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Conference Papers | 2002 Conference Papers SIMPLE
SOLUTIONS TO MANGANESE PROBLEMS
Jeff Roscoe, Plant
Operator, North
East Region Water Authority
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ABSTRACT
Soluble manganese (Mn) in potable water supplies disinfected
with strong oxidants such as free chlorine causes dirty
black water, which in turn causes staining and damage
to consumer-clothing, linen, goods etc. Tests performed
at different sites using PACl and Alum, produced results
that indicated PACl provided better Mn removal than
Alum where 95% removal was achieved compared to 0%.
In water with low levels of Mn, the management of Mn
can be an option by using weaker oxidising disinfectants
such as chloramines, which can be used in place of more
radical oxidants such as free chlorine.
KEY
WORDS
Mn (Manganese), NEW (North East Water), PACl (polyaluminium
chlorohydrate)
1.0
INTRODUCTION
Benalla,
Tungamah and St. James are towns supplied water from
water treatment plant's (WTP) operated by North East
Water (NEW). These towns have from time to time been
affected by black water events, which along with health
issues causes staining. Reticulation flushing is required
which increases plant operational costs, whilst NEW
incurs dry cleaning and sometimes replacement costs
for damaged or stained goods. Also consumer confidence
in the safety of the water and the Authority is diminished.
Tests on raw water taken at Tungamah, St. James and
Benalla indicated levels of total soluble manganese
above the guideline level of 0.1mg/L. Mn is removed
at different places using varied processes and treatments
such as aeration, chemical oxidation, coagulation /
flocculation and filtration and management of low Mn
levels is sometimes an option. This paper looks at a
range of simple solutions NEW trialed to solve the manganese
problems encountered at the NEW sites mentioned above.
2.0
BACKGROUND
2.1 Benalla water supply
The township of Benalla was until Christmas 1998 supplied
with unprocessed water with an average turbidity of
5 NTU from a closed catchment and a settling dam system.
The only chemical treatment was disinfection using chlorine
gas with a minimum contact time of 35 minutes during
peak demands. December 1998 saw the commissioning of
a 19 ML per day capacity Dissolved Air Floatation -
Filtration(DAFF) water treatment plant, located approximately
12 kilometres south of the town. The plant uses liquid
alum as the coagulant, LT20 polymer as the flocculation
aid, and soda ash for pre and post pH correction. The
filtered water is disinfected using chlorine gas dosed
to provide a residual of 0.2-0.5 mg/L. Also on site
are provision for powder activated carbon, potassium
permanganate and aqueous ammonia treatment.
Figure
1: Benalla water supply and treatment process
Tungamah
and St. James are small isolated towns that are supplied
water from a stock and domestic channel during the irrigation
periods. On site storage dams suffice during the closed
irrigation period. The raw water, which has an average
turbidity of 70 NTU but can reach levels of 400 NTU,
is chemically treated with liquid alum batched to 50
% w/w from dry powdered alum. It is then settled in
a 3 ML storage basin before being transferred to a similar
sized basin for storage and further chemical treatment,
such as pH correction and disinfection on it's way to
the reticulation via a tower.
Figure
2: Tungamah and St. James water supplies
3.0 DISCOVERY OF MANGANESE
ISSUES
During
October 1999 after just ten months of operation since
the commissioning of the new DAFF plant, daily consumption
increased in Benalla from 2.5 ML per day to 8 ML per
day within a two-week period with the onset of warmer
weather. This tripling of demand in such a short time
frame discoloured the reticulation water which in turn
made the phones light up with complaints multiplied
by the publics belief that because the new treatment
plant was believed by the consumers to prevent these
black water events and rightly so. The sharp increase
in demand scoured oxidised deposits of manganese off
the walls of the 12 kilometre pipelines that feed the
Benalla reticulation.
It was soon determined that even though the maximum
total soluble manganese levels reached during the summer
period were only 0.1 mg/L and 0.05 mg/L during the winter
period. Long detention times, up to 3 days and disinfecting
using chlorine to achieve free chlorine residuals was
causing the build up of the manganese deposits during
the lower demand periods.
Tungamah and St. James were experiencing increasing
incidents of manganese related, back water events in
their reticulations. Once again, because of the build
up of oxidised manganese deposits in the pipelines as
a result of the effect of disinfecting with chlorine
gas to achieve free chlorine residuals, the ensuing
customer complaints triggered an ongoing regime of weekly
flushing in an attempt to clean the system and supply
acceptable water.
Table 1: Average raw water
characteristics
Table
2: Average treated water characteristics - pre improvements
Table
3: Jar test results using alum (unfiltered) and pH corrected
with soda ash
4.0
SOLUTIONS TO MANGANESE PROBLEMS INVESTIGATED
Initial brainstorming by a group consisting of the immediate
operators, the treatment coordinator, the treatment
manager and some external liaisons such as the Water
Industry Training Centre covered a range of potential
solutions reported in literature and included, optimising
the current coagulant and aids, pre-treatment with potassium
permanganate, the trial of alternative coagulants and
finally detention times and current and alternative
disinfection treatments and processes.
