A bio-electrochemical reactor coupled with adsorber for the ...

Water Research 36 (2002) 3092–3102

A bio-electrochemical reactor coupled with adsorber for theremoval of nitrate and inhibitory pesticide

Z. Felekea, Y. Sakakibarab,*aDepartment of Chemistry, Addis Ababa University, Addis Ababa, Ethiopia

bDepartment of Civil Engineering, Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo 169-8555, Japan

Received 15 May 2001; received in revised form 14 November 2001; accepted 26 November 2001

Abstract

In this study, the treatment characteristics of nitrate and toxic pesticide by a combined bioelectrochemical reactor

(BER)/adsorption process was investigated. Experimental results showed that the disappearance of NO3� in BER was in

accordance with the applied current. NO2� was not detected in the effluent, but the production of N2O was increased

with increasing IPT loading. In the presence of IPT, up to 30% of the nitrate nitrogen consumed was converted to N2O,

while more than 95% converted to N2 in the absence of IPT. In adsorption column, IPT was efficiently removed onto

either granular activated carbon or silicone resin so as to meet the guideline value (40 mg/l) and to reduce the N2O

accumulation. A simplified kinetic model that considers the sequential reduction of nitrate and inhibition of the N2O

reduction step by pesticide as well as Langmuir adsorption isotherm was developed and used to evaluate the process

performance. Theoretically predicted effluent concentrations were in good agreement with the observed results for

nitrate, nitrite, N2O, N2 and IPT. It was considered that high removal performance of nitrate and pesticide by the

combined process is attributable to high affinity of adsorbates for IPT in comparison with relatively large inhibition

constant ðKiÞ: r 2002 Elsevier Science Ltd. All rights reserved.

Keywords: Combined process; BER; Adsorption; Drinking water; Nitrate; Pesticide; Inhibition

1. Introduction

Contamination of groundwater and surface water by

nitrate and pesticides originating from non-point

sources such as agricultural fields has become a growing

and common environmental problem in many parts of

the world [1]. Excess nitrate in drinking water may cause

methemoglobinaemia also called a blue-baby disease, in

new-born infants and pesticide residues have been

associated with cancer, nervous system disorders, birth

defects, and male sterility [2].

Pesticide removal processes include adsorption, air

stripping and membrane technologies. Among these, the

adsorption onto granular activated carbon (GAC),

powdered activated carbon (PAC) and polymeric resins

seems the most feasible technique for the removal of

organic pesticides even at very trace concentration levels

[3]. Ion exchange (IE), reverse osmosis (RO), electro-

dialysis (ED) and biological denitrification reactors (BR)

are the most common methods reported for the removal

of nitrate [4]. For simultaneous removal of nitrate and

pesticides, combined IE/GAC [5] and combined mem-

brane bioreactor/PAC processes [6] were proposed.

However, there are limitations to the available technol-

ogies because of build up of dead-end products, non-

selectivity for target contaminants and high treatment

cost.

Biological denitrification seems the most promising

process for nitrate removal. Complete reduction from

nitrate (NO3�) to nitrogen gas (N2) involves four

consecutive steps (Eq. (1)) that are catalyzed by different

enzymes [7]. The enzymes are susceptible towards

changes in pH, DO, carbon and energy source dose*Corresponding author. Tel/fax.: +81-3-5286-3902.

E-mail address: [email protected] (Y. Sakakibara).

0043-1354/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved.

PII: S 0 0 4 3 - 1 3 5 4 ( 0 1 ) 0 0 5 3 8 - 3

and the presence of inhibitors [8], which may cause the

accumulation of intermediates.

NO�3 -NO�

2 -NO-N2O-N2: ð1Þ

Heterotrophing denitrifying processes using metha-

nol, ethanol or acetic acid have been applied in full-scale

plants for the treatment of drinking water [9]. An

extensive post treatment is required to meet the effluent

quality standard for drinking water. Autotrophic drink-

ing water denitrification process by using hydrogen gas

as an electron donor was also investigated [10–13]. The

hydrogen is harmless, and in contrast to other sub-

strates, no further steps are needed to remove excess

substrate and the production of excess biomass is

desirably very low [11]. Nevertheless, its low solubility,

and hazardous (explosive) nature during use, transpor-

tation and storage may limit the use of hydrogen gas in

denitrification reactors. Furthermore, control of pH by

introducing an acid equivalent is required, which is

common to the traditional biological processes.

