Photo Catalytic Removal of Contaminants from ...

International Journal of Applied Environmental Sciences ISSN 0973-6077 Volume 8, Number 2 (2013), pp. 113-128 © Research India Publications http://www.ripublication.com/ijaes.htm

Photo Catalytic Removal of Contaminants from Secondary Treated Municipal Wastewater in a Continuous Recirculation Reactor Anna J. Cheriyan1*, Feroz Shaik1, Mahad Said Ali Bawaain2 and Jyoti P. Sarkar3 1

Caledonian College of Engineering, Post Box No 2322, CPO Seeb, PC 111, Sultanate Of Oman. 2 Department of Civil and Architectural Engineering , Sultan Qaboos University, PO Box 33 , PC 123, Al Khodh, Sultanate of Oman. 3 Chemical Engineering Department, N.I.T.Durgapur-713209, West Bengal, India. E-mail: *[email protected]

Abstract In this work, we have tried to show that photocatalysis can be effectively applied to degradation of organics in secondary treated municipal wastewater, when applied as a thin film coating on a glass reactor. The samples were taken from the outlet of the secondary clarifier in the municipal waste water plant located at Al Khoudh, Muscat, Sultanate of Oman. The experiments were done under exposure to natural solar radiation that is abundantly available in this area. The influence of flow rate, radiation incidence angle, exposure time, presence of reflector and type of catalyst was investigated. Differences in the degradation efficiency have been observed under different experimental conditions. Keywords: COD, Photocatalyst, coating, solar radiation, Total Coliforms.

1. Introduction The conventional method widely used for the treatment of wastewater involves chlorination. However the carcinogenic, mutagenic and odorous risks posed due to the formation of chlorinated by products are enormous [1]. Subsequently, dechlorination

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of the treated water will be required so as to maintain the low Chlorine levels in aquatic organisms [2]. Photocatalytic treatment is an emerging area that is being researched on for application to treatment of wastewater [3]. Many investigators have discussed the importance of Photo catalysis as an important alternative method for the treatment of polluted water [4-5]. Studies have been carried out in the last decade employing photo catalysis for treatment of wastewater from secondary treatment [6]. The photocatalytic treatment mechanism adopts a heterogeneous catalytic procedure, where the catalyst is in a different phase and hence separation of the catalyst for reuse is possible. The electronic structure of semiconductor is characterized by a valence band that is filled and an empty conduction band. On excitation by UV, of appropriate wavelength, a hole electron pair is created. The stored energy is wasted by recombination in the absence of suitable scavengers [7]. In the presence of organics , the oxidation occurs in this process, through an attack of hydroxyl radical OH•, which has a rate constant billions of times higher than normal rate constants, using air as the oxidant [8]. In the present work we have studied the photo catalytic degradation of organics and removal of Total Coliforms in secondary treated wastewater under various experimental conditions. The effect of varying the radiation incidence angle, effluent flow rates, kind of photocatalyst and exposure time on the degradation efficiency was evaluated. The photocatalyst was fixed to a support.

2. Materials and Methods 2.1 Chemicals All chemicals were procured from standard organizations. Two photocatalysts were employed for the experiments. Titanium dioxide was supplied by Merck and Titanium isopropoxide was supplied by Aldrich.. 2.2 Sample collection 5 litre containers, rinsed with 0.1 N Nitric acid followed by rinsing twice with de ionized water, were used for sample collection. Secondary treated wastewater samples were collected from the clarifier outlet in the municipal waste water plant located at Al Khoudh, Muscat, Sultanate of Oman. This plant employs activated sludge process and settling tanks for the secondary treatment. The samples for testing of total coliforms were collected in sterile 100 ml bottles. All samples were transferred immediately to the lab and stored under refrigerated conditions at 4⁰ C. 2.3 Preparation of photo catalyst coating One catalyst coating was prepared by the standard method followed by many researchers [9-11]. A slurry was prepared using TiO2 powder, Ethanol and Nitric acid. The mixture was sonicated to get uniform slurry. The cleaned glass tube was coated with slurry by the ‘dip coating’ method at a very slow rate.The coated tube was then air dried for half an hour. The process of coating was repeated four times to increase the amount of catalyst that gets attached to enhance the activity [12]. The glass tubes

