Conference Papers | 2001 Conference Papers
POLYALUMINIUM COAGULANTS IN WATER TREATMENT
Peter Gebbie, Fisher Stewart Pty Ltd
coagulants are finding increasing use in potable water
treatment plants throughout Australia, with polyaluminium
chloride (PACl) in particular now having wide application.
This paper reviews the properties and advantages of
using these chemicals with particular reference to experience
at Daylesford, the Grampians region, Swan Hill and Tidal
River, all in Victoria.
Polyaluminium chloride (PACl), Aluminium chlorohydrate
(ACH), Coagulants, Water Treatment
(aluminium sulphate) is the most commonly used coagulant
in Australian water treatment plants, low cost being
its major attraction. Alum however, has a number of
coagulation pH range: 5.5 to 6.5,
addition of alkalinity to the raw water is often
required to achieve the optimum coagulation pH,
particularly for soft, coloured surface waters that
are common in Australia,
aluminium levels in the treated water can often
exceed acceptable limits, and
floc produced is particularly fragile. This is especially
important if a coagulant is required to maximise
colour removal in a microfiltration-based water
Alum reacts in water to produce aluminium hydroxide
and as a by-product sulphuric acid is also formed. The
metal hydroxide precipitates out of solution and entraps
neutralized charged dirt particles (turbidity), as well
as coagulating soluble colour and organics by adsorption.
sulphuric acid produced reacts with alkalinity in the
raw water to produce carbon dioxide, thus depressing
a number of alternative aluminium-based coagulants have
been developed for water treatment applications.
These compounds have the general formula (Aln(OH)mCl(3n-m))x
and have a polymeric structure, totally soluble in water.
The length of the polymerised chain, molecular weight
and number of ionic charges is determined by the degree
of polymerisation. On hydrolysis, various mono- and
polymeric species are formed, with Al13O4(OH)247+ being
a particularly important cation. A less predominant
species is Al8(OH)204+.
These highly polymerised coagulants include the following:
chloride (PACl, n=2 and m=3),
chlorohydrate (ACH, n=2 and m=5), and
chlorohydrate (PACH): similar to ACH.
practice, there is little difference between the performance
of ACH and PACl in water treatment applications, even
though ACH is more hydrated.
3.0 ADVANTAGES OF POLYALUMINIUM
An important property of polyaluminium coagulants is
their basicity. This is the ratio of hydroxyl to aluminium
ions in the hydrated complex and in general the higher
the basicity, the lower will be the consumption of alkalinity
in the treatment process and hence impact on pH.
Various suppliers of ACH and PACl in Australia express
the basicity of their product as a percentage e.g. Omega
MEGAPAC-23 (40.2% w/w aluminium chlorohydrate) has a
basicity of 82% (Omega Chemicals, 2000).
The polyaluminium coagulants in general consume considerably
less alkalinity than alum. They are effective over a
broader pH range compared to alum and experience shows
that PACl works satisfactorily over a pH range of 5.0
important advantage of using polyaluminium coagulants
in water treatment processes is the reduced concentration
of sulphate added to the treated water. This directly
affects SO4 levels in domestic wastewater.
A raw water with a sulphate level of 3 to 5 mg/L will
typically have a SO4 concentration of 15 to 25 mg/L
following treatment with alum. The amount of soluble
sulphate present in domestic wastewater is now also
significantly increased and this can result in elevated
hydrogen sulphide production in the sewerage system,
leading to odour and corrosion problems.
1: Typical aluminium-based coagulants used in water
one water treatment plant in the Otway region of Victoria,
polyaluminium chloride replaced alum and in so doing
SO4 levels in the treated water were reduced from 27
to 4 - 5 mg/L. Previously, alum was dosed at 45 to 55
mg/L at this plant. The change to PACl had a major impact
on SO4 levels in the sewage, with reduced odour problems
evident at several pump stations in the sewerage system.
Table 1 summarises principal characteristics of commercially
available polyaluminium coagulants. Details for alum
and sodium aluminate are also included for comparison.
Other advantages of polyaluminium coagulants include
levels of residual aluminium in the treated water
can be achieved, typically 0.01-0.05 mg/L,
and ACH work extremely well at low raw water temperatures.
