Pesticide removals in the nitrifying expanded-bed filter at drinking water treatment plant

Journal of Science and Technology in Civil Engineering, NUCE 2021

PESTICIDE REMOVALS IN THE NITRIFYING

EXPANDED-BED FILTER AT DRINKING WATER

TREATMENT PLANT

Nguyet Thi-Minh Dao^a,b, The-Anh Nguyen^c, Viet-Anh Nguyen^b,

Mitsuharu Terashima^a, Hidenari Yasui^a,∗

^aFaculty of Environmental Engineering, The University of Kitakyushu,

1-1 Hibikino, Wakamatsu, Kitakyushu City, Fukuoka, Japan

^bInstitute of Environmental Science and Engineering, National University of Civil Engineering,

55 Giai Phong road, Hai Ba Trung district, Hanoi, Vietnam

^cFaculty of Water Resources Engineering, Thuy Loi University,

175 Tay Son street, Dong Da district, Hanoi, Vietnam

Article history:

Received 12/11/2020, Revised 10/12/2020, Accepted 11/12/2020

Abstract

The occurrence of pesticides even at low concentrations in drinking water sources might induce potential risks

to public health. This study aimed to investigate the removal mechanisms of eight pesticides by the nitrifying

expanded-bed filter using biological activated carbon media at the pretreatment of a drinking water plant. The

field analysis demonstrated that four pesticides Flutolanil, Buprofezin, Chlorpyrifos, and Fenobucard, were re-

moved at 82%, 55%, 54%, and 52% respectively, while others were not significantly removed. Under controlled

laboratory conditions with continuous and batch experiments, the adsorption onto the biological activated car-

bon media was demonstrated to be the main removal pathway of the pesticides. The contribution of microor-

ganisms to the pesticide removals was rather limited. The pesticide removals observed in the field reactor was

speculated to be the adsorption on the suspended solids presented in the influent water. The obtained results

highlighted the need to apply a more efficient and cost-effective technology to remove the pesticide in the

drinking water treatment process.

Keywords: biological activated carbon; drinking water treatment; nitrifying expanded-bed filter; pesticide re-

moval.

1. Introduction

In Vietnam and worldwide, intensive cultivation and increasing application rates of fertilizers,

pesticides, herbicides, and other related crop protection products are being practiced to meet the

growing food demand and assure food security. Accordingly, the degradations of surface and ground-

water, soil, and air quality were observed due to the release of surplus pesticides and herbicides to the

environment. The occurrence of pesticides was recorded in the river and groundwater in Hanoi, Hai

Phong, Da Nang and Ho Chi Minh Cities [1 ], as well as in private and public drinking water sources

^∗Corresponding author. E-mail address: hidenari-yasui@kitakyu-u.ac.jp (Yasui, H.)

1

Dao, T.-M. N., et al. / Journal of Science and Technology in Civil Engineering

in the Mekong Delta in Vietnam [2]; however, their concentrations were lower than the Vietnamese

drinking water standard.

Scientific evidence has pointed out the public health threats due to the combination of xenobiotic

chemicals, even at low concentrations, or the links between organochlorine residues and cancers [3].

Therefore, regulations and guidelines take effect at national and international levels to monitor and

control the occurrence and thresholds of emerging chemicals in drinking water. In 2017, the World

Health Organization (WHO) had provided the guideline values for 31 pesticides presented in drink-

ing water, which were of health significance [4]. In Europe, a proposal for a revised drinking water

directive has been adopted since 2018, in which a maximum concentration of 0.1 µg/L for individual

pesticides and 0.5 µg/L for total pesticides were regulated [5]. In the United States, the Environ-

mental Protection Agency has identified 18 types of pesticides and herbicides with their maximum

contaminant levels in the National Primary Drinking Water Regulations [6]. In Vietnam, the Min-

istry of Health has recently listed 27 pesticides with their maximum limits in the National Technical

Regulation on Drinking Water Quality (QCVN01-1:2018/BTY) [7].

