Chemical Engineering Journal 243 (2014) 7–13

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Chemical Engineering Journal

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Ballast water treatment using UV/TiO2 advanced oxidation processes:An approach to invasive species prevention

1385-8947/$ - see front matter � 2014 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.cej.2013.12.082

⇑ Corresponding author. Tel.: +86 10 58112456; fax: +86 10 58112842.E-mail address: hukf@ccs.org.cn (K. Hu).

Nahui Zhang a, Kefeng Hu b,⇑, Baohua Shan c

a Marine Engineering, Dalian Maritime University, 1 Linghai Road, Dalian, Liaoning 116026, Chinab China Classification Society, 9 Dongzhimen Nan Da Jie, Beijing 100007, Chinac Ahead Ocean Technology Co., Ltd., 6 Caishi Street, Dalian, Liaoning 116026, China

h i g h l i g h t s

�We investigated the UV/TiO2 process for ballast water treatment.� UV/TiO2 treatment had the potential to inhibit plankton regrowth.� The advantages of UV/TiO2 at low UV dose (i.e., 75%) were much greater.� The concentrations of disinfection by-products formed during UV/TiO2 treatment were very low.

a r t i c l e i n f o

Article history:Received 10 September 2013Received in revised form 24 December 2013Accepted 26 December 2013Available online 4 January 2014

Keywords:Ballast waterUV/TiO2 processDisinfection by-productsHydroxyl radicals

a b s t r a c t

Spread of marine invasive species (MIS) via ships’ ballast water causes global biotic homogenization andextinctions. In this study, a UV/TiO2 ballast water treatment system (BWTS) was designed to reducetransport of MIS. UV dose profiles simulated by computational fluid dynamics indicated that, dependingon the flow rate and UV light intensity, the average applied UV dose was 260 mJ/cm2. Combined UV/TiO2

treatment produced excess hydroxyl radicals that were confirmed by high performance liquid chroma-tography using a trapping agent. We then compared the effectiveness of UV/TiO2 BWTS against UV-aloneBWTS at two different UV doses (i.e., 100% and 75%). We found that, even though UV alone reduced theabundance of all tested organisms, UV/TiO2 significantly reduced the abundance of all groups and led to agreater reduction than UV alone. Each trial should include the storage of treated ballast water for at least120 h according to Guideline G8 of the International Maritime Organization. Comparison tests after 120 hof storage period showed that the plankton densities of treated water for UV/TiO2 treatment in theP50 lm group further deceased from 4 to 2 individuals (ind.)/m3, whereas those for UV-alone treatmentincreased from 6 to 57 ind./m3, revealing that UV/TiO2 treatment had the potential to inhibit planktonregrowth, but UV alone did not. Although the efficacy of both strategies increased after 120 h of storageperiod, the densities of microbes in discharge samples from the UV/TiO2 treatment were significantlylower than those in samples from UV-alone treatment. Moreover, the advantages of UV/TiO2 at low UVdose (i.e., 75%) were much greater and the concentrations of disinfection by-products (e.g., trihalome-thanes, haloacetic acids) were observed in levels of 0.01–1 lg/L.

� 2014 Elsevier B.V. All rights reserved.

1. Introduction

Ballast water is absolutely essential to the safe and efficientoperation of modern shipping, providing balance and stability toun-laden ships. Annually over 10 billion tons of ballast water aretransferred among ports [1]. An estimated 7000 marine and coastalspecies travel across the world’s oceans every day in ballast tanksand 84% of the world’s 232 marine ecoregions have reported find-ings of invasive species [2]. The introduction of marine invasive

species (MIS) into new environments is regarded as one of fourmajor risks to global marine environmental safety [3], and ballastwater discharge has been identified as a leading vector for MIS[4–7].

To minimize the associated impacts on health, environment andeconomy, the International Maritime Organization (IMO) adoptedthe International Convention for the Control and Management ofShips’ Ballast Water and Sediments in 2004 [8]. It will enter intoforce 12 months after the date on which 30 countries representing35% of the gross tonnage of the world’s merchant shipping haveratified it. To date, a total of 37 countries representing 29% of theworld’s merchant tonnage have signed [9]. In the near future, the

8 N. Zhang et al. / Chemical Engineering Journal 243 (2014) 7–13

maximum concentrations of living organisms that can be releasedwith ballast water must not exceed D-2 discharge standards of theIMO: ships shall discharge (1) less than 10 viable organismsP50 lm per m3; (2) less than 10 viable organisms P10 to<50 lm per mL; (3) less than 1 colony forming unit (cfu) of toxico-genic Vibrio cholerae (O1 and O139) per 100 mL; less than 250 cfuof Escherichia coli per 100 mL; and less than 100 cfu of intestinalEnterococci per 100 mL [8].

