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Conference Papers | 2001 Conference Papers OPTIMISATION
OF PHOSPHORUS REMOVAL AT WINMALEE SEWAGE TREATMENT PLANT
Nelly M.Guevara and Greg Searle -
Treatment Management Team,
Sydney Water Corporation
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ABSTRACT
An
optimisation programme was conducted at Sydney Water
Winmalee Sewage Treatment Plant to determine the cost
effective phosphorus removal in the sequenced batch
reactors (SBR).
The different phosphorus removal mechanisms were evaluated
and it was found that, simultaneous biological and chemical
reactions were responsible for the phosphorus removal.
The biological phosphorus removal represented the 32%
whereas the chemical and adsorption mechanisms were
represented by the 68% of the total phosphorus.
The
addition of iron into the flow splitter reduced the
SBR effluent phosphorus concentration from 1 mg/L to
<0.5 mg/L and the 50 percentile final effluent total
phosphorus was 0.1 mg/L, this was achieved with significant
chemical cost reduction.
KEYWORDS
Activated sludge, biological phosphorus removal, chemical
phosphorus removal, Spent Pickle Liquor (SPL), Sequenced
Batch Reactor (SBR), Ortho Phosphorus (Ortho-P), Total
Phosphorous (TP), Phosphorus Accumulating Organisms
(PAO)
1.0
INTRODUCTION
Winmalee
Sewage Treatment Plant was commissioned in May 1998
and it treats an average dry weather flow of 16 ML/d.
Figure 1 shows its schematic process diagram and it
consists of the following:
| » |
Conventional
preliminary treatment for screenings and grit removal. |
| » |
Sequenced
Batch Reactor (SBR) activated sludge for solids
and nutrients removal. The facility uses batch activated
sludge processing with sequenced periods of aeration.
This permits the use of a single vessel to accomplish
both biological degradation reactions and solid-liquid
separation. |
| » |
The
plant has four SBR basins arranged to operate in
two pairs. Each SBR operates with sequential phases
of aeration, settle and decant. |
| » |
Tertiary
treatment (ie. Filtration, chlorination and dechlorination)
and |
| » |
Sludge
dewatering process. |
Figure 1: Schematic Diagram
of Winmalee STP
The
SBRs at Winmalee STP were designed for biological and
chemical phosphorus removal by the addition of alum
at a concentration of 6 - 8 mgAl/L. An optimisation
programme was carried out since September to enhance
the phosphorus removal in the SBRs process by replacing
alum with SPL (Ferrous solution) which is a by-product
of steel manufacture at 160 g/L of Iron.
The SPL was dosed into the flow splitter where the screened
degritted sewage flows upward, this action provides
a turbulent flow which results in a suitable degree
of mixing for the SPL and phosphorus chemical reaction.
This
optimisation was based on studies carried out by scientists
and researchers about the interactions between chemical,
adsorption and biological phosphorus removal in activated
sludge. During the optimisation, trials have been performed
to determine the different phosphorus removal mechanisms,
these are:
| » |
Phosphorus
removal by chemical precipitation. |
| » |
Phosphorus
removal by adsorption. |
| » |
Phosphorus
removal biologically by cell growth and PAO |
1.1
Testing methods
All analysis were conducted on-site, using colorimetric
methods for ortho-P, TP and total iron.
2.0
RESULTS AND DISCUSSION
The
chemical phosphorus removal was optimised to determine
the optimum SPL dose and it was found that, 50% of phosphorus
removal is carried out in the early stages of the process
particularly in the flow splitter with the remaining
50% occurring in the SBRs. Not all the phosphorus is
removed by the addition of the SPL certain varieties
of microorganisms utilise phosphate as an energy source
for cell growth.
Jar
tests and plant trials were carried out to found the
optimum SPL dosage and found that, 8 -10 mgFe/L was
the optimum dosage to achieve SBR ortho-P of <0.5 mg/L.
The amount of phosphorus required for cell growth was
calculated from the amount of phosphorus in the organic
biomass wasted per day and it was found to be 42kg/d
which represents 2.5 mgP/L of the incoming total phosphorus.
The
phosphate adsorption capacity of the iron complex in
the SBRs was measured in a laboratory scale trial and
was found to be 0.00136 mgP/mgFe/d.
At this optimum operation the 50%ile SBR effluent ortho-P
was <0.5 mg/L. Fig 2 shows the improvement in phosphorus
removal using SPL.
Figure 2: SBR effluent ortho-P
versus SPL dose
The
conversion to SPL for phosphate precipitation has overall
been a positive move for the plant because the following
benefits:
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Increases
the average density of the activated sludge and
improve its settleability. |
| » |
Reduces
the alkalinity consumption, therefore reduces the
need to add caustic to buffer alkalinity and pH. |
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Low chemical usage and chemical costs savings. |
| » |
Low SBR effluent ortho-P concentration. |
The
Figs. 3 and 4 shows the alum usage and the chemical
costs reduction respectively during the optimisation
trials.
Figure
3: Daily alum usage
Figure 4: Monthly chemical expenditure
3.0
CONCLUSIONS
The following conclusions are drawn from this optimisation
trial.
| » |
The
addition of iron as SPL increased phosphorus removal
, the SBR effluent ortho-P concentration improved
from ~1mg/L to <0.5 mg/L. |
| » |
Simultaneous
biological and chemical phosphorus removal was observed,
it was found that, 60% of TP removed was by chemical
precipitation, 25 % biologically for cell growth,
8% by adsorption and the remaining 7% biologically
by Phosphate Accumulating Organisms. |
| » |
The
alum usage for phosphorus removal was reduced by
80% with savings for the business of $120,000/year. |
4.0
REFERENCES
He, Q.H., Leppard, G. G., Paige, C. R. and Snodgrass,
W. J. (1996). Transmission electron microscopy of a
phosphate effect on the colloid structure of iron hydroxide.
Wat. Res. 30(6) 1345-1352.
Roske,
I. Schonborn, C. and Bauer, H. (1995) . Influence of
the addition of different metals to an activated sludge
system on the enhanced biological phosphorus removal.
Int. Revue Ges. Hydrobiol. 80(4), 605-621.
Roske,
I. and C. Schonborn, (1994) . Interactions between chemical
and advanced biological phosphorus elimination. Wat.
Res. 28(5) 1103-1109.
Sedlak,
Richard, (1991) Editor of Phosphorus and Nitrogen removal
from Municipal Wastewater, Second Edition
Spatzierer,
G., Ludwig, C. and Matsche, N. (1985). Biological phosphorus
removal in combination with simultaneous precipitation.
Wat. Sci. & Tech. 17, 163-176. >DOWNLOAD
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