A part of BMT in Energy and Environment
Sunshine Coast Regional
Council Flow Through
Coagulation/Flocculation
Sediment Basin Design
Assessment
R.B17503.001.02.doc
July 2009
Sunshine Coast Regional
Council Flow Through
Coagulation/Flocculation
Sediment Basin Design
Assessment
Offices
Prepared For:
Sunshine Coast Regional Council
Prepared By:
BMT WBM Pty Ltd (Member of the BMT group of companies)
D:\687307931.DOC
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DOCUMENT CONTROL SHEET
BMT WBM Pty Ltd
BMT WBM Pty Ltd
Level 11, 490 Upper Edward Street
Brisbane 4000
Queensland Australia
PO Box 203 Spring Hill 4004
Document :
R.B17503.001.01.doc
Project Manager :
Dr Joel Stewart
Client :
Sunshine Coast Regional Council
Client Contact:
Maurice Mathews
Tel: +61 7 3831 6744
Fax: + 61 7 3832 3627
ABN 54 010 830 421
www.wbmpl.com.au
Client Reference
Title :
Sunshine Coast Regional Council Flow Through Coagulation/Flocculation Sediment
Basin Design Assessment
Author :
Dr Joel Stewart
Synopsis :
This report presents analysis and modelling of estimated performance a
coagulation/flocculation sedimentation basin for the Sunshine Coast Regional Council
REVISION/CHECKING HISTORY
REVISION
DATE OF ISSUE
CHECKED BY
ISSUED BY
NUMBER
0
25th June 2009
MGH
JPS
1
22th July 2009
MGH
JPS
2
23th July 2009
MGH
JPS
DISTRIBUTION
DESTINATION
Sunshine Coast Regional Council
BMT WBM File
BMT WBM Library
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REVISION
0
1
2
doc
doc
doc
1-
1-
1-
doc
doc
doc
3
CONTENTS
I
CONTENTS
Contents
i
List of Figures
ii
List of Tables
ii
1
INTRODUCTION
1-1
2
METHODOLOGY
2-1
2.1
Twin compartment design
2-2
2.2
Variable off-take outlet weir configuration
2-3
2.3
Inline dosing for coagulation/flocculation
2-4
2.4
‘Overdesign’ rainfall `analysis
2-7
3
RESULTS
3-1
3.1
Detention basin performance
3-1
3.2
Comparison with field data
3-4
3.3
‘Overdesign’ rainfall analysis
3-5
3.3.1 Frequency of basin operation under ‘design’ conditions and under
‘overdesign’ conditions
3-5
3.3.2 How much rainfall is associated with ‘design’ conditions and ‘overdesign’
conditions
3-6
3.3.3 How much runoff may be associated with ‘design’ conditions and
‘overdesign’ conditions
3-7
3.3.4 What is the likely sediment load associated with ‘design’ and ‘overdesign’
conditions
3-8
3.4
Discussion
3-8
4
CONCLUSIONS
4-1
5
REFERENCES
5-1
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LIST OF FIGURES
II
LIST OF FIGURES
Figure 1-1
Demonstration site basin performance
1-2
Figure 2-1
Basin showing fore-bay and extended detention pond
2-2
Figure 2-2
Variable off take weir
2-3
Figure 2-3
Basin depth/outflow relationship
2-4
Figure 2-4
Flocculent dosing apparatus
2-6
Figure 2-5
Performance parameter (N0/N1) and basin outflow
2-6
Figure 3-1
Basin outflow concentration with coagulation flocculation and settling3-2
Figure 3-2
Relationship between flow weighted outflow and inflow concentration 3-3
Figure 3-3
Modelled and monitored basin performance
3-4
Figure 3-4
Nambour daily rainfall
3-5
Figure 3-5
Nambour 5-day rainfall depth
3-5
Figure 3-6
5 day rainfall depth vs. frequency
3-6
Figure 3-7
5 day rainfall depth vs. wet period frequency
3-6
Figure 3-8
Design rainfall depth vs. proportion of rainfall outside the design event3-7
Figure 3-9
Modelled runoff proportion vs. 5-day rainfall depth
3-8
Table 2-1
Example values of KA and KB (Bratby 2006)
2-5
Table 3-1
Modelled Sediment Removal Efficiency for Type F Soils
3-1
Table 3-2
Modelled Sediment Removal Efficiency for Type D Soils
3-1
Table 3-3
Basin performance for settling and flocculation assisted basins
3-3
LIST OF TABLES
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1-1
INTRODUCTION
1
INTRODUCTION
This investigation is to model the performance (efficiency) of design sedimentation basins undergoing
continuous flow through and coagulation/flocculation in the Sunshine Coast area. This is achieved
through use of the previously developed continuous 6-minute time step rainfall runoff and
sedimentation basin model (BMT WBM 2008).
The sedimentation basin configurations used in previous modelling assessments were based upon
guidelines available in the Sunshine Coast Regional Council Erosion and Sediment Control Manual
(The Manual). The three model scenarios previously undertaken included:

Scenario 1: No bypass strategy: All catchment flows pass through the basin and basin overflow
is controlled by broad crested weir;

Scenario 2: Basin dewatering: A basin dewatering trigger was included for this scenario whereby
following a 24 hour period of no runoff the basin would be dewatered and made ready for the
next event with a nominal volume of water in the bottom of the basin; and

Scenario 3: Basin dewatering plus basin bypass: This scenario adopted the dewatering trigger
as described above in addition to a bypass trigger whereby if the detention basin is full and has
not been dewatered then all further catchment runoff is diverted around the basin (untreated)
until the basin can be dewatered (24 hours of no runoff).
Modelling for the above scenarios showed that sediment removal rates of between 80% and 90%
may be achievable however, for type D and F soils, the 50mg/L goal is unlikely to be achieved
without additional measures due to the large portion of small particle size sediment unable to be
naturally settled during large events.
Since undertaking the original sedimentation basin modelling work in 2008, the Sunshine Coast
Regional Council, in conjunction with a developer, have implemented a best practice New Zealand
design sedimentation basin including continuous flow through design and coagulation/flocculation.
This basin has been monitored through the summer of 2008/2009 and the outlet sediment
concentration assessed by the Sunshine Coast Regional Council. The results of this monitoring
program showed that with coagulation/flocculation, the 75ntu and 50mg/L target can be successfully
met at this trial site for a well maintained basin. These monitoring results are shown in Figure 1-1.
The figure shows that under the design coagulant dosing conditions, basin outflow turbidity regularly
complied with the 75ntu target. Up to 125mm/day of rain was recorded for this short monitoring
program and the complete data set is provided in Appendix A.
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1-2
INTRODUCTION
Figure 1-1
Demonstration site basin performance
The aim of this investigation is to upgrade the existing sedimentation basin model to reflect the
configuration and performance of a best practice basin undergoing continuous flow through with
coagulation/flocculation (Scenario 4). This report describes the model upgrade, parameter sets,
modelling and model results.
This report also provides calculations on the potential efficacy of the 75ntu and 50mg/L target
applying only to events below the design event. Daily rainfall records for Nambour have been used to
calculate the frequency of rainfall events considered to be ‘overdesign’ and estimated sediment loads
associated with these events.
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METHODOLOGY
2
2-1
METHODOLOGY
Sedimentation basin performance for the Sunshine Coast Regional Council was assessed using
spreadsheet analysis and modelling based on sedimentation basin sizing and soil erosion potential
as described by the Manual For Erosion and Sediment Control (SRSC, 2008) and the sedimentation
basin model described in BMT WBM (2008). The previously developed sedimentation basin model
contains the following features:

6-minute time step rainfall runoff modelling for 1 year of operation;

Sedimentation basin water balance controlled by overflow weir and/or pump-out;

