Advanced oxidation processes driven by solar energy can be an efficient method in removing organochlorine compounds from river water especially in tropical environments like the Caribbean region. The feasibility of solar photocatalytic degradation of an organochlorine compound, namely trichloroethanoic acid (TCA), which is commonly used in the Caribbean islands of Trinidad and Tobago, was separately assessed using titanium dioxide and zinc oxide as photocatalysts in suspended solution. Overall the prototype solar photoreactor operated and performed efficiently for the photodegradation of TCA. This study showed that a basic photocatalytic oxidation method for treating water using solar energy as the primary driver gives enhanced decomposition rates of the organochlorine compound when coupled to the additional application of the two separate semiconductor photocatalysts. The results further showed that for varying concentrations of TCA and photocatalysts alike, the organochlorine compound could be completely photocatalytically degraded using short exposure times under the applied influx of solar radiation. This means that this process could be optimised by judicious use of sensors so that dosage rates of the photocatalyst could be altered with variations in influent contamination levels, and the exposure time in the reactor could be altered according to daily variations in solar radiation intensity.

INTRODUCTION

The availability of ample, safe and inexpensive potable water is indeed proving to be one of the most critical problems for many developing countries today, especially small island developing states (SIDS) such as found in the Caribbean region. The shortage of community water supplies, their actual or potential pollution from anthropogenic sources, inadequate treatment and the resultant spread of associated diseases are still unresolved problems facing this region. Surface waters of island nations like Trinidad and Tobago are frequently contaminated by point source pollution (i.e. wastewater discharges from the paint manufacturing industry) and diffuse/non-point source pollution (i.e. run-off from land with herbicidal application, etc.). The cost of treating large volumes of water/wastewater is prohibitively expense if using traditional disinfection, degradation and removal methods (e.g. ultraviolet (UV) lamps, powdered activated carbon/granular activated carbon, etc.). Novel, cost-effective solutions are needed that are fully or partial self-sustaining. Solar energy is in abundance in SIDS like Trinidad and Tobago. Solar radiation promotes the degradation of most water pollutants by various disinfection mechanisms. Indirect photodegradation or classical sensitised photolysis with its singlet oxygen mechanism is active in destroying many toxic compounds with which they can form energy absorbing complexes. Although the use of natural light (as sunlight) for water treatment, especially to drive degradation processes, is a recent technological development (Bockelmann et al. 1995; Goslich et al. 1997), the capture and storage of solar energy by photochemical processes has been a mainstream and active area of research for many years (Calkins et al. 1976). Solar radiation and solar photochemical processes are becoming increasingly appreciated for their potential as water treatment technologies especially in the tropical developing country context (Cooper et al. 1998; Zhou & Smith 2002). The use of solar energy and its associated technologies are still very much under-exploited in the southern Caribbean islands of Trinidad and Tobago when compared with the amount of solar energy available due to their geographical location (i.e. 11°N latitude). In fact they experience an average solar incidence of approximately 3,000 h/year (Tota-Maharaj & Meeroff 2013). Moreover, some dissolved and suspended species can produce highly reactive oxidising free radicals, such as the OH radical, which when produced in high enough concentrations act as scavengers of any venomous impurities in water (Ibhadon & Fitzpatrick 2013). Hence, natural sunlight can be used by itself to degrade organic compounds but judicious use of small concentrations of photocatalysts can greatly enhance these degradation effects. Consequently the aim of this project was to develop such potential solutions for SIDS nations such as Trinidad and Tobago.

