DESALINATION

ELSEVIER Desalination 156 (2003) 193-I 97 www.elsevier.com/locate/desal

Treatment of synthetic industrial wastewater with UV/TiO, and RO using benzene as a model hydrocarbon

Nader M. Al-Bastaki Department of Chemical Engineering, College of Engineering, University of Bahrain, I? 0. Box 32038, Bahrain

Tel. +973 782122; Fax +973 684844; email: naderbsk@eng.uob.bh

Received 17 February 2003; accepted 21 February 2003

Abstract

Contamination of water resources with hazardous chemicals is one of the main concerns of all private and public sectors involved in the water production for drinking, irrigation and domestic use. The conventional disinfection process using chlorination adds to the complexity of the problem as some of these chemicals are precursors for disinfection by products such as trihalomethanes. This paper describes the work done to study the eff%ziency of two types of processes in removing a model hydrocarbon, namely, benzene, which was added to water at a concentration of 100 ppm. The two processes investigated were RO and UVITiO,. The RO process was based on a FilmTec SW30 spiral wound membrane, which was operated at pressures of 20,40 and 60 bar. The UV/TiO, employed was PhotoCat, manufactured by Purifies, Canada. The heart of the PhotoCat unit was 32 low pressure, low intensity mercury vapor lamps connected in series. Gas chromatography was used to analyze the results. The results showed that the UV/ TiO? resulted in a removal of 99.28%. However, the RO performance was less impressive with amaximum separation of 82.3%. At 40 bar, a negative permeability was observed, as the concentration of benzene in the permeate was higher than the feed. This latter result shows that care has to be taken with hydrocarbons polluting the feed water to RO units, since in certain cases the product water will more polluted than the feed.

Keywords: UVITi02: RO; Wastewater treatment; Permeability; pH

1. Introduction

Many concerns have been expressed recently with respect to the traditional drinking water and wastewater treatment processes, which have

encouraged the consideration of alternative or modified processes. One of these concerns is the formation of disinfection by-products (DBPs), particularly, trihalomethanes (THM) which may result due to the reaction of chlorine with the

Presented at the European Conference on Desalination and the Environment: Fresh Water for All, Malta, &8 May 2003. Ewopeau Desalination Socie@, International Water Association.

00 I 1-9 164/03/$- See front matter 0 2003 Elsevier Science B.V. All rights reserved PII: SO01 l-91 64(03)00341-2

194 Y M. Al-Bastaki /’ Desalination 156 /2003) 193~-197

naturally occurring humic and fulvic acids [I]. Membrane processes, such as microfiltration (MF) and ultrafiltration (UF), and photocatlytic processes are possible methods of overcoming the shortcomings of traditional water treatment methods.

Recently, UF and nanofiltration (NF) mem- branes have been employed in large-scale com- mercial plants to treat surface and river water as a replacement of the traditional methods based on sedimentation and media filters [2,3]. More- over, the use of MF and UF membranes for the treatment of sewage waste water has evolved lately, in the form of membrane bioreactors [4,5]. In addition, RO and NF processes have been considered for the removal of herbicides and pesticides [6,7].

An increasing number of studies have been reported in the recent literature about the applica- tion of UV/TiO, for the treatment of waste water. The emphasis has been on the decomposition of hydrocarbons [8,9] herbicides [ IO,1 I] and other industrial wastes such as dyes [ 12,131 and heavy metals [ 14, I 51.

2. Experimental setup and procedure

2.1. Water samples

Synthetic waste water samples were prepared by mixing benzene with distilled water. The concentration of benzene in all of the samples

PHOTO- CATALYTIC

PRE FILTER COALESCER REACTOR

,

ACCUMULATOR

/ / SEPARATION

was 100 ppm. The concentration of the samples was measured using a CC with a thermal con- ductivity detector (TCD) and an HP 3390A Reporting Integrator.

2.2 Experimental setup

The water samples were treated with two different processes, namely? reverse osmosis (RO) and UVITiO,. Each process utilized small scale, industrial elements. The membrane process (RO) was operated manually while the lJVITi0, process was a fully automated small pilot scale plant and could be operated either manually or by a computer.

The UV/TiO, apparatus used in this work was a PhotoCat unit manufactured by Purifies, Canada. The unit is a small scale mobile pilot plant, located at the Research Deanship laboratory, University of Bahrain. The process utilizes TiO, catalyst powder suspended in water. A schematic diagram of the process is shown in Fig. I. The reactor section employs 32 quartz tubes, which generate UV light at a wavelength of 365 nm. The total added length of the low pressure UV lamp tubes is 48 m. The water flow rate is adjustable up to 12 I/min. The effective volume ofthe liquid within the reactor, which is exposed to UV, is about 3.77 I. An inlet pre-filter removes large suspended solids while a coalescer is used to remove oil and grease. A dedicated ceramic filter separates the catalyst fi-om the suspension after exposure to UV.

SLURRY FEED PUMP

Fig. I. Schematic diagram of the PhotoCat UV/Ti02 photocatalytic reactor.