The conventional drinking water treatment process alone, which includes coagulation/flocculation,

sedimentation, and filtration, could partly remove the pesticides [8]. Available technologies to remove

such persistent chemicals from drinking water sources are chemical oxidation, chemical precipita-

tion, membrane, activated carbon adsorption and/or biofilter. Traditionally, granular activated carbon

(GAC) was effectively used to adsorb the pesticides and other organic pollutants due to its irregu-

lar crevices and porous particle shape that bind those specific contaminants [9]. Thuy et al. (2013)

demonstrated that pesticides (Chlorpyrifos, Diazinon, and Carbofuran) could be adsorbed on the low-

cost GAC generated from local products (bamboo and coconut shell) at the pilot-scale upgraded from

a typical drinking water treatment plant (WTP) in Vietnam [8]. However, a major set-back pertains to

GAC saturation overtime when all of its available adsorption sites could not bound with either organic

matters and/or microorganisms [9]. Consequently, frequent regeneration or continuous dosing of GAC

are needed to renew the adsorption capacity [10, 11]. The biological activated carbon (BAC) process

has received much attention to overcome such limitations [9 –13]. This process uses GAC as filtration

media to physically remove undesired matters. As the GAC media is gradually exhausted, micro-

bial colonization is developed on the surface of the media [9 ]. This naturally occurring biofilm can

biodegrade a wide variety of contaminants such as organic carbon, organic/inorganic substances, and

disinfection by-products precursors [9, 12, 14]. In this way, the service life of the BAC bed could be

extended from 6-12 months to several years [9, 15]. The pesticide removals using BAC process were

reported in [10, 16–18], in which the removal mechanism was thought to be simultaneous adsorption

and biodegradation. The biotransformation of persistent compounds such as pesticides at trace level is

possibly due to the cometabolism, in which pesticides might be biodegraded by non-specific enzymes

generated by the primary substrate metabolism [19]. Some parameters such as the type of activated

carbon, hydraulic retention time, and backwashing regime on the performance of BAC process [20];

however, a critical role of ammonia-oxidizing bacteria (AOB) in the enhancement of micropollutants

removal in nitrifying activated sludge of wastewater treatment [21 –24 ] was focused in this study. Nev-

ertheless, there is still questionable if the nitrifying biofilms could remove the pesticides in drinking

water treatment.

In addition to the pesticide concurrence, the presence of ammonium (NH_x-N), which is the total

of NH₄-N and free NH₃, in the surface water is of major concern for the operator of WTPs. Nitri-

fying expanded-bed filter using BAC media is widely used at the pretreatment of drinking water in

Japan and recently installed at the pilot and full-scale in several WTPs in Vietnam to remove NH_x-

2

Dao, T.-M. N., et al. / Journal of Science and Technology in Civil Engineering

N, dissolved manganese, and organic substances [25]. Because the process focuses on the biological

activities rather than the adsorption of media, the spent GAC could be used without regeneration or

frequent adding virgin adsorbent, making this process a very cost-effective option. In addition to the

treatment efficiency for NH_x-N and organics that were previously demonstrated [14], this study aimed

to investigate the possibility of degrading the pesticides of the BAC process. Based on the promising

results in the full-scale reactor receiving river water, a lab-scale reactor was installed to study the

pesticide removal mechanism and the contribution of nitrifying bacteria to the removal efficiency.

2. Materials and methods

2.1. Full-scale and lab-scale nitrifying expanded-bed biofilter

A full-scale expanded-bed reactor filled with GAC (average diameter 0.4-0.5 mm, uniformity

coefficient 1.7) was installed in 2015 as the pretreatment process of Vinh Bao WTP in Hai Phong,

Vietnam, as shown in Fig. 1. The packed-bed was of dimension (W × L × H = 2.6 × 2.6 × 1.5 (m)).

The reactor was continuously operated under the designed hydraulic loading of 15.5 m³/m²/h. The

packed-bed height of 1.5 m was fluidized at the expanded-bed height of 2 m, showing a volume of

13.5 m³for the expanded-bed media. The raw water was taken from Chanh Duong River, a branch of

Luoc River, then transported through an irrigation and drainage channel before feeding to the reactor.

The water intake quality and regime were largely affected by agricultural activities. An intensive

monitoring campaign was carried out during the peak period of the rainy season (from 3 to 10 August

2018) to investigate the pesticide removal efficiency of the reactor.

Figure 1. Experimental setup for full- and lab-scale nitrifying expanded-bed reactor

A fluidized-bed column with a cross-section area of 85 cm²was set up in the laboratory at the

University of Kitakyushu, Japan. BAC media was taken from an operating nitrifying expanded-bed

reactor at Anou Water Treatment Plant in Kitakyushu, Japan, and immediately transferred to the lab-

scale reactor. The column was operated at the similar hydraulic working conditions of 15.5 m³/m²/h,

in which the packed-bed height of 1.5 m was also expanded to 2 m, showing a volume of 17 L for the

expanded-bed media. The hydraulic regime of the lab-scale reactor was assumed to be identical to the

full-scale reactor.

