Performance Evaluation of Photo-Fenton and Fenton ...

International Review of Chemical Engineering (I.RE.CH.E.), Vol. 5, N. 4 ISSN 2035-1755 July 2013

Performance Evaluation of Photo-Fenton and Fenton Processses for Dairy Effluent Treatment Carla C. A. Loures, Hélcio J. Izário. Filho, Gisella R. Lamas Samanamud, André L. Souza, Rodrigo F. S. Salazar, André L. C. Peixoto, Oswaldo L. C. Guimarães Abstract – This study aimed to evaluate the efficiency of Photo-Fenton and Fenton processes in reducing organic matter of dairy effluent. An Orthogonal Array L9 Taguchi was used to determine optimal conditions of acidity media, temperature, Fenton concentration and UV radiation intensity. Reaction time was set up to 60 min. Optimized parameters were: pH 3.0, temperature, Fenton reagent concentration and UV radiation at the highest level. The Dissolved Organic Carbon percentage reduction (DOC) was 91 %. An effective degradation study was carried out, in which, the reduction percentage was found to be less than the most efficient DOC removal. A cost/benefit evaluation of the AOP process employed on the in natura dairy effluent treatment showed that the reagent consumption the main cost of the process. The highest efficiency experiment for the dairy effluent of this study had operational costs lower than US$ 0.50 to PhotoFenton and Fenton processes, respectively. Copyright © 2013 Praise Worthy Prize S.r.l. - All rights reserved.

Keywords: Dairy Effluent, Photo-Fenton, Taguchi Method, Economic Viability, Efficient Organic Degradation, Cost/Benefit Ratio

I.

Among biological methods, methanization is by far the most interesting since it transforms the organic matter in milk to methane, a compound well known for its unquestionable combustion properties. Moreover, high removal rates of the COD are obtained [11] even though this process does not eliminate phosphorus and nitrogen compounds contained in the dairy effluents [12]. Generally, biological effluent treatment processes (aerobic and anaerobic processes) offer the lowest cost per unit volume of wastewater treated or per kg of COD removed. The unit cost is normally lower than $5 per m3 for biological processes and higher than $10 per m3 for AOP. Consequently, biological processes are the preferred choice of the industry for wastewater treatment [13]. However, effluents containing non-biodegradable compounds pose a challenge to biological treatment schemes. Biological processes do not normally remove nonbiodegradable chemicals and in some cases, high concentration of such chemicals may inhibit the biological processes resulting in poor performance. Non-biodegradable compounds can be recalcitrant and/or toxic to microorganisms. The presence of such compounds requires non-biological processes for effective elimination and AOP have such a capability. Lately, new techniques have been proposed in order to solve some biological system limitations and improve the organic matter degradation and other compounds on different effluents [14], [15]. Advanced Oxidation Processes (AOPs) are very efficient technologies for

Introduction

The dairy industries are associated with generation of large amounts of wastewater. Water, in these industries, is used in all steps, such as: cleaning, sanitization, heating, cooling and washing floors [1]-[2]. A key feature of this sewage is bio organic matter and nonbiodegradable one due to the presence of organic compounds such as: fatty acids, esters, alcohols, aldehydes, ketones, amines and other nitrogen compounds [3]-[4]. Wastewaters from agro-industries are characterized by high chemical oxygen demand (COD) due to their high level of organic contents [5]. The dairy industry is particularly concerned: as a matter of fact it generates a huge amount of wastewaters: approximately 0.2 L to 10 L of waste per liter of processed milk [6]. In most cases, these effluents are not treated and are simply thrown into rivers where they contribute to eutrophication by phosphorus and nitrogen compounds [7]-[9]. Treating dairy effluents is thus of crucial importance not only for the environment, but also for the purpose of recycling water for use in industrial processes [10]. Nowadays, many physicochemical and biological methods are used to treat dairy effluents, with the particular aim of reducing the volume of the produced sludge. The physicochemical processes suffer the disadvantage that reagent costs are high and the soluble COD removal is low [5]. Moreover, chemical treatments could induce a secondary pollution due to the fact that chemical additives may contaminate the treated water. Manuscript received and revised June 2013, accepted July 2013

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some organic compounds that are especially difficult to degrade. During the past decade, studies using AOPs have been widely researched for different types of wastewater, including membranes enhanced by nanotechnologies [16]-[20]. AOPs refer specifically to processes that can produce hydroxyl radicals (•OH). Due to high standard reduction potential (1), this radical can oxidize a variety of organic compounds to CO2, H2O and inorganic ions from hetero-atoms [21]: •OH + e- + H+ → H2O Eo = 2.730 V

Finally, signal-to-noise (SN) in the Taguchi method can be used to optimize the process and to reduce the process variability [31]. The aim of this study consisted in evaluating the efficiency of bench-scale reactors from dairy effluent treatment by AOP – Photo-Fenton process. An economic evaluation of the Photo-Fenton process was also performed to balance the relation between energy and reagent consumption taking into account the chemical process only.