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were then calcined at 350⁰ C for 30 minutes. The coated tubes were then washed with deionized water prior to experimental use. Another catalyst coating was prepared by stirring 20 ml Titanium isoproxide in 150 ml isopropyl alcohol for 30 mins and then keeping the mixture for 30 minutes [13].The glass tubes were coated using the dip coating method. 2.4 Solar Photo reactor A solar photo catalytic glass tube reactor was constructed to perform the experiments on municipal wastewater under the weather conditions at Sultan Qaboos University, Al Khoudh, Oman. The photocatalytic experiments were performed under direct exposure to solar radiation at the terrace of Civil Engineering block, Sultan Qaboos University from 09.30AM to 02.30 AM every day during the months of July and August 2011. The borosilicate glass tube coated with TiO2 coating was placed at the focus of a reflector. A peristaltic pump was used for feeding and recirculating the effluent through the system. The reactor was operated in recycling mode. The glass tube reactor was mounted on a platform which could be tilted, so that the desired radiation incidence angle can be fixed (Figure 1).

Figure 1: Schematic of the experimental set up. The outer diameter of the glass tube was 10 mm. The surface area of the solar reflector is 0.09 m2. The volume handled is 1 litre and the illuminated volume is 0.025 l. 2.5 Experimental procedure The municipal water was diluted with distilled water in a 50: 50 ratio by volume. Before the start of experiments, the solution was circulated for 1 hour in the dark for evaluation of initial adsorption. The reactor was then uncovered and the photocatalytic process started. In the experiments, the angle of radiation incidence varied between 6⁰ to 22⁰, the flow rates varied between 0.003 m3/hr to 0.007 m3/hr. For a fixed angle and for a particular flow rate, the effluent was recirculated throughout the system for a 5 hour exposure with reflector and without it. Samples were collected every 1 hour and

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centrifuged before analyzing for parameters COD, TOC, pH, conductivity, Total Coliforms, Nitrite, Nitrate, Sulphate, Chloride ,Phosphate and alkalinity. 2.6 Analytical determination Total Organic Carbon (TOC ) was measured by the direct injection of the filtered samples into Shimadzu-5050A TOC analyzer. COD was measured using the standard reflux method [14] using a Merck Thermoreactor TR 620. Anion concentrations were measured using ion exchange chromatography( Dionex DX-120 ). Conductivity and pH measurements were done on Arion make model. The UV radiation intensity was measured using a SUNAAY make radiometer. Alkalinity was determined by titration with Sulphuric acid using Methyl Orange as an indicator.

3. Results

Decrease in Coliforms, %

3.1 Total Coliforms The initial waste water had a concentration of 2419 MPN/ ml of Total Coliforms. Treatment with photolytic oxidation gave a reduction of 96 % after exposure to 5 hours of solar radiation, when TiO2 coating was used, with the flow rate at 0.007 m3/hr and an inclination of 15⁰ without a reflector. This reduction percentage was increased to 99.86 % when a reflector was used for the same conditions. For comparing the efficiency in terms of type of coating, use of glass tube with coating made from Tianium isopropoxide gave a reduction of 98.5 % for the above conditions without a reflector. The graphs below show the trend described above. It is probable [15-17] that the damage to the cell is initiated by the permeability increase created in the cell wall. This will allow the intracellular contents to diffuse out of the cell and get oxidized by the photo catalysis mechanism, thus resulting in cell death. 120

0.003 m³/hr 0.005 m³/hr 0.007 m³/hr

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Figure 2: Variation in Coliforms at different flow rates, with an inclination of 15˚ in the presence of solar radiation on TiO2 coated glass reactor.

Decrease in Coliforms, %


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Figure 3: Variation in Coliforms at different flow rates , at an inclination of 15 ˚ , in the presence of solar radiation on TiO2 coated glass reactor with a reflector

Decrease in Coliforms , %

It could be observed that coating the tube with Titanium isopropoxide gave a better removal percentage for the Total coliforms for a selected experimental condition as can be seen from the figure shown below (Figure 4.)