Researchers have recently proposed a bioelectrochem-

ical reactor (BER) in which autotrophic denitrification

was stimulated with the passing of electric current [14].

In BER hydrogen gas is produced by electrolysis of

water (Eq. (2)) and autotrophic denitrifying microor-

ganisms were directly immobilized on the surface of the

cathode. The process is highly selective for the reduction

of nitrate to nitrogen gas with simultaneous neutraliza-

tion by carbon dioxide (Eq. (3)) at the anode [15].

10 H2Oþ 10 e�-5 H2 þ 10 OH�; ð2Þ

2:5 Cþ 5H2O-2:5 CO2 þ 10 Hþ þ 10 e�: ð3Þ

Autotrophic denitrification of nitrate to nitrogen gas

using hydrogen gas as an electron donor may proceed

stepwise as follows:

2 NO�3 þ 2H2-2 NO�

2 þ 2 H2O; ð4Þ

2NO�2 þ 2H2-N2OþH2Oþ 2OH�; ð5Þ

N2OþH2-N2 þH2O: ð6Þ

Net reaction:

2 NO�3 þ 10 e� þ 6H2O-N2 þ 12 OH�: ð7Þ

Overall reaction in BER:

2 NO�3 þ 2:5 CþH2O-N2 þ 2:5 CO2 þ 2 OH�: ð8Þ

According to Eq. (7), 5mol of electrons are needed to

reduce 1mol of nitrate to nitrogen gas.

In a former study [16], the effectiveness of a combined

BER/adsorption process to treat nitrate and trace

pesticide was demonstrated using Isoprothiolane (IPT),

dithiolidinemalonate fungicide as a test pesticide. How-

ever, in a subsequent study [17], it was also demon-

strated that a significant amount of nitrous oxide was

produced in BER when loaded high concentrations of

IPT.

This study was conducted to evaluate the performance

of the combined BER/adsorption process at different

pesticide loading conditions. Nitrogen balance based on

the stepwise reductions of nitrate to N2 was taken to

clarify the reduction step inhibited by pesticide.

Furthermore, a simplified kinetic model was developed

based on a non-competitive inhibition model and was

used to evaluate the process performance in conjunction

with adsorption and reactor mass balance equations.

Some experimental results obtained in former study by

Feleke et al. [16] and additional experimental and

theoretical results in subsequent studies were used to

analyze and evaluate the performance. Gas chromato-

graphy equipped with electron capture detector (ECD)

was used to take precise N2O measurements.

2. Materials and methods

2.1. BER and electrochemical experiments

The experimental apparatus used in this study is

shown schematically in Fig. 1. The effective liquid

volume and the surface are of the cathode of BERs

were 0.205 l and 251 cm2, respectively. Details of the

experimental apparatus were reported elsewhere [18].

Prior to the commencement of the present study, three

BERs (R1, R2 and R3) had been operated in parallel for

over 2 years, receiving continuous input of ground water

containing NO3� (24mg/l as N) and trace inorganic

nutrients. Similar denitrification performance of the

BERs in the absence of pesticide had been confirmed

[15].

To investigate the effect of pesticide in this study, R1

and R2 were operated at feed pesticide concentration of

about 215 and 1200 mg/l, respectively (Run I). R3 was

used as a control (no pesticide loading). Hydraulic

retention time ðyÞ was maintained at about 10 h, and

constant electric current of 5.0mA, which is stoichio-

metric amount needed for complete reduction of nitrate

in the feed, was applied through out the experiments. All

reactors were connected to gas collectors where the gas

phase was allowed to equilibrate with the liquid phase.