Flocs formed from alum at low temperatures settle
very slowly, whereas flocs formed from polyaluminium
coagulants tend to settle equally well at low and
at normal water temperatures,
less sludge is produced compared to alum at an equivalent
lower doses are required to give equivalent results
to alum. For example, a dose of 12 mg/L PACl (as
100%) was required for treatment of a coloured,
low turbidity water (Otway region, Victoria) compared
to similar performance obtained when using an alum
dose of 55 mg/L, and
increase in chloride in the treated water is much
lower than the sulphate increase from alum, resulting
in lower overall increases in the TDS of the treated
From Table 1, it will be noted that polyaluminium coagulants
are typically twice the price of liquid alum on per
kilogram aluminium basis. However, lower doses of the
coagulant and lower pre- and post-treatment alkali doses
can still make its use economical.
chloride solution (10% Al2O3) is stable for 4 to 5 months
when stored at less than 50oC and is so ideal for bulk
storage and dosing installations.
One possible disadvantage in using ACH/PACl relates
to the removal of dissolved organic carbon (DOC) from
water. It is well documented that effective DOC removal
is possible with alum, particularly when coagulating
at lower pH values using so-called "enhanced coagulation".
Alum appears to be a superior coagulant as far as removal
of humic and fulvic colour constituents are concerned.
A higher coagulation pH is adopted with polyaluminium
coagulants and it possible that removal of THM percussors
may not be as complete as with alum. The following examples
illustrate that this depends on the particular raw water
in question and in many cases may not be an issue.
Daylesford Water Filtration Plant is a new 8 ML/d in-filter/dissolved
air flotation plant constructed by Vivendi Water/US
Filter, with process and detailed design, engineering
and procurement provided by Fisher Stewart.
The plant treats highly coloured raw water from either
the Wombat or Bullarto Reservoirs. Raw water characteristics
for the supply from Wombat Reservoir are detailed in
Table 2. The water is also very corrosive with a pH
which can be as low as 6.4 and a calcium carbonate precipitation
potential (CCPP) value of - 17.5 mg/L CaCO3 (at 20oC).
this water supply has a true colour of 30 Pt/Co units
and a turbidity of 2.6 NTU, although on occasions the
colour can approach 100 Pt/Co units. The temperature
of the raw water can be as low as 5oC in winter months.
For raw water at 20oC with a true colour of 60 Pt/Co
units, turbidity 3.0 NTU and pH 6.7, WaterQual (a water
treatment and quality assessment model developed by
the author at Fisher Stewart) was used to compare the
predicted performance of alum versus PACl. A chlorine
dose of 1.5 mg/L for disinfection and a target treated
water pH of 7.5 were adopted for each case.
For an alum dose of 45 mg/L and a coagulation pH of
6.9, pre- and post-treatment doses of caustic soda of
17.9 and 4.7 mg/L were required (total 22.6 mg/L).
2: Raw Water Analysis, Wombat Reservoir at Daylesford
projections compare well with the results of jar-test
investigations carried out to determine the treatability
of raw water supplies at Daylesford using alum (GHD
An equivalent PACl dose of 12 mg/L as 100% was adopted
and used in the WaterQual model.
Addition of caustic soda at 3.4 mg/L was necessary to
achieve a coagulation pH of 6.9. A post-treatment alkali
dose of 4.7 mg/L was required in this instance (total
dose 8.1 mg/L).
of the treated water for each coagulant option determined
from WaterQual are given in Table 3.
3: Treated Water Quality Predicted Using WaterQual,
Wombat Reservoir, Daylesford, at 20oC
The advantages of using PACl in regard to the effect
on treated water TDS and sulphate levels are apparent.
Note also a small improvement in the CCPP value of the
treated water when using PACl.
The chemical doses predicted from WaterQual when using
PACl compare very well with actual requirements at the
Daylesford Water Filtration Plant.
at Daylesford has also confirmed the suitability of
PACl, with very effective flocculation observed at the
low water temperatures noted during plant start-up in
July 2000 (5-10oC).
Estimated chemical costs using alum amount to $38.1/ML,
compared to $41.3/ML with PACl. For an annual treated
water production of 1200 ML, this translates to a saving
$3900 per annum in favour of alum. The operational advantages
of using PACl, particularly in cold months, make it
an attractive coagulant for Daylesford.
THM level in the treated water is generally 40-50 mg/L
when treating raw water with a true colour of 90-100
Pt/Co units. This is well below the current AWDG recommendation
of 250 g/L. In this instance DOC removal appears to
water for Swan Hill is abstracted from the Murray River
and has typical quality characteristics given in Table
4: Typical Raw Water Analysis at Swan Hill
Alum doses in the range 30 to 60 mg/L have been used
to treat this water, which has a true colour of 20-30
Pt/Co units and a turbidity of 20 to 40 NTU under normal
river flow conditions.