3

Dao, T.-M. N., et al. / Journal of Science and Technology in Civil Engineering

In the plant design, the hydraulic loading of the reactor was almost fixed because the linear ve-

locity of the fluidized-bed was almost stable to maintain the bed expansion while the height or water

level was also fixed as water flowed by gravity from the intake to the used point. Hence, the designed

hydraulic loading of 15.5 m³/m²/h was kept in the monitorings of both full- and lab-scale reactors.

2.2. Studied pesticides

In this research, eight pesticides from different classes that are frequently used in a wide range of

crops in Vietnam were selected for monitoring. The occurrence of these pesticides was reported in

[1] in Chanh Duong River in 2011. Among them, two pesticides Atrazine and Chlorpyrifos are listed

in the Vietnamese National technical regulation on Domestic Water Quality (QCVN 01-1:2018/BYT)

for allowable concentrations of 100 and 30 µg/L, respectively. The list of targeted pesticides and some

of their physical properties were presented in Table 1.

Table 1. Some physical characteristics of targeted pesticides and maximum allowable limits

in drinking water regulations

Molecular

weight

(g/mol)

Octanol-Water

Maximum

No

Pesticide

Class

Chemical formula

Partition Coef_♣ficient allowable limit

log K_ow

(µg/L)

1

2

3

4

5

6

7

8

Atrazine

Fenobucard

Flutolanil

Isoprothiolane

Chlorpyrifos

Fipronil

1,3,5-triazine

Carbamate

Carboxamide

Dithiolane

Organophosphate

Fiprole

C₈H₁₄ClN₅

C₁₂H₁₇NO₂

C₁₇H₁₆F₃NO₂

C₁₂H₁₈O₄S₂

C₉H₁₁Cl₃NO₃PS

C₁₂H₄Cl₂F₆N₄OS

C₁₉H₁₇ClN₄

215.69

207.27

323.31

290.39

350.57

437.14

336.82

305.44

2.5

2.79

3.7

3.3

4.7

4

100^1,2, 3³, 0.1⁴

0.1⁴

30^1,2, 0.1⁴

0.1⁴

Fenbuconazole

Buprofezin

Triazole

Unclassified

3.23

4.3

0.1⁴

C₁₆H₂₃N₃SO

0.1⁴

^♣: The Pesticide Manual: a world compendium [26].

¹: National technical regulation on Domestic Water Quality (QCVN01-1:2018/BTY), Ministry of Health, Vietnam.

²: Guidelines for Drinking-water Quality, WHO (2017).

³: National Primary Drinking Water Regulation, EPA (2009).

⁴: Proposal for a revised drinking water directive, European Commission (2018): maximum concentration of 0.1 µg/L for

individual pesticide and of 0.5 µg/L for total pesticides.

2.3. Continous and batch experiments on pesticide removal mechanisms

As for the full-scale reactor in the field study of 7 days, 18 pairs of influent and effluent water

samples (ID #1 to #18) were taken for pesticide analysis at AM 8:00, AM 12:00, and PM 16:00 every

day, aiming to investigate the representative samples in different moments of a day. The organic and

nitrogenous pollutions were also investigated by monitoring the dissolved oxygen (DO) and NH_x-N

in the influent and effluent of the nitrifying expanded-bed filter.

In the laboratory, a lab-scale reactor was continuously fed with synthetic influent water composed

of 1.0 mgN/L as ammonium nitrogen, 0.5 mgP/L as phosphate, 3-5 mg DOC/L mixed from acetone

and ethanol, pesticides solution at the range of ng/L, and dechlorinated tap water in 2 months. The

influent water was mimic the river water quality observed in the field monitoring for nitrogen and

organic substrates, with a supplement of phosphorus nutrient for biomass growth. The pesticide stock

solution was prepared in acetone and kept in an amber glass bottle at room temperature. The influent

and effluent samples were collected every 2 to 3 days for pesticide analysis. Similarly, DO and NH_x-N

4

Dao, T.-M. N., et al. / Journal of Science and Technology in Civil Engineering

were also monitored to evaluate the biomass activity in the reactor. The phosphorus nutrient in the

reactor was not checked, while the DOC was occasionally measured to see the growth of heterotrophs;

however the accuracy was rather limited at this low range of concentration. The pH in the influent was

sometimes checked, and a value around 7.0 was often observed. When the DO was stably consumed

in the reactor, suggesting a development of the biomass on the BAC granules, a series of batch ex-

periments were carried out to examine the pesticide removal mechanisms. The first batch experiment

aimed to estimate the contribution of microbial activity to the pesticide removal using a microbial

inhibitor of sodium azide. A series of Erlenmeyer flasks of 500 mL were filled with a solution similar

to the influent in the continuous experiment. Fresh BAC was taken from the lab-scale reactor and