Sedimentation basin settling zone is represented by up to 6 continuously stirred tank reactors
(CSTRs) in series to represent mixing and approximation of plug flow;

Particle settling estimated through Stokes Law; and

Particle size distribution of sediment inflow approximation as described by Skaggs et al (2001).
This project requires the extension of the sedimentation basin model to account for the following
features as described by a best practice sedimentation basin demonstration operating on the
Sunshine Coast over the 2008-2009 summer (Appendix A).
Basic basin features include:

Basin sizing based on 75th, 80th and 90th percentile 5 day rainfall depths across a 0.64 ha
catchment and dimensions based on MSC (2007) basin design guidelines;

Basin outflow is controlled by overflow weir, manual drawdown, bypass or low flow drawdown (or
a combination of the above) depending on the scenario to be modelled.

Sediment inflow concentrations (and therefore design sediment storage volume) were based on
20th percentile TSS EMCs calculated from the RUSLE for the expected range of conditions in the
Sunshine Coast region (Appendix B).
The general features of the basin used in the following analysis that differ from the previous
sedimentation basin model configuration are as follows:

Twin compartmental design of fore-bay and extended detention pond with different operating
depths;

Variable off-take outlet weir configuration; and

Inline dosing for coagulation and flocculation of colloidal particles.
The methods used to reconfigure the sedimentation basin model presented in BMT WBM (2008) are
described in the following sections. Details regarding general configuration of the sedimentation basin
model, particle size distributions, rainfall runoff model and basin sizing can all be found in the report
R.B17032.001.02 (BMT WBM 2008) attached as Appendix B.
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2-2
METHODOLOGY
2.1
Twin compartment design
The original sedimentation basin model was upgraded by adding a second basin routing and settling
module to the original single compartment model.
The twin compartment basin shown in Figure 2-1 has the approximate fore-bay dimensions: 10m
wide x 4m long x 1m deep. The fore-bay is designed for quick cleanout and intercepts all runoff from
the disturbed areas of the site. Outflow from the fore-bay is via a broad crested weir approximately
8m long. Rainfall triggered coagulant dosing is introduced to the fore-bay where mixing under
turbulent flow takes place. The majority of coarse sediment will likely settle in the fore-bay.
Figure 2-1
Basin showing fore-bay and extended detention pond
The fore-bay is represented in the sedimentation basin model as a separate storage, with inflow and
sediment mass time series from the rainfall runoff model. Overflow from the sediment basin is via a
weir modelled on a weir equation (Street et al. 1996) and modelled water height above the basin full
level. The sedimentation basin settling zone is represented by continuously stirred tank reactors
(CSTRs) to represent mixing. Water and sediment are tracked through the storage with time and the
outflow concentration of sediment is determined through the application of:

Stokes law enhanced by flocculation efficiency equation (described in Section 2.3)

Mixed outflow concentration of sediment (before settling),

Mean residence time of the outflow volume; and

Approximation of particle size distribution.
The model calculates overall trapping efficiency and total outflow concentration of suspended
sediment.
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2-3
METHODOLOGY
2.2
Variable off-take outlet weir configuration
The original sedimentation basin model included a variety of dewatering options including bypass,
overflow weir and opportunistic dewatering via pumping rules. The current best practice basin design
incorporates a flow through structure with dewatering based on three floating boom weirs
approximately 2m wide as shown in Figure 2-2.
The extended detention sedimentation pond with floating weir outflow is 2m deep when operating at
full capacity and has a 1m deep settling zone and 1m dynamic zone. Each floating weir is set at
different levels within the operating zone at the nominated 1m, 1.4m and 1.8m levels.
The outflow weirs float and collect water from just below the water surface, therefore the outflow from
the basin is modelled on a sharp crested weir (Street et al 1996) of constant head. Outflow from each
weir has been calculated and summed to provide a relationship between water depth in the basin
(head) and outflow as shown in Figure 2-3.
Figure 2-2
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Variable off take weir
2-4
METHODOLOGY
Figure 2-3
2.3
Basin depth/outflow relationship
Inline dosing for coagulation/flocculation
Inline coagulant dosing apparatus for a best practice basin is undertaken via rainfall depth based
dosing. Rainfall collected from the roof of a small onsite shed housing pump system and battery is
used to trigger and deliver measured doses of coagulant to the fore-bay of the sediment basin (Figure
2-4). Sediment laden runoff from the catchment is mixed with the coagulant as it enters the fore-bay
or primary settling pond and is mixed through turbulence. No baffles are included in the fore-bay to
assist with mixing.
Modelling of the coagulation/flocculation process is undertaken via Equation 1 (performance
parameter) and Equation 2 (Bratby 2006)
Performance _ parameter 
n0 1  K A GT