This study uses a well-defined pilot-scale system to measure the technical feasibility and performance of photocatalytic degradation of the organochlorine compound trichloroethanoic acid, also commonly known as trichloroacetic acid (TCA). TCA is a well-known herbicide commonly used in the Caribbean region which was of special interest in this study since it can be degraded using natural-solar UV light in addition to heterogeneous photocatalysis processes (Ramlakhan 2011). TCA (has a chemical formula of CCl3COOH) is a stable and widely distributed chlorinated compound. The production of various types of organochlorine compounds in the chlorination of drinking water is well known. It is generally accepted that the reaction between chlorine and aqueous humic material in water is responsible for the production of the major portions of these organochlorine compounds. TCA is one of the main products that have been detected in water courses across the nation of Trinidad and Tobago. In fact, TCA has been shown to be present in urban air, in rainwater, in soils and in various river water locations across the Caribbean region (Ramlakhan 2011). There are many possible sources and reasons for TCA contamination, chiefly among them is that it was widely employed as a herbicide from the 1950s right through to the 1970s. Furthermore, chlorination of humic matter has been shown to produce TCA in normal tap water supplies (Miller & Uden 1983; Uden & Miller 1983; De Leer et al. 1985). In addition, dichloroacetic acid as well as TCA are two of the more common by-products of chlorination of drinking water supplies (Coleman et al. 1984). Thus, the discovery of chlorinated organic by-products has raised questions regarding the health effects of chronic exposure to chlorine and its by-products in drinking water.

TCA degrades in soils typically within 21–90 days depending on the rate of application and the underlying soil conditions (Miller & Uden 1983; Uden & Miller 1983). Water solubility has been recognised as an important determinant of the environmental behaviour of TCA. TCA can also be formed in the natural environment itself in many ways such as when it is produced photo-oxidatively as chlorinated ethens and ethenes are converted to trichloroacetylchloride and finally to this acid (Johnson & Jensen 1986). Hence, it can be found in the environment across Trinidad and Tobago today because it is a metabolite of different chlorinated substances such as chlorine and acetic acid. TCA is ubiquitous in river water and its concentration is highly variable throughout many rivers. Consequently, TCA was selected as the model pollutant compound in this study because:

  • 1. it has a very low vapour pressure and is soluble in water at any concentration (i.e. high water solubility of 1,200 g/L at 25 °C);

  • 2. it is found as a pollutant in natural surface water sources across Trinidad and Tobago (Ramlakhan 2011);

  • 3. it is easy to analyse and quantify since the degradation product (i.e. Cl ions) is never lost to evaporation (Bowden et al. 1998).

Thus TCA was used as a model pollutant in this study with de-ionised water to test the photocatalytic decomposition efficacies of titanium dioxide and zinc oxide as photocatalysts, respectively. Therefore, this project assessed under experimental conditions the disappearance of this common herbicide and the degree of mineralisation achieved by the two photocatalytic disinfectants when used in the presence of sunlight.

In summary, this research work compares and contrasts the toxic effects associated with titanium dioxide and zinc oxide water suspensions using TCA as the model pollutant. Thus the specific objectives of the study were:

  • 1. to determine the concentrations of photocatalyst in suspension that achieved the optimal degradation rate of TCA;

  • 2. to determine the relationship between natural light (as solar energy) and how it stimulates these photochemical reactions.

MATERIALS AND METHODS

The pilot plant was designed, constructed and tested at The University of the West Indies (UWI), Department of Mechanical Engineering, St Augustine, Trinidad. Photocatalysis experiments were carried out using a flat-bed, non-concentrating reactor placed on the roof of the mechanical engineering department at UWI. The photochemical reactor design, as illustrated in Figure 1, is a fixed-bed tubular reactor (non-concentrating) consisting of several lengths of serpentine shaped tubing constructed from borosilicate glass, and supported by an aluminium metal frame which also acts as a crude solar reflector. The angle of inclination was set at 12° to approximate the latitude of Trinidad and Tobago in order to allow optimal solar irradiation take-up. This orientation of the solar reactor and its set inclination angle from the horizontal provides the necessary conditions for maximum influx of solar radiation (Parra et al. 2001). The photoreactor surface area was (1.08 m2) and consisted of twelve borosilicate glass tubes mounted on a polished aluminium reflector sheet. The dimensions of the glass tubing were as follows: total length, 12 m; outer diameter, 19 mm; wall thickness, 1 mm; volumetric capacity, 5 L. The facility prevented air-gap formation by inducing an upward flow of floating bubbles. The reactor is fed by water (mixture of photocatalyst) with a stirrer to avoid precipitation of solids and to ensure the homogeneity of feedstock as shown in Figures 1 and 2. The raw water tank was simply a 40 L plastic tank insulated with fibreglass. An electronic metering pump was selected to control the flow rate through the photochemical reactor. A digital radiometer was used to measure the intensity of solar ultraviolet A long-wave (UV-A) radiation falling on the surface of the photochemical reactor while the increase in chloride ion concentration in the outflow liquid was measured using an ion selective electrode and ion chromatography which allows low concentrations of chloride ions to be detected once the meter has been calibrated. This increase is used to infer the degradation of the TCA in the influent as described below.

