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Conference Papers | 2002 Conference Papers
CONTROL OF ALGAE IN POTABLE WATER SUPPLY - BEMM RIVER
SHADECLOTH TRIAL
Kristine Hunter, Technical
Officer Water Quality,
East Gippsland Water
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ABSTRACT
Bemm River is a small coastal community located in East
Gippsland. High numbers of algae have been consistently
experienced in the Bemm River Storage Basin over a number
of years. These occurrences resulted in taste and odour
complaints as well as interfering with the effectiveness
of the disinfection process. In 2000 East Gippsland
Water initiated an innovative proactive response to
the algae problems in Bemm River. Of the three growth
limiting factors of algae - nitrogen, phosphorus and
light - it was decided that light would be the easiest
parameter to control. This control was achieved via
the construction of a 95% UV blockout shadecloth structure
covering the entire surface of the storage basin. Monitoring
to date has shown an apparent decrease in algae numbers
on a seasonal basis.
KEY WORDS
Algae,
Taste and Odour, Shadecloth
1.0
INTRODUCTION
The Bemm River water reticulation scheme was installed
between 1981 and 1983. The system consisted of a river
offtake pump (Bemm River) and a 6.8ML clay lined, rock
beached "turkey's nest" storage basin and a 3 km gravity
supply main to the township of Bemm River. Not long
after the completion of the scheme, the township experienced
a gradual decline in industry and population (350 in
1975) for a variety of reasons. Currently there is around
170 people living in Bemm River (69 connections). The
population peaks at ~400 during holiday seasons.
Detention
time in the gravity supply main is about 4 days with
possibly 5-6 days for customers at the extreme ends
of the system. The detention time in the storage basin
is also significant. Even conservatively estimating
70% of the storage to be available, and doubling the
daily consumption rate, there is 120 days (4 months)
of storage detention.
There was not a lot of raw water data available prior
to 2000, as the historic monitoring program consisted
mainly of quarterly sampling for basic physical-chemical
parameters. From April 2000 a new monitoring program
was implemented that increased the frequency of monitoring
these parameters and in February 2001 an intensive monitoring
program was instigated to assess the impact of the shadecloth
cover.
Given
the above information it is not hard to see how the
Bemm River storage basin became an ideal environment
for algae growth - low turnover time, sunlight and nutrients.
This proved to be the case. Anecdotal evidence suggests
high algae numbers occurring during the Summer months
for many years. Whenever staff noticed a problem with
the water (usually cloudiness or colour change) or earthy/musty
taste and odour complaints triggered an investigation,
an algae sample was taken and, if necessary, the storage
basin was manually dosed with sodium hypochlorite.
During a discussion on the algae issue the suggestion
of covering the storage basin was raised as a long term
solution. The suggestion being based on the premise
that the limiting factors for algae growth were, basically,
nutrient availability and light. A cover would also
limit the amount of windblown debris and access by birds.
Limiting nutrients or treating the basin for nutrient
removal were not viewed as viable options however limiting
light availability was a feasible option. A floating
cover or a solid roof structure were dismissed as options
due to inherent problems with both in terms of maintenance,
possible bacterial growth, security, longevity and cost.
The
concept of a shadecloth cover was raised as an off-the-cuff
remark and ensuing discussions found merit in the idea.
A surf of the Internet revealed a number of companies
specialising in large shadecloth structures. Several
of these companies were approached and asked for comment
on the viability of the idea, both in terms of achieving
the limiting light level as well as structural and economic
feasibility.
1.1
Design of shade cloth cover
As
the goal of the project seemed to be able to be realistically
achieved, contract documents and specifications were
drawn up and the works advertised. The following is
a brief summary of the information provided to the tenderers:
Area
to be covered:
The horizontal area to be covered has been estimated
from the existing plan as 2582 square metres. The shade
cloth cover is to cover the basin including a 300mm
strip beyond the top of the internal embankment.
Vertical
clearance:
The lowest point on the underside of the cover is to
have a minimum vertical clearance above the full water
level surface of 500mm and above the top of bank of
300mm. The top of bank level is RL 59.70 metres AHD
and top water level is RL 59.15 metres AHD.
Shape
of vertical section of cover:
The Authority has a preference for a cover design which
arches in a convex shape over the basin in order to
promote shedding of bird droppings and leaves etc off
the cover.
