第11卷第4期 湿 地 科 学 Vol.11 No.42013 年 12 月 WETLAND SCIENCE Dec. 2013

Ammoniacal Nitrogen and Organics Removal ModellingAmmoniacal Nitrogen and Organics Removal Modellingin Vertical Flow Wetlands Treating Strong Wastewatersin Vertical Flow Wetlands Treating Strong Wastewaters

Tanveer Ferdous Saeed1, Abdullah Al Muyeed1, Guangzhi Sun2

(1. Department of Civil Engineering, Ahsanullah University of Science and Technology, Dhaka, Bangladesh;

2. School of Engineering and Physical Sciences, James Cook University, Townsville, QLD 4811, Australia)

Abstract:Abstract: This paper reports a comparative evaluation between 2 kinetic models for predicting nitrification and biode-

gradable organics (BOD5) removal rates in 5 vertical flow (VF) wetland systems, that received strong wastewaters (i.e.

tannery, textile and municipal effluents). The models were formulated by combining first order and Monod kinetics,

with continuous-stirred tank reactor (CSTR) flow approach. The performance of the 2 models had been evaluated with

3 statistical parameters: coefficient of determination (R2), relative root mean square error (RRMSE), and model efficien-

cy (ME). The statistical parameters indicated better performance of the Monod CSTR model (over first order CSTR ap-

proach), for correlating ammoniacal nitrogen (NH4+—N) and BOD5 removal profiles across VF systems. Higher

Monod coefficient values (from Monod CSTR model) coincided with greater input NH4+—N and BOD5 loading, and

experimentally measured removal rate (g/(m2·d)) values. Such trends indicate that NH4+—N and BOD5 removals in the

VF systems were mainly achieved via biological routes. On the other hand, the rate constants (from the first order

CSTR model) did not exhibit such correlations (of Monod coefficients), elucidating their inefficiencies in capturing

overall removal mechanisms. The interference of organics removal on nitrification process (in VF wetlands) was identi-

fied through Monod coefficients. The deviation between NH4+—N and BOD5 Monod coefficients imply incorporation

of both coefficients, for calculating the area of a single VF bed. Overall, closer performance of the Monod CSTR mod-

el for predicting NH4+—N and BOD5 removals indicate its potential application, as a design tool for VF systems.

Keywords:Keywords: constructed wetlands; continuous-stirred tank reactor (CSTR); modelling; Monod kinetics; vertical flow;

wastewater

CLC number:CLC number: X703.1 Document code:Document code: A Article ID:Article ID: 1672-5948(2013)04-421-12

11 IntroductionIntroduction

The predominant aerobic environment inside

the media of the vertical flow (VF) wetland systems

often promotes nitrification, and biodegradable or-

ganics (BOD5) removal from wastewaters (Garcia etal, 2010; Cooper et al, 1996; Vymazal, 2005; Zhao

et al, 2011). The operational characteristics (i.e. in-

termittent loadings) of the VF wetlands develop

such aerobic conditions inside the bed matrix,

through alternate drying and wetting periods. Atmo-

spheric oxygen is diffused through the media pores

(of VF systems) during dry period, that is trapped

by the incoming wastewater (during wet period) to

support nitrification and BOD5 removal (Haberl etal, 1995).

Despite VF wetlands often accelerate nitrifica-

tion and BOD5 removals (from wastewater), contra-

dictory removal rates (of such pollutants) are often

observed (Sun et al, 1998; Sun et al, 2006). Oxygen

consumption difference (between the 2 metabo-

lisms) is the critical factor that often controls such

performance disparity (Saeed and Sun, 2012). Inade-

quate sizing of the VF wetland beds could be respon-

sible for not supporting substantial oxygen diffusion

Received date:Received date: 2013-05-07; revised date:revised date: 2013-08-21

Corresponding Author:Corresponding Author: Tanveer Ferdous Saeed, PhD, assistant professor, field of interest: environmental engineering. E-mail: tanveer.ce@aust.

edu; tanveer@alumni.ait.asia

湿 地 科 学 11卷

(the major source of oxygen inside the bed), re-

quired to accelerate simultaneous removal of both

pollutants (Mitchell and McNevin, 2001).

Mathematical modelling of pollutant removal

is an effective tool that may determine the required

size of a single VF bed to support simultaneous nitri-

fication and organics removal. Current wetland mod-

elling approach is primarily based on first order Kic-

kuth equation, K-C* models, and mechanistic tools

(Rousseau et al, 2004; Langergraber et al, 2009).