washed by deionized tap water several times to remove the remaining substrates. The wet BAC was

placed on a paper to remove the water attached to the surface, then weighted 10 mg before putting it

in each mesh bag. In the first group, where the microbial activity was promoted, the BAC bags were

placed into the flasks, and oxygen gas was continuously injected. In the second group, which inhibited

the biomass, together with the BAG bags, 500 mg/L of sodium azide was added in, and nitrogen gas

was continuously purged. The third group of control flasks was designed as the inhibited flasks with-

out the presence of BAC. All the flasks were operated under gentle stirring. The samples were taken

for pesticides and NH_x-N analysis at the starting time in the controlled flasks, and after 1, 2, 3 and 4

hours of the experiment for the other flasks. The first batch experiment was carried out in triplicate.

The second batch experiment aimed at investigating the association of the nitrification reaction rate to

the pesticide removals. While the controlled flasks were designed exactly as mentioned above, in two

groups of experimental flasks, the NH_x-N concentrations were varied at 1 and 5 mgN/L and oxygen

gas was continuously injected. Rather than changing the NH_x-N as above, the third batch experiment

aimed at estimating the impact of biomass concentration to the pesticide removals by using different

BAC amounts of 10 mg and 30 mg. The statistical analysis was carried out for the data obtained from

the triplicated batch experiments using a programming language R developed by the R Core team

(CRAN project) [27].

2.4. Analytical procedures

As for the pesticides, the glass sampling bottles were prewashed by acetone, purified water, and

the water samples. After collecting, 500 mL of water sample was filtered by glass-fiber filters (What-

man, 47 mm, Grade GF/F, prewashed with purified water, and dried at 105 °C in 3 hours). The filtrate

was passed through a PS-2 Sep-Pak cartridge with a pore size of 80 µm (Waters Associates, USA, pre-

conditioned with acetone, methanol, and purified water) at the flow rate of 10 mL/min, then finished

by adding 20 mL of purified water. The cartridges were stored at 4 °C and delivered to the labora-

tory for further analysis. The cartridges were dried by air for 1 hour. They were eluted by 5 mL of

acetone, then concentrated to 1 mL under a gentle nitrogen stream. Next, 5 mL of hexane was added,

and the concentration continued until the eluate volume was reduced to 0.9 mL. Finally, the eluates

were spiked with 100 µL of internal standards solution (10 µg/mL, Sigma-Aldrich, Japan) before be-

ing analyzed using the Gas Chromatography-Mass Spectrometry (GC-MS, QP-2100 Plus, Shimadzu,

Japan). The measurement conditions of GC-MS can be referred to in [28]. As for the continuous and

batch experiments of the lab-scale reactor, the pesticide analysis was carried out with the same pro-

cedure as mentioned above. Duplicated samples were taken, and the average values were reported for

data analysis.

Regarding other parameters, in Vinh Bao WTP, DO was continuously recorded every 15 minutes

using optical DO sensors (FDO 925-P probes and Multimeter 3430, WTW, Germany). NH_x-N was

5

Dao, T.-M. N., et al. / Journal of Science and Technology in Civil Engineering

occasionally measured using reflectometric with test strips (RQflex 10, Merck, Germany) with the

measuring ranges of 0.16–5.4 mg NH_x-N/L. In the laboratory, the DO was measured every 2-3 days

using the same materials as mentioned above, while NH_x-N was analyzed according to #4500-NH₃F

in Standard Methods [29].

3. Results and Discussions

3.1. Pesticide removals in the full-scale reactor

In the whole campaign, the water quality was degraded during the first 3 days and then signifi-

cantly improved in the last 4 days due to the release of freshwater from Luoc River. As shown in Fig. 2,

the average influent DO was found to be 5.50 (SD = 0.56) mgO₂/L, while the NH_x-N concentration

was relatively high at around 4.27 (SD = 0.94) mgN/L between day 0 and day 3, and then dropped

to about 1.58 (SD = 1.28) mgN/L in the remaining day. The biological activities of the BAC bed in

this monitoring campaign was mathematically modelled and described in a published paper, in which

about 0.76 (SD = 0.38) mgN/L and 2.81 (SD = 1.51) mgCOD/L were removed in the first 3 days

due to limited influent DO and almost removed in the last 4 days when DO was sufficient [14]. The

nitrifying expanded-bed filter could remove some pesticides to a certain extent, as indicated in Fig. 3.