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Figure 4: Variation in Coliforms in glass reactor coated with two different catalysts in the presence of solar radiation, without reflector at an angle of 15˚ and 0.005 m3/hr flow rate.

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3.2 TOC and COD The experimental study showed a removal of 80 % of the initial TOC, after 5 hours of exposure to solar radiation with 0.007 m3/hr flow rate at 15⁰ inclinations with TiO2 coating without using a reflector (Figures.5-6). The removal percentage increased to 88 % in the presence of a reflector, for the same experimental conditions and solar radiation exposure time. Around 75 % of the COD was removed without a reflector and the use of reflector enhanced the removal by 12 % for the same experimental conditions (Figures.7-8). The effect of coating type is apparent from the figure shown below (Figure 9.). The coating done with TiO2 showed better removal efficiency for TOC and COD.

Figure 5: Variation in TOC at different inclinations, with a flow rate of 0.007 m3/hr in the presence of solar radiation on TiO2 coated glass reactor. One of the experimental conditions varied was flow rate. Three flow rates- 0.003 m3/hr, 0.005 m3/hr and 0.007 m3/hr were considered for a particular angle of tilt. From the experiments, it can be seen that a flow rate of 0.007 m3/hr gave the maximum change in the measured parameters.

Figure 6: Variation in TOC at different flow rates , at an inclination of 15 ˚ , in the presence of solar radiation on TiO2 coated glass reactor with a reflector


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Similarly the angle of inclination was fixed at 6⁰, 15⁰ and 22⁰ varied by tilting the experimental platform. For a fixed flow rate, the reactor inclined at an angle of 15⁰ gave the optimum performance with regard to change in the monitored parameters. angle 6°

Decrease in COD , %

angle 15° angle 22°

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Decrease in COD , %

Figure 7: Variation in COD at different inclinations, with a flow rate of 0.007 m3/hr in the presence of solar radiation on TiO2 coated glass reactor. angle 6° angle 15° angle 22°

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Figure 8: Variation in COD at different inclinations, with a flow rate of 0.007 m3/hr in the presence of solar radiation on TiO2 coated glass reactor with a reflector.

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Decrease in COD ,%

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Figure 9: Variation in COD in glass reactor coated with different catalysts in the presence of solar radiation, without reflector at an inclination of 15 ⁰ and at 0.005 m3/hr flow rate.

Increase in Sulphate, %

3.3 Anion concentrations The organic contaminants containing Sulphur atoms may discharge Sulphate ions on degradation. The release may be initiated by the attack of the reactive hydroxyls radicals on the Sulphonyl groups of the organic compounds [18]. It was observed that there was a 70 % increase in the concentration of Sulphate ions after photo catalytic treatment for 5 hours exposure at a flow rate of 0.007 m3/hr and an inclination of 15⁰ without any reflector. Use of a reflector leads to a 75 % increase in Sulphate concentration. There was not much influence of the coating type on the sulphate ion concentration increase. The graphs below show the trend described above (Figure 10-11). 70 65 60 55 50 45 40 35 30 25 20 15 10 5 0

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Figure 10: Variation in Sulphate at different inclinations, with a flow rate of 0.007 m3/hr in the presence of solar radiation on TiO2 coated glass reactor.

Increase in Sulphate, %

Photo Catalytic Removal of Contaminants from Secondary Treated 75 70 65 60 55 50 45 40 35 30 25 20 15 10 5 0

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Figure 11: Variation in Sulphate at different flow rates , at an inclination of 15 ˚ , in the presence of solar radiation on TiO2 coated glass reactor with a reflector.

Increase in Fluoride, %

Organophosphorous compounds on degradation produce Phosphate ions at high pH [18]. At high concentrations of these inorganic ions the mineralisation rate can be decreased due to the adsorption of the ions on the photo catalyst sites. The variations in the concentrations of the anions Phosphate, Fluoride, Chloride and Nitrate can be seen in the following graphs for the different experimental conditions.