The former 2 models produce simple exponential in-

let-outlet concentration profiles across a wetland

system; the latter approach often includes numerous

fixed and guessed parameters (Rousseau et al, 2004;

Giraldi et al, 2010).

In a recent study, Saeed and Sun (2011) devel-

oped a kinetic model that encountered over-simplis-

tic postulations of the first order equations, and com-

plicated approaches of the mechanistic models. The

model combined Monod kinetics with continu-

ous-stirred tank reactor (CSTR) flow approach, and

linked input-output NH4+—N and BOD5 profiles

more closely across VF wetland systems. However,

that model was investigated with one type of waste-

water (i.e. medium strength synthetic domestic

wastewater). Wider application of the developed

equation, to accomplish modelling and designing of

the VF systems is inherently dependent on some ba-

sic characteristics, such as: (a) ability of the model

to predict removal rates from different types of

wastewaters; and (b) preciseness to capture the bio-

logical routes in VF systems, with different structur-

al configurations, arrangements, locations and load-

ings.

This paper provides conceptual analyses of

the first order and Monod CSTR models, for predict-

ing NH4+—N and BOD5 removal rates in VF wet-

lands, with different structural compositions, media,

loadings, that received 3 types of strong wastewa-

ters such as tannery, textile, and municipal effluents.

The main objectives of this paper are two-folds: (a)

to investigate the potential application of Monod

CSTR model as a design tool for VF wetlands

(through comparative evaluation); and (b) to identi-

fy the critical factors (through kinetic modelling),

that influence the bio-degradation routes of the key

pollutants (i.e. NH4+—N and BOD5) of VF systems.

22 Materials and MethodsMaterials and Methods

22..11 Data collectionData collection

Data sets had been collected from 5 VF sys-

tems (of 4 hybrid systems), that were employed to

provide treatment of tannery, textile, and municipal

wastewaters in Dhaka, Bangladesh. These hybrid

systems were operated between September, 2011

and January, 2013. For tannery hybrid system, data

sets had been collected from the first and last stage

VF wetlands. In case of textile hybrid systems, data

sets had been collected from 2 first stage VF wet-

lands (of 2 identical parallel hybrid systems). Data

sets had also collected from the first stage VF wet-

land of a hybrid system that treated municipal waste-

water. Table 1 summarizes the configurations and

operational arrangements of these hybrid systems;

detailed descriptions of such systems, and experi-

mental evidences are available elsewhere (Saeed

and Sun, 2011; Saeed et al, 2012; Saeed and Sun,

2013).

Table 2 illustrates statistical analyses of the en-

vironmental parameters profile, along with NH4+—N

and BOD5, and COD removal performances across 5

VF systems (of 4 hybrid systems, see Table 1), that

TableTable 11 Brief configurations of the experimental vertical flow wetland systemsBrief configurations of the experimental vertical flow wetland systems

Target

wastewater

Tannery

Textile

Municipal

Hybrid

system

VF-HF-VF

VF-HF

VF-HF-SF

System

quantity

One

Two

One

Experimental

scale

Pilot-scale

Lab-scale

Pilot-scale

Main media of

vertical flow systems

Coco peat and gravel

Sugarcane bagasse

Saw dust and coal

Hydraulic

loading

60 mm/d

566-5 660 mm/d

204-306 mm/d

Loading type

Intermittent

Intermittent

Intermittent

422

4期 Tanveer Ferdous Saeed et al: Ammoniacal Nitrogen and Organics Removal Modelling

treated tannery, textile, and municipal wastewaters.

According to Table 2, these raw wastewaters can be

classified as strong wastewaters in terms of organic

strength (George et al, 2002). Such data sets had

been incorporated to examine the validity of the

models, for predicting nitrification and BOD5 remov-

al (from high strength wastewaters) in VF wetlands.

22..22 Assumptions and formulations of the kineticAssumptions and formulations of the kinetic

modelsmodels

In this paper, the formulation of the kinetic

models for correlating inlet and outlet NH4+—N and

BOD5 concentrations across a VF wetland was

based on first order and Monod kinetics. According

to the assumption of the first order kinetics, concen-

tration of a reactant (i.e. pollutant) in a reactor is the

restraining factor, where as presence of catalysts (i.

e. microorganisms) is not considered to be limiting

(Mitchell and McNevin, 2001). On the other hand,

Monod kinetics postulates a closer inter-relationship

between substrate (i.e. pollutant) availability, and

growth of biomass inside a reactor (Bitton, 1994).