Pre-treatment dosing with lime is only required when
alum doses greater than 30 mg/L are required. The coagulation
pH is usually 6.3 to 6.5 and a post-treatment dose of
10-15 mg/L lime is required to give a treated water
pH of 7.1 to 7.3.
Lower Murray Water Authority has recently changed over
from using liquid alum to ACH (MEGAPAC 23) at the Swan
Hill Water Treatment Plant. As a consequence, the practice
of pre- and post-treatment dosing with lime to adjust
pH and alkalinity has now been discontinued.
ACH doses 20% of those for alum (as Al2 (SO4)3.18H2O)
are required, i.e. 6 to 12 mg/L as 100% ACH.
raw water has a pH of 7.4 to 7.7 and following ACH addition,
the coagulation pH is 7.2-7.5, reducing to 7.0 to 7.2
following chlorination (gas, 1.5 mg/L dose).
treated water using ACH is slightly more aggressive:
CCPP -10.9 to -13.3 mg/L compared to -10.7 mg/L with
alum, at 20oC.
By adding a small dose of lime (0.5-1.5 mg/L) at the
inlet of the plant and coagulating at a slightly higher
pH (7.6), the corrosivity of the treated water following
chlorination can be improved to give a CCPP of -9.8
mg/L at 20oC.
changeover has allowed Lower Murray Water to defer planned
capital expenditure in upgrading lime storage and dosing
facilities at the Swan Hill Water Treatment Plant.
There has been no noticeable change in the level of
THM's in the treated water at Swan Hill since changing
over to ACH. Typically the total concentration of THM's
in the treated water is 35-40 g/L, even when treating
raw water with relatively high colour levels. Further,
there are no discernable levels of taste- and odour-causing
compounds in the treated water (Neaves 2000).
6.0 GRAMPIANS REGION
is currently being used at three of six recently constructed
in-filter/DAF water treatment plants in the Grampians
region of Victoria; at Birchip, Charlton and Rainbow.
Raw water to these three townships principally comes
from the Wimmera-Mallee Channel system. Typically, the
raw water has the following average characteristics:
pH 8.5 - 8.7, TDS 500 to 600 mg/L, true colour 10 to
15 Pt/Co units and turbidity 1.5 to 2.5 NTU. The high
pH and TDS levels make treatment of this particular
water difficult and if alum is used, doses are higher
than would be normally expected; often 60 to 100 mg/L.
the Charlton raw water supply- pH 8.5, true colour 8
Pt/Co units and turbidity 2.5 NTU- an alum dose of 110
mg/L was required for effective treatment in laboratory
jar-tests compared to 40 mg/L using PACl. The coagulation
pH was 6.4 with alum and 7.2 with PACl, illustrating
the reduced impact PACl has on pH. Lower residual Al
levels in the treated water were also achieved using
PACl (GHD 1998).
of the water supply at Murtoa (also in the Grampians
region) using PACl was also found to be effective. This
water supply is also largely derived from the Wimmera-Mallee
Channel and has a TDS of around 550 mg/L and a pH of
8.6. Alum doses typically in the range of 50 to 70 mg/L
were found to be required, with supplemental addition
of sulphuric acid needed to achieve a desirable pH and
so avoid excessive coagulant doses. By contrast, PACl
doses required to give equivalent treatment were only
16 to 22 mg/L or a third of the alum dose required,
without the need for pre-treatment pH correction (USF
The above results are consistent with actual plant operating
experience at the treatment plants at Birchup, Charlton
and Rainbow, where typically ACH is dosed at approximately
Sulphuric acid is used at these three sites to reduce
the pH of the raw water to approximately 7.8. However,
no post-treatment addition of alkali is required to
correct the alkalinity of the treated water following
raw water quality at Rainbow is very similar to that
at Birchip. The results of jar-tests (GHD 1998) and
projections from WaterQual comparing the performance
of alum and ACH at Rainbow are summarised in Table 5
for raw water with a TDS of 720 mg/L and pH 8.4, at
The coagulation pH adopted for alum was 6.7 and for
ACH 7.6, whilst the target treated water pH was 7.5
in each case. A chlorine dose of 1.5 mg/L was assumed
in each case for disinfection.
alum, the sulphate level in the treated water increases
from 17.9 to 56.8 mg/L, whilst for ACH the corresponding
increase is only 2.3 mg/L.