An efficient way to achieve this aim is the physical and/orchemical treatment of ballast water on board [7]. To prevent theintroduction of MIS via ships’ ballast water, different treatmentmethods have been proposed, such as electrolysis, ultraviolet(UV) radiation, chemical methods (e.g., chlorine, ozone, calciumhypochlorite, peracetic acid, acetic acid). Each of the methodsmentioned above has its own advantages and disadvantages. Forexample, electrolysis and chemical treatment are considered tobe effective but they also cause corrosion of ballast tanks, intro-duce hazardous chemicals, and generate undesirable by-products[10]. UV radiation is physical treatment with no chemical doses,but its disinfection efficiencies are claimed limited [11–13]. Inaddition, UV exposure is usually done at both intake and dischargeof ballast water, due to its low ability to inhibit microorganismsreproduction and its effectiveness degrades with cloudy or turbidwater that restricts UV light penetration [14]. Therefore, the devel-opment of more effective and environmentally friendly treatmenttechnologies has become an urgent issue.

Recently, UV-based advanced oxidation processes (AOP) (e.g.,UV/TiO2, UV/Ag–TiO2/O3, UV/H2O2) have been investigated fortheir treatment efficacy [15–18]. UV radiation combined withTiO2 leads to an AOP with improved oxidative properties due tothe in situ formation of hydroxyl radicals (�OH). The mechanismof �OH formation is described as follows: (1) irradiation withphotons that have their energy greater than the bandgap energy,generally leads to the formation of an electron/hole pair in TiO2

particle (Eq. (1)) [19,20]; (2) valence band holes (hþvb) have beenshown to be powerful oxidants, whereas conduction band elec-trons (e�cb) can act as reductants. The holes in the valence bandcan react with surface-bound water and hydroxide groups to give�OH (Eqs. (2) and (3)) [20]; and (3) the electron can be transferredto the dissolved oxygen to give superoxide radical anion (O��2 ) (Eq.(4)) [20]. O��2 and its protonated form subsequently dismutate toyield hydrogen peroxide (H2O2) or peroxide anion (Eq. (5)). Thephotocatalytic reduction of H2O2 by e�cb then leads to the produc-tion of next �OH (Eq. (6)) [21,22].

TiO2 þ hv ! hþvb þ e�cb ð1Þ

H2Oþ hþvb ! �OHþHþ ð2Þ

OH� þ hþvb ! �OH ð3Þ

O2 þ e�cb ! O��2 ð4Þ

2O��2 þ 2Hþ $ 2HO�2 ! H2O2 þ O2 ð5Þ

H2O2 þ e�cb ! �OHþ OH� ð6Þ

For disinfection purposes, especially ballast water treatmentprocesses, it is necessary to monitor and control the applied doseof active substances that has a general or specific action on oragainst harmful aquatic organisms and pathogens. There are twoinactivation mechanisms for UV/TiO2 treatment, one is UV radia-tion, the other is �OH produced from photocatalytic reaction. Forpractical UV disinfection devices, the characterization of UV radia-tion profile is important, since UV radiation does not spreadhomogenously in water and zones where the radiation is weaker

may exist within the reactor. On the other hand, the further oxida-tion by �OH of bromide in seawater would lead to the formation ofhypobromous acid (HOBr) that is generally quantified as totalresidual oxidant (TRO) and presents a real interest for disinfectionas they could inhibit bacterial regrowth after treatment [23]. If �OHis formed, brominated organic compounds (e.g., bromate, tribro-momethane, tribromoacetic acid) can arise from bromide as men-tioned above.

In this study, the effectiveness of a laboratory-based ballastwater treatment systems (BWTS) using UV/TiO2 technology wereevaluated in the inactivation of MIS via ballast water, by comparingthe effectiveness of UV/TiO2 versus UV alone. UV dose was calcu-lated using a computational fluid dynamics (CFD) model. In orderto confirm the assumption presented in the introduction above,�OH measurement and bromide oxidation products by quantifica-tion of TRO, bromate, trihalomethanes (THMs), haloacetonitriles(HANs) and haloacetic acids (HAAs) were also monitored.