Based on such assumptions, the kinetic equations of

first order and Monod kinetics have been illustrated

through equations dC/dt=-kvCout and dC/dt=-Kmax(Cout/

(Chalf +Cout)) respectively; Cout (mg/L) is outlet pollut-

ant concentration; kv (1/d) is volumetric rate con-

stant; Kmax (g/(m3·d)) is maximum volumetric pollut-

ant removal rates; Chalf (mg/L) is half saturation con-

stant of the pollutant.

The flow pattern inside a packed reactor can

follow either of the 2 extreme approaches: (a) CSTR;

or (b) plug flow pattern. Since the experimental pilot

and lab-scale VF wetlands were operated under alter-

nate wetting and drying mode, it was likely that the

flow path was diverged (from the bulk direction) by

the unsaturated media while flowing downwards to-

wards outlet, thereby resembling CSTR flow pattern

(Kadlec, 1994; Wynn and Liehr, 2001; Sklarz et al,2010). This is also supported by recent modelling

studies (Giraldi et al, 2010; Saeed and Sun, 2011),

where the authors closely matched nitrogen and or-

ganics removal profiles assuming CSTR approach in

VF wetlands. In this study, an experimental VF wet-

land had been assumed as a single CSTR reactor, ex-

pressed through equation dC/dt+Cin/τ=Cout/τ; Cin (mg/L)

is inlet pollutant concentration; τ (d) is hydraulic re-

tention time.

Combining 3 equations above mentioned yields

first order CSTR model (in terms of areal rate constant

K1 (m/d)), and Monod CSTR model (in terms of maxi-

mum pollutant removal rates K2, g/(m2·d)) for predict-

ing the removal dynamics in a single VF wetland

TableTable 22 The concentration profiles across vertical flow wetland systems (The concentration profiles across vertical flow wetland systems (nn==5656))

Tannery

wetlands

Textile

wetlands

Municipal

wetlands

parameters

Minimum

25% Percentile

Median

75% Percentile

Maximum

Minimum

25% Percentile

Median

75% Percentile

Maximum

Minimum

25% Percentile

Median

75% Percentile

Maximum

pHin

7.5

7.6

7.8

8.2

9.2

6.6

6.7

6.9

7.0

7.1

6.6

6.9

7.0

7.1

7.3

pHout

5.7

6.6

6.9

7.2

7.5

6.6

6.9

7.1

7.2

7.5

6.5

6.7

6.7

6.8

7.1

Ehin

(mV)

-490.0

327.8

160.0

92.7

79.0

-245.0

-225.0

-193.5

-21.0

-5.0

-273.0

-120.3

-43.5

83.7

223.0

Ehout

(mV)

-403.0

173.3

64.5

53.2

151.0

-118.0

-61.7

-7.5

22.7

53.0

26.0

41.2

100.0

179.0

245.0

NH4+—Nin

(mg/L)

20.0

35.0

57.5

120.0

160.0

90.0

100.0

125.0

180.0

400.0

20.0

42.5

95.0

160.0

240.0

NH4+—Nout

(mg/L)

5.0

10.0

27.5

56.2

85.0

15.0

31.2

52.5

80.0

140.0

12.0

16.0

42.5

103.0

160.0

BODin

(mg/L)

80.0

270.0

1 170.0

3 625.0

11 000.0

2 100.0

2 200.0

2 625.0

2 950.0

4 250.0

500.0

1 000.0

1 875.0

2 500.0

3 625.0

BODout

(mg/L)

48.0

60.0

300.5

718.8

2 300.0

275.0

512.5

625.0

750.0

950.0

100.0

275.0

475.0

575.0

900.0

CODin

(mg/L)

700.0

1 356.0

3 200.0

9 875.0

20 500.0

4 500.0

6 000.0

14 050.0

18 900.0

21 300.0

2 375.0

3 500.0

3 688.0

4 875.0

6 400.0

CODout

(mg/L)

110.0

207.5

820.0

3 900.0

11 100.0

2 400.0

3 200.0

3 625.0

4 225.0

10 750.0

1 050.0

1 213.0

1 375.0

1 750.0

2 400.0

423

湿 地 科 学 11卷

reactor, as expressed in equations K1=q(Cin-Cout)/Cout,

q (m/d) is hydraulic loading rate, and K2=q(Cin-Cout)

(Chalf+Cout)/Cout respectively (Saeed and Sun, 2011).

