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Supplementary information for:
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Treatment efficiency of a wet detention pond combined with
filters of crushed concrete and sand – a Danish full scale study
of stormwater
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Melanie J. Sønderup1,2,*, Sara Egemose1, Timm Bochdam2, Mogens R. Flindt1
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1) Department of Biology, University of Southern Denmark, Campusvej 55, 5230 Odense M
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2) Arwos A/S, Forsyningsvejen 2, 6200 Aabenraa
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*Corresponding author: Melanie J. Sønderup, e-mail: melanie@biology.sdu.dk, phone: +4565502768.
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Relations between water level and flow measurements
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The best relation (R2 = 0.9398) between water level in the neutralization well and manual flow
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measurements out of the well is given by a Sigmoidal relation with 3 parameters:
Flow out of concrete filter
Qconcrete 
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a
 depth  x 0  
1  exp  

b


a  8.893
b  1.5207
x 0  6.4585
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Flow out of sand filter
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There is a strong linear correlation (R2 = 0.9843) between the flow (manual measurements) out of the
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concrete filter and the relation between flow (manual measurements) out of the concrete filter and the
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sand filter (in percentage):
Qconcrete
 100%  a  Qconcrete  b
Qsand
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a   9.4705
b  91.5352
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This means that the flow out of the sand filter is almost identical (0.8 L s-1 or 82 %) to the flow out of
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concrete filter at low flow (1 L s-1), whereas the difference is far more pronounced at high flow out of the
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concrete filter (8 L s-1) compared to the sand filter (1.3 L s-1 or 16 %).
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Flow out of the pond and into both filters
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Water level loggers placed in both filters measured water level changes, which are then calculated into
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volume changes from the known surface area and bank slopes. As the system operates by mass
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conservation, the inlet flow to the filters is calculated from addition of the outlet flow and the volume
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change over time. The calculated flow was validated by manual flow measurements.
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Overflow from the pond
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Overflow occurs whenever the water level in the pond reaches the overflow level, which is a 1.25 m long
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wall inside the outlet well. The water level in the pond was measured by a logger placed in the outlet well.
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The overflow is calculated from free fall and kinetic/potential energy, whenever the water level reaches the
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overflow level:
E pot  E kin ; where E pot  m  g  h and
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E kin  0.5  m  v 2
g h
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Qoverflow  v  l overflow  actual water depth  overflow level 
v
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Flow into the pond
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The water level logger placed in the outlet well from the pond measured water level changes in the pond,
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which is then calculated into volume changes from the known surface area and bank slopes. As the system
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operates by mass conservation, the inlet flow to the pond is calculated from addition of the outlet flow, the
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overflow and the volume change over time. The calculated flow was validated by manual flow
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measurements.
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Flow in and out of the oil separator
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The oil separator is only in use, when the water level in the grit chamber reaches a certain level. The
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relation between manual flow measurements out of the oil separator (3.0-4.5 L s-1) and water level in the
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grit chamber is described by a linear regression:
Qoil separator  a  depthgrit chamber  b
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a  0.0223
b  1.1961
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Flow into the grit chamber (runoff from catchment)
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As the system operates by mass conservation, the inlet flow to the grit chamber is calculated from addition
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of the flow into the pond and oil separator. The calculated flow was validated by manual flow
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measurements. Runoff from the catchment was compared to precipitation data from a local rain gauge, to
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separate the inlet flow into precipitation and additional sources like groundwater.
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Tables and figures
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Table S1:
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Linear correlations between precipitation (amount and duration) and retention of SS, PP and TDP in % (n =
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5). The table contains the equation for the linear correlation, the R2 value for the regression line and the p-
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value for the linear regression.
Precipitation
Amount [mm]
Duration [h]
Parameter
SS
PP
TDP
SS
PP
TDP
SS %
PP %
TDP %
SS %
PP %
TDP %
Equation
= 105.130 - (3.542 * mm)
= 105.869 - (4.734 * mm)
= 101.475 - (3.199 * mm)
= 91.059 - (1.956 * h)
= 86.540 - (2.558 * h)
= 84.196 - (1.272 * h)
R2
0.814
0.815
0.636
0.992
0.951
0.402
p-value
0.036
0.036
0.106
<0.001
0.005
0.251
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Figure S1:
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Measured pH values from March 2013 to March 2014 in outlet from the pond = inlet to the filters, outlet
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from the concrete filter before and after neutralization and pH in the receiving water. Horizontal lines
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indicate periods without measurements. Neutralization was stopped after 210 days.
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