Figure 1

(a) Photograph of front view of solar photochemical reactor, illustrating the glass tubing, aluminium reflector frame and the variable flow pump set-up. (b) Photograph showing side view of the solar photochemical reactor, illustrating the photocatalyst influent entry point and storage tanks for inflow and outflow water.

Figure 1

(a) Photograph of front view of solar photochemical reactor, illustrating the glass tubing, aluminium reflector frame and the variable flow pump set-up. (b) Photograph showing side view of the solar photochemical reactor, illustrating the photocatalyst influent entry point and storage tanks for inflow and outflow water.

Figure 2

Schematic process-flow diagram from: stage (i) photocatalyst mixing; stage (ii) inflow and flow through; and final stage (iii) effluent from solar photoreactor.

Figure 2

Schematic process-flow diagram from: stage (i) photocatalyst mixing; stage (ii) inflow and flow through; and final stage (iii) effluent from solar photoreactor.

Throughout the experiments conducted on the solar photochemical reactor, deionised water was used together with the TCA to create the influent waste stream, because its purity and lack of other trace elements meant any results produced would be highly accurate for the breakdown of the selected model pollutant. The degradation of TCA obeys the stoichiometry depicted in Equation (1). 
formula
1
From Equation (1), 1 mole of TCA dissociates completely to produce 3 moles of chloride ions. The photochemical decay rates and other relevant values were computed using the Chick–Watson (C–W) kinetic model as shown in Equation (2) where N is the outflow concentration of chloride ions, N0 is the theoretical concentration of TCA chloride ions, K is the inactivation rate constant (m2/Wh), i is the intensity of received solar UV-A radiation (W/m2), T is the time of exposure to solar UV-A radiation (h) and e = 2.7182. Thus the actual TCA photodegradation rate is actually measured by detecting the increase in chloride ion concentration in the outflow stream as shown in Equation (3), and this rate can be compared with the theoretical maximum rate which can be deduced by stoichiometry via Equations (1) and (2). 
formula
2
 
formula
3
The pH of the solution ranged from 1.2 to 2.7 based on initial concentrations of TCA at 1–5 mmol/L. The United States Environmental Protection Agency (USEPA 2009) Method 557 was used for the determination of the TCA concentrations. The extraction technique using ion chromatography and conductivity was used to quantify TCA concentrations with a detection limit of 0.45 μg/L as described by USEPA (2009). The reagent, TCA, was tested at three varying concentrations corresponding to typical amounts found in river waters across the Caribbean region. These were 1, 3 and 5 mmol/L of TCA, respectively. A 10 mmol initial sample was prepared in a 1 L volumetric flask and via dilution with deionised water the required concentrations of 1, 3 and 5 mmol/L of TCA were prepared. For each of the respective concentrations of TCA ranging from 1–5 mmol, the oxidation and degradation processes via solar UV-A were carried out using varying dosages of the two photocatalysts, titanium dioxide and zinc oxide, ranging from 1.0 to 3.0 mg/L respectively. TiO2 (nano-particle powder, 21 nm particle size) and ZnO (nano-particle powder <100 nm particle size) were selected for this study as standard materials in the field of photocatalytic reactions, containing anatase and rutile phases. The two photocatalyst powders (TiO2 and ZnO) were used for the oxidation and degradation processes because of its chemical stability, ready availability, reproducibility and activity as a catalyst for oxidation processes. The photocatalyst titanium dioxide and zinc oxide powders were obtained from Sigma-Aldrich, St Louis, Missouri, USA.

