Post on 17-Jul-2020
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Performances of French Vertical Flow
Constructed Wetlands Under Tropical
Climates: Robustness and Reliability
of the System.
Rémi LOMBARD LATUNE, Lucas PELUS, Nicolas FINA,
Fréderic l’ETANG, Bernard Le GUENNEC and Pascal MOLLE
Big Sky Montana, 23rd august 2017
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French overseas territories context
Issues
High population growth
Lack of maintenance
Climate (tropical rains)
Sewer (clear water intrusion,
H2S)
Price (limited financial capacity,
spare parts on importation)
Sludge management
Needs
Compact
Simple to operate
Hydraulic load variation
acceptance
Decentrilized
Limit opex and capex
Limit sludge production
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VFCW adaptation
footprint reduction (2 0.8 m²/p.e.):
• no modification of the surface
area in operation/p.e.
• 1 single stage, 2 filters in parralel,
• Heliconia psittacorum
• Cana indica
• Cyperus involucratus.
Tropical design Substitution species
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Rainy events
acceptance
(hydraulic overloads)
Performances of French VFCW tropical design …
Robustness and Reliability ?
Nitrogen removal
Organic loads
variations
VFCW adaptation
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Taupinière treatment plant (Martinique island)
• 900 p.e.
• Objectives : 90% removal COD,
BOD5, TSS and 80% TKN
• Operating since October 14
• Siphon (4m3 2.5 cm/batch)
VFCW :
• 2 lines with 2 VFCW US/S
(0.8m²/p.e. 4x180 m²)
• Low load 1 line in operation
Simplified trickling filter (0.1m²/p.e.)
• Recirculation loop 100-300%
pump
• Decanted sludge 1st stage
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VFCW unsaturated / saturated (0.8 m²/p.e.)
• Unsaturated filtering layer : 40 cm 2/4 mm gravel
• Transition layer : 15 cm 11/22 mm gravel, with aeration pipes (0.25 lm/m²)
• Saturated layer : 40 – 60 cm 20/40 mm gravel
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Simplified trickling filter (0.1 m²/p.e.)
Illu
str
atio
n: E
pu
rNa
ture
• 150 cm of pumice stones
• 2 feeding networks working alternatively
• HLR: 1.5 m/d
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Monitoring
• On-line flow measurement at each
treatment step
• 29 24-hours flow-composites sample
campaigns (Nov 14 – May17),
on each treatment step
• Parameters analyzed : COD, BOD5,
CODd, TSS, NTK, NH4, NO3, NO2,
PO4, PT.
• 6 rainy sampling campaigns identified
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Experimental settings
1/ Without resting period, 1 year, average 32% of nominal BOD5 load
Alternation restored, 1.5 year :
2/ Normal conditions, average 85% of nominal BOD5 load
3/ Overload conditions, average 164% of nominal BOD5 load
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Raw waste water
Qin (m3/D) COD (mgO2/L) BOD5 (mgO2/L) TSS (mg/L) TKN (mgN/L) NH4 (mgN/L) PT (mgP/L) Dry Rain Dry Rain Dry Rain Dry Rain Dry Rain Dry Rain Dry Rain Min. 30,1 50,6 612 385 340 170 200 291 62,7 31,0 48,4 21,2 5,85 4,20 Aver. 38,9 72,0 1011 725 512 368 400 381 100,3 61,4 76,0 44,4 10,37 6,85 Max. 63,9 101,7 1394 1051 680 580 700 519 130,0 87,1 110,0 67,9 14,80 9,52 S.D. 11,1 21,4 176 268 94 158 127 77 15,2 25,8 13,3 21,2 1,92 2,46
Raw wastewater characteristics during dry (n=19) and rainy (n=6) campaigns.
Values from the scaling up period (n=4) have been removed.
• High variability in pollutants concentration and hydraulic flow
• During rainy events, variability and pollutants loads increase
• Hydraulic load doubled !
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0
10
20
30
40
50
60
70
80
90
100
0.0 0.5 1.0 1.5 2.0 2.5 3.0
Cu
mu
lati
ve p
erc
en
tage
(%
)
Hydraulic load on the operating bed (m/d)
Low loads (32% ofBOD5 nominal load)
Normal loads (85% ofBOD5 nominal load)
Overloads (164% ofBOD5 nominal load)
Hydraulic nominal load
Nitrification weakeninglimit
Hydraulic loading rates
• Normal loads : 15% of the events are overloaded and 2% are above
nitrification weakening limit (Prost-Boucle et Molle, 2012).
• Overload conditions : 95% of the values are over nominal load, 15%
above nitrification limit.
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0102030405060
-40 60
Scaling up period
Whole bed dry events
Whole bed rainy events
Half bed dry events
Half bed rainy events
Nominal load
100%
80%
60%
0
10
20
30
40
50
60
0 20 40 60Applied load (g/m²/d)
TKN
0
50
100
150
200
250
300
350
0 100 200 300Applied load (g/m²/d)
TSS
0
100
200
300
400
500
600
0 200 400 600
Trea
ted
load
(g/
m²/
d)
Applied load (g/m²/d)
COD 100% 100%
100% 90% 90%
80% 80% 80%
60%
• COD removal is about 90%, variability increases with overloads
• Rainy events decrease efficiency to 80%
• TSS removal is over 90%. Rainy events bring high TSS load.
Removal efficiency
• TKN shows limits of the system.
• > 25 mgN/m²/d, removal rate decrease.
• During rainy events, for nominal load, filters can’t achieve 60% removal.
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70%
75%
80%
85%
90%
95%
100%
BOD5 COD TSS TKN NT
Low loads, VFCW
Low loads, VFCW+TF
Normal loads, dry events, VFCW
Normal loads, dry events, VFCW+TF
Normal loads, rainy events, VFCW
Normal loads, rainy events, VFCW+TF
Overloads, dry events, VFCW
Overloads, dry events, VFCW+TF
Overloads, rainy events, VFCW
Full system performances
• TSS removal objective achieve on rainy events with one stage overloaded
• BOD5 removal objective achieve on rainy events with one stage at regular load
• 2nd stage is required for COD and TKN objectives. During 1 rainy event, with
high overloads (165% BOD5), COD removal can’t be achieved.
• This configuration don’t maximize total nitrogen removal : simplified trickling
filter perform total nitrification, but as recirculation loop doesn’t include the 1rst
stage, NO3 produced is not denitrified.
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
BOD5 COD TSS TKN NT
Low loads, VFCW
Low loads, VFCW+TF
Normal loads, dry events, VFCW
Normal loads, dry events, VFCW+TF
Normal loads, rainy events, VFCW
Normal loads, rainy events, VFCW+TF
Overloads, dry events, VFCW
Overloads, dry events, VFCW+TF
Overloads, rainy events, VFCW
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Conclusions
• Tropical design (0.8 m²/pe) validated !
• Rainy events provide high overloads, values up to 2.75 m/d
(8 times the nominal loads) at least accepted
• Stong and steady performances on TSS and C removal even
at 165% of nominal load
• Nitrification performances decrease with overloads
• In case of lack of alternation, perfromances remains steady
for low loads conditions.
COD BOD5 TSS TKN TN
% mgO2/L % mgO2/L % mg/L % mgN/L % mgN/L
VFCWs/us 85 125 90 25 90 25 60 40 60 50
VFCWs/un+TF 90 75 95 15 95 15 90 6 70 35
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Thank you for your attention !