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36 IWA Specialist Group on Use of Macrophytes in Water Pollution Control: Newsletter No. 37

DESIGN AND PERFORMANCE OF THE WETLAND WREATMENT SYSTEM AT 

THE BUFFALO NIAGARA INTERNATIONAL AIRPORT

Scott Wallace and Mark Liner

Naturally Wallace Consulting, Vadnais Heights, Minnesota, USA

(scott.wallace@naturallywallace.com, mark.liner@naturallywallace.com)

Abstract

The wetland treatment system at the Buffalo Niagara International Airport has been in 

operation for two years now, and represents one of the major applications of intensified 

wetlands for treatment of glycol-based deicing fluids. The full-scale system is designed to 

remove 4,500 kg/d of biochemical oxygen demand (BOD5). The wetland design was based 

on pilot testing conducted in 2007, which established degradation rate coefficients for both 

aerated and non-aerated modes of operation.

Aeration was found to profoundly affect treatment performance. When aerated at 0.85 m

3

air per hour (per m3of wetland bed), the volumetric (2TIS) BOD5 removal rate constant 

averaged 5.4 day−1 with a temperature coefficient (θ) of 1.03, based on experiments 

conducted at 22°C and 4°C. In contrast, the non-aerated wetland had a rate coefficient of 

0.55 day−1.

Operation of the full-scale system during the 2009/2010 deicing season revealed nutrientlimited conditions that resulted in polysaccharide slime formation within the wetland bed. A 

system for nutrient addition was updated and expanded for the 2010/2011 deicing season, 

which resulted in successful operation at BOD5 loadings of over 20,000 kg/d, which is 

almost five times greater than the original design. During the 2010/2011 deicing season, 

BOD5 removal averaged 98.3%.

Keywords

aircraft deicing, airport, ethylene glycol, propylene glycol, intensified wetland, subsurface 

flow wetland

INTRODUCTION

In addition to other contaminants found in stormwater and snowmelt at airports (e.g., fuel & 

lubricant residuals, oil & greases, airport stormwater runoff contains large amounts of aircraft 

de-icing fluids during periods when de-icing and anti-icing operations are carried out. The 

ADFs may be ethylene glycol (EG), or propylene glycol (PG) based and are applied as water 

solutions The volumes of de-icing fluids used depend on weather conditions and the type of 

aircraft involved (Higgins and Maclean, 2002).

Controlling and treating stormwater runoff from airport facilities is a matter of increasing 

environmental concern and is now having a major effect on the design and operation of 

airports. Within the United States and Canada, new regulatory standards are being 

implemented for the management of de-icing fluids used to remove ice from aircraft during 

winter operations. Preferred chemicals are EG and PG, which in the 50% concentrations 

commonly used for aircraft de-icing and anti-icing, has a BOD5 of approximately 

200,000 mg/L. Although this is diluted by ice-melt, BOD5 concentrations in excess of 

20,000 mg/L are common. These high influent concentrations, combined with the inherently 

variable nature of winter storms and the low temperature of de-icing runoff, challenges the 

operation of conventional mechanical treatment plants.
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IWA Specialist Group on Use of Macrophytes in Water Pollution Control: Newsletter No. 38 37

Constructed wetlands are an attractive option for treatment of deicing runoff, because when 

operated in a subsurface mode, the risk to airfield operations is minimal and the systems 

require much less operation & maintenance than conventional systems.

PILOT TESTING

Pilot testing was carried out at Campus D‘Alfred of the University of Guelph in Ontario, 

Canada. . The tests were carried out indoors in 1 m

3

vessels configured as mixing tanks 

(containing mixers and heaters), open aerated basins (AB cells, simulating aerated tanks and 

containing fine bubble aeration spargers), non-aerated vertical subsurface flow (VSSF) 

constructed wetland cells and aerated VSSF intensified (IW) test cells. The wetland cells 

were vegetated with common reed (Phragmites sp.) taken from ditches near the test unit. Test 

runs were preceded by month-long calibration and acclimatization periods.