Nitrification in a wetland reactor is accom-

plished via two inter-related routes: conversion of

NH4+—N to NO2-N, followed by transformation into

NO3-N (Kadlec, 2009). For the first step of nitrifica-

tion process (i.e. conversion of NH4+—N to NO2-N),

the half saturation constant value (Chalf for Nitro-

somonas degradation) can be considered as 0.05 mg/L

(Verstraete and Vaerenbergh, 1986) in K2 equation.

For BOD5 removal, half saturation degradation con-

stant (Chalf of K2 equation) is generally considered

as 60 mg/L (George et al, 2002).

22..33 Statistical analysesStatistical analyses

The statistical analyses undertaken in this

study include: (a) regression analyses; (b) paired

test; and (c) modified index of agreement (MIA), as

illustrated in the following paragraphs.

Regression analyses: the accuracy of the mod-

els (K1 and K2 equations) for predicting nitrification

and BOD5 degradation in VF wetland systems had

been examined by statistical parameters, coefficient

of determination (R2), relative root mean square er-

ror (RRMSE), and model efficiency (ME). The defi-

nitions and mathematical expressions of these pa-

rameters refer to the literature bySun and Saeed

(2009).

For regression analyses K1 and K2 equations

had been arranged in a general form, as illustrated

in equation K=f(Cin, Cout, q)/Cout.

The general form allowed accomplishing a

comparative evaluation between actual data (from

the VF systems), and the data predicted by the mod-

els, through the statistical parameters: R2, RRMSEand ME. The value of f(Cin, Cout, q) and (Cout) in gener-

al equation 6 can be calculated, from the perfor-

mance data of each wetland (for regression analy-

ses). As such, 3 sets of data can be obtained from

each VF bed for each parameter.

Paired test: in order to investigate the difference

between NH4+—N and BOD5 rate constants (m/d) and

Monod kinetic coefficients (g/(m2·d)), derived from

first order and Monod CSTR models respectively,

statistical analyses had been carried out using soft-

ware GraphPad Prism (version 5.03). Various statis-

tical tests (Kolmogorov-Smirnov test, D’Agostino

and Pearson omnibus normality test, and Shap-

iro-Wilk normality test) were performed, to check

whether data distribution approximated to normality

(data was log-transformed if necessary). The results

(data approximated to normality) were accepted

when α=0.05.

The statistical analyses in this paper include

paired t test and Mann Whitney test. The choice of

these two tests was dependent on whether the data

approximated to normality. If data approximated

normality, NH4+—N and BOD5 rate constants and ki-

netic coefficients (derived from first order and

Monod CSTR models) were analyzed through

paired t test. Such analyses illustrated statistical dif-

ference (p<0.05) between the rate constants, and

Monod coefficients for a given parameter (for exam-

ple NH4+—N). Similar analyses were also per-

formed between NH4+—N vs. BOD rate constants,

and NH4+—N vs. BOD Monod coefficients for iden-

tifying the statistical difference (p<0.05) between

such kinetic values. If data did not approximate to

normality Mann Whitney test was performed, in-

stead of paired t test.

Modified Index of Agreement (MIA): the ac-

curacy of the kinetic coefficients (calculated from

the models) for predicting NH4+—N and BOD5 re-

moval in VF wetlands had been compared with ex-

perimentally measured removal rates employing

MIA parameter. The mathematical expression of

MIA refers to the literature by Sklarz et al (2010).

MIA values range between 0-1. A higher MIA value

indicates closer correlation between the measured

and predicted values.

33 Results and DiscussionResults and Discussion

33..11 Performance of the kinetic modelsPerformance of the kinetic models

First order CSTR model for predicting NH4+—N

and BOD5 removal: Fig.1 represents the perfor-

mance of the first order CSTR model, as indicated

424

4期 Tanveer Ferdous Saeed et al: Ammoniacal Nitrogen and Organics Removal Modelling

by 3 statistical parameters (R2, RRMSE and ME),

when arranged in the general form for predicting

NH4+—N and BOD5 removal (from strong wastewa-

ters) in VF wetlands. Overall NH4+—N and BOD5

rate constants had been obtained from the slope of

the regression line, passing through the origin.

As observed in Fig.1, lower statistical correla-

tion values (i.e. R2 and ME) indicated the inefficien-

cies of the first order CSTR model, for predicting

NH4+—N and BOD5 removals in VF systems, that re-

ceived tannery, textile and municipal wastewaters.

However, the model matched BOD5 removal perfor-

mance data effectively (as indicated by the statisti-

cal parameters), across the VF wetland that received

municipal wastewater.