2. Materials and methods

2.1. Test facility and experimental design

Natural seawater used as influent water was supplied fromDalian Harbor in the Yellow Sea, China, and was kept in the holdingtank (Fig. 1). Salinity, temperature and pH of influent water were32.5 PSU, 23.4 �C and 8.15, respectively. Plankton densities wereadjusted by adding concentrated natural populations from theharbor into the influent water; the target densities for organismsP50 lm and P10 to <50 lm were at least 105 individuals (ind.)/m3 and 103 cells/mL, respectively [24].

To determine if a UV/TiO2-based BWTS more effectively reducesthe introduction of MIS via ballast water than a UV-alone BWTS,we conducted two comparison trials. The only difference betweentested systems was that one utilized UV-alone treatment, while theother utilized an AOP technology involving UV plus a TiO2 nano-thin film. UV-alone treatment was conducted in a cylindrical reac-tor with a 200 mm inner diameter and a height of 390 mm. Fivenatural quartz sleeves holding five low pressure (LP) UV lampswith power of 600 W (Beasun Electronic Co., Ltd., Beijing, China)were placed at the center of reactor. UV/TiO2 treatment was carriedout in the same UV reactor with a TiO2 nanothin film encircling theUV lamps.

Two paired trials corresponding operating conditions of UValone and UV/TiO2 were conducted. For each trial, the influentwater directed to tank 1 was treated by UV irradiation alone,whereas that directed to tank 2 was treated with UV/TiO2 (Fig. 1)and the effectiveness of the two different BWTS on organismdensities was measured. According to Guideline G8 of the IMO,each trial should include the storage of treated ballast water forat least 120 h. Therefore, treated water was stored in tanks for120 h, after which the UV/TiO2 tank was discharged directly andthe UV-alone tank was subjected to another round of UV radiationtreatment and then discharged.

2.2. UV dose determination and adjustion

Disinfection level and operation cost are determined directly byUV reactor geometry configuration [25]. UV dose is determined byboth incident radiation and exposure time in UV reactor (Eq. (7)).

UV dose ðmJ=cm2Þ ¼ incident radiation ðmW=cm2Þ� exposure time ðsÞ ð7Þ

Since it is impractical and cost-ineffective to apply bioassaytests to evaluate the performance of large-scale UV reactors, CFDcoupled with irradiation model simulation was used to simulate

S.P.1Holding tank

Influent water

Filter

UV/TiO2reactor

S.P.2, T0h

S.P.3, T0h

S.P.4, T120h

S.P.6, T120h

Discharge

Discharge

Tank 2

Treated water

Treated water

Tank 1UV

reactor

Flow

Sample

Dm

Diso

Magnification of sampling point

UV reactorS.P.5, T120h

Fig. 1. Schematic representation of experimental design comparing effectiveness of ballast water treatment system (BWTS) using UV/TiO2 versus UV alone. S.P.1–S.P.6 denotesampling points. T0 h and T120 h represent time scale from hour 0 to hour 120. Diso and Dm are the diameters of the sampling point opening and the main flow in the line,respectively.

N. Zhang et al. / Chemical Engineering Journal 243 (2014) 7–13 9

movement of organisms through the UV reactor, calculate theirexposure time to UV light, and compute UV dose received byvarious sections of the population of organisms. Fig. 2 shows theUV dose calculated by CFD model, and UV dose used in this studywas 260 mJ/cm2. The CFD model was applied to design and opti-mize the UV reactor geometry configuration before the test. UVintensity was measured with a UV irradiance meter (LinshangTechnology Co., Ltd., Shenzhen, China) during the test. Withoutchanging flow rate, electrical ballasts (Beasun Electronic Co., Ltd.,Beijing, China) were always applied to adjust the power of LP UVlamps and further to change UV intensity.

Fig. 2. UV dose simulated by CFD model.

2.3. Sample collection, identification and quantification of planktonand bacteria

Six sampling points (S.P.1–S.P.6) and a diagram of the samplingpoints installed in a pipe are shown in Fig. 1. Samples for the influ-ent water were collected from S.P.1, while samples immediatelyafter treatment (T0 h) were collected from S.P.2 and S.P.3 for theUV-alone and UV/TiO2, respectively. Samples for UV/TiO2 after120 h of storage were collected from S.P.6 (T120 h), while samplesfor before and after UV-alone re-treatment were collected fromS.P.4 and S.P.5 (T120 h), respectively. Since the capacity of BWTS is20 m3/h, the total time needed for one test cycle was 30 min.Therefore, triplicate samples were collected at a sequence of begin-ning T5 (5 min), middle T15 (15 min), and end T25 (25 min).