(1)

II.

2+

Photo-Fenton’s reaction (UV/Fe + H2O2), has demonstrated to be one of the most efficient process for effluent treatment due to high oxidation power. When Fe (III) complexes are irradiated, a central-orbit electron is promoted and a ligand bonds to a central metal, called Ligand-to-Metal Charge Transfer (LMCT), which implies in the reduction of Fe (III), Fe (II) and ligand oxidation (2), forming hydroxyl radical [22]: Fe(OH)2+ + hv → Fe2+ + •OH

II.1.

Experimental

Sampling and Preservation

The effluent studied comes from a dairy factory in Vale do Paraiba region, city of Guaratinguetá, São Paulo. This sample was collected from the production line that feeds the treatment plant before the acid pretreatment. 400 L of raw effluent were collected and stocked in a cold chamber at 4 ºC. Due to complex characteristics some methodologies were adapted to improve the accuracy and precision of analytical results [31]. To determine COD in dairy effluents, some adjustments were made to 5220 D Closed Reflux colorimetric method of [32]. To measure Biochemical Oxygen Demand (BOD), Winkler’s modified method was used. For COD and BOD determinations, preliminary tests with diluted deionized water were used due to specific concentrations on each methodology. The dissolved organic carbon (DOC) determination was performed in a TOC-VCPH Total Organic Carbon Analyzer (high sensitivity) from Shimadzu based on high temperature catalytic oxidation. The statistic and graphic analysis were done using software Minitab R15. Ferrous ions mass and H2O2 estimation used in the Photo-Fenton treatment To determine the preliminary mass proportion between H2O2 and Fe2+ of Fenton reagent, the H2O2 weight (30 % w/w) necessary to the radical formation •OH as shown in (3) [16]:

(2)

Fe2+ generated by irradiation in the presence of hydrogen peroxide reacts promoting the sequence of Fenton’s reaction. In the catalytic reaction, a cycle of regenerated Fe2+ should be established. The use of Fe2+/Fe3+ in the presence of hydrogen peroxide under irradiation is called Photo-Fenton. Generally, the optimum molar ratio H2O2/Fe2+ recommended for a Fenton treatment is from 10 to 40 [23], [24]. The hydrogen peroxide dosage is important to obtain the best degradation efficiency while the concentration of Ferrous ions is important to the reaction kinetics [24]. However, the excess of any of these compounds may decrease the degradation efficiency, since both H2O2 and Fe2+ can capture hydroxyls radicals [25]. The H2O2 / Fe2+ stoichiometric ratio depend on physical-chemicals characteristics of the wastewater. In the same way, design of experiments has been widely used to optimize process parameters and to enhance the quality of products by applying engineering and statistics concepts [26]. In 1987, Taguchi [27] developed and published a statistical tool for designingof-experiments (DOE) to meet the above requirements. The Taguchi technique provides an efficient and systematic method to optimize designs for performance, quality and cost [28]. It has been successfully employed in designing reliable, high quality products at relatively low costs [29]. The advantage of employing Taguchi technique has been summarized by [30]. Primarily, the method requires limited number of experiments to conduct the experimental design. Another important point is that many different variables can be examined simultaneously. This means that predominant parameters can be investigated deeply whereas, secondary parameters can be overlooked. Therefore, time, energy and resources can be saved.

C + 2 H2O2 → CO2 + 2 H2O

(3)

From the theoretical mass of H2O2 a ratio of H2O2/Fe2+ was estimated for each level of Fenton reagent. Seeking for a better mass proportion, some preliminary experiments were conducted under the following conditions: pH = 3, 0; temperature = 25 ºC; reaction time = 2 h; UV irradiation = 28 W, and aliquots taken every 30 min. It was verified that degradation efficiency of the pollutant linearly grows with an increase in the H2O2 dosage [33]-[34]. However, in some cases, the increase in concentration of peroxide does not cause a significant increase in the degradation efficiency [29]-[30] and might even promote the reduction in the process efficiency [30], since it acts as scavenger.