Four pesticides, which were Flutolanil, Buprofezin, Chlorpyrifos and Fenobucard were removed at

82% (SD = 6.03), 55% (14.50), 54% (SD = 18.31), and 52% (SD = 12.77), respectively. The other

pesticides were not considerably removed at removal rates lower than 50%. Comparing to those reg-

ulated in Vietnamese drinking water standard, the concentrations of Atrazine and Chlorpyrifos were

lower than the threshold values. However, the pesticides Atrazine, Fenbuconazole and Isoprothiolane

were found at elevated concentrations of 1,000 ng/L, 6,000 and 400 ng/L respectively, which were

far higher than their reported values in Chanh Duong River in 2011 [1]. Hence, prevention measures

should be taken to protect this specific drinking water source.

(a) Dissolved oxygen

(b) Ammonium

Figure 2. Influent and effluent dissolved oxygen and ammonium in the full-scale reactor in Vinh Bao WTP

(red circle = influent, white triangle = effluent)

In the full-scale reactor, it was unclear if the pesticides were removed due to adsorption or

biodegradation. As adsorption is principally an exchange process, it is largely influenced by the

physicochemical characteristics of both adsorbents which was BAC and the adsorbates, which were

the pesticides. As for the BAC bed, there were opposing viewpoints related to its adsorption capacity

over time. Some researchers believed that the physical adsorption capacity of BAC would be exhausted

6

Dao, T.-M. N., et al. / Journal of Science and Technology in Civil Engineering

Figure 3. Influent and effluent pesticides and removal efficiencies in the full-scale reactor in the rainy season

in Vinh Bao WTP (red circle = influent, white triangle = effluent, bar = removal efficiency)

after 2 to 3 months [15], or after six months of operation [9]. However, others proved that adsorption

in BAC was still partly responsible for micro-pollutants removal after four years of filtration [10]. In

this study, the life-service of the BAC bed reached more than 6 years at the time of the experiment,

and it was unable to tell if there were still available adsorption sites on the BAC surface. For the pes-

ticides, it was reported that the adsorption capacity could be enhanced in case (i) the small molecular

size or low molecular weight, (ii) hydrophobic molecule, expressed by high Octanol-water distribu-

tion coefficient (log K_ow) and (iii) the electrostatic interactions with the GAC surface [20]. As shown

in Table 1, all the pesticides could be considered as hydrophobic (log K_ow> 2). However, there was

no correlation between the removals of the pesticides and the log K_ow. A similar lack of correlation

between the physicochemical properties of the pesticides and their adsorption onto GAC was previ-

ously reported at a very diluted concentration (ng/L) [10 ]. Therefore, it was difficult to evaluate the

adsorption of those pesticides based on the obtained results in this study.

In the dynamic simulation of the biological activities in the reactor, the available DO was rec-

ognized as a critical factor controlling the activities of nitrifiers and heterotrophs in the biofilm

[14]. However, no correlation could be drawn between the pesticide removals and DO consump-

tion. As related to the micropollutants, previous studies have observed higher reductions when the

nitrifying activated sludge system worked at a higher nitrogen loading rate (> 1 gNH_x-N/gVSS.d)

[21, 22]. Because the micropollutants often presented at trace level, which was insufficient to sustain

the biomass growth, their removals were probably due to the action of ammonium monooxygenase

enzyme through the cometabolism of the main substrates [22]. In this study, no correlation was found

between the pesticide removals and the influent NH_x-N. Even though the role of nitrifiers could not

be confirmed, it should be noted that the reactor was operated at a much-lowered nitrogen rate com-

pared to those reported in wastewater treatment. Furthermore, as indicated in the simulation [14],

the activity of nitrifiers was rather limited due to the dominance of the heterotrophs in the biofilm

at limited DO conditions. Therefore, further studies should be conducted under laboratory-controlled

conditions to reveal the degradation pathway of pesticides in the nitrifying expanded-bed filter.

Another factor might explain the pesticide removals in the field study. As demonstrated in [30 ],

the pesticides might adsorb on the suspended solids (e.g. clay particles) and colloids presented in the

river water. In this study, the suspended solids concentration were found around 54.0 (SD = 13.22)

7

Dao, T.-M. N., et al. / Journal of Science and Technology in Civil Engineering

mg/L during the monitoring campaign [14]. All the four pesticides with removal rates higher than

50% also have high values of log K_ow, highlighting the possibility of adsorption onto the surface of

suspended solids while no more accessible adsorption sites could be offered from the BAC bed.