The following chemical costs (delivered to site) were
used to calculate the operating cost of each treatment
liquid (47% w/w), $200/t
liquid (40.2% w/w), $1100/t
soda: liquid (46% w/w), $600/t
acid: liquid (34% w/w), $900/t, and
(gas): $1600/t (980 kg drums).
5: Predicted Performance of Alum v ACH at Rainbow
the alum option, total chemical costs amount to $76.8/ML,
whilst for ACH $63.5/ML; a saving of approximately 17%.
This example illustrates how lower operating costs can
be realized when using ACH coagulant compared to alum.
Superior quality treated water is also produced with
respect to TDS, CCPP and SO4.
7.0 TIDAL RIVER
Fisher Stewart has participated in the delivery of water
treatment and reticulation facilities at Tidal River,
Wilson's Promontory NP through its role as consultant
to Parks Victoria. Raw water supplied to Tidal River
is derived from a small weir and off-take. The volume
of the weir and area of the contributing catchment is
relatively small and subsequently, there can be substantial
changes to the raw water quality during rainfall events.
is treated using a 5L/s (0.4 ML/d) Aquagenics "AquaPac"
packaged water treatment plant. Initially liquid alum
and caustic soda were used in the treatment regime.
The water was found to be difficult to treat and in
an attempt to improve plant performance, PACl was trialed
in lieu of alum (DELTREX AC100S).
The coagulant was found to be very effective and since
changing over to PACl one of the most noticeable (and
unexpected!) advantages noted has been the increased
"robustness" of the water treatment process. Previously
when using alum, plant performance was adversely affected
after heavy rain. PACI has shown an ability to much
better deal with these changes. The chemical's ability
to coagulate over a wider pH range is of enormous benefit
in this instance.
major benefit of using PACI has been a considerable
reduction in the volumes of sludge trucked off-site
to disposal. This has had a major impact on plant operating
costs. Pre-treatment alkalinity adjustment using caustic
soda has been greatly reduced since changing over to
PACl. Table 6 is a comparison between alum and PACI
at Tidal River. A similar treated water quality is achieved
in both instances with the CCPP higher in the case of
PACl. An estimated $24/ML (or 14%) saving in total chemical
costs is possible using PACl. Chemicals are delivered
to site in 15 and 200L packages. This, combined with
the remote location of Tidal River, explains the high
cost of chemicals delivered to the site.
6: Predicted Performance of Alum v PACl at Tidal River
in the treated water is 6.0 down from 27.6 mg/L, reducing
the potential for odour production from wastewater generated
at Tidal River.
coagulants- ACH and PACl- can often give significant
advantages over alum, including:
residual aluminium levels in the treated water,
treated water quality including lower TDS and sulphate
levels and possibly higher CCPP values, and
In many cases, post-treatment pH adjustment using an
alkali is not required, reducing the overall capital
cost of the plant as well as improving operator amenity
and reducing maintenance requirements.
increased competition in the marketplace, the unit cost
of polyaluminium coagulants will probably decrease in
the future, making conversion from alum to ACH/PACl
more attractive and, more widespread in Australia.
information suggests that THM formation will not be
compromised when using ACH/PACl but this should be first
confirmed in the laboratory with jar-tests.
And finally, a word of advice: when changing over from
alum to PACl/ACH, it is important to make sure chemical
storage tanks, dosing pumps and piping are all thoroughly
flushed out with clean water to avoid forming an "aluminium
AWWA Coagulation Committee (1989), JAWWA, 81,
GHD (1996), Daylesford Water Supply, Report on Water
Quality Improvement Works: Central Highlands Water Authority,
GHD (1998), Design and Construction Specifications
for Birchip, Charlton and Rainbow Water Treatment Plants:
Grampians Regional Water Authority, Melbourne, Australia
Neaves, K. (2000) Lower Murray Regional Water Authority,
Private Communication, Mildura Omega Chemicals
(2000), Megapac 23 Information Leaflet, Melbourne,
Filter (1998), Private Communication, Melbourne
Carbonate Precipitation Potential, mg/L CaCO3
Dissolved Solids, mg/L
Peter Gebbie is a Senior Engineer in the Water Industry
Group at Fisher Stewart, Melbourne. He is responsible
for process design and detailed engineering tasks
associated with water and wastewater treatment projects.
Tel: (03) 8517 9268