For analysis of organisms P50 lm in size, triplicate (i.e., T5, T15

and T25) 20 L samples were collected from S.P.1 for influent water,while triplicate 1 m3 samples were collected from and S.P.2 andS.P.3 for treated water immediately after treatment (T0 h), fromS.P.4, S.P.5 and S.P.6 at treated water discharge sampling event(T120 h), respectively. Water samples were concentrated throughplankton net with a mesh size of 50 lm (diagonal dimension) into60 mL specimen bottle. Five drops of stain stock solution addedinto specimen bottle for 30 min staining, and formalin solutionadded to fix the samples. Then retained organisms were enumer-ated using total count method under a biological microscope(Olympus BX61, Tokyo, Japan) at 40–100� magnification [26].

For analysis of organisms P10 to <50 lm in size, triplicate 1 Lsamples were collected from S.P.1, while triplicate 10 L sampleswere collected from S.P.2 and S.P.3 for treated water immediatelyafter treatment (T0 h), from S.P.4, S.P.5 and S.P.6 at treated waterdischarge sampling event (T120 h), respectively. One and 10 L watersamples were concentrated through plankton net with a mesh sizeof 10 lm (diagonal dimension) into 25 and 60 mL specimen bottle,respectively. Five drops of stain stock solution added into specimenbottle for 15 min staining, and formalin solution added to fix thesamples. The pretreated samples were left to settle for 24 h, andsupernatants were extracted and transferred to a 50 mL samplecontainer. Shacked the subsample enough before transferring acertain amount to a counting chamber with cover glass for obser-vation using a microscopic counting under a biological microscope(Olympus BX61, Tokyo, Japan) at 200� magnification [26].

Further, 0.7 L water samples were collected in triplicate at eachsampling event for microbial analysis. Heterotrophic bacteria wereenumerated using the agar spread plate method with autoclavedmarine broth 2216E. The culture dishes were then closed andplaced upside down in a 37 ± 0.5 �C incubator for 48 h [26,27].Escherichia coli were cultivated in an incubator at 44 ± 0.5 �C for

100

150

200

250

300

4-HBA

UV treatment aloneUV dose = 260 mJ/cm2

(a)

AU)

10 N. Zhang et al. / Chemical Engineering Journal 243 (2014) 7–13

24 h, and intestinal Enterococci were cultivated at temperature of35 ± 2 �C and incubated for 24 h. E. coli and intestinal Enterococciwere check using standard most probable number (MPN) protocols[26,28]. Total Vibrio sp. was determined by placing the filter onTCBS cholera medium agar plates which were incubated at 37 �Cfor 18 h. Physiological and biochemical test kits (API 20E) wereused for further identification [26].

0

50

10 12 14 16 18 20 22 24

0

50

100

150

200

250

300

3,4-DHBA

(b)

4-HBA

3,4-DHBA

Abso

rban

ce (m

Retention Time (min)

UV/TiO2 treatment

A: UV dose = 260 mJ/cm2

B: UV dose = 221 mJ/cm2

C: UV dose = 195 mJ/cm2

AB

C

Fig. 3. HPLC chromatograms of 4-HBA and 3,4-DHBA: (a) UV radiation alone, and(b) UV/TiO2 treatment. Mobile phase consisting of 20% (v/v) methanol and 80% (v/v)0.01 M phosphoric acid was used to separate 3,4-DHBA.

2.4. Chemical analysis

Three replicated of 5 L water samples were collected at eachsampling event for chemical analysis. A Waters high performanceliquid chromatography (HPLC) (Waters Corp. Milford, MA, USA)equipped with a 2998 photodiode array detector and a Nova-PackC18 column (5 lm, 250 � 4.6 mm I.D.), was employed indirectly todetect �OH after a trapping reaction with 4-hydroxybenzoic acid(4-HBA). Through the determination of hydroxylated derivativesof 4-HBA, 3,4-dihydroxybenzoic acid (3,4-DHBA), the concentra-tion of �OH was evaluated relatively [29]. Detection was at210 nm with a mobile phase composed of a 0.01 M HPLC gradephosphoric acid:HPLC grade methanol mixture (80:20 in volume)at a flow rate of 1 mL/min.

Bromate was analyzed by ion chromatographic method using aion chromatograph system (DIONEX ICS-1500, Thermo FisherScientific Inc., USA) following US EPA method 317.0 [30]. THMs,HANs and HAAs were analyzed according to US EPA methods551.1 and 552.3, using a gas chromatograph (GC) with electroncapture detection (Agilent 7890A, Agilent Technologies Inc., USA)[31,32]. Injections of 2 lL samples were introduced via split/split-less injector onto a GC column (HP-5MS, 30 m � 0.25 mm I.D. with0.25 lm film thickness, J&W Scientific Agilent).