3.2. Pesticide removals in the lab-scale reactor

a. Continuous experiment

As mentioned above, there was biomass developed on the used BAC at the start up period in the

lab-scale experiment. As shown in Fig. 4, there was about 5 mgO₂/L consumed at day 0 and a small

amount of NH_x-N was removed. After 10 days of acclimation, the influent NH_x-N of 1.11 (SD = 0.13)

mgN/L was removed entirely, and about 7.81 (SD = 0.26) mgO₂/L was stably consumed. Because the

DO consumption for nitrification was stoichiometrically 4.57 mgO₂/L for 1 mgN, it was believed that

both nitrifiers and heterotrophs bacteria were stably settled onto the BAC media. Regarding the pesti-

cides, a very high removal was observed during the first several days. However, as shown in Fig. 5, the

removal efficiencies were gradually decreased. After 66 days of the experiment, most of the pesticide

(a) Dissolved oxygen

(b) Ammonium

Figure 4. Influent and effluent dissolved oxygen and ammonium in the lab-scale reactor

(red circle = influent, white triangle = effluent)

Figure 5. Influent and effluent pesticides and removal efficiencies in the lab-scale reactor

(red circle = influent, white triangle = effluent, bar = removal efficiency)

8

Dao, T.-M. N., et al. / Journal of Science and Technology in Civil Engineering

could not be removed or having removal rates lower than 20%. In the same period, better removal

rates of around 40% were observed for Fenbuconazole and Buprofezin. Regarding the increasing

trend of the effluent pesticides, it was speculated that the pesticide removals of Fenbuconazole and

Buprofezin would reduce further as the experiment continued. While the activities of microorganisms

were confirmed, the decreasing removal of the pesticides suggested that their removals might result

from the adsorption rather than biodegradation. It was likely that there were very limited adsorption

sites on the BAC media after 2 months of the continuous experiment. To study the pesticide removal

mechanisms, a series of batch experiments were carried out to evaluate the contribution of biomass

to pesticide degradation.

b. Batch experiments: Contribution of biomass to pesticide removals

In the first batch experiment, 500 mg/L NaN₃successfully inhibited the biomass activities, as

shown in Fig. 6(a) where the influent NH_x-N was kept almost constant during 4h of the experiment

in control and inhibited flasks. In the non-inhibited flasks, as the DO was continuously purged in, the

influent of 1 mg/L of NH_x-N was removed entirely at the end of the batch experiment. As indicated in

Fig. 7, most of the pesticides showed no significant difference (p-value < 0.05) between the control,

(a) Inhibition of bacteria

(b) Variation of NH_x-N

(c) Variation of BAC weight

Figure 6. Ammonium concentrations in the batch experiments

Figure 7. Influent and effluent pesticides and removal efficiencies in the lab-scale reactor (red circle =

influent, white triangle = effluent, reported data = average value of triplicate measurements)

9

Dao, T.-M. N., et al. / Journal of Science and Technology in Civil Engineering

inhibited, and non-inhibited flasks in triplicated batch tests (Table 2). Further, the variation of pes-

ticide concentrations in time were minor, except for Chlorpyrifos and Buprofezin. The results were

consistent with those observed in the continuous experiment, confirming that the bacteria can not

degrade the targeted pesticides and the removals would stop when the BAC media were saturated. As

for Chlorpyrifos and Buprofezin, their concentrations were decreased in time, as proven by the high-

lighted p-values. However, higher degrees of decline was observed when BAC media were present.

It was noticeable that the log K_owwere 4.7 and 4.3 for Chlorpyrifos and Buprofezin, which are the

highest values among those of targeted pesticides, suggesting the possibility of their adsorption onto

the BAC media.

Table 2. Statistical analysis for pesticide concentrations in a triplicated batch experiment

p value

No

Pesticide

1h

2h

3h

4h

1

2

3

4

5

6

7

8

Atrazine

Fenobucard

Flutolanil

Isoprothiolane

Chlorpyrifos

Fipronil

0.11

0.19

0.14

0.09

0.13

0.11

0.12

0.59

0.22

0.67

0.70

0.16

0.51

0.63

0.07

0.89

0.82

0.09

0.10

0.06

0.49

0.48

0.01

0.45

0.62

0.17

0.11

0.02

0.32

0.01

0.001

Fenbuconazole

Buprofezin

Figure 8. Pesticide ratios after 4 hours of batch experiment with variations of ammonia and biological

activated carbon weight (C_tand C₀: concentrations of experimental and control flasks at 4h)

In the second and the third batch experiments, the nitrifiers activities were improved when the

influent NH_x-N was increased from 1 to 5 mgN/L, or the weight of BAC media was increased from

10 to 30 mgBAC, as seen in Fig. 6(b) and Fig. 6(c). However, neither changing the nitrification

rates nor the biomass concentration induced any considerable influence on the pesticide removals.