The pilot facility was configured as a mixing tank; two open-water AB cells; one non-aerated 

VSSF CW cell containing gravel 0.3 m deep and; and two aerated gravel-bed VSSF 

intensified wetland cells (IW1 and IW2) containing gravel 0.8 m deep. There were two 

separate sets of cells which, after initial calibration and acclimation periods at room 

temperature, operated in parallel at the two design basis target temperatures. The set-up for 

high temperature operations consisted of a mixing tank and an AB cell followed by parallel 

CW and IW cells. (These were considered as two separate trains.) The set up for low 

temperature operations consisted of an AB Cell and one IW cell in series. These latter two 

vessels were located inside a walk-in refrigerator at the Alfred Pilot Unit (Train 3). The initial 

configuration of the pilot facility is shown in Figure 1.

Figure 1. Pilot test facility schematic

Results from the four testing periods (Runs A, B, C, D) and the three process trains (Trains 1, 

2, 3) are summarized in Table 1 (Wallace et al., 2007).

Train 2

Train 1

Mixing Tank

AB Cell

Hi Temp

IW Cell

Lo Temp

CW Cell

Hi Temp

AB Cell 

Lo Temp

IW Cell

Hi Temp

Walk-in Refrigerator

SW from ADF

Storage Tanks

Train 3
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38 IWA Specialist Group on Use of Macrophytes in Water Pollution Control: Newsletter No. 37

Table 1. Average BOD5 concentrations (mg/L); percent removal is listed in parenthesis

Train 1 Train 2 Train 3

Run Mixing 

Tank 

Influent

AB Effluent

High Temp.

IW Effluent

High Temp.

AB Effluent

High Temp.

CW Effluent

High Temp.

AB Effluent

Low Temp.

IW Effluent

Low Temp.

A 1,491 542 (64) 8 (99) 542 (64) 212 (61) 649 (56) 27 (96)

B 1,524 Not Used 56 (96) 257 (83) 119 (54) 679 (55) 21 (97)

C 773 Not Used 25 (7) 177 (77) 29 (84) 325 (58) 10 (7)

D 694 Not Used 4 (99) 130 (81) 34 (74) Not Used 24 (97)

Since the different treatment trains were operated at different flow rates and temperatures, 

inspecting percent removals based on concentration (as summarized in Table 2) does not 

provide a complete understanding of treatment performance. The data were analyzed using a 

tanks-in-series (TIS) flow model using a relaxed parameter, P (Kadlec and Wallace, 2009)

that accounts not only for the number of tanks but weathering of the BOD5 as the PG 

degraded into daughter products. 

Calculated rate coefficients for the aerated IW cells for the high temperature runs are 

summarized in Table 2, based on a flow model with P = 2 (this is an approximate value based 

on previous tracer testing of aerated intensified wetland cells):

Table 2. Aerated intensified wetland rate coefficients for high temperature runs

Run

Average BOD5 (mg/L)

k2TIS(d−1Influent Effluent )

A 542.3 7.5 9.15

B 1,524.0 55.7 5.17

C 773.0 25.0 5.60

D 694.0 4.0 14.84

Average 8.69

The higher results from Run D where the aeration rate was increased from 0.85 m

3

air per 

hour (per m

3

of wetland bed) to 2 m

3

of air per hour indicate that even further improvement 

of treatment could be possible. A similar set of BOD5 IW rate coefficient calculations were 

made for the low temperature operations and these are presented in Table 3:

Table 3. Aerated intensified wetland rate coefficients for low temperature runs

Run

Average BOD5 (mg/L)

k2TIS(d−1Influent Effluent )

A 648.8 26.5 4.81

B 679.3 21.0 5.72

C 325.0 10.3 5.63

D 694.0 23.5 5.41

Average 5.39

For Runs B and C, the data can be used to determine the Arrhenius constant, θ, based on the 

following equation:

 20

20

T

T

k k  

where kT is the rate constant at temperature T = T°C and k20 is the rate constant at 20°C. 

