Integrated ozone–photo–Fenton process for the removal of pollutant from industrial wastewater☆

P.Asaithambi*,Baharak Sajjadi,Abdul Raman Abdul Aziz*

Department of Chemical Engineering,Faculty of Engineering,University of Malaya,50603,Malaysia

1.Introduction

Organic,inorganic and biorefractory organic pollutants are main water pollutants.Different types of organic effluents that are known to cause serious environmental problems are produced by the pulp and paper industry[1,2],leather industry[3,4],sugar industry[5],dairy industry[6],rubber processing industry[7],agricultural industry[8],distillery industry[9–12],food industry[13],textile industry[14–16],olive processing industry[17],wood based industry[18],etc.Amongst these industries,the distillery industry[12]generates a large amount of wastewater,which has a considerable impact on water bodies.The effluent from the distillery industry is colored and has high chemical oxygen demand(COD),biological oxygen demand(BOD),total solids(TS),total dissolved solids(TDS),total suspended solids(TSS),total volatile solids(TVS)and other organic,inorganic matters such as chlorides,sulfates,total nitrogen,potassium,phosphorous,sodium and calcium which causes pollution when discharged into water bodies without proper treatment[19].Much research has been carried out on different treatment methods of the distillery industrial effluent,including biological flocculation[20],biotreatment[21],biological treatment using thermotolerantPediococcus acidilactici[22],ozone-based advanced oxidation[23],micro filtration[24],reverse osmosis[25],activated carbon[26],ozonation[27],electrochemical degradation[28],electrocoagulation[29],UV photocatalytic[30],cavitationally induced process[31],ultrasound and ozone assisted biological processes[32].These methods are associated with some disadvantages such as high operating cost,lower pollutant removal efficiency,transfer of pollutants from one phase to another.

Some effective and environmental friendly alternatives to conventional oxidative methods have been developed for water treatment.They are generally referred as advanced oxidation processes(AOPs)and they mainly involve UV light in the presence of hydrogen peroxide(H2O2)orozone(O3),UV-nearvisible light in the presence of Fenton reagent,ultrasound,etc.These methods degrade and decolorize pollutants by using highly oxidizing hydroxyl radicals(?OH)radicals that are formed at ambient temperature and atmospheric pressure within the treatment systems.The principal mechanism of AOPs includes generation of very powerful and non-selective oxidizing agents and free hydroxylradicals(?OH)to destroy hazardous organic and inorganic pollutants.A single AOP usually does not completely remove pollutants.Consequently,combining two or more AOPs enhances hydroxyl radical generation,which leads to higher oxidation rates[33].For example,a combination of the photo-Fenton with ozonation process maximizes the removal efficiency of pollutants with minimal operational cost.

To the best of our knowledge,there is limited research on the use of hybrid AOPs for industrial effluent treatment.Kusicet al.did a comparative investigation of the efficiency of several ozone-and/or UV-based processes for the mineralization of phenol.The highest mineralization efficiency was achieved by the UV/H2O2/O3process[34].Catalkaya and Kargi employed different AOPs to treat pulp mill effluent.They found that TiO2-assisted photo-catalysis yielded higher total organic carbon(TOC)and toxicity removals compared to the other AOPs[35].Wuet al.studied isopropyl alcohol degradation and its major degradation intermediate-acetone.The results showed that the UV/H2O2/O3process was the most efficient compared to the other AOPs such as the O3,O3/UV,H2O2/UV,H2O2/O3processes[36].Wu and Ng investigated decolorization of C.I.Reactive Red 2 using ozone-and photo-based AOPs.They observed that the UV/O3/H2O2/Fe3+system were the most suitable for high pollutant removal with minimum power consumption[37].Lucaset al.used a Fenton reagent(H2O2/Fe2+)for the removal of COD from olive mill wastewater in a batch reactor[23].Chandrasekara Pillaiet al.treated terephthalic acid wastewater by ozonation catalyzed process with Fe2+,H2O2and UV irradiation.The combined O3/H2O2/Fe2+/UV process yielded high pollutant removal percentage of around 90%at 240 min compared to other AOPs[38].Hadavifaret al.demonstrated the Fenton and photo-Fenton processes for the treatment of alcohol distillery industries.They concluded that higher removal efficiency was achieved in the photo–Fenton(18%to 97%)process compared to the Fenton(5%to 47%)process alone[39].Most of the previous research focused on pollutant removal efficiency from industrial effluent and wastewater.In the photo-and ozone-based AOPs,electrical energy per order is an important parameter from the economical point ofview.Besides,most of the previous research work focused on removal of pollutants from industrial effluent and wastewater using single AOP.It is not economical to use a single AOP for industrial wastewater treatment due to its high operating cost and lower pollutant removal efficiency.Therefore,it is suggested to integrate these technologies with other methods such as the photo,ozonation and Fenton processes.However,to the best of our knowledge,the removal of color and COD and determination of electrical energy per order of distillery industrial effluent by different AOPs such as the UV/H2O2,Fe2+/H2O2,UV/Fe2+/H2O2and O3/UV/Fe2+/H2O2processes have not been reported yet.