To examine the long-term performance under stressed

operating conditions, BER was operated by changing

the applied electric current from 0 to 10mA and at the

same time, by varying IPT loading (Run II). A control

electrochemical reactor without denitrifying biofilm

(R4) was also operated at different electric current to

determine if direct electrochemical reactions of nitrate

and IPT can occur in this system under experimental

conditions used for continuous experiment. In abiotic

experiment, groundwater was fed initially into the

reactor for about 20 days without any pretreatment.

Z. Feleke, Y. Sakakibara / Water Research 36 (2002) 3092–3102 3093

Thereafter, the reactor and recycling line were

disinfected by injecting benzalkonium chloride solution

to eliminate any residual microbial activity and a sterile

distilled water containing only about 20mg/l of NO3�

and 350 mg/l of isoprothiolane was fed. The result

obtained from this abiotic experiment was compared

with that of biological experiment. The details of

experimental conditions were summarized in Table 1.

Teflon tubing (NICHIAS Corp.) with different diameter

were used at feed, recycling and effluent lines of the

process.

2.2. BER/adsorption experiments

A combined process consists of two unit processes,

BER and packed adsorption bed, in series as shown in

Fig. 1. After evaluating the performance of BER in the

presence of IPT, IPT was loaded to three BERs and their

effluent streams were fed to adsorption columns (Run

III). The effluents of R1 and R2 were introduced into the

columns packed with activated carbon, and that of R3

was fed into a column packed with silicone resin. IPT

loading into BERs was about 366mg/l d (feed concentra-tion 150 mg/l). The columns are made up from acrylic

pipe with internal diameter of 3.5 cm. The effective

height of the adsorption column bed was 60 cm. The

BERs were operated at constant electric current of

2.5mA and the adsorption columns were operated at

constant pesticide loading. Nitrate and isoprothiolane

concentrations were monitored at the inlet and outlet of

the BERs and the adsorption columns. The composition

of influent groundwater and effluent from the combined

process is also given in Table 1.

2.3. Analytical methods

All samples were filtered through a 0.2-mm cellulose

nitrate membrane filters (ADVANTEC, Toyo Roshi

Ltd.) prior to analysis. Procedures of the preparation of

sample for analysis were performed as described in the

standard methods [19]. Nitrate and nitrite were mea-

sured with an ion chromatograph (IC 7000-Series II,

Yokogawa Analytical Systems). Gas production rate

from the reactors was measured by collecting the

produced gas in a gas-sampling bag (Tedlar bag)

and measuring the total volume on a daily basis.

N2, O2, CO2 and H2 gas compositions were analyzed

by gas chromatograph equipped with thermal

conductivity detector (Shimadzu, Series GC-14B). N2O

emission was measured by gas chromatograph

equipped with a Porapack Q column and ECD

(Shimadzu GC-14B).

Stock isoprothiolane solution was prepared by

dissolving pesticide grade isoprothiolane (Wako

Pure Chemical Industries Ltd.) in ethanol (99.5%). A

set of five calibration standards was prepared from

the stock solution. Samples were extracted using

solid phase extraction (SPE) column (EnvirElut Pesti-

cide, Varian). The samples extracted with SPE were

eluted with dichloromethane. The extraction method

had an efficiency of 98% in the concentration range 2–

2000 mg/l.Isoprothiolane was analyzed by gas chromatograph

(Shimadzu GC-17A), equipped with ECD and fused

silica capillary column (Shimadzu) with DB-5 statio-

nary phase. The GC oven was heated from 501C to

1901C at 201C/min, from 1901C to 2101C at 301C/min

and finally to 2801C at 101C/min. GC/MS analysis of

Fig. 1. Schematic of experimental apparatus.

Z. Feleke, Y. Sakakibara / Water Research 36 (2002) 3092–31023094

isoprothiolane was also made with a quadrapole mass

spectrometer (Shimadzu GCMS-QP5050A) with GC

inlet and electron impact ionization (EI). The GC

column oven was heated from 501C to 1201C at 301C/

min, 1201C to 2701C at 151C/min and finally held at

2701C for 2min. The type and dimension of the column

is similar to that used in GC-ECD.

