|
Conference Papers | 1998 Conference Papers DESIGN
AND OPERATION OF AN OZONE/BAC WATER TREATMENT PLANT
Wayne Hill Operator,
Edenhope Water Treatment Plant
Reg McMillan Manager
Service Quality,
Grampians Region Water Authority
 DOWNLOAD
(pdf
27K)
ABSTRACT
Edenhope
in Western Victoria draws its water supply from Lake
Wallace - a natural shallow eutrophic lake. The supply
has historically suffered from severe blue-green algal
blooms over extended periods. Blooms have involved both
Microcystic and Anabaena species.
Raw water dissolved organic carbon (DOC) levels are
also very high - typically above 20 mg/L. A new advanced
water treatment plant has been constructed to provide
a reliable and safe treated water supply even under
difficult raw water conditions. The plant utilises the
ozone/biological activated carbon (BAC) process for
the first time at a significant level in Australia.
This
paper describes the raw water quality issues associated
with Lake Wallace and details the basis of process selection
and the design criteria for the treatment plant. Initial
operating results are provided.
KEY WORDS
Blue-green
algae, toxins, ozonation, granular activated carbon,
biological activated carbon (BAC).
1.0 INTRODUCTION
Lake Wallace water can be characterised as moderately
coloured with a high dissolved organic content. Typical
raw water quality is shown in Table 1. Of note is the
particularly high total organic carbon (TOC) content.
Table
1: Raw Water Quality
The Lake is subject to severe blue-green algal blooms
which have necessitated shut-down of the supply and
introduction of emergency bore water supplies due to
confirmed toxicity of the blooms. Levels of up to one
million cells of the blue-green algae Microcystis aerogonosa
per mL have been measured.
Grampians
Region Water Authority took over jurisdiction for the
Edenhope water supply in 1995 and commenced a program
to improve the water supply to ensure that a safe and
reliable water quality was delivered to its customers.
2.0 TREATABILITY
2.1
Coagulation/Filtration
Jar
tests undertaken on the raw water indicated typical
doses of alum of between 80 and 150 mg/L. This resulted
in TOC removal rates of around one-third. This indicated
that conventionally treated water would still contain
more than 10 mg/L of DOC. In addition, the raw water
taste and odour was unchanged through this type of treatment.
2.2
Ozonation
One
of the primary objectives of the plant is to provide
surety of removal of algal toxins. Published literature
to date suggested that ozone would be effective in destroying
Microcystin hepatotoxins, but data on PSP toxins which
are produced by some Australian blue-green algae was
limited.
Ozone
testing was undertaken by the Australian Water Quality
Centre in Adelaide using coagulated and filtered Lake
Wallace water which was spiked with various algal toxins.
This
test work showed that the design ozone capacity was
sufficient to:
-
Fully oxidise Microcystin LR
- Completely
oxidise most PSP toxins. The worst result was a 96%
removal - still satisfactory
- Reduce
taste and odour compounds but not fully remove them.
This
work confirmed that basis of the process design.
Testing
for bromate formation showed levels below the detection
levels of Australian laboratories. However this was
higher than guideline levels and further work is being
undertaken in this regard.
2.3
Granular Activated Carbon
A small pilot GAC filter was set up to carry out subjective
tests on Lake Wallace treated water. These demonstrated
its effectiveness in removing the residual tastes and
odours in the water.
3.0
TREATMENT PLANT DESCRIPTION
Based
on the problems experienced with the Lake Wallace supply
and the results of the tests previously described, the
following treatment process steps were selected.
A summary of treatment plant design criteria is as follows:
| Nominal
Plant Capacity |
3
ML/d |
| Coagulant
Dose Range |
Alum
50-200 mg/L |
| Flocculation
Stages |
2 |
| Flocculation
Time |
20
minutes total |
| Flotation
Rate (inc. DAF recycle) |
8
m/hr |
| Filtration
Rate (inc. DAF recycle) |
9
m/hr |
| Filter
Media |
Mono
medium sand |
| Disinfection |
Sodium
Hypochlorite |
| pH
Adjustment |
Soda
Ash |
Advanced Treatment Module
| Ozonation
pH |
Not
greater than 6.5 |
| Ozonation
Capacity |
800
kg/hr |
| Maximum
Ozone Dose |
9
mg/L @ 2 ML/d |
| Ozone
Contact Time |
5
minutes @ 2 ML/d |
| BAC
Vessels |
2
No. |
| Empty
Bed Contract Time |
15
minutes @ 2 ML/d |
| Activated
Carbon |
Coal-based |
4.0 OPERATION
The
full plant was commissioned on 22nd January 1998 and
produced a clear water totally free from taste and odour.
In the first stage of treatment, raw water is sampled
and monitored in the plant's laboratory to establish
the correct chemical dosage. Alum is added and mixed
with the raw water in a tank to help dirt, colour and
algae particles form into a floc. With air injected
from below, the water passes over a weir to the flotation/filtration
tank.
The floc, with impurities entrapped, rises with the
air to form a scum (float) which is periodically skimmed
off to a sludge tank and discharged to sewer. About
90 percent of the foreign matter in the water is removed
by flotation. The remaining 10 percent is trapped as
water passes through sand filter beds in the bottom
of the tank.
Algal
toxins released from decaying Blue Green Algae and tastes
and odour originating from algae and other organic matter
are not always removed in the first treatment stage
and hence a further advanced treatment stage with ozone
and biologically activated carbon is necessary to ensure
water delivered to consumers meets all Australian Drinking
Water Quality guidelines and is pleasant for consumption.
In this second stage, ozone is generated from air to
produce a very strong oxidant which, when bubbled through
the water, breaks down organic molecules and micropollutants
including algal toxins. Bench testing of filtered Lake
Wallace water which had been "spiked" with various algal
toxins was carried out at the Australian Water Quality
Centre in Adelaide and confirmed that relatively low
doses of ozone would destroy the algal toxins. Following
ozonation, water is then passed through beds of biologically
activated carbon which remove the organics and any residual
tastes and odours.
A low dose of chlorine is added prior to the fully treated
water entering the one million litre clear water storage
to prevent any bacteriological regrowth in the tank
and to enable control of biological activity in the
activated carbon filters when backwashing. pH adjustment
using Soda Ash is carried out prior to water entering
the clear water storage.
A further low dose of chlorine is added as high lift
pumps transfer the water from the clear water storage
to the elevated tank which provides water pressure to
the town.
5.0
CONCLUSION
Australia's
first significant use of the ozone/BAC process was implemented
at Edenhope, Victoria to overcome problems associated
with chronic blue-green algae in the source water. The
plant has been operating for 8 months producing consistent
quality of water with removal of all tastes and odours.
The plant has not had to operate to date under extreme
blue-green algae conditions. Bench tests carried out
during the design process indicate that it will be able
to perform during these events.
6.0
REFERENCES
1.
Kaeding, U.W. and Faulkner M.P.
"Ozonation of Treated Lake Wallace Water for the
proposed Edenhope Water Treatment Plant" 1997.
2.
National Health and Medical Research Council, Agriculture
and Resource Management Council of Australia and New
Zealand "Australian Drinking Water Guidelines"
1996.
3.
Gutteridge Haskins & Davey Pty Ltd "Report on Water
Treatment at Edenhope" 1996.
DOWNLOAD
(pdf
27K)
|