Monod CSTR model for predicting NH4+—N

and BOD5 removal: Fig.2 represents the perfor-

mance of Monod CSTR model when arranged in

general form, for predicting nitrification and organic

(BOD5) removals in VF wetlands. Overall NH4+—N

and BOD Monod coefficients had been calculated,

from the slope of the regression lines passing

through the origin.

As observed in Fig.2, Monod CSTR model

showed better performances for correlating NH4+—

N and BOD performance data across all VF systems

(as indicated by statistical parameters R2 and ME),

when compared with the first order CSTR model

(see Fig.1). Such findings illustrated the efficacy of

Monod based model, to predict NH4+—N and BOD5

removal (from strong wastewaters) more accurately

in experimental VF wetlands.

Note: the dotted lines are 95% confidence band, indicating the band contains true regression fit line.

Fig.Fig.11 Regression of first order CSTR model for predicting nitrification and BODRegression of first order CSTR model for predicting nitrification and BOD55 removal in vertical flow wetlandsremoval in vertical flow wetlands

Note: the dotted lines are 95% confidence band, indicating the band contains true regression fit line.

Fig.Fig.22 Regression of Monod CSTR model for predicting nitrification and BODRegression of Monod CSTR model for predicting nitrification and BOD55 removal in vertical flow wetlandsremoval in vertical flow wetlands

425

湿 地 科 学 11卷

33..22 Comparative evaluation of the modelsComparative evaluation of the models

Better performance of Monod CSTR model

(over first order approach), for linking NH4+—N and

BOD5 profiles jeopardized the postulation of excess

biomass presence (according to first order kinetics)

in VF wetlands. In fact a closer correlation existed

between substrate availability and increase of bio-

mass (assumption of Monod kinetics) in the experi-

mental VF wetland reactors, thereby representing a

closer reflection of microbiological degradation.

These findings were further supported by

Fig.3 and Fig.4, that represented the correlation

trends between the rate constants (obtained from

first order CSTR model-equation 4), Monod coeffi-

cients, and input NH4+—N, BOD5 loadings. Accord-

ing to Fig.3, NH4+—N and BOD5 rate constants did

not exhibit positive correlation with corresponding

input loading increments (in VF wetlands). In con-

trast, NH4+—N and BOD5 Monod coefficients illus-

trated positive trend with corresponding loading in-

crements (see Fig.4). Such findings denote greater

removals at enhanced input loadings probably due

to higher availability of substrate that stimulated

biomass growth in experimental VF wetland reac-

tors. Similar trends were also reported with experi-

mental values (i.e. input-removal rate profiles) in

other wetland studies (Calherios et al, 2009; Zhao

et al, 2009).

It should be noted that the correlation values

of Monod KBOD vs. input BOD5 loading plots were

higher, when compared with NH4+—N Monod coef-

ficients vs. input NH4+—N loading plots (for tan-

nery, textile and municipal VF wetlands, see Fig.4).

As such, it could be stated that increase of BOD5 re-

moval rates was more linear with loading increment

(in the experimental VF reactors).

Fig.Fig.33 Correlation plots of NHCorrelation plots of NH44++——N and BODN and BOD55 rate constants vs. input loadingsrate constants vs. input loadings

Fig.Fig.44 Correlation plots of NHCorrelation plots of NH44++——N and BODN and BOD55 Monod coefficients vs. input loadingsMonod coefficients vs. input loadings

426

4期 Tanveer Ferdous Saeed et al: Ammoniacal Nitrogen and Organics Removal Modelling

Such phenomena could be described through the me-

tabolism kinetics of the nitrifying and heterotrophic

biomass. In an aerobic VF wetland system, nitrifica-

tion rates are often lower (over aerobic organic deg-

radation), due to slower growth rate of autotrophic

microbes (Grady et al, 1999). It could be possible

that, such slower growth rate of the autotrophic nitri-

fiers could not be matched with enhanced NH4+—N

loading increments, which the resulting deviated ni-

trification rates. This was also justified by the scat-

tered Monod coefficients values (KNH4+—N) from the

general trend line, at upper NH4+—N loading ranges

(see Fig.4).

33..33 Pollutant removal rates vs. rate constant andPollutant removal rates vs. rate constant and

Monod coefficient profilesMonod coefficient profiles

Fig.5 and Fig.6 indicated the correlation pro-

files of nitrification and organics removal rates

(measured experimentally) vs. corresponding rate

constants, and Monod coefficients in VF wetlands.

As observed in Fig.5, lower R2 correlation values

were observed between nitrogen and organics re-

moval rates vs. corresponding rate constants, regard-

less incoming wastewater type. Such lower correla-

tion values imply that the rate constants were incapa-

ble, to predict the experimentally measured removal

rates. These findings are in agreement with the ob-

served inefficiencies of the first order kinetic model,

as indicated by the regression parameters (see Fig.1).