n1 1  K B G 2T
n0 = number concentration of primary particles at time T=0
n1 = number concentration of primary particles at time T
KA = Constant
KB = Constant
G = Root mean square velocity gradient (s-1)
T = time in seconds
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Equation 1
2-5
METHODOLOGY
G
g  hL
T 
Equation 2
g = weight per unit mass (9.81 N/kg)
hL = head loss (m)
T = time in seconds
v = kinematic viscosity ( = 10-6 m2/s at 20 degrees)
Values of KA and KB have been adopted form literature as presented by Bratby (2006) and shown in
Table 2-1. for the current application, KA and KB have been selected from “Natural turbid water –
Alum” in Table 2-1. These values produce a lower value of n0 / n1 for the current application and are
therefore conservative.
The model calculates G for every time step based on average head loss of 0.1m across the basin
and the calculated detention time for each parcel of water flowing from the basin. A plot showing a
time series of the performance parameter and basin outflow vs. time step is shown in Figure 2-5. This
plot shows that the performance parameter exceeds 10 (inflow concentration is 10 times the outflow
concentration) for all but the largest flow events. The performance parameter does not fall below 2 for
the largest modelled event.
Table 2-1
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Example values of KA and KB (Bratby 2006)
2-6
METHODOLOGY
Figure 2-4
Figure 2-5
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Flocculent dosing apparatus
Performance parameter (N0/N1) and basin outflow
METHODOLOGY
2.4
2-7
‘Overdesign’ rainfall `analysis
Table 6-2 in The Manual (MSC 2007) lists appropriate 2-day and 5-day rainfall depths (mm) for
locations throughout the Sunshine Coast. These rainfall depths are commonly considered to be
‘design rainfall depths’, rather than a guide for basin sizing. Consequently rainfall events exceeding
these ‘design rainfall depths’ are considered ‘overdesign’ and therefore not subject to assessment
against the 50mg/L/75 NTU performance criteria.
The purpose of the overdesign rainfall analysis is to provide an estimate of:
a) How often is a basin likely to be operating under ‘design’ conditions and under ‘overdesign’
conditions;
b) How much rainfall is associated with ‘design’ conditions and under ‘overdesign’ conditions;
c) How much runoff is likely to be associated with ‘design’ conditions and under ‘overdesign’
conditions; and
d) What is the likely sediment load associated with ‘design’ conditions and ‘overdesign’ conditions.
Approximately 100+ years of daily rainfall data from Nambour was analysed to calculate the
frequency of rainfall events considered to be ‘overdesign’ (Table 6-2 from MSC 2007), rainfall depths,
approximate runoff volumes and associated sediment loads. The rainfall runoff model used for the
sediment basin analysis was used to perform this assessment.
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3-1
RESULTS
3
RESULTS
3.1
Detention basin performance
Detention basin performance is measured in terms of:

Efficiency (total annual load trapping efficiency) determined by mass balance (load in minus load
out); and

