOMES
This project provided Aqwest with feasible and
practical option to improve water quality at Tech
WTP, including the potential for longer term cost
benefits.
Aqwest is also undertaken a number of strategic
projects to improve the overall future security of
the water supply for the Bunbury community.
Therefore there are a number of proposed and
potential projects which may have impacts on the
overall system configuration. Aqwest is currently
reviewing these potential projects alongside the
Tech WTP aeration options to ensure that
resources are allocated to provide Aqwest with
best overall outcomes.
The outstanding chemistry questions are still being
considered, and further research may be
undertaken to resolve the unusual behaviour and
difficulty in stripping CO2.
CONCLUSION
The investigation into improving pH and stability at
the Tech WTP provided a number of unforseen
challenges, which could due to the difficult
combination of high iron and high CO2.
However after numerous trials an option has been
developed that will not only improve water quality
but that could provide a cost saving. Aqwest is
currently reviewing the larger master plan for water
treatment to determine how and when this project
will be furthered.
ACKNOWLEDGMENT
Nicholas. D, Carlyle G. Management Of Copper
Corrosion in Regional Australia.
Pontius, F. 1990, Forth edn. Water Quality and
Treatment; A Handbook of Community Water
Supplies, American Water Works Association.
Figure 12: Examples of Aeration Pilot Plant Results
Figure 13: Pilot Plant Analysed Data - Optimal Results