As shown in Fig. 8, after 4 hours of the batch experiments, the pesticide concentrations were not

significantly different in the control and experimental flasks. The observed maximum differences

10

Dao, T.-M. N., et al. / Journal of Science and Technology in Civil Engineering

were around 20% for Flutolanil, Isoprothiolane, and Fenbuconazole. While the role of nitrifiers in

nitrifying activated sludge system was confirmed regarding the pesticide degradation [21 –24], their

insignificant contribution to the pesticide removals observed in this study might due to the different

biomass composition in oligotrophic biofilm compared to those in wastewater treatment. Again, only

Chlorpyrifos and Buprofezin showed a consistent decrease of concentrations in time. As for the others,

the slight variations of concentration might be due to the analytical errors and instrument sensitivity,

which were acceptable at these ranges of concentration.

From the batch experiments, the pesticide removals in the nitrifying expanded-bed reactor using

BAC media was revealed. The adsorption was thought to be the main removal pathway of the pesti-

cides. The used BAC media showed good pesticide removal efficiencies at the first period; however,

most of the removals were gradually decreased to lower than 20% after 2 months of continuous op-

eration. Although the nitrifiers were effective in removing the NH_x-N, their contribution to pesticide

removals was rather limited. The removals of some pesticides observed in the field reactor might

result from the adsorption on the suspended solids presented in the influent water

4. Conclusions

In this study, the pesticide occurrence and removals were monitored in a full-scale nitrifying

expanded-bed reactor using BAC media. Although the recorded concentrations might not be a concern

for the WTP operators following current Vietnamese regulation, detailed monitoring programs on

pesticides would be needed to protect the supply source from these pollutants. The full-scale reactor

receiving river water could remove four pesticides, which were Flutolanil, Buprofezin, Chlorpyrifos,

and Fenobucard at removal rates of 82%, 55%, 54%, and 52%, respectively.

While previous studies on wastewater treatment have reported a positive association between ni-

trification rates and pesticide removals, this research successfully demonstrated that the nitrifying

expanded-bed reactor using BAC media was not effective in removing the pesticides in drinking wa-

ter treatment. In the controlled laboratory conditions, the batch experiments using microbial inhibitors

or changing the nitrification rates and biomass concentrations showed insignificant differences in the

removals of trace pesticides. Consequently, the adsorption onto BAC media was the main removal

pathway for pesticides, which was quickly saturated after 2 months of operation. The obtained re-

sults highlighted the need to apply a more efficient and cost-effective technology targeting pesticide

removals in drinking water treatment.

Acknowledgements

This work was supported by JSPS Grants-in-Aid for Scientific Research (18K11739, 19H01160,

and 19H02278), Japan.

References

[1] Hanh, D. T. (2015). Occurrence of organic micro-pollutants in the aquatic environment in Vietnam.

Doctoral Thesis, the University of Kitakyushu, Japan.

[2] Chau, N. D. G., Sebesvari, Z., Amelung, W., Renaud, F. G. (2015). Pesticide pollution of multiple drinking

water sources in the Mekong Delta, Vietnam: evidence from two provinces. Environmental Science and

Pollution Research, 22(12):9042–9058.

[3] Carvalho, F. P. (2006). Agriculture, pesticides, food security and food safety. Environmental Science &

Policy, 9(7-8):685–692.

11

Dao, T.-M. N., et al. / Journal of Science and Technology in Civil Engineering

[4] World Health Organization (2017). Guidelines for Drinking-water Quality.

[5] European Commission (2018). Climate Change 2013 - The Physical Science Basis. Summary for Policy-

makers. In Intergovernmental Panel on Climate Change (Ed.), pp. 1–30.

[6] US Environmental Protection Agency (2009). Drinking Water Contaminants. National Primary Drinking

Water Regulations.

[7] QCVN 01-1:2018/BYT (2019). National technical regulation on Domestic Water Quality. Ministry of

Health, Vietnam.

[8] Pham, T. T., Nguyen, V. A., der Bruggen, B. V. (2013). Pilot-Scale Evaluation of GAC Adsorption Using

Low-Cost, High-Performance Materials for Removal of Pesticides and Organic Matter in Drinking Water

Production. Journal of Environmental Engineering, 139(7):958–965.

[9] Simpson, D. R. (2008). Biofilm processes in biologically active carbon water purification. Water Re-

search, 42(12):2839–2848.

[10] Rattier, M., Reungoat, J., Gernjak, W., Joss, A., Keller, J. (2012). Investigating the role of adsorption and

biodegradation in the removal of organic micropollutants during biological activated carbon filtration of

treated wastewater. Journal of Water Reuse and Desalination, 2(3):127–139.