N.M. Al-Bastaki / Desalination 156 (2003) 193-197 195

A 1 HP compressor is employed to compress air to 8.5 atm. The compressor periodically gene- rates a pressure pulse within the system to remove the catalyst from the ceramic filter and other parts within the system to keep it always in suspension. The compressed air in combination with automatic pneumatic valves assures this operation. The accu- mulator acts as a buffer tank and as a degassing device and plays a function in recycling the catalyst back to the reactor. The unit is also equipped with pH and flow control as well as with metering pumps for dosage of acid, base or oxidizing agents, as required. The feed and recirculation pumps are % HP.

Fig. 2 shows a simplified flow diagram for the RO process. The RO unit consisted of a high- pressure pump, a FilmTec SW30 membrane, feed and product tanks, pressure gauges and rotameters for flow rate measurement.

2.3. Experimental procedure

200 I of 100 ppm solution of benzene in water was placed in the feed tank of the UV/TiO, pro- cess. The product was recycled to the feed tank. Samples of the product were taken for analysis every 15 min. To study the effect of pH, experi- ments were also done at various pH values by adjusting the pH of the feed and monitoring the

FEED TANK

I’RESSURt PUMP

-i I k ROTOMl;TtR

/

L-l

1’ PRESSURt GAUGb CONCENTRATE

(RETENTATE) TANK

Fig. 2. Schematic diagram of the RO unit.

online pH measurement until a steady pH was obtained.

The RO process was operated at three pressures, namely, 20 bar, 30 bar and 60 bar. Samples of the permeate were taken for analysis of the concen- tration, and the permeate flux was measured using a rotometer as well as manually using a graduated cylinder and a stop watch.

3. Results and discussion

Fig. 3 presents typical GC outputs showing water and benzene peaks.The UV/TiO, photo- catalytic process is generally believed to be effective for treatment of hydrocarbons. The results shown in Fig. 4 appear to agree with this notion. The percent removal of benzene was calculated by measuring the concentration of benzene at various intervals, up to 45 min. The benzene concentration in the permeate was steady after I5 min of reticulation, with a removal percentage of about 99.28%. The production of CO, as a result of the oxidation reaction results in lowering the pH of the solution as can be seen in Fig. 5.

In the UV/TiO, process, UV light supplies energy to the TiO, catalyst and prompts the transport of an electron from the valence band to the con- ductive band producing a positively charged hole in the catalyst. The free electron is taken up by oxygen molecules to create superoxide radicals. The superoxide radicals react with the water molecules to produce hydroxyl radicals. Similarly, at the positive holes water molecules or hydroxyl ions are oxidized to produce additional hydroxyl radicals. The organic matter is oxidized at the catalyst surface and in the solution by any of the following routes: the hydroxyl radicals, the superoxide radical, direct oxidation of adsorbed organics at the positive holes or by the conductive band electron [16].

The benzene removal efficiency of RO appears to be less pronounced compared to the UV/TiO, as shown in Fig. 6. The maximum separation was

196 !V .Af. .dl-Bastaki IT Desalination 156 (2003) 193 197

Fig. 3. Typical CC output. This specific result was for the

test using RO. The sharp peak represents benzene.

1000 E E - 100 ppmfeed 5 100

.-

5 z 8 10 s

0 zi

: m 1

2 _/

iii 0.1

0 10 20 30 40 50

Time, min

Fig. 4. Concentration of benzene after treatment with UV/

TiO ,.

observed at 30 bar with a permeate concentration of 17.7 ppm or a removal of 82.3%. The results indicate that the rejection of benzene by the poly- amide composite SWRO FilmTec membrane varies with pressure. Negative permeability is noticed at a pressure of 20 bar. This is indicated by an increase in the concentration of benzene in the

9.0

8.8

8.6 I a

a.4

a.2

8.0

I-

4

0 IO 20 30 40 50

Time, min

Fig. 5. pH change with UV/TiOz treatment.

0 10 20 30 40 50 60 70

P, bar

Fig. 6. Concentration of benzene in the permeate for

different pressures of RO treatment.

permeate compared to the feed. The permeate flux, however, followed a linear trend with increasing pressure, as shown in Fig. 7. Due to the very low concentration of benzene in the feed water, the nonlinear flux of benzene with pressure has apparently not affected the linearity of the water flux vs. pressure curve.

4. Conclusions

The UV/TiO, photocatalytic process showed a much higher efficiency than RO in removing of

N.M. Al-Bastaki /Desalination 156 (2003) 193-197 197

1.4e-5

1.2e-5

Y l.Oe-5 2

& 8.0e-6

6.0e-6

4.0e-6

. 100ppmbenzene - hear regression

/

10 20 30 40 50 60 70

P, bar

Fig. 7. Variation ofthe permeate flow rate with RO pressure.

traces of benzene from the feed water. The maxi- mum removal rate was 99.28% for the UV/TiO, compared to 82.3% for RO. Moreover, the RO process showed a negative permeability at an operating pressure of 20 bar, resulting in a higher concentration of benzene in the permeate compared to the feed.

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