In contrast, the correlation plots of Monod co-

efficients KNH4+—N, KBOD vs. experimentally measured

removal rates exhibited higher correlation values

(see Fig.6). These results suggested that, Monod

Fig.Fig.55 Correlation plots of experimentally measured removal rates vs. rateCorrelation plots of experimentally measured removal rates vs. rate

constants in vertical flow wetlandsconstants in vertical flow wetlands

Fig.Fig.66 Correlation plots of experimentally measured removal rates vs. MonodCorrelation plots of experimentally measured removal rates vs. Monod

coefficients in vertical flow wetlandscoefficients in vertical flow wetlands

427

湿 地 科 学 11卷

coefficients were capable in predicting NH4+—N

and BOD5 removal routes more accurately in the VF

wetland reactors. In addition, higher correlation val-

ues (see Fig.6) also suggested that the removal

routes were primarily accomplished via microbio-

logical pathways.

Maximum fit values (R2=1.0) of Monod

NH4+—N coefficients vs. experimentally measured

removal rate increment plots (see Fig.6) elucidated

that, nitrification rates reached towards maximum

capacity in experimental VF wetlands (Mitchell and

McNevin, 2001). This is further supported by the de-

viation of NH4+—N Monod coefficients (from the

general trend line), at upper input loading ranges

(see Fig.4) probably due to the inefficiencies of the

nitrifying biomass, for accomplishing nitrification

(beyond maximum capacity).

The accuracy of Monod NH4+—N, and BOD5

kinetic coefficients (for predicting biological degra-

dation in VF wetlands), had further been investigat-

ed by incorporating MIA index. As such, Table 3 il-

lustrated MIA correlation values between Monod

NH4+—N and BOD5 kinetic coefficients and corre-

sponding removal rates (measured experimentally).

According to Table 3, higher MIA correlation values

(closer to 1.0) had been observed between predicted

(by Monod coefficients) and measured values, that

was also demonstrated by R2 values (Fig.6). In gen-

eral, these statistical analyses confirmed the capabil-

ity of the Monod CSTR model, for predicting micro-

biological routes more accurately in the experimen-

tal wetland systems.

TableTable 33 Modified Index of Agreement analysis between experimentallyModified Index of Agreement analysis between experimentally

measured removal rates and predicted valuesmeasured removal rates and predicted values

Monod CSTR model

KNH4+—N=q(Cin-Cout)(0.05+Cout)/Cout

KBOD=q(Cin-Cout)(60+Cout)/Cout

Target

pollutant

NH4+—N

BOD5

Values of Modified Index of Agreement

Tannery wetlands

0.99

0.92

Textile wetlands

0.99

0.87

Municipal wetlands

0.99

0.86

33..44 Influence of BODInfluence of BOD55 removal on nitrificationremoval on nitrification

Nitrification and biodegradable organic re-

movals in the wetland systems generally follow sim-

ilar pathways, but often exhibit contradictory remov-

al rates. In an aerobic VF wetland reactor, the avail-

able oxygen was utilized rapidly by the heterotro-

phic biomass due to faster kinetics, hence limiting

oxygen availability to support the metabolism of

slower growing autotrophic nitrifiers (Grady et al,1999). Previous research studies also reported lower

nitrification rates in the wetland systems, when in-

fluent BOD5 concentration in the wastewater ranged

between 193-366 mg/L (Sun et al, 1998; Wu et al,2011). Since influent BOD5 concentrations across

the experimental VF wetlands were substantially

higher (see Table 2) than the reported values, kinetic

investigation on such possible interference had been

carried out in this study.

Fig.7 and Fig.8 represented correlation plots

of NH4+—N, BOD5 rate constants, and Monod coef-

ficients vs. input BOD/NH4+—N ratio in VF systems

(treating strong wastewaters). As observed in Fig.7,

no clear correlation trend had been observed be-

tween inputs BOD/NH4+—N ratios vs. correspond-

ing rate constants, indicating the independency of ni-

trification rates on organics removal despite substan-

tial influent organics concentration (see Table 2).

However the correlation plots of NH4+—N,

BOD5 Monod coefficients vs. input BOD/NH4+—N

ratio (see Fig.8) represent contradictory scenario.