The mode of operation in which the photoreactor operated was single-pass only. In this approach, the reactor area and the flow rates were predetermined in order to accurately achieve the preferred destruction of the organic contaminants in this single-pass mode. This meant that the flow rates were strictly controlled so that they did not exceed 21 L/h during the experimental procedure.

RESULTS AND DISCUSSION

Heterogeneous photocatalytic processes use certain metal oxides that can readily generate hydroxyl radicals on the surface of particles when absorbing UV light. The anatase forms of titanium dioxide and zinc oxide have low band-gap energies of approximately 3.2 eV, which is almost equivalent to 400 nm wavelength of light. The principal reaction mechanisms for solar photocatalytic oxidation and degradation using metal oxides are shown in Figure 3.

Figure 3

The photocatalytic process with solar UV-A, light energy and metallic oxides (after Wang & Hong 1999; Wist et al. 2002; Herrmann 2005).

Figure 3

The photocatalytic process with solar UV-A, light energy and metallic oxides (after Wang & Hong 1999; Wist et al. 2002; Herrmann 2005).

Photocatalysts are very reactive in the presence of light and can directly react with organic contaminants absorbed onto the surface or indirectly via the formation of radicals such as and HO2 species. In addition hydrogen peroxide (H2O2) can be formed in photocatalytic processes by combining two HO2 species (Benitez et al. 1996). The formed H2O2 can then either participate in the radical chain reactions as a promoter/capturer or scavenger of hydroxyl radicals. Heterogeneous photocatalytic processes using metal oxides such as titanium dioxide and zinc oxide are an emerging technology in water and wastewater treatment. Their applications in oxidising refractory organic contaminants still remain mostly at the laboratory scale. A key point in using this technology is to select the proper heterogeneous catalyst. Further research work is required to develop more active photocatalysts to increase UV quantum efficiency and develop innovative technologies to prevent the potential loss of photocatalysts.

A differential toxicity exists between titanium dioxide and zinc oxide and are maybe related to the mechanisms depending on which of the particles act on the pollutant compound. It is well documented that these two compounds are photosensitive and produce reactive oxygen species (ROS) in the presence of light (Yeber et al. 2000; Fubini & Hubbard 2003; Kubo et al. 2005; Adams et al. 2006). However, there is only a limited amount of research on the direct correlation between photocatalytic ROS production and pollutant degradation activities for TCA as an herbicidal compound.

 Figures 4 and 5 are graphical plots that represent the degradation of TCA pollutant at 1 and 5 mmol/L concentrations respectively (i.e. representing low and high contaminant levels). These graphs show the actual mean values of the measured experimental data plotted with theoretical values derived from using the exponential expression shown in Equation (2). The mean solar intensity (W/m2) was 35.86 (±) 4.1 for the 15 month period of analysis. As can be seen there is a marked increase in photodegradation of TCA for both titanium dioxide and zinc oxide when compared to the values obtained from direct solar photolysis only. This indicates an amplified rate of TCA breakdown directly attributable to photocatalytic activity. The experimental tests carried out in the reactor had matching conditions of pH, photocatalyst dosage rate, initial concentrations of the acid and solar UV-A intensity. Both plots showed that the simultaneous kinetics of the breakdown of the TCA and the disappearance of the respective photocatalyst dose occurred at similar rates to those predicted according to the C–W kinetic model (Wang & Hong 1999; Wist et al. 2002; Herrmann 2005).