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II.2.

Degradation of Dairy Effluent by Photo-Fenton Process

proposed for this stage were: pH, temperature, concentrations of H2O2 and Fe2+ in Fenton’s reagent and UV radiation source. Table I shows selected levels variables per liter of effluent treated dairy. The levels selection were based on [23]. Most studies performed with Photo-Fenton showed pH of 3.0–4.0, which is considered outstanding preventing ferric salts hydrolysis. For this exploratory experimental design, a higher hydrogen ion concentration ratio (pH 3.0 to 5.0) was used to evaluate the effluent behavior under oxidative conditions in different values of acidity. Temperature values were selected considering two important aspects: average local temperature (~30 ºC) and, peroxide solution degradation temperature. The levels for Photo-Fenton’s reagents were based on preliminary studies. Low-pressure Hg lamps of 15 and 28 W were used for ultraviolet radiation, evaluating the radiation of Photo-Fenton process in the dairy effluent characterized by high concentrations of dissolved solids and high turbidity. The variable response was given in percentage of DOC reduction, according to the Eq. (4):

Photochemical treatment was performed in a semi batch reactor. Fenton’s reagent was added according to the following stock concentrations: 0.82 mol L-1 of FeSO47H2O, H2O2 at 30 % w/w. After thermal conditioning of the effluent, ferrous ions, and H2O2 solutions were added at the system, simultaneously. These additions were performed by dosing pump during 50 min in 1h of reaction. Sulfuric acid and NaOH (5.0 mol L-1, both) solutions were used to maintain the medium acidity during all reaction time. This control was performed by a potentiometer with borosilicate glass electrode. The electrode was kept in the reaction bath. The operational stages with Photo-Fenton/Fenton processes were as follows: 1°- 3.0 L of dairy effluent kept at room temperature, homogenized and placed in a glass container; 2º- Thermostatic bath and centrifugal pump were turned on; 3º- Temperature was adjusted according to experimental design; 4º- pH was regulated according to experimental records; 5º- Lamp was started to emit Ultraviolet (UV) radiation; 6º- In parallel, ferrous and peroxide solution were continuously added during 50 min of a total of 1 h reaction, where the highest rate of addition was 2.1 ml / min and the lowest 1.81 ml / min; 7º- After adjusting all parameters and setting the time control for 1 hour treatment, pH was kept constant; 8º- Aliquots (20 mL) were taken every 10 minutes; 9º- The pH of both rates were adjusted from 8.0 to 9.0 for ferrous ions precipitation and filtered in quantitative filter paper; 10°-Each sample of dairy effluent was submitted to determine concentrations of COD, H2O2 residual concentrations and DOC. Dilutions owing to pH adjustment (AOP and precipitation) were taken into account to the concentration calculation in all analytical determinations.

% reduction = 1- DOCt/DOC0 where: DOC0 = Initial DOC; DOCt = Treated DOC. II.4.

Study of the Effective Degradation of the Effluent After AOP

When an AOP with a Fenton reagent is applied, another operation must be performed afterwards, ensuring the removal of iron ions and conditioning. This consists of altering the pH from 8.0 to 9.0 of the treated solution (NaOH 5 mol L-1). Basically, this precipitation process consisted in the following steps: 1º- Alkaline precipitation of iron in an exact volume of treated effluent by AOP, filtration and drying of the precipitate in an oven at 150 ºC for 3 h; 2º- Maceration of dry residue in a porcelain capsule until a thin and homogeneous mass was obtained, using around 50 mg; 3º- Heat digestion of the mass with 2 mL of aqua regia solution and 10 mL of deionized water (ferrous ion oxidation); 4º- Concentrated sulfuric acid and excess of K2Cr2O7 1 eq L-1 solution added to the dissolved residual product reacting for 30 min under heat conditions by its own exothermic property; 5º- Titration of the excess of dichromate (after cooling) by a Ferrous II solution, previously standardized.