Success in meeting the 50mg/L target discharge concentration.
Basin performance has been modelled for soil types D and F (incorporating the different particle size
distributions described in Table 3-2 in BMT WBM (2008) and for all settling basin volumes described
in Table 3-3 of BMT WBM (2008). A total of 48 detention basin simulations have been undertaken for
the following analysis representing the median year rainfall and wet year rainfall pattern.
The detention basin operational scenario (Scenario 4) includes no bypass strategy, all catchment
flows pass through the basin and basin overflow is controlled by variable offtake weir. Continuous
coagulation/flocculation is undertaken according to Equations 1 and 2.
Basin efficacy results for Scenario 4 are provided in the following tables. Modelled basins met the
50mg/L target for some of the time, but were still shown to exceed this target from time to time (in
very high flow events only)
Predicted sediment removal percentages for Scenario 4 type D and F basins are provided in the
following tables. The data represents the proportion of total sediment load that is removed as a result
of coagulation/flocculation assisted settling.
Table 3-1
Modelled Sediment Removal Efficiency for Type F Soils
Basin Sizing Design Criteria
Removal
Efficiency (%)
Type F
Coolum
Kenilworth
Mapleton
Nambour
th
75 Percentile 5day event
80th Percentile 5day event design
90th Percentile 5day event
median
wet
median
wet
median
wet
94.3
92.9
94.5
94.1
94.1
93.6
94.1
93.9
94.9
93.4
95.0
94.7
94.7
94.2
94.6
94.5
96.1
94.8
96.3
96.2
96.1
95.6
96.1
96.1
Table 3-2
Modelled Sediment Removal Efficiency for Type D Soils
Basin Sizing Design Criteria
Removal
Efficiency (%)
Type D
Coolum
Kenilworth
Mapleton
Nambour
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th
75 Percentile 5day event
median
wet
93.9
92.5
94.1
93.7
93.6
93.2
93.6
93.4
80th Percentile 5day event
median
wet
94.6
93.1
94.7
94.4
94.3
93.8
94.2
94.1
90th Percentile 5day event
median
wet
95.9
94.6
96.1
96.0
95.9
95.3
95.8
95.9
3-2
RESULTS
Basin performance with coagulation/flocculation is compared against basin performance with stokes
law settling in Figure 3-1. The model output shows estimated TSS concentrations in the
coagulation/flocculation basin outflow for a Type D soil may still exceed the 50mg/L limit during high
flow events. The potential water quality improvement of the coagulation/flocculation basin over the
settling only basin is considerable however and shows that meeting guideline values of 50mg/L TSS
is much more likely under Scenario 4 conditions.
Figure 3-1
Basin outflow concentration with coagulation flocculation and settling
The model results indicate that achievement of the 50mg/L target may not be met during some
periods of basin operation. An estimate of overall basin performance however may be obtained by
calculating the flow weighted concentration of modelled TSS from the basin. This is calculated by
summing the total mass of sediment passing through the basin divided by the total outflow and is a
readily available output from the model.
The flow weighted TSS concentration from the basin is sensitive to inflow concentration, basin sizing
and the selection of parameters used to calculate the performance parameter (Equation 1). A
sensitivity analysis has not been undertaken to investigate these relationships, however presented
below is a plot showing modelled flow weighted TSS concentration for the 80th percentile 5 day event
sized basin for type D soils in Nambour under settling only and coagulation/flocculation assisted
settling for typical inflow sediment concentrations of 50 -2000 mg/L.
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3-3
RESULTS
Figure 3-2
Relationship between flow weighted outflow and inflow concentration
Figure 3-2 shows that without coagulation/flocculation, the 50 mg/L limit is unlikely to be obtained for
most type D developing sites as inflow TSS concentrations are likely to exceed 250 mg/L. The
coagulation/flocculation basin performs much better in the model achieving guideline levels with
inflow TSS concentrations of 1000 mg/L.
Overall performance of the coagulation/flocculation basin compared with settling only is shown in
Table 3-3 for type D and F basins for the 80th percentile 5 day designed basin. The table shows
significantly higher performance of flow through coagulation/flocculation assisted basins over
conventional settling for type F and type D soils.
Table 3-3
Removal
Efficiency
(%)
Coolum
Kenilworth
Mapleton
Nambour