The present work focused on combinations of AOPs such as the UV/H2O2,Fe2+/H2O2,UV/Fe2+/H2O2and O3/UV/Fe2+/H2O2processes for the removal of color,COD and electrical energy per order from distillery industrial effluent.According to our experimental results,the O3/UV/Fe2+/H2O2process achieved complete pollutant removal with minimum electrical energy per order.Therefore,this process was used in the study for the treatment of industrial effluent.Effects of various operating parameters such as H2O2concentration(30 to 180 mmol·L?1),Fe2+concentration(0.05 to 0.80 mmol·L?1),effluent pH(2 to 11),COD concentration(1500 to 6000 mg·L?1)and effect of UV power(8 and 16 W)on color removal,COD removal and electrical energy per order were studied.

2.Material and Methods

2.1.Characterization of distillery industrial ef fl uent

The effluent was collected from nearby distillery premises at Kuala Lumpur,Malaysia.The main characteristics of the effluent were pH:4.1–4.3;COD:80000–90000 mg·L?1;BOD:7000–8000 mg·L?1;TSS:15.44 g·L?1;TDS:5550–5750 mg·L?1;color— dark brown;odor—burnt sugar.The chemicals used in the experiments to adjust the pH value were H2O2(50 wt%),FeSO4·7H2O,H2SO4and NaOH.All the chemicals were of analytical grade and purchased from Merck Specialist Private Limited.Only double distilled water was used for preparing the needed solutions.

2.2.Methods

The experimental setup for the present investigation is schematically shown in Fig.1.The experimental setup consisted of an ozone reactor and a photo-chemical reactor.The photo-chemical reactor was made of borosilicate glass with a net capacity of 600 ml.The reactor was surrounded with a water jacket to remove the heat produced by the lamp and to maintain a constant temperature.The reactor was covered with an aluminum foil to prevent light leakage from the reactor.The reactor was placed on a magnetic stirrer in order to maintain a uniform concentration.The reactor had inlet ports for feeding reactants and for withdrawing sample with measuring temperature at the top.The source of UV irradiation was 16 W low-pressure mercury vapor lamp with the maximum emission of 254 nm,placed in a quartz tube.The lamp tube was immersed in the solution for treatment.

O3was generated using a lab-scale O3generator and O3was bubbled into the photo reactor through a ceramic diffuser for the O3based AOPs.The flow rate and concentration of O3were controlled at 10 L·min?1and 2 g·h?1.The O3concentration was determined using an iodimetric method.All the experiments lasted around 4 h and 10 ml of samples was collected from the sampling port at different time intervals.The samples were then quenched with Na2SO3and filtered using a filter paper to determine the color removal(UV–visible absorbance at the wavelength of λ=290 nm)and COD removal(closed re flux method).

2.3.Analysis

2.3.1.Color removal

The color removal was calculated using Eq.(1).

where,Abs0andAbstare the absorbance atinitialtime and any timetfor corresponding wavelength,λmax.

2.3.2.COD removal

The COD removal was calculated using Eq.(2).

where,COD0and CODt(in mg·L?1)are the chemical oxygen demand(COD)at timet=0(initial)andt(reaction time)respectively.

2.3.3.Electrical energy per order evaluation

The main operating cost of the ozone–photo–Fenton process is associated with electrical energy perorderduring the process.The equations to identify electrical energy per order(EE/O)are discussed below.