Seal
between cover and top of bank:
Contractor to provide in tender submission details of
how the shade cloth cover is to be sealed between outside
edge of cover and top of bank to prevent ingress of
light, vermin and wind blown material.
Rock
beaching:
The rock beaching around the top of bank may have to
be removed to prevent potentially harmful contact with
the shade cloth.
Internal supports:
It is proposed that the cover clear span the basin with
no support columns or other structural elements in the
basin.
Compliance
with standards:
Design and construction of the shade cloth cover is
to comply with all relevant Australian Standards or
where no Australian standard is available either USA
or International standards.
Maintenance
access below cover:
Access by O&M staff for maintenance of the basin is
to be provided in the shade cloth cover by allowing
a section to be easily released and rolled back and
then following maintenance work be reinstated by Authority's
O&M staff.
Light
exclusion:
The shade cloth is to exclude a minimum of 98% of sunlight
and have a minimum guaranteed life of 20 years.
The
contract was awarded to Super Span Pty Ltd and works
commenced in February 2001 and were completed in May
2001. Over the next 12 months there was some maintenance
of the structure required as the shadecloth stretched
more than was anticipated and some of the supporting
poles had to be realigned.
The
'skirt' around the bottom of the structure also had
to be reinstated as it had been weighted down with rock
beaching and the movement of the structure had resulted
in tearing of the shadecloth. A few other minor holes
/ tears also occurred where the shadecloth was in contact
with some of the structures in and around the basin
(level gauge board, concrete inlet pit) however these
were all repaired with minimal fuss and effort.
2.0
DISCUSSION
An
indication of the water quality of the Bemm River storage
is shown in Table 1. The data presented in the table
consists of samples taken either from Bemm River or
from the Bemm River storage (both sites classified as
raw / source water). As can be seen, the water quality
is quite good - not too highly coloured or turbid, oxygenated
and low levels of iron. The nutrient data was only gathered
for 12 months and so it is unknown whether these are
typical base levels or seasonally elevated.
Table
1 : Indication of Source Water Quality (1993 - 2002)
An intensive monitoring program was instigated in February
2001 and ran for 12 months. Biological, microbiological
and physical-chemical parameters were monitored in the
storage basin to determine whether (a) there was any
affect on algae numbers and (b) there was any effect
on water quality as a result of the installation of
the shadecloth cover.
A series of siphon lines were installed to enable samples
to be taken from the storage basin with minimal fuss
and effort on the part of field staff. These lines were
suspended from the floating arm offtake structure using
weights and anchor lines and were positioned such that
a sample could be taken from the bottom (~ 50 cm from
the bottom), middle (~2m from the bottom) and surface
(~ 30 cm from the surface) by simply turning on a tap.
Note: at full supply the basin is 4.5m deep. The results
of this study are shown below. Note that the data has
not been analysed statistically, interpretation of results
in based on 'eye-balling' of data and graphs. Viewing
the data, there was little difference discernable between
the bottom or surface samples in terms of overall values
and trends of parameters - only the surface results
are presented graphically in this paper.
From
looking at the graphs presented below, it can be seen
that there appears to have been minimal impact on the
physical-chemical parameters by the installation of
the shadecloth cover.
Figure
1 : Bemm River Storage Basin (Surface) - Nutrients and
Total Organic Carbon
Figure 2 : Bemm River Storage (Surface) - Physical -
Chemical Parameters
In
terms of the impact on nutrients and total organic carbon
for both the surface and bottom samples taken, there
appears to be have been a decrease in the levels of
total organic carbon since the installation of the shadecloth.
This is assumed to be a reflection of the barrier to
windblown dirt / debris into the storage basin. The
total phosphorus levels appear to have been unaffected
however the total nitrogen levels appear to be rising
slightly.
Of
the total nitrogen components analysed, the most significant
increase was in oxidised nitrogen (NOx - nitrates and
nitrites), sometimes referred to as 'available' nitrogen
in terms of nitrogen required for plant growth.