Additionally, salinity, pH, temperature, dissolved oxygen andoxidation reduction potential (ORP) were measured by an YSIMultiparameter Water Quality Sonde (YSI-6600 V2, YellowSprings, OH, USA). Total suspended solid (TSS) was measured bya gravimetric method [33]. Total organic carbon (TOC) was pro-cessed following US EPA method 415.3 [34]. TRO was determinedby a colorimetric N,N-diethyl-p-phenylenediamine method basedon US EPA method 330.5 [35].

3. Results and discussion

3.1. �OH characterization

�OH is known to cause damage to protein, lipid, DNA and RNA,leading to tissue injury and cell death [36]. Measurement of �OHformation is thus essential to confirm its existence and understandits role involved in organisms inactivation process. While it isknown that measurement of �OH is very difficult due to its highreactivity and low abundance in aqueous solution. In this study,4-HBA that has proven to overcome some of the difficulties inher-ent to salicylate was used as �OH trapping agent [29], and HPLC wasthen employed to characterize �OH indirectly by determining itssingle reaction products with 4-HBA, 3,4-DHBA. To confirm �OHexistence during UV/TiO2 treatment process, an additionalexperiment was carried out at laboratory. Using the same UV dose,3,4-DHBA formation from the UV/TiO2 treatment process wascompared to its formation from the UV irradiation alone. The char-acteristic one peak of 3,4-DHBA was clearly observed upon HPLCchromatogram after UV/TiO2 treatment (Fig. 3b), indicating that�OH was really generated on the surface of TiO2 membrane underUV radiation. In contrast, no 3,4-DHBA peak were significantlydetected after UV radiation alone (Fig. 3a).

In addition, the change of peak height of 3,4-DHBA at differentUV doses is also shown in Fig. 3b. There is no significantly

difference in peak height of 3,4-DHBA, indicating the amount ofproduced �OH is not changed with the present applied UV doses,which is consistent with the results reported by Bahnemann etal. [37].

3.2. Efficiency of UV/TiO2 BWTS

Initial plankton densities of influent water in the P50 lm andP10 to <50 lm groups were 5.85 � 105 ind./m3 and 5000 cells/mL, respectively. The densities of microbes were relatively highin all samples (Table 1). Two paired tests were conducted at twodifferent UV doses (i.e., trial 1 = 100% UV dose and trial 2 = 75%UV dose).

The results indicated that no Vibrio cholera or intestinal Entero-cocci were present in any treated water samples (Table 1). Thedensities of organisms in all groups were significantly reducedimmediately after UV-alone or UV/TiO2 when compared to thatof initial influent water; however, the decreases in response toUV/TiO2 were significantly higher than those in response to UValone (Table 1). The inactivation ratios for UV/TiO2 tests decreasedby about 3-log for heterotrophic bacteria and 5-log for E. coli imme-diately after treatment from an initial bacterial concentration ofaround 106 cfu/100 mL in influent water (Table 1).

In addition, the treated water (i.e., after UV-alone and UV/TiO2

treatment) was held in the tanks 1 and 2 (Fig. 1) for 120 h to ensurethat biotic factors, such as biological re-growth, DNA repair inorganism and resting stages, would not develop during testing.And the effects of BWTS on organism concentration were testedby comparing the treated water to the untreated water in orderto eliminate the factor of natural death causing by survival envi-ronmental conditions changes. After the storage period of 120 h,the plankton densities of untreated water (i.e., the rest of the influ-ent water in the holding tank) in the P50 lm and P10 to <50 lmgroups were around 3.7 � 104 ind./m3 and 500 cells/mL (Table 1),respectively, indicating that there was no obvious reduction inuntreated water when compared to initial influent water, and thetest cycle was valid as the untreated water with viable organismdensity exceeding 10 times the maximum permitted values ofRegulation D-2.1 (i.e., greater than 100 viable organisms P50 lmper m3, and greater than 100 viable organisms P10 to <50 lm

Table 1Plankton densities in the P50 lm and P10 to <50 lm groups and microbes in influent water, treated water after UV irradiation alone (T0 h and T120 h), and treated water after UV/TiO2 treatment (T0 h and T120 h). Mean ± standard deviation of replicate measurements (n = 3).