3. Results and discussion

3.1. Effect of IPT on nitrate reduction

A one-dimensional biofilm-electrode model developed

in a former study [20] showed that under hydrogen

limited condition, the net denitrification rate is a linear

Table 1

Experimental conditions: (A) operating conditions; (B) composition of influent groundwater and effluent from the combined process at

2.5mA and (C) reactor dimensions

(A) Experiment Operating

condition

BER Abiotic reactor

R1 R2 R3 R4

(Run I) Denitrification/inhibition IPT (mg/l) 215 1200 0 FExperiment y (h)a 10 10 10 F

I (mA) 5 5 5 F

(Run II) Long-term denitrification and

comparison with abiotic reactor

IPT (mg/l) (100–300)b (10–160)b 100–1200 400

y (h) (10)b (10)b 10 10

I (mA) (2–2.5)b (2–2.5)b 0–10 0–10

(Run III) combined treatment IPT (mg/l) 150 150 150 Fy (h) 10 10 10 FI (mA) 2.5 2.5 2.5 FColumn Carbon Carbon Silicone F

(B) Parameter Average concentration in Run III

Influent Effluent (mg/l)

NO3�-N 22.5 10.2

NO2�-N NDc 0.01

SO42�-S 8.1 7.6

PO43�-P 1.9 1.7

Cl� 17.6 17.3

Na+ 49.3 48.3

K+ 8.0 8.0

Ca2+ 23.5 19.3

Mg2+ 8.5 6.4

NH4+-N NDc NDc

pH 6.9 6.6

IPT 0.15 o0.002d,

o0.006e

(C) Reactor dimensions

(BERs (R1–R4))

Effective liquid volume=0.205 l,

Area: cathode=251 cm2, anode=160 cm2

(Adsorption columns)

Empty bed volume=0.58 l

(3.5 cm ID� 60 cm length)

aHydraulic retention time.bData were shown elsewhere [16].cNot detected.dGAC.eSilicone.


function of electric current. This condition is considered

to exist when electric current density is less than about

0.1mA/cm2 and thickness of denitrifying biofilm is

larger than roughly about 100mm [20]. Furthermore, the

linear relation of Eq. (9) was experimentally verified in

former studies by Sakakibara et al. [18] and Feleke et al.

[15]. The net denitrification rate without inhibition may

be expressed by:

Jnet ¼iC

5 F; ð9Þ

where ic is current density at cathode and Jnet is the net

denitrification rate of nitrate to nitrogen gas.

For hydrogen limited condition in BER and the

inhibition of a specific step (Reactions 4–6) by toxic

substance, the rate may be expressed by including non-

competitive inhibition term:

JjP;R ¼ic

njF

Ki

Ki þ Ci; ð10Þ

where JjP;R is either the production or reduction rate of

constituent j; nj is the number of moles of electrons

utilized for the reduction of step j; Ci is the concentra-

tion of an inhibitory compound and Ki is the inhibition

constant.

IPT inhibition may occur at higher loading. Since

values for pesticide inhibition constants were not

available, Ki for IPT was obtained by measuring the

rate of production of an intermediate at different

pesticide loading and making mass balance on the rate

of production and reduction as expressed by

JN2ORjci¼0JN2OP � Jx

¼ 1þ Ci=Ki; ð11Þ

where Jx is the observed rate of N2O production in BER

(corresponding to the difference between the production

rate and reduction rate of N2O), JN2OP is the production

rate of N2O, and JN2ORjCi¼0 is the reduction rate of N2O

at Ci ¼ 0; respectively. The above equation is obtained

by substituting Eq. (10) into a steady-state mass balance

equation for N2O and then rearranging to collect the

inhibition term into right-hand side of the equal sign.

A set of steady-state mass balance equations for

nitrogen species are given in Table 2. Fig. 2a shows the

influent and effluent concentration of nitrate and nitrite

for the three reactors at applied current of 5.0mA,

which corresponds to the stoichiometric current needed

to reduce all nitrate to nitrogen gas according to Eq. (7).