4.1
Potassium permanganate trial at Benalla
After jar testing revealed that optimising the existing
coagulant, oxidant and pH variations would not reduce
the manganese concentration in the treated water at
Benalla WTP potassium permanganate was introduced as
a pre-treatment to oxidise the soluble manganese. Figure
3 below shows the theoretical way to oxidise the manganese
using potassium permanganate. Also given is a range
of pH levels and approximate times needed for the oxidation
and the flocculation stages.
Figure
3: Theoretical permanganate dosing for manganese removal
The
detention times changed with the characteristics of
the water but in general the higher the pH the quicker
the reaction.
Permanganate
dosing was applied at the Benalla WTP with poor results.
The configuration of the WTP meant that the permanganate
was being dosed at the same time as the Alum, giving
no time for the permanganate to oxidise manganese before
coagulation. It was found necessary to increase the
pH to almost 8.5 to achieve manganese oxidation but
this pH level was too high for the liquid alum to flocculate
optimally. It was concluded that potassium permanganate
would not be a solution unless major capital works in
the form of an oxidisation contact tank were installed.
Other solutions to Benalla's problems were sought.
4.2
Disinfection changes at Benalla
During the manganese incidents in Benalla, low free
chlorine residuals were being encountered at the extremities
of the reticulation especially during low demand periods.
A decision was made after the necessary community and
business consultation, to change our disinfection process
to chloramination in an attempt to achieve more persistent
residuals. The resultant change of disinfection methods
not only produced stabilised and longer lasting residuals
but the use of the less radical chloramine disinfectant
did not oxidise the manganese and cause the dreaded
manganese deposits, even in long detention times. The
manganese stayed in soluble form and didn't appear to
build up in pipe biofilms. It also has to be said that
during extended low demand periods, the treated water
storage levels are lowered in the 10 ML storage tank
to reduce detention times and maintain total chlorine
residuals but it also helps maintain low heterotrophic
plate counts, which need to be managed when dealing
with chloraminated water held longer than 72 hours.
4.3
Polyaluminium Chlorohydrate Coagulant Trials
Jar testing using a range of different coagulants on
Tungamah and St James raw water in a separate project
designed to assess alternative coagulants revealed a
most unexpected result. It was found that PACl provided
better Mn removal than Alum where up to 95% removal
was achieved opposed to little or no removal using alum.
Once this discovery was made, PACl was also jar tested
successfully for removing manganese from Benalla's raw
water, and manganese test results were reviewed at the
Moyhu WTP where PACl had been in use for 12 months.
Table
4: Jar test results using PACl
Table
4 above shows at optimum doses of PACl, good removal
of iron and manganese is achieved with minimal affect
on alkalinity, which in turn helps, maintain a good
pH level.
Based
on these results, Tungamah was converted to a PACl coagulant
plant. Collected ongoing data from this site since changing
to PACl as a coagulant shows consistent lowering to
acceptable levels of manganese and in turn occurrences
of black water events in the reticulation have been
eliminated. Since changing coagulant to PACl the frequency
of flushing the reticulation has now been lowered to
3 monthly for turbidity reasons (no filtration) rather
than for managing the old manganese issues. St. James
is in the process of changing over to PACl as a coagulant.
Table
5: Average treated water characteristics - post improvements
It
is unclear how PACl manages to remove manganese from
these raw waters. The chemical make up of PACl - Al2(OH)nCl6-n
would provide some chlorine to oxidise the Mn (not huge
amounts but maybe enough to kick the reactions along
a bit). More in-depth research needs to be done to confirm
exactly how PACl removes manganese.
Another
benefit that arose from the change of coagulant was
that the pH correction process required when using liquid
alum was no longer necessary which in turn reduced associated
chemical and labor costs and storage and handling issues
etc.
5.0
CONCLUSIONS
It was found in jar tests and real scale trials that
PACl was an efficient means of manganese removal in
the raw waters of St James and Tungamah. Mn removal
rates were found to be up to 95% better using PACl in
comparison to Alum. The raw water quality of Benalla
did not show the same Mn removal rates as when PACl
was applied to the raw water at St James and Tungamah,
which leads us to believe that the increased turbidity
also plays some role in causing the Mn to be bound in
the floc.
This
caused two different approaches to be used when assessing
the issues, where Benalla was viewed in a more control
and management sense rather than one of forced removal
of Mn as was the case in Tungamah and St James. Since
the changes to the plants have been made all towns are
now consistently well below the guideline value for
Manganese of 0.1 mg/L and black water complaints are
now a thing of the past.
6.0
ACKNOWLEDGEMENTS
I would like to acknowledge the assistance of the people
who I work with - Mark Welsh, Rex Humphries, Mike Leak,
Mark Samblebe, Jim Little, Trevor Reaper and Julie and
the team in the office.
Also
Steve Wilson and Ken Herbert from the Water Industry
Training Centre for letting me pick their brains on
different occasions and finally my wife Helen and sons
Chris and Ben who created the pictures of the town water
supplies for me.
7.0
REFERENCES
Water Industry Training Centre - Manganese report (2000)
Mike Leak - Treatment Manager NEW
Mark Samblebe - Treatment specialist NEW DOWNLOAD
(pdf
550KB)
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