Figure 4

Photodegradation of TCA at 1 mmol/L concentration as a function of solar UV-fluence with direct photolysis (light energy, hv) and photocatalysts TiO2 versus ZnO (sample number n = 60).

Figure 4

Photodegradation of TCA at 1 mmol/L concentration as a function of solar UV-fluence with direct photolysis (light energy, hv) and photocatalysts TiO2 versus ZnO (sample number n = 60).

Figure 5

Photodegradation of TCA at 5 mmol/L concentration as a function of solar UV-fluence with direct photolysis (light energy, hv) and photocatalysts TiO2 versus ZnO (sample number n = 60).

Figure 5

Photodegradation of TCA at 5 mmol/L concentration as a function of solar UV-fluence with direct photolysis (light energy, hv) and photocatalysts TiO2 versus ZnO (sample number n = 60).

The photocatalysts have been shown to be highly effective for each of the three TCA concentrations used (Table 1) showing the results obtained for the photodegradation of TCA at 3 mmol/L concentration (i.e. medium level of contamination). For all three cases of TCA pollution, the decay rates for direct solar photolysis disinfection were significantly less than the decay rates when photocatalyst was added. From the analysis of variance, statistical analysis using a sample number of 60, the pollutant (TCA) concentrations at 1 and 5 mmol/L, with direct sunlight treatment showed strong statistical variations (P < 0.01), whereas between 3 and 5 mmol/L and 1 and 3 mmol/L this was less strong (P < 0.05).

Table 1

Inactivation rate constants, K (min−1), for TCA at 3 mmol/L for direct photolysis (light energy only), and photocatalyst TiO2 and ZnO at varying concentrations of 0.5–3.0 mg/L

Inactivation rate constants K (min−1)
Photochemical processesTiO2ZnOPhotocatalyst dosage (mg/L)
Direct photolysis 0.041 0.043 – 
Photocatalyst 0.052 0.055 0.5 
0.064 0.061 1.0 
0.083 0.087 1.5 
0.091 0.098 2.0 
0.052 0.066 2.5 
0.048 0.056 3.0 
Inactivation rate constants K (min−1)
Photochemical processesTiO2ZnOPhotocatalyst dosage (mg/L)
Direct photolysis 0.041 0.043 – 
Photocatalyst 0.052 0.055 0.5 
0.064 0.061 1.0 
0.083 0.087 1.5 
0.091 0.098 2.0 
0.052 0.066 2.5 
0.048 0.056 3.0 

During the experiments, the two photocatalysts showed strong oxidising reactions with minimal production of disinfectant by-products at concentrations <2 mg/L. The mechanisms of solar photocatalyst treatment of contaminated water in the presence of the photosensitised metal oxides occur by electron transfer reactions and negative hydroxyl radical (OH) generators (Ibhadon & Fitzpatrick 2013; Tota-Maharaj & Meeroff 2013). Free hydroxyl radicals act as scavengers and are responsible for the modelled pollutant's putrefaction. In other words, the process kinetics is controlled by the generation rate of the intermediary species that are in turn responsible for the substrates photo-oxidation.

Table 1 also details the results obtained for the photodegradation of TCA at 3 mmol/L concentration when tested against a range of varying concentrations of titanium dioxide and zinc oxide from 0.5 to 3.0 mg/L of photocatalyst concentration. As is apparent that a dosage of 2.0 mg/L was found to be the optimal value for the treatment with both titanium dioxide and zinc oxide. This specific concentration of either photochemical produced the fastest decomposition rates but if even higher dosages were to be added to the influent, the reactor's surface eventually became opaque and there was a subsequent decrease in the rate of photochemical disinfection as also depicted in Table 1. This opaqueness is due to unintended intermediate by-products being generated at higher photocatalyst dosage.