II.3. Experimental Design for Dairy Effluent Degradation by AOP – Taguchi’s L9 Orthogonal Array Method A factorial statistic planning was performed to optimize the parameters represented by Taguchi’s L9 orthogonal array, in duplicate, enabling a more simple and standardized method of fractional factorial experiments [36]. Independent variables and factors

TABLE I CONTROL FACTORS AND LEVELS OF AN EXPLORATORY STUDY FOR DAIRY EFFLUENT TREATMENT WITH THE PHOTO-FENTON PROCESS BY USING TAGUCHI’S L9 ORTHOGONAL ARRAY METHOD Factor Symbology Level 1 Level 2 Level 3 Temperature (ºC) A 20 30 35 pH B 3.0 4.0 5.0 Fenton’s reagent C 0.255 mol H2O2 L-1 + 0.294 mol H2O2 L-1 + 0.343 mol H2O2 L-1+ + 0.0108mol Fe2+ L-1 + 0.0143 mol Fe2+ L-1 + 0.0215 mol Fe2+ L-1 UV D Without 15 W 28 W



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The dosage result according to the chemical equivalence is expressed in mg C Kg-1, considering that the dichromate had reacted only with the organic load present in the weighted mass. The organic load obtained in the precipitate is subtracted from the organic load determined by DOC analysis from the filtrated solution being its result the effective organic load degraded.

confirming the increase of dairy effluent biodegradation through this treatment. Color and turbidity results were satisfactory, making the visual characteristics of the treated effluent as clear and colorless. They presented a removal of 95.5 % and 98.8 %, respectively. With the reduction of organic load and consequently of the concentration of total solids (total removal), the effluent did not show any characteristic smell of the dairy effluent. Also, there has been an oxidation of the nitrogenous compounds, with possibly a NOx formation, since the concentration of nitrates and nitrites were not changed. In all experiments by AOP, after 60 min time of reaction, the oxidized effluent showed no residual concentration of peroxide, which shows a correct determination of H2O2 levels in the experiment. Oxidized effluent, in all AOP experiments and after reactional time of 60 min, did not show residual peroxide which suggests a correct determination of H2O2 levels in the design of experiment.

III. Results and Discussions III.1. Characterization of in Natura Effluent Physical-chemical analysis of dairy effluent samples were conducted to COD, BOD5, DOC, BOD5/COD, pH, turbidity and color determinations according to [32]. As shown in Table II, in which the results of organic and inorganic parameters used in dairy effluent analysis after being submitted to photochemical treatment are displayed to the following experimental conditions: pH 3,0, temperature at 35 ºC, Fenton reagent in the proportions 0.343 mol H2O2 L-1 + 0.0215 mol Fe2+ L-1. In general, the physicochemical results were expressive, with significant reductions in percentages. Regarding the organic load, there has been a DOC degradation of 90.86%, showing a significant efficiency in the oxidation process of the dairy effluent, even at high concentrations of interfering ions such as chloride [37], [38]. According to the Federal Law and the State of São Paulo Law, there is not a specific concentration value of COD; however, it is recommended a BOD value of < 60 mg L-1 or minimum efficient reduction of 80 % for treatment process. In general, homogeneous photocatalytic treatments of DOC and COD parameters were efficient. Another important parameter to be analyzed is the relation BOD5/COD. According to [36], this relation is different for several residues, which can be altered, specially, by biological treatment. The relation shows effective oxidation degradation in a determined organic load. Biodegradability was evaluated [39], which refers to a relation of BOD5/COD > 0.4 characteristically for biodegradable effluent. Thus, Table II shows that the result obtained by AOP (Photo-Fenton) reached 0.69

III.2. Delineating Experiments The delineating experiment was conducted to evaluate the photocatalytic reaction of the dairy effluent relative to the concentration of Fenton reagent (mH2O2 mFe2+), shown below. Therefore, a concentration of H2O2 (0.343 mol L-1 ) was fixed owing to its best result for DOC and Chemical Oxygen Demand (COD) percentage degradation considering the absence of peroxide residual after 2 h of reaction. For a better assessment, the results of DOC and COD were compared after the treatments as can be seen on Table III. The experimental conditions of these delineating experiments were: pH 3.0, temperature 25 °C, Fenton reagent in accordance as can be seen in Table III and ultraviolet radiation of 28 w. It can be observed in Table III that the photocatalytic oxidation reaction during the first 60 min presented the highest range of reduction of the two response factors being 78 % and 50 % for COD and DOC, respectively (the maximum of reduction after two hours of reaction was 86 % of COD and 53 % of DOC).