Based on these results, the value of θ was calculated to be approximately 1.03.
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IWA Specialist Group on Use of Macrophytes in Water Pollution Control: Newsletter No. 38 39

The BOD5 removal rate coefficients for the non-aerated CW cell were also calculated based 

on a flow model with P = 4. A higher value of P was assumed because there is less internal 

mixing in non-aerated cells. Results are summarized in Table 4:

Table 4. Non-aerated constructed wetland rate coefficients for high-temperature runs

Run

Average BOD5 (mg/L)

k2TIS(d−1Influent Effluent )

A 542.3 212.3 0.68

B 257.0 119.0 0.27

C 177.0 29.0 0.73

D 129.5 33.5 0.51

Average 0.55

NUTRIENT LIMITATIONS

Operation of the full-scale treatment wetland indicated that nutrient-limited conditions 

occurred during the 2009/2010 de-icing season, since propylene glycol (C3H8O2) and 

ethylene glycol (C2H6O2) do not contain nitrogen, phosphorus and other nutrients. The 

nutrient demands for bacterial growth can be estimated using the factors summarized in 

Table 5.

Table 5. Approximate nutrient requirements for bacterial growth (Grady et al., 1999)

Nutrient Approximate nutrient requirement 

(g/kg of biomass produced)

Nitrogen 85

Phosphorus 17

Potassium 10

Calcium 10

Magnesium 7

Sulfur 6

Sodium 3

Chloride 3

Iron 2

Zinc 0.2

Manganese 0.1

Copper 0.02

Molybdenum 0.004

Cobalt < 0.0004

One major unknown parameter in treatment wetlands is the yield ratio of microbial growth, 

which can be defined as the fraction of the influent biochemical oxygen demand (BOD) 

converted to microbial biomass. For suspended-growth activated sludge systems, this 

parameter can be easily measured and is often quite high; in the range of 1.0 to 0.7.

Treatment wetlands are attached-growth systems with a very long solids retention time where 

microbial predation would presumably substantially reduce the yield ratio. In one study of a 

pilot-scale, fill-and-drain subsurface flow wetland, the yield ratio was estimated to be 0.068 

(Austin et al., 2007). With such a low yield rate, this would imply that treatment wetlands 

would not require large amounts of nutrients to maintain a mature microbial community 

under steady-state conditions.
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40 IWA Specialist Group on Use of Macrophytes in Water Pollution Control: Newsletter No. 37

Although the Buffalo design had anticipated the need for nutrient addition, establishment of 

effective treatment was slow, and the system began to experience foaming associated with the 

formation of polysaccharide slimes by the resident bacteria. Foaming and slime formation is 

known to occur at wastewater treatment plants subject to severe nutrient limitations (Arias et 

al., 2008; Ayati and Ganjidoust, 2008).

Owing to the observed slime formation at Buffalo, estimates on the yield ratio had to revised 

upwards, and was finally estimated by the authors to be at least 0.3. This had profound 

implications on the rate of nutrient addition, as the Buffalo wetland was designed to process 

4,500 kg/d of BOD; a yield ratio of 0.3 implies 1,350 kg/d per day of microbial biomass 

production. Based on the information in Table 5, this amount of biomass production would 

require 115 kg/d of nitrogen and 23 kg/d of phosphorus, in addition to micronutrients. After 

feeding the appropriate amount of nutrients, slime formation ceased and treatment 

performance improved dramatically.

FULL-SCALE OPERATION

Operation of the full-scale treatment wetland was continued through the 2010/2011 de-icing

season (with improved nutrient addition). The full-scale system consists of four subsurface 

flow wetland beds (saturated vertical downflow), each sized at 4,640 m

2 with a bed depth of 

1.5 m. The distribution piping is buried under an insulating level of compost such that no 

water is exposed during the treatment process (Figure 2).

Figure 2. Full-scale treatment wetland at Buffalo Niagara International Airport.