As shown, the effluent nitrate concentration was

decreased gradually and then approached steady state

after about 14 days of operation. The nitrate consump-

tion rate in the three reactors was nearly the same

regardless of different IPT loading. Nitrite, which is

more toxic than nitrate itself, was not detected in all

reactors. These results indicate that the reductions steps

of nitrate to nitrite and then to nitrous oxide were not

affected by isoprothiolane in the loading range applied.

The predicted effluent nitrate concentrations, assum-

ing 100% current efficiency were also shown in the same

figure. The effluent nitrate concentration for the three

reactors approached the predicted values after about 14

days of operation.

Fig. 2b shows the nitrous oxide production character-

istics of the three reactors. After confirming the nitrous

oxide in the gas phase is nearly the same for all reactors

in the absence of IPT, different pesticide loads were

applied starting from day 4. The results in Fig. 2b shows

that the total flux of nitrous oxide in the liquid and gas

phases increased drastically for the reactors being fed

with IPT. However, for the control reactor (R3), nitrous

oxide production rate was very low. Furthermore, the

increase in nitrous oxide concentration was in propor-

tion to the IPT load. The IPT feeding was stopped from

day 41 onwards and it can be seen that nitrous oxide in

the gas phase was dropped to the same level as that of

control reactor. This result suggests that isoprothiolane

may inhibit the reduction of N2O to N2. The estimated

value of Ki obtained by plotting the reciprocal of the

maximum reaction rate against the inhibitor concentra-

tion was about 10mg/l. This procedure may also be

applicable to evaluate the inhibition of any steps of

denitrification. By coupling a simplified kinetic model

(Eq. (10)) with a completely mixed flow reactor model

(equations given in Table 2), the flux of N2O from the

three reactors was estimated. The parameter values used

for calculation are given in Table 3.

The N2 gas production rate was decreased as pesticide

loading increases (Fig. 2c). The predicted nitrogen gas

Table 2

Steady-state mass balance equations

Nitrate:

CNO�3 f � CNO�

3

y� acJNO�

3¼ 0

Nitrite:

CNO�2 f � CNO�

2

yþ acJNO�

2 P � acJNO�2 R ¼ 0

N2O :CN2Of � CN2O

yþ acJN2OP � acJN2OR �

PN2OQg

RTV¼ 0

N2 :CN2f � CN2

yþ acJN2P �

PN2Qg

RTV¼ 0

Where Cj is the molar concentration of substrate j; ac is specific

surface area of the cathode, y is hydraulic retention time, Qg is

gas production rate, V is the effective reactor volume, Pj is the

partial pressure of gas constituent j; R is gas constant, and T is

the absolute temperature. The subscript f represents feed

solution.


production rate is also shown, which is in close

agreement with the observed values. The slight dis-

crepancy is possibly due to the assumption of 100%

current efficiency used for calculation. The observed

current efficiency was about 80%. N2O accumulation

and the reduction in N2 production were related to the

increase in IPT loading, suggesting that this pesticide

inhibited a single step of the reduction of nitrous oxide

to nitrogen gas. This is undesirable from the point of

view of complete reduction of nitrate to nitrogen gas and

also the current utilization efficiency for the specific step

will be reduced.

The nitrogen balance was made based on measure-

ments of NO3�-N and NO2

�-N in the liquid phase, and

N2 and N2O in the gas phase. The concentration of

gaseous components in the liquid phase were not directly

measured; but estimated by using Henry’s law assuming

that equilibrium was established between the two

phases. Although the concentration of NO was not

monitored in this study, it was shown that this

Fig. 2. Disappearance of nitrate and production of nitrous oxide and nitrogen gases in BERs operated at different IPT loading

conditions (Run I).


intermediate has a very high affinity for its reductase and

its accumulation may not be significant [25]. Fig. 3

shows the percentage of nitrogen recovered as different

nitrogenous compounds along with the total mass

balance for the three reactors. When the influent IPT

concentration was 1200 mg/l, about 30% of the feed

nitrogen was converted to N2O. The production rate of

N2O in the control reactor was very small (about 4%).

