1

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

2

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

3

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

4

Rainy events

acceptance

(hydraulic overloads)

Performances of French VFCW tropical design …

Robustness and Reliability ?

Nitrogen removal

Organic loads

variations

VFCW adaptation

5

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

6

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

7

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

8

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

9

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

10

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 !

11

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.

12

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.

13

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

14

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

15

Thank you for your attention !