Exponential photodegradation curves representing the decrease in TCA concentration as a function of solar UV-A fluence for concentrations of 5 mmol/L, were based on mean values recorded from the photoreactor's performance. For the degradation of TCA, δ had the following exponential relationships with δ = 100e−0.074iT, δ = 100e−0.057iT and δ = 100e−0.049iT, for concentrations of titanium dioxide ranging from 1.0, 1.5 and 2.0 mg/L, respectively. For zinc oxide, δ = 100e−0.085iT, δ = 100e−0.062iT and δ = 100e−0.040iT, with the same dosage concentrations. In fact the two photocatalysts showed a high efficacy rate when used with natural sunlight with TCA reductions ranging from 95 to 99% for concentrations as low as 1.0 mg/L. In addition, both the photocatalysts were found to be photobleaching and were very effective in this regard at even low concentrations (i.e. less than 2.0 mg/L). However, for higher concentrations (i.e. greater than 3.0 mg/L of photocatalyst), again the reactor's surface eventually became opaque and photochemical disinfection was subsequently limited. Direct photolysis inactivation rate constants were found to be 0.034 and 0.046 min−1 while degradation rates using titanium dioxide ranged from 0.046 to 0.078 min−1 and for zinc oxide from 0.053 to 0.081 min−1, respectively. One-way analyses of variance (ANOVA) statistical tests were carried out between the inflow and outflow pH, as well as the degradation rates constants for both photocatalyst with P > 0.01, indicating no significant statistical differences. Elmolla & Chaudhuri (2010) found that the pH can be influenced by both TiO2 and ZnO with effects on the photo-oxidation activity caused by electrostatic interactions between the active semiconductor (TiO2 or ZnO) and the substrate molecules (in this case TCA). At concentrations of 1.0–2.0 mg/L there were no statistical significance between natural light and the photocatalyst (P > 0.05).

As solar intensity increases, exposure time had to be decreased proportionally to keep the product of time and intensity or fluence constant. Hence, the flow rate through the system was altered and regulated by using a pump with variable speed. This meant percentage reduction of TCA residuals tended to vary with the time of day and hence with the prevailing solar intensity or fluence. As shown in Table 2 the average solar UV-A radiation level occurring on the reactor was 45 W/m2 with a corresponding mean photodegradation rate constant K of value 0.068 m2/W h. The rise in temperature of the test influent wastewater stream did not exceed 11.3 °C although solar radiation may have caused some potential side-effects such as an enhanced TCA-water reaction and enhanced germicidal action of any active TCA residuals. Since the rise in temperature of the water between the inlet and outlet ports of the reactor did not exceed 10 °C when experiments were run over a three hour period, it can be quite confidently assumed that the major photodecomposition effect is due to the solar radiation alone and not due to thermal impacts. The accumulated energy, Ehv, corresponds to the solar energy reaching the reactor's surface and is another way of measuring the reactor's efficacy when degrading this organic compound. For all concentrations of TCA test runs, the mean accumulated energy was calculated to be 23.4 KJ/l as shown in Table 2. This energy is directly related to the rate of degradation of the acid and follows first-order kinetics. The results of this study highlights the potential for a sustainable option of safe treatment and removal of hazardous compounds which, if released into the surface or ground water, can have detrimental effects on the health of ecosystems. These photochemical technologies and applications in developing countries with adequate sunlight and high risk of chemical contamination which occurs in the Caribbean region could become a reality after some refinements to the design, field testing and assessment of costs and acceptability of this renewable technology. In an effort to reduce overall system cost; a low-cost coconut husk filter was incorporated to aid in the water filtration and purification process. Coconut husk and fibres are readily available as a free waste product across Trinidad and Tobago. This further post-solar photochemical disinfection treatment is described by Tota-Maharaj & Meeroff (2013) and uses coconut husk and coconut fibres as a filtration medium which acts as an adsorbent for efficient catalyst removal.