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Basin performance for settling and flocculation assisted basins
Basin Sizing Design Criteria
Type F
Type D
80th Percentile
80th Percentile
5-day event
5-day event
Settling only
Flocculation Settling only
Flocculation
assisted settling
assisted settling
85.9
94.9
84.7
94.6
85.6
93.4
84.5
93.1
85.2
95.0
84.0
94.7
84.9
94.7
83.6
94.4
3-4
RESULTS
3.2
Comparison with field data
The modelled basin performance has been compared with the field data presented in Appendix A
(Figure 3-3). Although turbidity (ntu) can not be directly compared with TSS (mg/L) the monitoring
results indicate that the model may be under predicting the sediment removal. Further monitoring
should be undertaken on a range of basins in the region to confirm that this is the typical case before
model refinement is undertaken. In the present study however the model results may be considered
conservative based on the limited performance data currently available.
Figure 3-3
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Modelled and monitored basin performance
3-5
RESULTS
3.3
‘Overdesign’ rainfall analysis
The daily rainfall record for Nambour is shown in Figure 3-4. The 5-day rainfall depth has been
calculated from the daily record by summing rainfall totals for each 5 day period for every day in the
record. This plot is shown in Figure 3-5 in relation to the Nambour 5-day design depth of 43.4 mm.
This time series of 5-day rainfall depth has been used to calculate the statistics presented below.
Figure 3-4
Figure 3-5
3.3.1
Nambour daily rainfall
Nambour 5-day rainfall depth
Frequency of basin operation under ‘design’ conditions and
under ‘overdesign’ conditions
The calculated frequency of overdesign conditions for Nambour is 12.1% of all days (12 days out of
every 100) or approximately 44 days every year as shown in Figure 3-6. In terms of a proportion of all
‘wet’ or rainy periods, the proportion of events exceeding the design rainfall depth is 26.1% as shown
in Figure 3-7.
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3-6
RESULTS
Figure 3-6
Figure 3-7
3.3.2
5 day rainfall depth vs. frequency
5 day rainfall depth vs. wet period frequency
How much rainfall is associated with ‘design’ conditions
and ‘overdesign’ conditions
As Figure 3-5 suggested, many large rainfall events would be considered ‘overdesign’ under the
current interpretation of the 43.4mm design guideline. Figure 3-8 presents the relationship between
design rainfall depth and the total amount of rain (rainfall depth) associated with design and
overdesign conditions. The 43.4mm design rainfall depth captures 36% of total rainfall depth. The
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3-7
RESULTS
majority of rainfall (63.3%) would be considered ‘overdesign’ under the current interpretation of the
guideline and therefore not subject to the 50mg/L/75 NTU target.
The design rainfall depth would have to be significantly increased to capture higher proportions of the
rainfall.
Figure 3-8
3.3.3
Design rainfall depth vs. proportion of rainfall outside the design event
How much runoff may be associated with ‘design’
conditions and ‘overdesign’ conditions
The rainfall runoff model used to generate inflows to the sedimentation basin model was used to
assess typical daily runoff from daily rainfall. This model operates on a 6 minute time step, therefore
there can be significant variability between daily total rainfall and daily total runoff. Two separate
analyses were undertaken corresponding to infiltration rates of 6mm/hr and 3mm/hr to account for the
potential range in generated runoff.
Figure 3-9 shows that just 15%-20% of all modelled runoff (1 year simulation only) is associated with
‘design’ 5-day rainfall depths less that 50mm. The vast majority of runoff (in a volumetric sense) is
therefore likely to be associated with ‘overdesign’ rainfall events.
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3-8
RESULTS
Figure 3-9
3.3.4
Modelled runoff proportion vs. 5-day rainfall depth
What is the likely sediment load associated with ‘design’
and ‘overdesign’ conditions
In the current modelling framework, sediment concentration is modelled using an event mean
concentration (EMC) which is constant for any given model run. Therefore the likely sediment load
associated with ‘design’ and overdesign rainfall is directly proportional to runoff results presented in
Section 3.2.3.
The model therefore indicates that 80%-85% of all sediment load delivered to the sedimentation
basin is likely to be exported during ‘overdesign’ events.
3.4
Discussion
The results show:

The potential water quality improvement of the coagulation/flocculation basin over the settling
only basin is considerable and model results show that the 50mg/L TSS guideline is much more
likely to be met under coagulation/flocculation conditions.

The 50mg/L limit may be exceeded under scenario 4 in some high flow instances, however the
result depends heavily upon the inflow concentration and particle size distribution of sediment;

The coagulation/flocculation basin is likely to achieve a flow weighted discharge concentration
less than 50mg/L with inflow TSS concentrations of 1000 mg/L or less; and

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Basin performance (removal efficiency) is very similar between type D and F soils.
3-9
RESULTS
In all cases without coagulation/flocculation, sediment concentrations in the outflow exceeded the
50mg/L requirement. This is due to the modelled particle size distribution of the type F and D soils
containing large fractions of non-settleable material. The addition of a flocculating agent applied
through online dosing provides the means to remove these small particles that would otherwise not
settle through gravity alone.
Many factors may influence the performance of coagulation/flocculation processes including mixing. It
is important that the coagulant is well mixed through the water to facilitate the flocculation process,
and that the opportunity for flocs to come into contact with one another in the settling zone is
facilitated. In the water/wastewater industry, compartmentalisation through baffling is used to assist
the development of flocs. Theoretical and process based studies on coagulation/flocculation
behaviour indicate better performance for reactor compartments in series than single compartment
reactors (Bratby 2006) highlighting the benefits of the multiple compartment configuration of current
best practice basins subject to coagulation/flocculation treatment processes.
The importance of flow through design basins has been highlighted through the rainfall ‘overdesign’
analysis presented above. The interpretation of the design rainfall depth figure as a trigger to indicate
when the 50mg/L target should be complied with is unlikely to produce the desired sediment removal
outcome. Current analysis indicates that the 50mg/L target does not need to be met approximately
12% of the time (44 days per year), however these times correspond with the highest rainfall and the
greatest proportion of runoff and therefore sediment export. 80-85% of runoff and therefore sediment
export is likely to occur during these 44 days per year.
Modelling shows that the well designed basin can achieve performance criteria even during relatively
high flow events indicating that the design rainfall depth can be significantly increased without making
the 50mg/L target impossible to achieve. A 5-day ‘design’ rainfall depth of 100mm-200mm may be a
more appropriate compliance trigger.
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4-1
CONCLUSIONS
4
CONCLUSIONS
This aim of this investigation was to model the operational performance of a sedimentation basin as
configured under the Sunshine Coast Regional Council Erosion and Sediment Control Manual
undergoing inline coagulation/flocculation and flow through design. The continuous model previously
developed by BMT WBM (2008) was modified to accommodate the best practice features:

Twin compartmental design of fore-bay and extended detention pond with different operating
depths;