Electrical energy per order represents a major fraction of the operating costs.Simple figures-of-merit based on electric energy consumption can be very useful and informative.A thorough understanding of the overall kinetic behavior of organic and inorganic industrial effluents is necessary for describing meaningful electrical efficiencies.Electrical energy per order can be defined as kW·h(kilo watt hour)of electrical energy required to reduce the concentration of a pollutant by 1 order of magnitude in 1 m3of contaminated water.The electrical energy per order EE/O(kW·h·m?3)can be calculated by using the following Eq.(3).

where,EE/O is the electrical energy per order(kW·h·m?3),Pelis the electrical power input(kW),tis the irradiation time(min),Vis the volume of effluent used(L),C0andCtare the initial and final effluent COD concentrations(mg·L?1),respectively.

The color and COD removals of industrial effluents were investigated using the pseudo first-order kinetic model,as shown in Eq.(4)

kis the pseudo first-or derrate constant for the decay of effluent concentration(min?1).

Fig.1.Schematic diagram of ozone–photo–Fenton system.

Combining Eqs.(3)and(4)gives an equation for the electrical energy determination in the following form:

The total electrical energy per order of ozone–photo–Fenton system can be calculated by using Eq.(6).

where,EE/OUV,EE/OO3—electrical energy per order for the photo and ozonation processes.

3.Results and Discussion

3.1.Comparisons of UV/H2O2,Fe2+/H2O2,UV/Fe2+/H2O2,O3/UV/Fe2+/H2O2

Previous research has shown that the ozonation(O3)and photo(UV)processes are not very effective for complete removal of pollutants when used separately[40–42].However,when they are combined with the Fenton process,they have a significant synergistic effect on pollutant removal,mainly due to considerable amount of?OH that is produced from the O3,UV and Fe2+/H2O2processes.The treatment process using this combination starts with the photolysis of ozone,which produces hydroxyl radicals(?OH),as shown in the following equations.

Oxidation of H2O2catalyzed by Fe2+via the classical Fenton's reaction could be one of the main sources of?OH generation.This reaction could be further catalyzed through photo reduction of Fe3+and its reaction with O3,as shown in the following chemical reactions[38].

The color removal percentages,COD removal efficiency and electrical energy per order of four AOPs—UV/H2O2,Fe2+/H2O2,UV/Fe2+/H2O2and O3/UV/Fe2+/H2O2are compared and shown in Fig.2(a)and(b).The figures show that the color and COD removals was about 70.75%,52%,100%,100%and 62.75%,49%,95%,100%for the UV/H2O2,Fe2+/H2O2,UV/Fe2+/H2O2,O3/UV/Fe2+/H2O2processes respectively.Based on Fig.2(a)and(b),it was evident that the color and COD removal efficiencies were significantly higher for the UV/Fe2+/H2O2and O3/UV/Fe2+/H2O2processes compared to the UV/H2O2and Fe2+/H2O2processes.The above results indicated that introducing H2O2,Fe2+and O3into the UV system led to a significant increase in the color and COD removals.This could be attributed to the available parallel pathways to generate abundant?OH radicals for efficient color and COD removals from distillery effluent within a shorter reaction time.

The economic feasibility of hybrid ozonation and photo processes is associated with electrical energy per order.The minimum electrical energy per order of 0.015 kW·h·m?3was required for the removal of color(100%)and COD(100%)in the O3/UV/Fe2+/H2O2process.The other hybrid processes such as the UV/H2O2and UV/Fe2+/H2O2processes required high power consumption for color and COD removals compared to the O3/UV/Fe2+/H2O2process.

3.2.Effect of experimental parameters on O3/UV/Fe2+/H2O2 process

The effects of selected operating parameters of the O3/UV/Fe2+/H2O2process,including H2O2and Fe2+concentration,effluent pH,COD concentration and UV power on decolorization and degradation of distillery industrial wastewater were investigated in this study and the results are discussed below.

Fig.2.Comparison of various AOPs such as UV/H2O2,Fe2+/H2O2,UV/Fe2+/H2O2 and O3/UV/Fe2+/H2O2 on(a)color and COD removals and(b)electrical energy per order.