Sampling points Influent water T0 h T120 h

UV alone UV/TiO2 Untreated water UV alone (before)d UV alone (after) d UV/TiO2

S.P.1 S.P.2 S.P.3 S.P.1 S.P.4 S.P.5 S.P.6

Trial 1 (100% UV dose) a

Organisms P50 lm (ind./m3) 585,333 ± 2517 6 ± 2 4 ± 2 37,567 ± 551 57 ± 10 7 ± 2 2 ± 2Organisms P10 to <50 lm (cells/mL) 5010 ± 76 8.14 ± 1.20 4.07 ± 0.12 476 ± 57 51 ± 8 4 ± 1 0.80 ± 0.12Heterotrophic bacteria (cfu/100 mL) 5,266,667 ± 208,167 2333 ± 321 1500 ± 400 390,000 ± 20,000 14,066 ± 251 3730 ± 404 9633 ± 513E. coli (MPN/100 mL) 3,366,667 ± 152,753 46 ± 3 <2 3333 ± 208 414 ± 25 12 ± 3 16 ± 3Intestinal Enterococci (cfu/100 mL) 14 ± 3 NDc NDc NDc NDc NDc NDc

Vibrio cholera (cfu/100 mL) NDc NDc NDc NDc NDc NDc NDc

Trial 2 (75% UV dose) b

Organisms P50 lm (ind./m3) 584,667 ± 1528 9 ± 3 4.52 ± 0.67 36,500 ± 200 50 ± 8 8 ± 3 2.33 ± 0.58Organisms P10 to <50 lm (cells/mL) 4967 ± 40 9.8 ± 1.1 4 ± 1 524 ± 10 74 ± 10 8.82 ± 1.10 2.12 ± 0.08Heterotrophic bacteria (cfu/100 mL) 5,233,333 ± 208,167 5700 ± 265 1733 ± 321 410,000 ± 26,458 19,600 ± 2480 7170 ± 550 10,400 ± 2000E. coli (MPN/100 mL) 3,400,000 ± 300,000 256 ± 15 6 ± 1 3633 ± 416 633 ± 16 19 ± 3 22 ± 6Intestinal Enterococci (cfu/100 mL) 13 ± 1 NDc NDc NDc NDc NDc NDc

Vibrio cholera (cfu/100 mL) NDc NDc NDc NDc NDc NDc NDc

a Denotes UV dose is 260 mJ/cm2.b Denotes UV dose is 195 mJ/cm2.c Denotes not detected.d Denotes without and with secondary UV radiation.

N. Zhang et al. / Chemical Engineering Journal 243 (2014) 7–13 11

per mL) according to Guideline G8 of the IMO [24]. The densities oforganisms in treated water in the P50 lm and the P10 to <50 lmgroups for UV/TiO2 treatment decreased further after the storageperiod of 120 h, whereas those for UV-alone treatment (withoutsecondary UV radiation) increased (Table 1).

These results are particularly promising given the fact that thecomplete inactivation was attributed to both �OH treatment andUV radiation, and the possibility of organism regrowth and darkrepair of DNA were reduced. In contrast, the densities of heterotro-phic bacteria and E. coli with strong regrowth ability increased after120 h of storage period, with the increases in UV-alone treatmentwithout secondary UV radiation being significantly higher thanthose in the UV/TiO2 treatment (Table 1). Similar results were ob-served in trial 2.

In trial 2, the density of organisms in all groups immediatelyafter UV/TiO2 at low UV dose (i.e., 75%) treatment was still far low-er than that of the proposed D-2 discharge standard of the IMO,whereas the density of organisms in the UV alone almost failedto meet the performance standard. These findings indicate that,at low UV dose, the effects of UV/TiO2 are much greater than thoseof UV alone (Table 1). Our findings are very important to a realworld application, as luminous decay, fouled quartz sleeves, andhigh turbidity and low clarity water can restrict UV light penetra-tion, leading to reduce UV dose reaching the organism and poorperformance of UV-alone or UV/TiO2 BWTS. As a result, the UV/TiO2 treatment process may be particularly beneficial when thereare challenging poor water conditions at the point of ballast wateruptake and/or poor working conditions for the BWTS. On the otherhand, from the inactivation test results, it can be notice that after120 h of storage period, secondary UV radiation is necessary forUV alone treatment to meet the D-2 discharge standard of theIMO [24], whereas it is not necessary for the UV/TiO2 treatment(Table 1).

The energy cost was calculated from the UV lamp power (P) andtreatment time (t). One assumption made was that the normalUV-alone BWTS ran on 85% rated power of UV lamp to effectivelymeet the D-2 discharge standard, while that for UV/TiO2 BWTS was75% rated power. Therefore, the energy cost for UV alone in onetest cycle was 0.85P�2t (85%P with twice treatment), while thatfor UV/TiO2 was 0.75P�t (75%P with once treatment). With theabove-mentioned results, it can be concluded that for the sametreatment processing conditions, UV/TiO2 BWTS can provide

approximately 56% energy savings than UV-alone BWTS in thisstudy.

