Proceedings of ERE 2013
Modelling of Pollutant Removal Rates in Constructed 
Wetlands under Varying Hydraulic Loading Rates
Udukumburage RS, Ariyasena MADN, Rupasinghe GGMBMB and *
Karunarathne S
Department of Earth Resources Engineering, University of Moratuwa 
* Corresponding author - Shiromi27@yahoo.co.uk
Abstract: A subsurface horizontal flow constructed wetland located at 
University of Moratuwa was observed from January 2012 to November 2012 in 
order to derive empirical equations for the treatment efficiencies of BOD, COD, 
Nitrate-N, Nitrite-N and Total Phosphorous with varying hydraulic loading 
rates. Source of polluted water was the grey water effluent from the Staff 
canteen which is located near the constructed wetland. BOD, COD, Nitrate-N, 
Nitrite-N and Total Phosphorous, turbidity and conductivity 
were taken to ascertain the water quality. Overall treatment efficiency of the 
wetland cell is above 75% for all parameters. Regression technique was used to 
derive the empirical equations. IBM SPSS statistics -20 software was used in 
analysing the experimental data. The derived equations exhibit a high level of 
accuracy since predicted data and actual data shared a strong correlation of 
79.1% for COD, 93.2% for BOD, 85.1% for Nitrate-N, 84.1% for Nitrite-N and
measurements
86.2% for Total Phosphorous. Constructed wetlands in tropic environments 
capable of using these equations with relevant adjustments to the original.
are
Keywords: BOD, COD, Nitrate-N, Nitrite-N, Total phosphate
Grey water can be treated by means 
of physical, chemical, and biological 
treatment techniques. Activated
filtration, 
chlorination, 
filtration and sedimentation are 
some of such waste water treatment 
methods. Also waste water is treated 
to a certain degree by natural
1.Introduction
Fresh water has become a scarce
resource as a result of unmanaged
man-made
sludge, trickling 
neutralization,andexploitation 
pollution. Waste water is basically 
two types, black and grey, 
categorized according to the 
consisting impurities. Black water 
consists of human excreta while grey
Karunarathne S. B.Sc.Eng.(Hons) (Moratuwa). 
M.Sc.(Saitama), Ph.D.(Saitama). MASCE(USA). 
C.Eng.. M.I.E.(S.L) Head of the Deportment 
Earth Resources Engineering. University of 
Moratuwa
Udukumburage RS, Ariyasena MADN and 
Rupasinghe
Undergroduate students in the Department of 
Earth Resources Engineering, University of 
Moratuwa.
water consists of other nutrients, 
heavy metals and micro-organisms. 
Effluents from industries, domestic 
and agriculture are majorly grey 
water. Discharging these to water 
ways create a bad impact on human 
health and aquatic environment 
(Mayo and Bigambo,2005).
GGMBMB. Final year
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Proceedings of ERE 2013
2.Material and Methods
The horizontal flow sub surface 
constructed wetland at the 
University of Moratuwa has been 
used for the testing purposes 
throughout the project.
mashesmechanisms. Bogs, swamps,
different types of 
which
and fens are 
natural wetlands 
considered as the kidneys of nature 
as they remove pollutants through 
natural processes. Some of these 
applied in
are
principles 
constructing artificial wetlands as 
These constructed wetlands 
(CW) incur low cost and low 
maintenance but effective in 
removing pollutants, due to simple 
and
(Economopoulou and Tsihrintzis, 
2003). Vegetation, soils, types of 
substrate, organic and hydraulic 
loading and hydraulic retention time 
are the most important design and 
operational parameters of 
constructed wetlands. These CWs
are
2.1 Experimental Setting
The grey water discharged from a 
university staff canteen was fed 
through the wetland system. Pre- 
treatment comprised of the removal 
of suspended solids and a grease 
trap to separate and remove the oil 
and grease which are not treated by 
the wetland itself.
Table 1 - Characteristics of the pilot 
scale constructed wetland
well.
operationreliable
are engineered designs (Vymazal, 
2011). Therefore the design 
parameters can be varied for the
ValueParameter
Q - flow rate 
O - porosity 
d - depth 
L - length 
W - width
1.428
m3/day
0.435
0.75 m
required efficiency and for localized, 
climatic conditions. Hydraulic 
loading rate of constructed wetland 
is an important parameter that 
determines the pollutant removal 
efficiency of the same. Only a few 
studies have
4.00 m
been done
investigate the impacts of varying 
hydraulic loading rates on the 
treatment efficiencies and they are 
also limited to temperate climatic 
regions and therefore the 
cannot be translated to the Sri 
Lankan conditions 
the objective of the 
to investigate the
to 2.16 m 
2.643 m3 •
Ve effective
volume
8.64 m2A - surface area
results
The original design flow rate of 1428 
m3/ day, was determined using the 
rule of thumb. The removal 
efficiencies for certain parameters 
this hydraulic loading rate were 
lower than 60%. Hence the upper 
limit of the varying hydraulic 
loading rate was maintained below 
1.6 m3/day and the lower limit was 
maintained above 0.5 m3/day.
directly. Hence
present study is 
. . , validity of
empirical equations of removal 
for v
at
— rates
anous parameters, derived from 
the experimental analysis.
ERE 2013 Page 64
Proceedings of ERE 2013
Experimental Procedure
The water flow measurements 
taken from the inlet and the outlet of 
the constructed wetland in weekly 
basis from February to November 
2012. A plastic barrel of a known 
volume was used in collection of the 
water while the time taken was 
measured to fill it to measure the 
hydraulic loading rate.