3.2.1.Effect of H2O2 concentration

The oxidation of industrial effluent by the O3/UV/Fe2+/H2O2process was tested with the feed amounts of oxidants(H2O2)chosen from the preliminary experiments.The results are shown in Fig.3.It can be ascertained from Fig.3 that the H2O2concentration varied from 30 to 120 mmol·L?1and the color and COD removals increased from 53.06%to 100%and 40.26%to 91.65%,respectively within 3 h.The COD removal slightly decreased from 91.65%to 90.43%by further increasing the concentration of H2O2from 120 to 180 mmol·L?1.This phenomenon,which is known as?OH scavenging by H2O2,has been widely reported in the literature[43,44].The reactions are shown in Eqs.(13)–(15).

Fig.3.Effect of H2O2 concentration on color and COD removals and electric energy per order in O3/UV/Fe2+/H2O2 process(conditions:COD concentration:3000 mg·L?1;Fe2+concentration:0.40 mmol·L?1;effluent pH:7;UV lamp:16 W and 254 nm;O3 flow rate:10 L·min?1 and reaction time:3 h).

The electrical energy per order was calculated based on different concentrations of H2O2shown in Fig.3.The electrical energy per order decreased from 0.95 to 0.51 kW·h·m?3with increasing H2O2concentration from 30 to 120 mmol·L?1.It was due to generation of more?OH radicals.However,the electrical energy per order tend to increase slightly from 0.51 to 0.55 kW·h·m?3with increasing concentration of H2O2from 120 to 180 mmol·L?1,which may be attributed to coincident?OH consumption at high H2O2concentrations[45].

3.2.2.Effect of Fe2+concentration

The amount of Fe2+concentration in the O3/UV/Fe2+/H2O2process is an important parameter affecting the oxidation processes.Fe2+acts as a catalyst,does not precipitate in the reaction and enhances the O3/UV/Fe2+/H2O2process.Therefore,with increasing Fe2+iron,the surface of iron and simultaneous production of free radicals increased,which eventually increased the color and COD removal efficiencies.The results are shown in Fig.4.The figure shows that when Fe2+concentration increased from 0.05 to 0.40 mmol·L?1,the color and COD removals increased from 88.52%to 100%and from 82.30%to 95.53%,respectively.Further increase in Fe2+concentration from 0.40 to 0.80 mmol·L?1slightly decreased the color and COD removals from 100%to 98%and 95.53%to 94.05%,respectively.It could be explained by redox reaction that?OH is either scavenged by hydroxyl radicals or Fe2+,as shown in the following equations[46].

Fig.4.Effect of Fe2+concentration on color and COD removals and electric energy per order in O3/UV/Fe2+/H2O2 process(conditions:COD concentration:3000 mg·L?1;H2O2 concentration:120 mmol·L?1;effluent pH:7;UV lamp:16 W and 254 nm;O3 flow rate:10 L·min?1 and reaction time:3 h).

The effect of Fe2+concentration on electrical energy per order is shown in Fig.4.Experiments with varying Fe2+concentrations from 0.05 to 0.80 mmol·L?1were conducted.When the concentration of Fe2+increased from 0.05 to 0.40 mmol·L?1,the electrical energy per order decreased from 1.21 to 0.51 kW·h·m?3.Further increasing the initial concentration of Fe2+from 0.40 to 0.80 mmol·L?1increased the electrical energy per order from 0.51 to 0.87 kW·h·m?3.The above results indicated that more hydroxyl radicals could be produced with optimum Fe2+and H2O2concentrations in the UV and O3processes.

3.2.3.Effect of pH

The optimum pH for removal of pollutants by using the Fenton and photo–Fenton processes is between 2 and 4.5[47–49].However,the ozonation process is more efficient at high pH values compared to under an acidic condition due to the presence of hydroxyl radicals[50].Fig.5.shows the effect of the effluent pH value during the use of O3/UV/Fe2+/H2O2process.It can be ascertained from the figure that the color and COD removals increased from 88.90%to 100%and from 80.76%to 95.45%with increasing effluent pH from2 to 7.Further increasing effluent pH from 7 to 11 slightly decreased the color and COD removals.

Electrical energy perorder of the O3/UV/Fe2+/H2O2process at different effluent pH was studied and the results are shown in Fig.5.It was observed that the electrical energy per order decreased from 1.19 to 0.51 kW·h·m?3with increasing effluent pH from 2 to 7,hence the electrical energy per order also increased from 0.51 to 0.96 kW·h·m?3with further increase of the initial effluent pH from 7 to 11.The above results indicated that more hydroxyl radicals were produced at neutral condition than at acidic and alkali conditions.

