Phosphorus cycling in a
sedimentation pond
of a constructed wetland
Santiago Clerici
Michael Krom, Robert Mortimer and Sally MacKenzie
WWT Slimbridge
•Effluents high in N, P, Organic Matter and suspended solids
•River Severn estuary downstream
River Severn
The treatment wetland
•Sedimentation pond to remove suspended matter
•PO4 and excess suspended matter taken up by reed beds Reserve
Ditch
Inflow
Cascade
100 mts
Failing of the treatment wetland
Excess P is being exported from the system
Palmer Felgate et.al 2011:
Sedimentation pond becomes source of P in summer
Collapse of algal blooms in early Summer:
P released by redox reactions when oxygen levels go down
P released by decomposition of algal matter
Objectives
To find the dominant processes controlling P cycling in the sedimentation pond in Spring and Summer
To know whether the sediment is a source or a sink of Phosphorus?
To find the major processes controlling P cycling in the sediment and whether they change seasonally?
Model of P cycling in the water column
Moles P / day
INLET
POND Part P
POND PO4
Part P Inflow
PO4 Inflow
Algal uptake
Respiration
Part P Outflow
PO4 Outflow
Resuspension
Settling
Sediment fluxes
Chlorophyll and Oxygen levels
Chl.a
0
150
300
450
Mar-11 Apr-11 May-11 Jun-11
date
Ch
l.a (
μg
/l)
Bloom
Crash
DO - March 2011
0
25
50
75
100
125
12:00 18:00 00:00 06:00 12:00 18:00
time
DO
(%
sa
tura
tio
n)
Bloom
DO - June 2011
0
25
50
75
100
125
12:00 18:00 00:00 06:00 12:00 18:00
time
DO
(%
sa
tura
tio
n)
Crash
Night hours
INLET
POND Part P
POND PO4
Part P Inflow PO4 Inflow
Algal uptake
Respiration
Part P Outflow PO4 Outflow
Settling
Resuspension
Sediment fluxes
Sampling P at inlet and pond
INLET
POND Part P
POND PO4
Part P Inflow PO4 Inflow
Algal uptake
Respiration
Part P Outflow PO4 Outflow
Settling
Resuspension
Sediment fluxes
Calculating water column fluxes
Obscured incubation
20
25
12:00 18:00 00:00 06:00 12:00 18:00
time
PO
4 (
μM
P)
Open incubation
20
25
12:00 18:00 00:00 06:00 12:00 18:00
time
PO
4 (
μM
P)
Only respiration
Photosynthesis + respiration
INLET
POND Part P
POND PO4
Part P Inflow PO4 Inflow
Algal uptake
Respiration
Part P Outflow PO4 Outflow
Settling
Resuspension
Sediment fluxes
Calculating sediment fluxes
PO4 in Benthic Chamber #1
March
0
1.5
12:00 18:00 00:00 06:00 12:00
time
PO
4 (
μM
P)
PO4 in Benthic Chamber #1
June
15
25
35
12:00 18:00 00:00 06:00 12:00 18:00
time
PO
4 (
μM
P)
P fluxes March
INLET
Part P Inflow
14
PO4 Inflow
2
Algal uptake
1
Part P Outflow
12
PO4 Outflow
1
Settling
4
Sediment fluxes
0.1/0.2
Fluxes in moles P / day
Pond Part P
14 moles
Pond PO4
2 moles
P fluxes June
INLET
Part P Inflow
16
PO4 Inflow
47
Algal uptake
2
Respiration
1
Part P Outflow
13
PO4 Outflow
55
Settling
6
Sediment fluxes
Fluxes in moles P / day
Pond Part P
9 moles
Pond PO4
35 moles
10
Sediment phosphorus cycle (after Slomp et al 1996)
Fe~P
Water
Sediment
Ca~P
To outlets
Water: it transports PO4
Solid: it retains P but it can be released it back into water
Solid: it retains P indefinitely
PO4
Organic P
Apatite P
Pore water
PO4
Description of the solid phase
-15
-10
-5
0
0.75 1
porosity
de
pth
(c
m)
sediment content
20x increase
-5
0
5
0 50 100
% O2 saturation
de
pth
(c
m)
-15
-5
5
0 400
uM NH4
de
pth
(c
m)
March
June
Organic P ↔ Pore water PO4
Increased respiration in June
Organic P (μ mole P / litre)
-16
-8
0
0 16000
de
pth
(c
m)
june
march
200 moles P released between March and June
PO4
Organic PPore water
PO4
Iron~P (μ mole P / litre)
-16
-8
0
0 16000
de
pth
(c
m)
june
march-15
-5
5
0 300
uM Fe
de
pth
(c
m)
March
June
Iron~P ↔ Pore water PO4
Increased dissolved iron in June
25 moles P released between March and June
Fe~P
PO4
Pore water
PO4
Ca2+
(μ mole Ca / litre)
-12
-6
0
0 8000
de
pth
(c
m) Apatite P (μ mole P / litre)
-16
-8
0
0 16000
de
pth
(c
m)
june
march
Apatite P ↔ Pore water PO4
Consumption of calcium in June
100 moles P consumed between march and June
PO4
Apatite P
Pore water
PO4
-15
-5
5
0 80
de
pth
(c
m)
Pore water PO4 ↔ Water column PO4
-15
-5
5
0 600
de
pth
(c
m)
PO4 (μM) - March PO4 (μM) - June
Calculated: 0.3 mole P / day Calculated: 7 moles P / day
Summary: control of P in the water column
Pond retains 15% of Particulate P, both in March and June
Algal bloom in March
Oxygen saturation
Algae and sediments take up SRP in March
Collapse of algal bloom
Oxygen levels very low in June
Large plume of SRP flows in, in June
Important release of P from sediments in June
Summary: release of P from sediments
Release of PO4 mainly by consumption of accumulated organic matter
Redox related dissolution of Fe also contributes to the release of PO4
Precipitation of apatite counteracts, in part, the release of PO4