[11] der Bruggen, B. V., Everaert, K., Wilms, D., Vandecasteele, C. (2001). The use of nanofiltration for the

removal of pesticides from groundwater: an evaluation. Water Supply, 1(2):99–106.

[12] Korotta-Gamage, S. M., Sathasivan, A. (2017). A review: Potential and challenges of biologically acti-

vated carbon to remove natural organic matter in drinking water purification process. Chemosphere, 167:

120–138.

[13] Alvarino, T., Suarez, S., Lema, J., Omil, F. (2018). Understanding the sorption and biotransformation of

organic micropollutants in innovative biological wastewater treatment technologies. Science of the Total

Environment, 615:297–306.

[14] Dao, N. T.-M., Nguyen, T.-A., Nguyen, V.-A., Terashima, M., Goel, R., Yasui, H. (2020). A mathe-

matical model of a nitrifying expanded-bed reactor for the pretreatment of drinking water. Biochemical

Engineering Journal, 158:107561.

[15] Servais, P., Billen, G., Bouillot, P. (1994). Biological colonization of granular activated carbon filters in

drinking-water treatment. Journal of Environmental Engineering, 120(4):888–899.

[16] Herzberg, M., Dosoretz, C. G., Tarre, S., Green, M. (2003). Patchy Biofilm Coverage Can Explain the

Potential Advantage of BGAC Reactors. Environmental Science & Technology, 37(18):4274–4280.

[17] van der Aa, L. T. J., Kolpa, R. J., Rietveld, L. C., van Dijk, J. C. (2012). Improved removal of pesticides in

biological granular activated carbon filters by pre-oxidation of natural organic matter. Journal of Water

Supply: Research and Technology-Aqua, 61(3):153–163.

[18] Liang, C.-H., Chiang, P.-C., Chang, E.-E. (2007). Modeling the behaviors of adsorption and biodegrada-

tion in biological activated carbon filters. Water Research, 41(15):3241–3250.

[19] Zearley, T. L., Summers, R. S. (2012). Removal of Trace Organic Micropollutants by Drinking Water

Biological Filters. Environmental Science & Technology, 46(17):9412–9419.

[20] Rattier, M., Reungoat, J., Gernjak, W., Keller, J. (2012). Organic micropollutant removal by biological

activated carbon filtration: a review. In Urban Water Security Research Alliance Technical Report.

[21] Tran, N. H., Urase, T., Kusakabe, O. (2009). The characteristics of enriched nitrifier culture in the degra-

dation of selected pharmaceutically active compounds. Journal of Hazardous Materials, 171(1-3):1051–

1057.

[22] Fernandez-Fontaina, E., Omil, F., Lema, J. M., Carballa, M. (2012). Influence of nitrifying conditions on

the biodegradation and sorption of emerging micropollutants. Water Research, 46(16):5434–5444.

[23] Rattier, M., Reungoat, J., Keller, J., Gernjak, W. (2014). Removal of micropollutants during tertiary

wastewater treatment by biofiltration: Role of nitrifiers and removal mechanisms. Water Research, 54:

89–99.

[24] Park, J., Yamashita, N., Wu, G., Tanaka, H. (2017). Removal of pharmaceuticals and personal care prod-

ucts by ammonia oxidizing bacteria acclimated in a membrane bioreactor: Contributions of cometabolism

and endogenous respiration. Science of The Total Environment, 605-606:18–25.

[25] Nakamoto, N. (2014). Progress in Slow Sand and Alternative Biofiltration Processes. Water Intelligence

12

Dao, T.-M. N., et al. / Journal of Science and Technology in Civil Engineering

Online, 13.

[26] Tomlin, C. D. S. (2013). The pesticide manual: A world compendium. Thirteen edition, British Crop

Production Council.

[27] Team, R. C. (2020). R: A Language and Environment for Statistical Computing. Retrieved from

https://www.r-project.org.

[28] Duong, H. T., Kadokami, K., Pan, S., Matsuura, N., Nguyen, T. Q. (2014). Screening and analysis of 940

organic micro-pollutants in river sediments in Vietnam using an automated identification and quantifica-

tion database system for GC–MS. Chemosphere, 107:462–472.

[29] APHA/AWWA/WEF (2017). Standard Methods for the Examination of Water and Wastewater. In Stan-

dard Methods (23th ed.). American Public Health Association, American Water Works Association, Water

Environment Federation.

[30] Pham, T. T. (2012). Assessment of Low Cost - High Performance Adsorbents for Safe Drinking Water

Production from Polluted Surface Water. Application in Northern Vietnam. Doctoral Thesis, KU Leuven,

Belgium.

13