Variable off-take outlet weir configuration; and

Inline dosing for coagulation and flocculation of colloidal particles.
The results show considerably better basin performance in terms of removal efficiency, particularly in
terms of small particle size removal. The 50mg/L TSS guideline is much more likely to be met under
coagulation/flocculation conditions, however this outflow concentration may still be exceeded from
time to time.
The coagulation/flocculation basin is likely to achieve a flow weighted discharge concentration less
than 50mg/L with inflow TSS concentrations of 1000 mg/L or less and is likely to remove 93%-95% of
sediment.
Under current interpretation of the design rainfall depth parameter (43mm for Nambour), rainfall
analysis indicates that the 50mg/L target does not need to be met approximately 12% of the time (44
days per year). These days are associated with approximately 80-85% of runoff and sediment export.
The adoption of the design rainfall depth parameter as a trigger for when a basin needs to achieve
the 50mg/L target may therefore be ineffective.
The design rainfall parameter is used to size a basin, but should not prescribe when it needs to
perform. Under best practice design, modelling indicates that the 50mg/L target may be achievable
over a wider design rainfall depth window.
D:\687307931.DOC
REFERENCES
5
5-1
REFERENCES
BMT WBM (2008). Sunshine Coast Regional Council Sediment Basin Design Assessment. Report
prepared for the Sunshine Coast Regional Council. BMT WBM Pty Ltd, July 2008. Ref
R.B17032.001.02.doc
Bratby, J., (2006) Coagulation and Flocculation in Water and Wastewater Treatment. Edition 2. IWA
Publishing, 2006. ISBN 1843391066, 9781843391067
Capelin M. A., (1987). Horticulture Land Suitability Study. Sunshine Coast South East Queensland.
Land Resources Branch, Department of Primary Industries. Queensland Government, Brisbane 1987
ISSN 0811-9007. AGDEX 526
Duncan, H. (1999). Urban Stormwater Quality: A Statistical Overview. CRC for Catchment Hydrology,
Monash University, Victoria: 99/3.
SRSC, 2008. Sunshine Coast Regional Council Manual for Erosion & Sediment Control. Sunshine
Coast Regional Council
Mitsch, W.J. & Gosselink, J.G. (2000). Wetlands. Third Edition, John Wiley and Sons Inc, Canada.
Street, R., Watters, G. and Vennard, J. (1996). Elementary Fluid Mechanics. 7th Edition, John Wiley
& Sons: Cleveland.
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DEMONSTRATION SEDIMENTATION BASIN
APPENDIX A: DEMONSTRATION SEDIMENTATION BASIN
D:\687307931.DOC
A-1
A-2
Date
15/02/2009
17/02/2009
18/02/2009
19/02/2009
23/02/2009
24/02/2009
10/03/2009
11/03/2009
12/03/2009
13/03/2009
15/03/2009
16/03/2009
17/03/2009
18/03/2009
19/03/2009
20/03/2009
22/03/2009
23/03/2009
24/03/2009
28/03/2009
29/03/2009
30/03/2009
2/04/2009
3/04/2009
4/04/2009
5/04/2009
6/04/2009
7/04/2009
8/04/2009
9/04/2009
10/04/2009
11/04/2009
12/04/2009
13/04/2009
18/04/2009
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Demonstration Basin Performance Data (SCRC)
Kunda Park
Outlet
Rainfall (mm)
Inlet
(NTU)
Coagulant
120
>3000
16 gypsum
17
>3000
18 gypsum
11 gypsum
45
83
9 gypsum
2246
14 gypsum
35
1380
8 CaCl
3
7
25
1290
9 CaCl
31
CaCl
1
1165
CaCl
26
11
3
14
10 CaCl
67
12 CaCl
83
31 CaCl
1
11
18 CaCl
8
7
17
15 CaCl
125
>3000
21 CaCl
10
8
121
>3000
CaCl
45
>3000
18 CaCl
2
2490
14 CaCl
0
0
1
22
1
16 CaCl
6
145 CaCl
BMT WBM 2008
APPENDIX B: BMT WBM 2008
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B-1
BMT WBM 2008
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B-2
BMT WBM 2008
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B-3
BMT WBM 2008
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B-4
BMT WBM 2008
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B-5
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B-6
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B-7
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B-8
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B-9
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B-10
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B-11
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B-12
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B-13
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B-14
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B-15
BMT WBM 2008
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B-16
BMT WBM 2008
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B-17
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B-18
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B-19
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B-20
BMT WBM 2008
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B-21
BMT WBM 2008
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B-22
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