The subsurface vertical flow wetlands have an internal array of aeration tubes to enhance 

oxygen transfer as a means of intensification. The enhanced oxygen transfer rate allows for 

the aerobic degradation of glycol compounds. Each wetland can be supplied with air up to 

140 m

3

/min, which allows an oxygen transfer rate approximately 60 times greater than what 

would be supplied through passive atmospheric diffusion.
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IWA Specialist Group on Use of Macrophytes in Water Pollution Control: Newsletter No. 38 41

During the 2010/2011 de-icing season, influent BOD5 concentrations were in excess of 

15,000 mg/L. Generally, the system had very low effluent values, which averaged 77 mg/L 

(based on a correlation between online total organic carbon measurements and laboratory 

BOD5 values), as shown in Figure 3.

Figure 3. Influent and effluent BOD5 values for the intensified wetland at Buffalo Niagara 

International Airport.

The wetland system also experienced BOD5 loadings well in excess of the original design 

parameters. While the design was based on a BOD5 loading rate of 4,500 kg/d, actual 

loadings exceeded 20,000 kg/d. BOD5 removals remained in the range of 90 to 100% (with 

an average removal rate of 98.3%), and reductions in treatment efficiency were only observed 

at the heaviest organic loadings (Figure 4).

Figure 4. BOD5 loadings and treatment efficiency for the intensified wetland at Buffalo 

Niagara International Airport
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42 IWA Specialist Group on Use of Macrophytes in Water Pollution Control: Newsletter No. 37

CONCLUSIONS

The Buffalo Niagara system has demonstrated that aerated, intensified subsurface flow 

wetlands are a viable technology for treatment of deicing runoff at airport facilities. The 

Buffalo system was successfully scaled up from pilot testing results, and has demonstrated 

high levels of treatment effectiveness even when subjected to organic loadings well in excess 

of the original design parameters.

The Buffalo Niagara wetland has served as a template for the upgrade and refurbishment of 

existing treatment wetlands at London‘s Heathrow airport and the airport facility in 

Edmonton, Alberta. However, the Buffalo facility has also demonstrated that nutrient 

limitations in deicing runoff can be a significant impediment to treatment of deicing runoff, 

and that nutrient addition is necessary to reach the full potential of wetland treatment 

systems.

REFERENCES

Arias, C.A., Vollersten, J., Lange, K.H., Pedersen, J., Hallager, P., Bruus, A., Laustsen, A., 

Bundensen, V.W., Nielsen, A.H., Nielsen, N.H., Wium-Andersen, T., Hvitved-Jacobsen, 

T., Brix, H. (2008). Integrating constructed wetland filters and wet detention ponds for the 

treatment of urban stormwater runoff. In: Proceedings of the 11th International 

Conference on Wetland Systems for Water Pollution Control, 1-7 November 2008. Billore 

S.K., Dass P., Vymazal J. eds. Vikram University and IWA: Indore, India. pp. 781.

Austin, D.C., Maciolek, D.J., Davis, B.M., Wallace, S.D. (2007). Damköhler number design 

method to avoid clogging of subsurface flow constructed wetlands by heterotrophic 

biofilms. Water Science and Technology 56(3), 7–14.

Ayati, B., Ganjidoust, H. (2008). Investigation of pollution sources in Anzali reserved 

wetland. In: Proceedings of the 11th International Conference on Wetland Systems for 

Water Pollution Control, 1–7 November 2008. Billore S.K., Dass P., Vymazal J. eds. 

Vikram University and IWA: Indore, India. pp. 116-122.

Higgins, J.P., Maclean, J. (2002). The use of a very large constructed sub-surface flow 

wetland to treat glycol-contaminated stormwater from aircraft de-icing operations. Water 

Quality Research Journal of Canada 37(4), 785–792.

Kadlec, R.H., Wallace, S.D. (2009). Treatment Wetlands, second edition. Boca Raton, 

Florida: CRC Press.

Wallace, S.D., Higgins, J., Liner, M.O., Diebold, J. (2007). Degradation of aircraft deicing 

runoff in aerated engineered wetlands. In: Multi Functions of Wetland Systems: An 

International Conference, 26-29 June 2007. University of Padova and International Water 

Association: Padova, Italy.