According to Fig.8, greater BOD5 Monod coeffi-

cient values coincided with greater input BOD/

NH4+—N values (in VF wetlands). Such trends de-

noted increase of organics removal rates with organ-

ic loading increment, and were in agreement with

the findings of Fig.4 (i.e. positive correlations be-

tween input loadings vs. Monod coefficient incre-

ments). Simultaneously, lower NH4+—N Monod co-

efficient values coincided with greater input BOD/

NH4+—N values (see Fig.8). At such greater ratios,

428

4期 Tanveer Ferdous Saeed et al: Ammoniacal Nitrogen and Organics Removal Modelling

substantial organics removal rates (due to higher

loading) could have depleted oxygen availability in

the experimental VF wetlands, thereby limiting

slower nitrification process.

33..55 Impact of redox potential and influent concentrationImpact of redox potential and influent concentration

Dissolved oxygen (DO) and redox potential

(Eh) values are 2 important environmental parame-

ters that often influence nitrification and biodegrad-

able organic removal rates in the wetland reactors.

However, Vymazal and Kröpfelová (2008) reported

that, redox potential values were better indicators of

nitrogen and organic removal pathways in the wet-

land systems. As such, redox potential profiles (see

Table 2) had also been incorporated in this paper, to

examine the prediction performance of the models.

Fig.9 and Fig.10 (correlation plots) illustrated

the impact of influent redox potential values (cou-

pled with influent NH4+—N and BOD5 concentra-

tion) on corresponding rate constants and Monod co-

efficients in VF wetlands. As observed in Fig.9, nei-

ther influent redox values, nor incoming concentra-

tions had clear influence on NH4+—N and BOD5

rate constants, despite such incoming parameters

were substantially variable.

The correlation plots of influent Eh, NH4+—N,

and BOD5 concentrations vs. corresponding Monod

coefficients (see Fig.10); indicated positive correla-

tion between increase of Monod coefficient values

and influent concentrations. Such results were in

agreement with the trend of input loading-Monod

coefficient profiles (see Fig.4). However, input load-

ing profiles exhibited more clear correlations (see

Fig.4), when compared with influent concentrations

(see Fig.10). This performance disparity could be

due to the inclusion of flow rate (into input load-

ing); an influential factor that often determines

Fig.Fig.77 Correlation plots of input BOD/NHCorrelation plots of input BOD/NH44++——N ratios vs. rate constantsN ratios vs. rate constants

Fig.Fig.88 Correlation plots of input BOD/NHCorrelation plots of input BOD/NH44++——N ratios vs. Monod coefficientsN ratios vs. Monod coefficients

429

湿 地 科 学 11卷

Fig.Fig.99 Correlation plots of rate constants vs. incoming concentration and redox potentialCorrelation plots of rate constants vs. incoming concentration and redox potential

Fig.Fig.1010 Correlation plots of Monod coefficients vs. incoming concentration and redox potentialCorrelation plots of Monod coefficients vs. incoming concentration and redox potential

wetland performance (Rousseau et al, 2004; Wong

and Somes, 1995).

Simultaneously, Monod coefficients did not

exhibit any clear correlation when plotted against in-

fluent Eh values (see Fig.10) illustrating that incom-

ing concentration was a critical factor that con-

trolled removal kinetics in VF wetlands. Greater oxi-

dized conditions inside the beds of the VF systems

(Saeed et al, 2012; Saeed and Sun, 2013), could

have encountered the impact of variable Eh values

(of the incoming wastewater).

33..66 Implication of kinetic coefficients for VF wetImplication of kinetic coefficients for VF wet--

land designland design

Table 4 indicated statistical comparison

(paired test-section 2.4) between rate constants vs.

Monod coefficients (for a given parameter), and

NH4+—N vs. BOD5 coefficients (for a given model).

As observed in Table 4, the comparison between

rate constants vs. Monod coefficients indicated sig-

nificant statistical differences (p<0.05) for both pa-

rameters (NH4+—N and BOD5), and were in agree-

ment with the performance differences observed in

Fig.1 and Fig.2.

Simultaneously, statistical analyses between

NH4+—N, BOD5 kinetic rate constants indicated no

statistical difference (see Table 4). Such observa-

tions were not in agreement with faster degradation

characteristics of the heterotrophic biomass; over au-

totrophic nitrifies (Grady et al, 1999). In contrast,

statistical comparison between NH4+—N and BOD5

Monod coefficients illustrated significant differenc-

es (p<0.05, see Table 4) for all VF wetlands, coincid-

ing with the established biological metabolisms.