The samples were collected in 
weekly basis for a particular flow 
rate from the inlet and outlet of the 
constructed wetland. The standard 
methods were implemented when 
collecting the water samples from 
both inlet and outlet of the wetland 
system. The departmental analytical 
laboratory was used in order to 
analyse BOD, COD, Total Nitrate 
Nitrogen, Total Nitrite Nitrogen, 
Total
conductivity etc.
2.2 varying hydraulic loads. Within the 
hydraulic loadings ranging from 
0.56 m3/day to 0.90 m3/day, the 
treatment efficiencies of BOD and 
COD do not produce significant 
differences. Hence, it can be said that 
the wetland still manage to operates 
effectively in spite of the increased 
hydraulic loadings with respect to 
BOD and COD removal.
were
^ 100% -----
;gjg 50% - -
CD
p
hCL)
E 0% 
j? 400650900 150400650 
Hydraulic loading rate (l/day)
Figure 1- Variation of the treatment 
efficiencies for varying hydraulic 
loading rates of the subsurface flow 
constructed wetland
3.2 Mathematical modelling of 
the hydraulic loading rate on 
pollutant removal efficiency
Derivation of the regression 
equations for each parameter and 
the addition of the effect of the rain 
as an independent variable to the 
already derived equations were the 
primary concerns. Hence multiple 
regression method was employed in 
order to come up with formulas for 
each parameter. A best fit 
comparison was carried out to 
determine the best suited 
relationship with and without of 
rainfall effects for each parameter by 
using the amount of data explained 
by the regression method (Pearson's
Phosphate (TP), pH,
3. Results and Discussions
3.1 Experimental analysis of 
the effect of hydraulic loading 
rate on pollutant removal 
efficiency
Increasing hydraulic loading rates 
from 0.56 m3/day to 1.6 m3/day 
have caused the BODs, COD, Nitrite- 
N, Nitrate-N and Total Phosphate 
treatment efficiencies to drop down 
by significant proportions. Figure 1 
shows the variation of the treatment 
efficiencies of BOD - 16%, COD - 
12%, Total Nitrate Nitrogen - 11%, 
Total Nitrite Nitrogen - 13% and 
Total Phosphate (TP) - 27% due to
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Proceedings of ERE 2013
Table 2 - Validation of the regression 
equations
correlation coefficient R-squared 
value).
The statistical data collected were 
analysed using 'IBM SPSS statistics 
-20' software. The regression 
equations obtained and verified for 
COD , BOD , Nitrate-N , Nitrite-N , 
TP removal with respect to varying 
hydraulic loading rates as given in 
equations from 1-5.
EcoD^Qr) = 0.696 * r -0.01 * Q +79.23 
.....0) *
EBOD,(Q,r) = “ 0.23 * r -0.012 * Q 
+94.039.... (2)
ENi,raMQ,r)= 0-987 * r +0.001 * Q 
+59.573...(3)
ENitrile/(Q,r) = 0.238 * r +0.014 * Q 
+51.075...(4)
Etp,(q,o = - 0.315 * r -0.012 * Q 
+58.862.... (5)
R2 value of
tested and
predicted
efficiencies
Parameter
.791COD
.932BOD
.851Nitrate-N
.841Nitrite-N
.862TP
4.Conclusions
The equations derived, for each of 
the above mentioned parameters are 
'empirical' and can be used for other 
horizontal 
constructed wetlands with similar 
parameters after further verification 
and proper adjustments are 
incorporated to the original 
equations. Verification process has 
shown that all the five parameters 
hold a strong relationship with 
'loading rate' and 'rainfall'.
Incorporation of the 'Rainfall' data 
has shown a critical improvement on 
each of those equations whereas 
percentage of the data explained has 
risen up in significant proportions. 
Higher standardized coefficients 
have proven the degree of 
importance of the loading rate and 
rainfall data on treatment efficiency 
of each parameter. An effective 
loading rate, which provides the 
necessary overall pollutant removal
subsurfaceflow
Whereas,
E(o,r)= Treatment Efficiency
Q = Hydraulic Loading Rate 
(1/ day)
R = Rainiall data (mm)
Derived regression equations were 
verified using a separate data set 
obtained from the experimental 
analysis and statistically tested for 
the correlation coefficient. According 
to the Table 2, almost all the 
parameters consist of 'R value' more 
than 0.75 and hence the relationships 
in between the 'Predicted Data' and 
'Test Data' are strong.
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Proceedings of ERE 2013
condition, can be determined by the 
derived equations.
The experimental analysis suggests 
the design surface area can be 
reduced by more than 25% and 
hence the constructed wetlands can 
be popularized among the Sri 
Lankan domestic environments
Acknowledgement
The authors are sincerely thankful to 
Dr. A.M.K.B. Abeysinghe, Mrs. 
P.T.N. Pathiraja, Mrs. M.W.P. 
Sandamali and all who helped in 
many ways for the success of the 
project.
References
Economopoulo, M.A., & Tsihrintzis, 
V.A., (2003). Design Methodology 
and Area Sensitivity Analysis of 
Horizontal Subsurface Flow 
Wetlands,Water 
Resources Management, 17, pp. 147- 
162,
Constructed
Kadlec, R.H., Knight, R.L., (1996), 
Treatment wetlands, Boca Raton, FL: 
CRC Press, pp. 893,
Langergraber, G., (2008). Modeling 
of Processes in Subsurface Flow 
Constructed Wetlands, Vadose Zone 
Journal, 7, pp.830-842,
Vymazal, J., (2011).Constructed 
Wetlands for Wastewater Treatment, 
Environ. Sci. Technol, 45,pp. 61-69,
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