3.3. Chemical analysis

As explained in the introduction, it is suspected that bromide isoxidized by �OH during oxidation of seawater, resulting in the for-mation of HOBr and disinfection by-products (DBPs). In order toaddress this DBPs formation potential when active substances(e.g., �OH, HOBr) are employed for ballast water treatment, the jointGroup of Experts on the Scientific Aspects of Marine EnvironmentalProtection-Ballast Water Working Group (GESAMP-BWWG) hassuggested a preliminary list of 18 chemicals to be assessed in allBWTS tests [38].

In this study, physicochemical analysis was performed whenthe maximum UV dose of 260 mJ/cm2 was applied. Salinity, pH,temperature, and dissolved oxygen did not vary before and aftertreatment (Table 2). ORP showed little enhancement after treat-ment (Table 2), indicating that residual oxidative species was pres-ent during seawater oxidation. Due to filtration, TSS and TOC levelsdecreased significantly from 19.4 to 3.8 mg/L and 13.7 to 8.4 mg/L,respectively (Table 2). The concentration of TRO was low (18 lg/L)after UV/TiO2 treatment, which is consistent with the finding thatno bromate was detected in any samples after oxidation of seawa-ter (Table 2).

The influent water contained low levels of trichloromethane(TCM), dibromochloromethane (DBCM), monobromoacetic acid(MBAA), tribromoacetic acid (TBAA), monochloroacetic acid(MCAA), dichloroacetic acid (DCAA) (Table 2). 8 compounds includ-ing trichloroacetic acid, bromochloroacetic acid, bromodichloroace-tic acid, chlorodibromoacetic acid, dibromoacetonitrile, dichloroacetonitrile, bromochloroacetonitrile and trichloroacetonitrile weredetected below the detection limits immediately after UV/TiO2

treatment, while tribromomethane (TBM), TCM, DBCM, dichloro-bromomethane (DCBM), MBAA, dibromoacetic acid (DBAA), TBAA,MCAA and DCAA were observed in levels of 0.01–1 lg/L (Table 2).Contrary to expectation, among the detected DBPs, the chlorinatedcompounds were more predominant than the brominated com-pounds; TCM (0.846 lg/L) and MCAA (1.04 lg/L) were found athigher concentrations than TBM (0.083 lg/L) and MBAA(0.703 lg/L), respectively. This may due to the fact that the concen-tration of hypochlorous acid produced from UV/TiO2 treatment of

Table 2Physicochemical indicators in influent water and treated water immediately after UV/TiO2 treatment (T0 h) with the maximum UV dose of 260 mJ/cm2. Mean ± standard deviationof replicate measurements (i.e., T5, T15 and T25) (n = 3).

Test indicators MDLa Influent water (T0 h) Treated water (T0 h)

Salinity (PSU) n/ab 32.5 ± 0.1 32.7 ± 0.1pH n/ab 8.11 ± 0.01 8.12 ± 0.01Temperature (�C) n/ab 23.4 ± 0 23.3 ± 0.1Dissolved oxygen (mg/L) n/ab 7.52 ± 0.07 7.41 ± 0.08ORP (mV) n/ab 173 ± 3.05 232 ± 2.58TSS (mg/L) 0.1 19.4 ± 0.06 3.8 ± 0.13TOC (mg/L) 0.2 13.7 ± 0.1 8.4 ± 0.2TRO (lg/L) 10 NDc 18 ± 0.4Bromate (lg/L) 0.11 NDc NDc

Tribromomethane (lg/L) 0.009 NDc 0.083Trichloromethane (lg/L) 0.005 0.122 0.846Dibromochloromethane (lg/L) 0.009 0.04 0.051Dichlorobromomethane (lg/L) 0.008 NDc 0.056Monobromoacetic acid (lg/L) 0.01 0.113 0.703Dibromoacetic acid (lg/L) 0.01 NDc 0.093Tribromoacetic acid (lg/L) 0.01 0.115 0.416Monochloroacetic acid (lg/L) 0.01 0.733 1.04Dichloroacetic acid (lg/L) 0.013 0.153 0.256Trichloroacetic acid (lg/L) 0.01 NDc NDc

Bromochloroacetic acid (lg/L) 0.01 NDc NDc

Bromodichloroacetic acid (lg/L) 0.008 NDc NDc

Chlorodibromoacetic acid (lg/L) 0.01 NDc NDc

Dibromoacetonitrile (lg/L) 0.006 NDc NDc

Dichloroacetonitrile (lg/L) 0.002 NDc NDc

Bromochloroacetonitrile (lg/L) 0.002 NDc NDc

Trichloroacetonitrile (lg/L) 0.002 NDc NDc

a Denotes method detection limit.b Denotes not applicable.c Denotes not detected.