TABLE II RESULTS OF PHYSICAL-CHEMICAL ANALYSIS OF IN NATURA EFFLUENT AND AFTER PHOTO-FENTON TREATMENT Effluent Releasing Parameters Literature Values References Patterns[36] (mg L-1) in natura Treatment Aspect Turbid Limpid Absence Color (mg Pt-Co L-1) 432 28.68 Absence ST (mg L-1) 5680 Absence 545-5720 [18] 8 pH 6.0 – 6.4 8.0-8.50 5.25-8.0 [18], [35] Odor Irritant Absence 60 COD (mg O2 L-1) 9000–10000 930-939 797-8000 [18], [35] BOD5 (mg O2 mg L-1) 2300 – 2500 643.0-652.0 1292-60000 [18] BOD5/COD 0.25 0.69 DOC (mg /L-1) 1513-1800 137.0-143.0 2500-5000 [18] Chlorides (mg L-1) 1301.8 27 -1 N-NH3 (mg L ) 158.0 0.03 0.25-657 [18] N oganic (mg L-1) 180.0 0.05 16.5 – 1048 [18], [35]



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TABLE III RESULTS OF COD AND DOC PERCENTAGE REDUCTION OF EXPLORATORY STUDY IN THE PHOTOCATALYTIC REACTION OF DAIRY EFFLUENT IN RELATION TO THE CONCENTRATION (MOL H2O2 L-1 + MOL FE2+ L-1) Fenton reagent % of reduction of COD x time (min) % of reduction of DOC x time (min) molH2O2 L-1 + mol Fe2+ L-1 Relation Molar 30 60 90 120 30 60 90 120 0,343+0,030 11.40 70 % 78 % 84 % 85 % 28 % 41 % 44 % 45 % 0,343+0,024 14.30 72 % 78 % 85 % 86 % 35 % 50 % 52 % 53 % 0,343+0,018 19.00 73 % 76 % 82 % 84 % 30 % 36 % 37 % 39 % 0,343+0,012 28.60 63 % 69 % 75 % 78 % 27 % 31 % 36 % 37 %

Therefore, it was decided to work with an L9 matrix with time reaction of 1 h and using as concentrations as shown in Table I for the respective levels 1, 2 and 3 of Fenton reagent of L9 array.

According to Table I, it can also be noticed that the temperature (level 1 = 20 ºC, level 2 = 30 ºC and level 3 = 35 ºC) and the measurement of UV radiation source in W (level 1 = absence, level 2 =15 and level 3 = 28) are significant. Aiming to obtaining the variable value response (DOC percentage reduction), the following configuration should be considered: high level for Fenton’s reagent and low level for pH. Table IV shows Analysis of Variance (ANOVA) involved factors in dairy effluent treatment with Photo-Fenton process, according to Taguchi’s L9 orthogonal array experiment. According to the analysis of variance on Table IV, all factors show significant effect (F > 2) in the percentage of DOC reduction, pH being the most significant, with F equal to 79.824 and p-value equal to 2 x10-6. Less significant are the concentration of Fenton (F= 13.911), UV radiation (F=7.156) and the temperature (F = 4.499). Thus, according to F tests values, the effect of pH is approximately 6 times more significant to Fenton’s variable, 11 times more significant to UV and 18 times more significant to temperature, based on DOC reduction in ANOVA. It was also observed the high confidence value given by the test of p-value with the same sequence of significance on test F, being the higher percentage superior to 99.99 % for pH and 95.60 % for Fenton’s reagent and temperature. According to Table V the experimental conditions showing more DOC percentage reduction (about 90.86 %) in the dairy effluent was experiment 7, with Fenton’s reaction equal to 0.343 mol H2O2 L-1 + 0.0215 mol Fe2+ L-1, ultraviolet radiation at 28 W, pH = 3.0 and temperature at 35 °C. It should be mentioned that better results treated with Photo-Fenton process, as shown on Table V, were obtained due to the pH between 3.0 and 4.0. It can also be observed that tests 5 and 7 presented high percentage of DOC reduction for the same concentration value of Fenton’s reagent and the lamp of same power.

III.3. DOC Results for Taguchi’s L9 Method Fig. 1 shows the study of the process variability by the graph of the average of the responses in relation to signal-to-noise (S/N). The results shown in Fig. 1 were calculated from the situation "bigger is better", since it seeks the highest percentage reduction of COT for each experimental condition of the matrix L9. Main Effects Plot for DOC Variation Data Means T

pH

80 70

Mean

60 1

2 Fent on

3

1

2 UV

3

1

2

3

1

2

3

80 70 60

Fig. 1. Percentage variation results of DOC reduction in factors used for dairy effluent treatment according to Taguchi’s L9 method

Taguchi’s treatment analysis based on the graphic of effects (Fig. 1) indicates that Fenton’s reagent factors (level 1= 0,255 mol H2O2 L-1 0.0108 mol Fe2+ L-1level 2 = 0.294 mol H2O2 L-1 + 0.0143 mol Fe2+ L-1 and level 3 =( 0.343 mol H2O2 L-1 + 0.0215 mol Fe2+ L-1) and pH (level 1 = 3.0, level 2 = 4.0 and level 3 = 5.0) were relevant factors in the process. Notice that the effluent degradation process has a better performance in pH with more acidity (3.0).