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Figure 12: Variation in Fluoride at different flow rates, with an inclination of 15˚ in the presence of solar radiation on TiO2 coated glass reactor.

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Increase in Fluoride, %

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Figure 13: Variation in Fluoride at different flow rates , at an inclination of 15 ˚ , in the presence of solar radiation on TiO2 coated glass reactor with a reflector. 0.003 m³/hr

Increase in Phosphate, %

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Figure 14: Variation in Phosphate at different flow rates, with an inclination of 15˚ in the presence of solar radiation on TiO2 coated glass reactor.


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Increase in Phosphate,%

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Figure 15: Variation in Phosphate at different flow rates , at an inclination of 15 ˚ , in the presence of solar radiation on TiO2 coated glass reactor with a reflector.

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Increase in Chloride, %

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Figure 16: Variation in Chloride at different flow rates, with an inclination of 15˚ in the presence of solar radiation on TiO2 coated glass reactor.


Increase in Chloride, %

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Figure 17: Variation in Chloride at different flow rates, at an inclination of 15 ˚, in the presence of solar radiation on TiO2 coated glass reactor with a reflector. 3.5 Conductivity and pH The conductivity kept increasing for the experimental conditions as can be seen in the figures shown (Figure 18-19) below. There was no much variation in the pH. The increase in conductivity may be due to the increase in the anion concentrations.

Increase in Conductivity , %

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Figure 18: Variation in conductivity at different flow rates, at an inclination of 15 ˚, in the presence of solar radiation on TiO2 coated glass reactor with a reflector.


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Increase in pH, %

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Figure 19: Variation in pH at different inclinations, with a flow rate of 0.007 m3/hr in the presence of solar radiation on TiO2 coated glass reactor. 3.6 Coating Characteristics Two identical borosilicate glass plates were prepared with the coatings using Titanium dioxide and Titanium isopropoxide and were subjected to photo catalysis under solar radiation for a period of 11 hours to observe changes in their surface structure.Scanning electron microscopy (JSM -5600) was used to observe the surface characteristics. The sample was prepared employing a standard sputtering technique. Surface morphology of the coatings shows that the coating formed by Titanium dioxide was homogeneous and no fractures were evident on the surface after its application as a photo catalyst (Figure 20-21). The coating prepared using Titanium isopropoxide shows a comparably uniform surface structure. However fissures could be observed on the surface structure after the experimental duration (Figure 22-23).

Figure 20: Scanning electroscopy image for the glass plate coated using Titanium dioxide before subjecting to photo catalytic treatment.

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Figure 21: Scanning electroscopy image for the glass plate coated using Titanium dioxide after subjecting to photo catalytic treatment.

Figure 22: Scanning electroscopy image for the glass plate coated using Titanium isopropoxide before subjecting to photo catalytic treatment.

Figure 23: Scanning electroscopy image for the glass plate coated using Titanium isopropoxide after subjecting to photo catalytic treatment.


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Conclusion The increase in efficiency with increase in flow rates can be because the contact with the coated catalyst increases with flow rate. The experimental results show that there is a scope for application of photo catalysis in real wastewater systems , probably after the tertiary stage of treatment to increase its potabilty. The photonic efficiency of the coatings prepared by Titanium Isoproxide route , when measured in terms of the COD and TOC reduction is less when compared to the coating prepared by using Titanium dioxide. This may be due to the amount of TiO2 that is deposited on the coating , when prepared by using Titanium isopropxide. However, uncertainty in results can be there due to many reasons like evaporation of the solvent, adsorption of the solvent on the reactor sites, difficulty in analyzing the complex municipal water mixture due to the its complex nature. The quantitative and qualitative evaluation of the intermediates that can be formed is an arduous task that calls for the use of sophisticated instruments. Hence the assessment of the pollution disappearance measured in terms of the TOC and COD alone in a short exposure time is insufficient to advocate the use of Photocatalyst methodology as there can be possibility of formation of toxic and bio resistant intermediate compounds.

Acknowledgement The authors would like to acknowledge the financial support received from “The Research Council” of Oman through grant number 43(RC/ENG/CAED/1101).

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