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0.1

0.3

0.4

0.6

0.8

0.9

1.0

1.2

HAA

s co

ncen

tratio

ns (µ

g/L)

TBM TCM DBCM DCBM

THM

s co

ncen

tratio

ns (µ

g/L)

MBAA DBAA TBAA MCAA DCAA

12 N. Zhang et al. / Chemical Engineering Journal 243 (2014) 7–13

seawater is higher than that of HOBr, which is different from ozon-ation and/or chlorination of ballast water treatment [7].

On the other hand, treated ballast water is generally depositedin ballast tanks prior to discharge, and the length of storage timedepends on route distance. Several authors have addressed the for-mation of DBPs in chlorinated and ozonated ballast water [7,39];however, there is a lack of information about the effects of storagetime to DBPs formation in treated ballast water. Fig. 4 shows thetime-dependent trends of DBPs formation immediately aftertreatment, as well as during the 120 h of storage period. Amongthe measured DBPs, the maximum concentrations of THMs andHAAs were observed immediately after treatment, after which theydecreased with storage time extension. The time-effect trends ofthe observed DBPs were dependent on their formation, stability,as well as TRO level [40]. THMs were relatively volatile, and theywere the final products in the presence of TRO. However, TRO levelproduced from UV/TiO2 treatment was very low (18 lg/L), it wasquickly consumed by organisms present in ballast water ratherthan reacting with natural organic matter to further produce DBPs.Hence, the concentration of DBPs decreased with increasing stor-age time. Decline rate of HAAs were significantly higher than thatof THMs, likely due to the fact that typical seawater environment(pH = 8.1) does not contribute to the formation of HAAs [41,42].

0 24 48 72 96 1200.0

Storage time (h)

Fig. 4. Concentration of DBPs as a function of storage time of treated samples (i.e.,after UV/TiO2 treatment) at salinity: 32.5 PSU, temperature: 23 ± 1 �C, pH: 8.15,applied UV dose: 260 mJ/cm2. Error bars represent the standard deviation ofreplicate measurements (n = 3).

4. Conclusions

Our trials provide preliminary data illustrating the benefits ofUV/TiO2 technology to prevent the introduction of MIS via ships’ballast water. Compared to UV alone, UV/TiO2 treatment resultedin significantly lower abundance of individuals in the P50 lmand P10 to <50 lm groups, and reduction of heterotrophic bacte-ria and E. coli to levels meeting the D-2 discharge standard of theIMO. Although the densities of heterotrophic bacteria and E. coliincreased after 120 h of storage period relative to immediately

after treatment, the increases following UV/TiO2 treatment weresignificantly lower than those following UV alone. Due to the effec-tive biological inactivation of UV/TiO2 BWTS at low UV dose (i.e.,

N. Zhang et al. / Chemical Engineering Journal 243 (2014) 7–13 13

75%), the operating conditions of the system can be set to admin-ister low UV doses under normal circumstances. Power compensa-tion would serve as an inherent back-up strategy if the BWTSshould fail for reasons such as increased flow rate, luminous decay,fouled quartz sleeves or turbid water with lower percentage trans-mittance. In addition, UV/TiO2 BWTS uses less energy than UValone because secondary treatment prior to ballast water dischargemust be conducted for UV-alone BWTS to meet the D-2 perfor-mance standard of the IMO. DBPs formation in the UV/TiO2 BWTSis of much less concern than in systems that employ oxidativemethods such as chlorine and ozone [7].

Various ballast water treatment technologies have been devel-oped over the last two decades. Currently, use of strong oxidation(e.g., sodium hypochlorite) has become the predominant technol-ogy, followed by UV radiation. Each of these methods have inher-ent advantages and disadvantages regarding effectiveness, costs,ship and crew safety, power and space requirements, and environ-mental soundness [43]. Our study clearly demonstrates the poten-tial benefits of UV/TiO2 BWTS to reduce invasion risk, and theresults presented herein indicate that UV/TiO2 technology is anefficient, environmentally friendly and relatively cost-effective toolfor onboard ballast water treatment.

Acknowledgements

The authors would like to thank Ahead Ocean Technology Co.,Ltd. for the collaboration and this work could not have been carriedout without its funding support.

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