Table 5 indicated overall NH4+—N and BOD5

Monod kinetic coefficients calculated by Monod

CSTR model in VF wetlands that received strong

430

4期 Tanveer Ferdous Saeed et al: Ammoniacal Nitrogen and Organics Removal Modelling

TableTable 44 Statistical results of the calculated kinetic parametersStatistical results of the calculated kinetic parameters

Comparison between first order

and Monod coefficients

Comparison between NH4+—N

and BOD kinetic coefficients

Kinetic coefficients

KNH4+—N (m/d) vs. KNH4

+—N (g/(m2·d))

KBOD (m/d) vs. KBOD (g/m2.d)

KNH4+—N (m/d) vs. KBOD (m/d)

KNH4+—N (g/(m2·d)) vs. KBOD (g/(m2·d))

p

Tannery wetlands

<0.000 1

0.001 4

NS

0.009 7

Textile wetlands

<0.000 1

<0.000 1

0.012 5

<0.000 1

Municipal wetlands

<0.000 1

<0.000 1

NS

<0.000 1

Note: NS means no significant difference; p<0.05 means significant difference.

wastewaters (i.e. tannery, textile, and municipal ef-

fluents). The calculated coefficients had also been

compared with NH4+—N and BOD5 Monod coeffi-

cients of VF wetlands, dosed with medium strength

domestic wastewater in Australia (see Table 5)

(Saeed and Sun, 2011). Overall, Table 5 suggested

(a) higher NH4+—N and BOD5 Monod coefficient

values for textile and municipal VF wetlands, when

compared with tannery systems; and (b) significant

deviation of KBOD, over KNH4+—N values for both strong

and medium strong wastewaters.

TableTable 55 Comparison of Monod NHComparison of Monod NH44++——N and BODN and BOD55 kinetickinetic

coefficients for strong and medium strength wastewaterscoefficients for strong and medium strength wastewaters

Monod

coefficients

(g/(m2·d))

KNH4+—N

KBOD5

Wastewater types

Tannery

(this study)

3.9

257.6

Textile

(this study)

68.5

2193.0

Municipal

(this study)

19.7

555.4

Domestic

(Saeed and

Sun, 2011)

14.2

54.3

Higher values of the kinetic coefficients as ob-

served in textile and municipal VF wetlands (over tan-

nery VF wetlands) indicated that, hydraulic flow varia-

tion across the systems could have played a major role

for such disparity (see Table 1), as influent NH4+—N

and BOD5 concentrations were higher across all VF

systems (see Table 2). Such findings jeopardized the

current approach of considering generalized rate con-

stants (Kadlec and Knight, 1996), for calculating the

bed area of wetland reactors despite hydrological vari-

ations (across different systems).

Simultaneously, significant deviation between

NH4+—N and BOD5 kinetic values (see Table 5) also

suggested incorporation of KNH4+—N coefficients, along

with KBOD. Such practices might produce 2 different

bed area values (for nitrification, and organic bio-

degradation) for a single VF system, thereby select-

ing the greater area to facilitate the biodegradation

routes of both pollutants. In general, closer matching

of NH4+—N and BOD5 profiles (by Monod model)

across VF systems (treating strong and medium

strength wastewaters), with different structural com-

positions and loading variations, elucidated the poten-

tial application (of the equation) for calculating the

area of a single VF wetland.

44 ConclusionsConclusions

Monod CSTR model demonstrated better cor-

relation values (as indicated by the statistical param-

eters R2 and ME) when compared with the first order

CSTR equation, for predicting nitrification and

BOD5 degradation in VF wetlands. Higher Monod

coefficient values coincided with greater input load-

ings, and experimentally measured removal rates.

On the other hand, rate constants (calculated from

the first order CSTR model) did not exhibit such pat-

terns indicating their inefficacy, for capturing over-

all biodegradation routes in VF wetlands.

The interference of organics removal on nitri-

fication process (in VF wetlands) was identified

through Monod coefficients that could not be depict-

ed by the rate constants. Significant deviation of

Monod KBOD5 (from Monod KNH4+—N values), indicates

incorporation of both coefficients for calculating the

bed area of a single VF wetland.

In general, Monod CSTR model predicted ni-

trification and BOD5 removal rates more accurately

in VF systems that were employed to provide treat-

ment of different strong wastewaters. Such perfor-

mance indicates that the model may be utilized as a

design tool for VF wetlands.

431

湿 地 科 学 11卷

AcknowledgementsAcknowledgements

The authors would like to acknowledge the

undergraduate students of Department of Civil Engi-

neering, Ahsanullah University of Science and Tech-

nology, Bangladesh for providing maintenance of

the wetland systems during the experimental period.

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