Fig.5.Effect of pH on color and COD removals and electric energy per order in O3/UV/Fe2+/H2O2 process(conditions:COD concentration:3000 mg·L?1;H2O2 concentration:120 mmol·L?1;Fe2+concentration:0.4 mmol·L?1;UV lamp:16 W and 254 nm;O3 flow rate:10 L·min?1 and reaction time:3 h).

3.2.4.Effect of COD concentration

The effect of COD concentration on color and COD removals of the distillery industrial effluent by the O3/UV/Fe2+/H2O2process and the associated electrical energy per order were investigated.Pollutant concentration is an important parameter in any effluent treatment techniques.The influence of COD concentration is shown in Fig.6.The figure showed that the color and COD removals decreased from 100%to 75%and from 100%to 63%with increasing COD concentration from 1500 to 6000 mg·L?1within 3 h.It was also observed that the electrical energy per order increased with increasing COD concentration.It was attributed to the fact that an increase in COD concentration increases the number of organic molecules but the adding of H2O2and Fe2+was constant,consequently,the generation of?OH radicals is constant.The results showed that the?OH radicals produced at high COD concentration were inefficient to absorb all the COD concentrations in the solution which decrease the color and COD removals.Increasing the number of organic molecules also obstructs the penetration of photons into the solution,therefore producing less?OH radicals[51].It was suggested that the optimal COD concentration for the O3/UV/Fe2+/H2O2process in the experiment was 3000 mg·L?1and the color and COD removal percentages reached 100%and 95.50%.

Fig.6.Effect of COD concentration on color and COD removals in O3/UV/Fe2+/H2O2 process(conditions:H2O2 concentration:120 mmol·L?1;Fe2+concentration:0.4 mmol·L?1;effluent pH:7;UV lamp:16 W and 254 nm;O3 flow rate:10 L·min?1 and reaction time:3 h).

3.2.5.Effect of UV power

The effects of UV power have important parameters to check performance of O3/UV/Fe2+/H2O2process.UV is mainly used for photolysis of H2O2and photo reduction of ferric ion to ferrous ion.The influence of UV on color and COD removals of distillery industrial effluent was investigated by using two different UV powers—8 and 16 W.The results are shown in Fig.7.The color and COD removals increased from95%to 100%and from90%to 100%within 3 h when the UV powers were 8 and 16 W,respectively.This could be attributed to increased number of photons that reacted with Fe3+ions,which increased the number of active species and formed hydroxyl radicals.The photolysis rate of H2O2and photo reduction of Fe3+reduced at low UV power.

3.3.Instrumental analysis

Treatment(color and COD removals)of distillery industrial effluent by the O3/UV/Fe2+/H2O2process was performed in this study.After a predetermined time interval,the samples were evaluated using UV/Vis spectra and the results are shown in Fig.8.The absorbance of UV/Vis peak decreased by increasing the reaction time of the O3/UV/Fe2+/H2O2system.The position of the peak in the UV/Vis region also changed from 301 nm to around 260 nm,which indicated formation of new smaller organic byproducts.According to the UV/Vis data,it might be attributed to the color and COD removals of the distillery effluent.

Fig.7.Effect of UV power on color and COD removals in O3/UV/Fe2+/H2O2 process(conditions:COD concentration:3000 mg·L?1;H2O2 concentration:120 mmol·L?1;Fe2+concentration:0.4 mmol·L?1;effluent pH:7;UV lamp:16 W and 254 nm;O3 flow rate:10 L·min?1 and reaction time:3 h).

4.Conclusions

In this study,the color and COD removals of distillery industrial effluent using the UV/H2O2,H2O2/Fe2+,UV/H2O2/Fe2+and O3/UV/Fe2+/H2O2processes,together with the associated electrical energy per order were investigated successfully.The results showed that the hybrid system of O3/UV/Fe2+/H2O2completely removed color and COD from the effluent within 3 h with minimum electrical energy per order.Various operating parameters of the hybrid AOPs(O3/UV/Fe2+/H2O2)were studied.It was concluded that the O3/UV/Fe2+/H2O2process was effective compared to other processes reported in the present investigation.The results of the O3/UV/Fe2+/H2O2process for removal of pollutants from industrial effluent showed that this hybrid process could be used as an efficient,effective and environmental friendly technique for complete removal of organic and inorganic pollutants,which increases the reusability of wastewater.

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