Table 2

Summary of the characteristics of the photochemical reactor and its mean performance for the photodecomposition of TCA for 1, 3 and 5 mmol/L concentrations

Photoreactor's characteristicsComputed numerical value
Solar UV-A intensity (W/m245 
Accumulated solar energy, Ehv (KJ/l) 23.4 
Primary degradation of TCA, δ (%) 100 
Photodegradation rate constant, K (m2/W h) 0.068 
Exposure time per sample (min) 35 
Inflow temperature (°C) 22.3 
Outflow temperature (°C) 33.6 
Flow rate (L/h) 18.5 
Photoreactor's characteristicsComputed numerical value
Solar UV-A intensity (W/m245 
Accumulated solar energy, Ehv (KJ/l) 23.4 
Primary degradation of TCA, δ (%) 100 
Photodegradation rate constant, K (m2/W h) 0.068 
Exposure time per sample (min) 35 
Inflow temperature (°C) 22.3 
Outflow temperature (°C) 33.6 
Flow rate (L/h) 18.5 

CONCLUSIONS AND OUTLOOK

This project investigated the use of solar photocatalysis with titanium dioxide and zinc oxide to degrade a control pollutant, namely TCA, which is a typical herbicide, by-product of chlorine disinfection and is often used in industries such as paint manufacturing. The modelled pollutant TCA photo-degraded at varying rates for both photocatalytic processes with all results indicating that the combined effect of natural light with photocatalyst always outperformed the effect of direct sunlight only. The prototype solar reactor operated and performed satisfactorily during the entire experimental procedure. Thus the various photochemical methods of water treatment which utilise solar energy have proven to be efficient in the degradation of TCA with the developed pilot plant proving to be a functionally simple technique for water disinfection using solar photocatalytic processes.

Initial trial results from this pilot unit suggest that zinc oxide marginally out-performed titanium dioxide as a photocatalyst for a low TCA concentration of 1 mmol/L but there were no significant differences in the performance. At a much higher TCA concentration of 5 mmol/L results were not as definitive, with both photochemicals performing best at their lowest concentrations of 1 mg/L with zinc oxide performing slightly better. The average optimal concentration of photochemical concentration used in this study was found to be 2 mg/L. At higher concentrations, the reactor's surface eventually becomes opaque (due to an increase in turbidity), and there was a subsequent decrease in the rates of photochemical disinfection achieved.

Future considerations should be given to further experiments looking at the potential for solar decontamination of water inoculated with other pollutant organisms and the use of alternative photocatalysts such as silicon dioxide and various other photochemicals (Adams et al. 2006). Further work and investigations would be conducted on possible simultaneous disinfection of microbial contaminates in surface waters. This photochemical facility could also be tested with treated sewage effluent used for irrigation, as livestock water or in hydroponic cultivation practices in Trinidad and Tobago.

This pilot-scale process could be further optimised by judicious use of sensors so that the dosage rate of the photocatalyst could be altered with diurnal variations in influent contamination levels, and the exposure time in the reactor altered according to the daily variations in solar radiation intensity. Design of systems should also take into account both single and multi-pass modes to increase exposure times while minimising the reactor's footprint and reuse of the photocatalyst. Thus, for a potential full-scale system, simple feedback and/or feed forward control or even model predictive control could be used to minimise photocatalyst dosages so they match incoming contaminant levels while exposure times are controlled to match changes in solar radiation incidence levels. This could be done by combining a variable speed pump system with a valve actuator system to allow changes in flow settings from single, dual or multi-mode pass settings.

ACKNOWLEDGEMENTS

The research was supported by Key-North Engineering Limited, St Augustine, Trinidad, West Indies. The authors would also like to thank the Desalination Company of Trinidad and Tobago and specifically their engineering manager, Tawari Tota-Maharaj, for his technical assistance and guidance. The authors wish to express gratitude to Dr Denver Cheddie from the Utilities Engineering Department, University of Trinidad and Tobago for support with measurements and analysis. Finally we would like to acknowledge Dr Krishpersad Manohar from the Department of Mechanical Engineering of the University of the West Indies for supporting this research initiative.

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