As there is no historic nutrient data for this site
it is not known whether this is a seasonal phenomena
or whether it is in response to the decreased numbers
of algae (see later discussion) - no algae / plant growth
to remove available nutrients from the water column
- or an increase in nitrogen levels as a result of decaying
matter in the storage basin. However, a time series
graph of total coliforms (refer Figure 3) as an indication
of the bacterial load in the storage basin does support
the theory that the dying algae did decompose and the
consequent bacterial activity converted some of that
biomass into available nitrogen which is now sitting
there with very little biological activity present in
the storage basin. With no further 'food' and minimal
input into the storage as pumping from Bemm River occurs
only 3 - 4 times per year, the bacterial load also decreased.
Monitoring for both nutrients and total organic carbon
is continuing on a monthly basis.
Figure 3 : Bemm River Storage
- Total Coliforms
Algae samples were taken from the surface (via the siphon
line) on a fortnightly basis for 12 months. The analyses
was qualitative and results were indicative of the abundance
of algae present. In order to graph the results, an
arbitrary numeric value was assigned to the results
as follows:
The algae species were then grouped into 'Green Algae',
'Blue-Green Algae' and 'Other', the latter category
covering species such as diatoms, cryptomonads, synedra
etc.
The
values of the individual species were summed into their
relevant categories for each month and then the three
categories plotted as a stacked bar chart to present
the overall abundance of algae in the storage basin.
The figures shown on the Y axis are indicative of abundance
only.
As
mentioned earlier, there was no formal algal monitoring
program prior to September 2000. This was also the time
when there was a change in laboratory service providers.
The two laboratories used different algae enumeration
methods and so, strictly speaking, the data is not comparable.
However, for the purposes of demonstrating the presence
of algae historically, the results from the first laboratory
were assigned a numeric value using the same logic as
in the above table and added to the stacked bar chart.
Most of the evidence for algae presence / blooms is
anecdotal from both the local community and the field
staff - there was very little formal sampling undertaken
prior to 1998.
The available algae data - historic and from the 12
month monitoring program - is presented below:
Figure 4 : Bemm River Storage
- Algae Data
From the available data it appears that the installation
of the shadecloth cover resulted in a decrease in algae
numbers and frequency of occurrence. This is also supported
by the nutrient and bacterial levels as discussed earlier.
Monthly monitoring is continuing to confirm that this
decrease in algae numbers is real and permanent and
not a reflection of a poor Summer season.
3.0
CONSTRUCTION OF SHADECLOTH COVER
The
construction of a shadecloth cover over a potable water
storage attracted media interest with two articles published
in the The Age "A Shade Over The Top? For Sure" (09/07/01)
and Weekly Times "Covering up keeps it clean" (31/10/01).
Super Span Pty Ltd also won First Prize in the 2002
ASCASPA (Australian Canvas and Synthetic Products Association
Inc) Awards for a Tension Structure (Shadecloth).
The
following are some photos taken before and during construction
of the structure in order to give you some appreciation
of the task undertaken by East Gippsland Water and Super
Span Pty Ltd:
4.0
CONCLUSION
The
aim of this project was to limit (or eliminate) algae
growth in the Bemm River Storage Basin and the data
to date has shown that this aim has been achieved in
that the levels of algae numbers has decreased and appears
to be remaining low.
In terms of the effect of the shadecloth cover on other
parameters in the storage basin, there has been little
effect on physical-chemical parameters however there
appears to have been an increase in total nitrogen levels
within the basin.
The
overall value of total nitrogen is still low but this
situation will continue to be monitored to determine
whether this increase is as a result of the die off
of algae and whether the levels will change (increase
/ decrease) or reach a plateau over time.
Another
positive aspect of the shadecloth cover is the potential
increase in efficiency of disinfection - there is now
little or no algae present in the water to provide a
'mask' for bacteria to avoid disinfection and, from
the microbiological load (in terms of total coliform
data), there are less bacteria to deal with.
5.0 ACKNOWLEDGEMENTS
I would like to thank the following people:
John Hutchison (Manager
Technical Services) - for having faith in a scatterbrained
idea!
Stuart
Cannon
(Super Span Pty Ltd) - for his vision and tenacity in
seeing this through
Gary
Boyd
(Technical Officer East Gippsland Water) - for his project
management, frequent visits to the site and photos
East
Gippsland Water Orbost Staff
- for their patience and effort in taking all of the
field data and samples Water Ecoscience - for all analytical
work
Jacque
Hocking
- for her patience in entering reams of incomprehensible
data!
Amanda
Smeldts and James Gourley
- for helping me to argue with the graphing package!
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