The nitrogen mass balance was satisfied within 5%

experimental error. The experimentally obtained mass

balances were agreed well with the model predictions

(Fig. 3).

These results also suggest that in the absence of IPT,

electric current can control the rates of each reaction

step. However, if IPT exists even at very low concentra-

tion, nitrous oxide reduction rate was decreased. IPT

and its transformation products were reported to inhibit

the activity of certain enzyme [21]. When the IPT

loading was stopped, the N2O production was decreased

to the same level of that of control reactor (Fig. 2),

indicating that the inhibition is reversible. Although the

mechanism of inhibition is not clear, it is considered that

the observed effect is probably due to non-competitive

inhibition of the enzyme involved in N2O reduction. The

yield of electron transfer can be reduced under such

inhibitory conditions. Further study will be needed

regarding the effect of the unutilized electrons on long-

term process performance.

Fig. 4 shows the influent and effluent concentration of

nitrate, nitrite and isoprothiolane at different applied

electric current as a function of time. As seen in the

figure, the effluent nitrate concentration varied depend-

ing on the electric current applied. The dotted line shows

the effluent nitrate predicted using a complete mixing

reactor model (Table 2). The calculated results for

effluent nitrate concentration at different electric current

are in close agreement with the experimental results.

Nitrite appeared in few cases at concentrations

p0.05mg/l, regardless of the variable electric current.

The effluent isoprothiolane concentration depended

on loading and no relation with electric current was

observed in the applied current range up to 10mA. The

removal efficiency of isoprothiolane in BER was very

small, particularly at lower influent concentrations. In

an attempt to identify the possibility of biological

degradation of isoprothiolane under denitrifying condi-

tion in the present study, the influent concentration was

increased up to 1200 mg/l, and maintained for about 40

days as shown in Fig. 4. During this period, the removal

efficiency of isoprothiolane was increased to about 33%.

However, the presence of metabolic products of

isoprothiolane could not be confirmed. Yamamoto and

Fukami [22] showed biological degradation of iso-

prothiolane under different conditions with the forma-

tion of different metabolic products. Recently,

Kishimoto et al. [23] reported biodegradation of

isoprothiolane in paddy field soils under aerobic and

anaerobic conditions. Their results also showed that

biological degradation rate of this pesticide was very

slow under anaerobic condition compared to that

observed under aerobic condition. Since the removal is

slow under anaerobic condition, combination of aerobic

process may enhance the decomposition of isoprothio-

lane. This can possibly be achieved either by direct

aeration of the adsorption bed or by producing oxygen

at the anode in BER. In this regard, recent study by

Sakakibara and Nakayama [24] indicated that in BER

equipped with multi-electrode system a highly oxidizing

zone can be developed around the anode, which could

possibly be used for aerobic degradation purpose. In the

electrochemical experiment using abiotic reactor (Table

1), no significant biological denitrification as well as

electrochemical decomposition of IPT were observed

under the condition used in this study.

Fig. 5 shows the observed and predicted nitrous oxide

production rate as a function of IPT loading in BER.

The trend of N2O production is almost a linear function

of IPT loading. These results indicate that to maintain

the complete reduction of nitrate to nitrogen gas,

pesticide loading should be limited.

3.2. BER/adsorption process performance

In a former study by Feleke et al. [16], it was shown

that IPT was effectively removed by adsorption columns

packed with GAC or silicon resine. The equilibrium

single-solute adsorption capacities obtained from batch

tests showed that the experimental data described well

with the Langmuir model for both adsorbents. The

maximum adsorption capacity ðqmÞ and adsorption

constant (b) were obtained at 12.05mg/g and 3.5

Table 3

Parameter values used for calculation

Parameter Value Unit

CNO�3f

1.715� 10�3 mol/l

CNO�2f

0 mol/l

CN2Of 9.51� 10�9 mol/l

CN2f 6.07� 10�4 mol/l

CN26.07� 10�4 mol/l

Ic 1.721 C/d/cm2

F 96487 C/mol

y 0.417 D

Ac 1224.39 cm2/l

Qg 1–20� 10�3 l/d

R 0.082 atm l/mol/K

T 293 K

V 0.205 L

KI 3.44� 10�5 mol/l�1


(mg/l)�1 for GAC and 0.47mg/g and 2.3 (mg/l)�1 for

silicon [16].

