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Adapting the technology of zero liquid discharge by willow bed
for leachates treatment
The context
• Wastewater treatment never reaches 100% efficiency at all time
• Sometimes, presence of residual contamination is not acceptable
Evapotranspirative constructed wetlands with zero liquid discharge
1
Evapotranspirative wetlands
• First zero discharge CW’s designed mainly for domestic wastewater treatment
• Typically planted of willow clones, known for high transpiration rates
Mean ET rates ~ 5 mm/d
Some studies report rates > 15 mm/d
(Frédette & Brisson, 2017) 2
ET wetlands for leachate treatment
Contaminated leachate can be phytotoxic and low in nutrients
Water input is entirely dependent on rainfall (variable volume) received over a surface larger than the ET wetland (large volume)
Boreal climate with very cold winter reduces the time of the operation season
3
Modelling - Evaluate the performance in different weather conditions and with
different conception designs - Propose guidelines for sizing and managing the system
Adapting the technology
Pilot scale - Monitor plant condition in
situ and measure ET capacity of the system
Experimentation scale
- Teste specific questions like what is the relative effect of contamination level and substrate type on evapotranspiration rate
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Pilot leachate treatment system
1. Open storage tank
2. Primary treatment 3. Polishing treatment
5
Willow bed conception
• Horizontal subsurface flow
• Sand and peat substrate
• Mean loading rate of 3 m³/d
• Salix miyabeana (SX67)
• Roots of 6 years old, stems of 3 years old
• 2.3 plants/m², fertilized in 2013, 2014 and 2017
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Water characteristics
0
200
400
600
800
As (μg/L)
Cr (μg/L)
Cu (μg/L)
D/F(TEQ/L)
N(mg/L)
0
200
400
600
800
As (μg/L)
Cr (μg/L)
Cu (μg/L)
D/F(TEQ/L)
N(mg/L)
0
200
400
600
800
As (μg/L)
Cr (μg/L)
Cu (μg/L)
D/F(TEQ/L)
N(mg/L)
As: Arsenic Cr: Chromium Cu: Copper D/F: Dioxins and furans TEQ: toxic equivalent 7
Plant conditions in situ
After 5 years of operation…
• Small concentration of metals in substrate (~10 and 15 mg/kg of Cr and Cu)
• Dioxins and furans in willow root zone (0.6 pg/g in substrate and 0.2 pg/g in roots), but no clear phytotoxic effect
• Signs of nutrient deficiency, early degenerescence of the leaf cover
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0
2
4
6
8
10
12
14
16
May June July Aug. Sept. Oct.
Plant conditions in situ
• Very high leaf area index (mean of 9 for the season)
• Individual leaf area show significant differences
LAI (
m²
leaf
/m²
soil)
Theoretical surface
Actual surface 9
LAI (
m²
leaf
/m²
soil
)
Evapotranspiration components
From May 27th to October 17th (2016)
• Open tank evaporation : 3.4 mm/d (pan evaporation)
• CW’s evapotranspiration : 3.8 mm/d (reference ET)
• Willow bed evapotranspiration : 28 mm/d (water balance) 3.1 mm d¯¹ m¯² of leaf
Constant and high
water availability
Very important oasis
effect
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Modelling the system
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Modelling evapotranspiration
• Open tank : open water evaporation calculation method
• CW’s : Penman-Monteith reference ET with crop coefficient
• Willow bed evapotranspiration : leaf parameter based equation Represents the few evapotranspiration constrains in wetlands
Requires minimal data to calculate
Parameters can be used for modelling
T = Ḡs · activeLAI · Kυ
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Modelling evapotranspiration
• Willow bed evapotranspiration : compute activeLAI Considering adaxial transpiration -> activeLAI = LAI * 1,2
0
2
4
6
8
10
12
14
16
May June July Aug. Sept. Oct.
R² = 0.91
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LAI (
m²
leaf
/m²
soil)
Modelling evapotranspiration
• Willow bed evapotranspiration : predict Ḡs
According to solar radiation, temperature and relative humidity
Mean measured = 414 mmol s¯¹ m¯² vs mean modelled = 412 mmol s¯¹ m¯²
300
350
400
450
500
100 200 300 400 500 600 700
Mo
de
lled
Ḡs
(m
mo
l s¯¹
m¯²
)
Measured Ḡs (mmol s¯¹ m¯²)
R² = 0.45
0
500
1000
0 200 400
Mean radiation flux (W m¯²)
0
500
1000
0 10 20 30
Mean temperature (°C)
0
500
1000
0 50 100
Relative humidity (%)
Me
asu
red
Ḡs
(mm
ol s
¯¹ m
¯²)
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Modelling evapotranspiration
• Willow bed evapotranspiration : compute seasonal T
From May 27th to October 17th (2016)
T = 26.7 mm/d
0
1
2
0 5 10 15
T/ET
LAI
R² = 0.75
15
0
10
20
30
40
50
60
70
80
90
100
0 50 100 150
ET (
mm
/d)
Days of operation
Transpiration Water balance
Reference ET
Modelling 2016 operation season
• Initial levels Open storage tank : 95% full
CW’s and WB : 70% full
• Willow bed size : 48 m²
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Open storage tank 2240 m²
Constructed wetlands 125 m²
Willow bed
48 m²
Rain 1480 m³
E 1051 m³
ET
ET 251 m³
Rain
Outflow 342 m³
Rain
Modelling 2016 operation season
Measured Modelled
CW’s influent (m³) 601 618
WB influent (m³) 565 575
WB effluent (m³) 313 342
WB ET (m³) 273 251
• Initial levels Open storage tank : 95% full
CW’s and WB : 70% full
• Willow bed size : 48 m²
0
10
20
30
40
50
60
0 50 100 150 200
Days of operation
Constructed wetlands Willow bed
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Modelling system operation: Zero discharge
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Open storage tank 2240 m²
Constructed wetlands 125 m²
Willow bed
48 m²
Rain 1480 m³
E 1051 m³
ET
ET 251 m³
Rain
Storage tank
141 m³
Rain
• Initial levels Open storage tank : 95% full
CW’s and WB : 70% full
Storage tank : 50 m³
• Willow bed size : 48 m²
Modelling system operation: Sizing
19
Open storage tank 2240 m²
Constructed wetlands 125 m²
Willow bed 100 m²
Rain 1480 m³
E 1051 m³
ET
ET 392 m³
Rain
Storage tank
113m³
Rain
• Initial levels Open storage tank : 95% full
CW’s and WB : 70% full
Storage tank : 80 m³
• Willow bed size : 100 m²
Willow bed 75 m²
Modelling system operation: Optimizing ET
20
Open storage tank 2240 m²
Constructed wetlands 125 m²
Rain 1480 m³
E 1051 m³
ET
ET 421 m³
Rain
Storage tank
115 m³
Rain
• Initial levels Open storage tank : 95% full
CW’s and WB : 70% full
Storage tank : 80 m³
• Willow bed size : 75 m²
Conclusions and perspectives
Willow ET in situ can be considerably higher than mean rates found in literature (high «oasis effect» and water availability)
Cumulative ET can exceed rainfalls in boreal climate, if waterflow is to be properly managed
Zero liquid discharge system using willow bed can be modelized for other climatic conditions with minimal data
ET volume can be optimized to reduce the size of the willow bed
Effect of factors like contamination and fertilization on ET rates still has to be addressed
21
Thank you!