Factors 1- Temperature 2 – pH 3 – Fenton 4 – UV Residual

TABLE IV VARIANCE ANALYSIS OBTAINED FROM AVERAGE DOC REDUCTION VALUES ACCORDING TO L9 ORTHOGONAL ARRAY FOR DAIRY EFFLUENT TREATMENT BY PHOTO-FENTON PROCESS Degrees of Sum of Squares Mean sum of Squares F Freedom 112.034 2 56.0168 4.49977 1987.436 2 993.7180 79.8242 346.342 2 173.1708 13.9106 178.171 2 89.0857 7.1562 112.040 9 12.4488


P 0.044199 0.000002 0.001765 0.013803


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TABLE V MEAN PERCENTAGE OF DOC REDUCTION FOR SPECIFIC FACTORS AND EXPERIMENTS ACCORDING TO TAGUCHI’S L9 ORTHOGONAL ARRAY AFTER DAIRY EFFLUENT TREATMENT BY FENTON AND PHOTO-FENTON’S PROCESSES Exp Temp (°C) pH Fenton (mg L-1) UV (W) meanDOC (%) 1 1 1 1 1 67.40 2 1 2 2 2 69.98 3 1 3 3 3 56.75 4 2 1 2 1 82.97 5 2 2 3 3 72.74 6 2 3 1 2 54.96 7 3 1 3 3 90.86 8 3 2 1 2 65.79 9 3 3 2 1 52.58 TABLE VI DOC MEAN PERCENTAGES AND EFFECTIVE DEGRADATION Mean of DOC Mean of effective degradation Difference in mean (%) DOC (%) DOC 67.40 53.9 13.5 69.98 55.3 14.7 56.75 42.6 14.2 82.97 63.9 19.1 72.74 55.3 17.5 54.96 47,0 7.96 90.86 69.0 22.0 65.79 50.3 15.5 52.58 41.6 11.0

[40] pointed pH out as a decisive factor for an efficient reaction. The result of this study is practically based on the value of pH adopted, independently from the type of effluent to be treated. Degradation speed tends to be higher than pH, which is around 3.0, and tends to diminish with the increase of pH due to formation of ferrous species catalytically decomposing hydrogen peroxide in oxygen and water, preventing the formation of hydroxyl radicals. The efficiency of Photo-Fenton process is related to the optimum quantities of ferrous and hydrogen peroxide to be used in the oxidative process as well as pH in the reaction media. Through this concept, we noted that experiments 1, 4 and 7, being performed with pH of 3.0 and concentration of Fenton’s reagent (0.255 mol H2O2 L-1 + 0.0108 mol Fe2+ L-1) and (0.294 mol H2O2 L-1 + 0.0143 mol Fe2+ L-1) and (0.343 mol H2O2 L-1 + 0.0215 mol Fe2+ L-1) had a DOC percentage removal of 65, 66, and 91%, respectively. These results show that the more quantity of species (concentration of oxidant agent and catalyst) used in Fenton’s reagent, the more the reaction efficiency is. Experiments 1 and 4 were performed without UV radiation, which could also indicate less efficiency of the process because the presence of UV radiation maximize hydroxyl radical’s reaction. After the statistic analysis of the values obtained in Taguchi’s L9 array, a linear regression model for the percentage of DOC reduction was adjusted with coded variable levels given in (4): DOC variation = 70.4 + 2.52 T – 12.8 pH+ + 5.37 [Fenton] + 3.85 UV

Thus, it can be noticed that the precipitation in alkaline pH had influenced the result of the employed process (Photo-Fenton), owing to the quantity of organic matter added to the precipitate. It can be observed as in experiment 7 that the PhotoFenton AOP real effective degradation, in percentage terms, was 69 % that is, a difference of around 22 % of DOC percentage degradation, a significant result. To the following processes to the AOP, specifically to the Fenton process, where only the filtrated is used, the total DOC reduction will be considered. III.5. AOP Degradation Rate To express DOC behavior reduction among Taguchi experimental replication planning, a first order fit exponential decay was used (5), obtaining correlations higher than 0.97 representing a good adjustment among its mean points. It can be pointed out that this mathematical adjustment has the function of evaluating the behavior of the organic load reduction to dairy effluent oxidation studied in relation to time:

(4)

Y  Y0  A1e

III.4. Effluent Effective Degradation After AOP Due to physical-chemical characteristics, a complementary study was performed with the effluent treated owing to quantify the total organic matter present (co-precipitation). After analytical characterization of the precipitate, the effective reduction of DOC of each experiment of the Taguchi L9 array is shown in Table VI. During the precipitation of ferrous ions, coprecipitated species (complexed anions or cations) were dragged by the hydroxide or adsorbed on the surface of Fe(OH)3 and/or Fe(OH)2 by electrostatic interaction.

 x / t1 

(5)

The exponential outlines of dairy effluent organic load degradation were shown in Fig. 2. In this block the highest DOC percentage degradation values were found. It is also significant the influence of pH in it. Good correlations were also found in the DOC mean adjustment results (R2 > 0.999), except for experiment 4 (R2 < 0.9), in which, the equation does not express a good mathematical adjustment. Lower deviation values between DOC mean results were also observed, being 2.4 %, 2.5 % and 3.5 % for experiments



285


4, 5 and 7, respectively, as showed in Fig. 2.

Taguchi L9 orthogonal array was carried out considering the chemical process only (Photo-Fenton). DOC reduction of optimized time for 3 L of effluent was considered in the final calculation of energy and reagent consume proportionally to the relation cost/benefit (less is better), in each experiment to Taguchi L9 array as shown in Table VII. Analyzing the results in Table VII, it can be observed that the best result is the one that shows a better cost/benefit ratio. On experiment 7, DOC percentage degradation was of 90.86 %, however, its cost/benefit value of 4.32 is the fourth smaller among the others. Another relevant factor is the cost of inputs (reagents) bigger in relation to the energy for all experiments. An individual analysis among Photo-Fenton and Fenton processes and its cost/benefit can be performed in relation to the final destination of the treated effluent in two specific evaluations. First of all, if the destination is the direct disposal to rivers, the investment cannot be any different of experiment 7. Even though it is lower than experiment 7, experiment 4 (Fenton Process) is still consistent to national laws (reduction higher than 80 %). The cost/benefit value being less than experiment 7. Specifically, if the effluent treated by Photo-Fenton is later treated biologically (AOP hybrid – activated sludge), the temperature parameter must be evaluated thoroughly for it is the one which makes the process more expensive. Treatments with lower temperature must be the most significant factor in the process. Experiments 1, 2 and 3, for example, used the oxidation reaction at 20 oC. Among them, experiment 2 is the one that shows better cost/benefit (4.03), showing the second best value in all 9 experiments of Taguchi planning. Being all the variables at intermediate levels and a DOC percentage reduction of 69.98 %. Assessing concomitantly the energy and reagents consume, experiment 6 may be considered the best one. In this experiment, even using intermediate temperature (30 oC), the cost/benefit value was 4.87, that was compensated by using the lower Fenton reagent (level 1) allied to pH 5.0 and UV lamp of 15 W. This experiment reached a DOC percentage reduction of 54.96 %.

Fig. 2. Experimental outlines of oxidation of dairy effluent by Photo-Fenton process of experiments 4, 5 and 7 of Taguchi design in relation to time and DOC means

It can be observed that there is no stationary phase in the reaction, that is, after a 60 min period of dairy effluent degradation, the reaction could still be carried on and the reduction of DOC be superior of 83 %. Experiment 5 had the lowest organic load reduction (72.4 %) of this block. It can be observed that the reaction is carried out in about 40 min (70 % of DOC reduction). In experiment 7, the outline of DOC degradation is steady by the end of the reaction (around 50 min), exactly at the end of H2O2 addition that can indicate a higher DOC percentage reduction (> 91 %) in case there is more peroxide in the reaction system. The pH values of this block in the lowest level (pH 3.0) presented the highest values of organic load degradation that confirms the Photo-Fenton process mechanisms even in an environment with high matrix complexity. For a process of 1 h that is only photoirradiated, using 3 L of dairy effluent, the percentages of DOC reduction were: 13.5 % with UV radiation of 15 W and 18.2 % with 28 W. These experimental results show the importance in combining a catalyzed oxidation power to the photolysis, especially when the organic load is high and of difficult degradation. III.6. Economic Evaluation of Photo-Fenton Process The economic evaluation (energy consume and reagents) of dairy effluent treatment according to