Fig. 6 shows the influent and effluent concentrations

of nitrate, nitrite and IPT of the combined BER/

adsorption process, in which additional treatment data

collected in continuous operation were included in the

former results reported by Feleke et al. [16]. The

calculation results were obtained using equations in

Table 2 and parameter values in Table 3. The effluent

IPT concentrations were calculated according to

Eq. (12) as a function of bed volumes of treated water,

where the adsorption beds were equally divided into 10

segments and a completely mixing condition was

assumed for each segment. The applied electric current

was 2.5mA which corresponds to the reduction of about

10mg-N/l achieved in many full-scale and pilot-scale

processes [9].

Z t

0

QðC0 � CeiÞ dt ¼ qmbCei

1þ bCei

Mi; ð12Þ

where Q is the flow rate (l/d), C0 is the feed IPT

concentration or the influent concentration of IPT to the

ith segment of the column (mg/l), Cei is the concentration

of IPT remaining in solution in the ith segment of the

column (mg/l), Mi is mass of adsorbent in the ith

segment of the packed bed (g) and t is time of operation

(d).

Fig. 3. Nitrogen mass balance for the three reactors operated at different IPT loading conditions.


Fig. 4. Performance of the BER (R3) at different electric current (Run II).

Fig. 5. Relationship between nitrous oxide production rate and isoprothiolane loading.


As shown in the figure, nitrate removal in the three

BERs is according to the complete mixing reactor

model. The effluent isoprothiolane concentration was

decreased below the detection limit of gas chromato-

graph (about 2mg/l) for the columns packed with

activated carbon and p6mg/l for the column packed

with silicone material. The breakthrough did not occur

for the three packed columns during the operation time

of 130 days, which corresponds to about 113 bed volume

of treated water.

One of the factors to keep low effluent concentration

of IPT before the breakthrough point is the adsorption

constant in Eq. (12), b, or the inversed parameter, 1/b.

The values of 1/b, which were 0.28 and 0.48mg/l for

GAC and silicon, respectively, are regarded as repre-

senting a degree of the affinity of adsorbent for IPT. The

lower the parameter 1/b, the higher the affinity is.

Higher affinity results in lower effluent concentration of

IPT. In this regard, a comparison of the values of 1/b

with the inhibition coefficient Ki(=10mg/l) in Eq. (10)

reveals that the combination of GAC or silicone resin

with BER is a promising process configuration in order

to reduce the inhibition effectively, because the values of

1=b were roughly about 20–40 times smaller than Ki:

4. Conclusions

The treatment performance of a coupled bio-electro-

chemical/adsorption process for denitrification and

pesticide removal was demonstrated and a mathematical

model to evaluate the performance was developed. In

BER, the disappearance of nitrate was stable at variable

electric current and isoprothiolane (IPT) loading condi-

tions. Nitrite was not accumulated, but nitrous oxide

production was increased with increasing isoprothiolane

loading, meaning that the reduction step of N2O to N2

was inhibited by IPT. The effluent concentration

of IPT from column packed either with GAC or

silicon material was low enough to meet the guideline

value (40 mg/l) and, at the same time, to reduce the

inhibition effectively, because the inversed adsorption

constant (1/b) in Eq. (12) is roughly about 20–40 times

smaller than the inhibition coefficient (Ki) in Eq. (10).

Furthermore, the experimental results were in good

agreement with the calculation results for nitrate, nitrite,

N2O, N2, and IPT in the effluent. In conclusion, it can

be said that the combined BER/adsorption process

proposed in this study is promising process to remove

nitrate and IPT.

Fig. 6. Performance of the combined BER/adsorption process (Run III).


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