TABLE VII VALUES OF ENERGY AND REAGENT CONSUME OF 3 L DAIRY EFFLUENT TREATED BY PHOTO-FENTON PROCESS OF EACH OPTIMIZED EXPERIMENT OF TAGUCHI L9 ARRAY Experiment

Energy Consume[41] US$ / 3L

Reagent Consume US$/3L

Total Value[42] US$/3L

Mean DOC Reduction %

Relation US$/%red.(x1000)

1 2 3 4 5 6 7 8 9

0.132 0.078 0.079 0.105 0.080 0.08 0.167 0.134 0.139

0.188 0.204 0.226 0.224 0.226 0.188 0.226 0.188 0.240

0.314 0.282 0.305 0.329 0.306 0.268 0.393 0.322 0.339

67.40 69.98 56.75 82.97 72.74 54.96 90.86 65.79 52.58

4.66 4.03 5.37 3.96 4.2 4.87 4.32 4.89 6.44



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IV.

Conclusion

DOC, COD and BOD5 percentage reduction were verified after the photochemical treatment, which varied from 91; 91 and 74 %, respectively. This demonstrates that a big part of organic load present in the dairy effluent was degraded by the photochemical process, which means a significant contribution of AOP process for the degradation of dairy effluent. The analysis of ANOVA from the Taguchi’s L9 experimental design, made it possible to identify the most significant factors in DOC percentage reduction. In this case, all factors were significant, being pH the one of higher influence. An effective DOC degradation showed a coprecipitation of organic species to the formed precipitate, in the treated effluent by Photo-Fenton and respectively alkalinisation, may vary 22.0 to 7.96 %. In relation to the evaluation of cost/benefit, two aspects were considered. First, if the treated effluent by AOP were released directly in superficial water (river), the variables and respective levels of the process should be: 30 oC, pH in 3.0, 0.294 mol H2O2 L-1 + 0.0143 mol Fe2+ L-1 and without UV irradiation, obtaining a DOC percentage reduction of 82.97 %. In case the effluent treated by Photo-Fenton is treated afterwards by a biological treatment, the best experiment in relation to cost/benefit must be processed in the following variables and respective levels: 30 oC, pH in 5.0, 0.255 mol H2O2 L-1 + 0.0108 mol Fe2+ L-1and UV irradiation of 15 W, being the DOC reduction of 54.96 %. In light of the significant results obtained via AOP, based on homogeneous photocatalysis (photo-Fenton and Fenton process), it can be said that this method has a significant potential of application for the treatment of dairy effluent, which combined with an activated sludge type biological process can degrade its organic load within the process efficiency values established by Article 18 (CETESB), enabling its disposal. Of the two processes and considering the best cost benefit, the Fenton process is more advantageous. The researchers of this review have worked with some environmentally important themes. Especially with oxidative processes applied to industrial effluents and wastes, considering homogeneous and heterogeneous catalysis, in batch and semi-continuous processes. They work with the statistical evaluation of experimental planning, and have also applied a mathematical model to determine the importance of each parameter to the model and its costs.

[2]

Acknowledgements

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To the laboratory of Environment coordinated by Prof. PhD Messias Borges Silva. To CAPES for the financial support.

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Rodrigo Fernando dos Santos Salazar. PhD in Science with emphasis in Analytical Chemistry at Federal University of São Carlos (Brazil, 2013). He has experience in Chemical Engineering with emphasis on wastewater treatment, and Chemistry, with an emphasis on Analytical Chemistry. Prof. Salazar is assistant professor at Franciscan University (UNIFRA) and in the State University of Rio Grande do Sul (UERGS) in the area of Sciences and Technology. E-mail: [email protected] André Luís de C. Peixoto was born in São José dos Campos, Brazil, in 1981. He received his degree as a chemical engineer in 2006 and obtained his MSc. degree in 2008 from the Engineering School of Lorena (University of São Paulo), Brazil. On September 2008, he became a member of the Center of Studies on Chemical Engineering as a PhD candidate and workes at Federal Institute of Education, Science and Technology (IFSP, Brazil). Mr. Peixoto is member of the American Institute of Chemical Engineering and the Brazilian Society of Chemical Engineering. E-mail: [email protected] Oswaldo Luiz Cobra Guimarães was born in Bananal, Brazil, in 1962. He received his degree as a chemical engineer in 1986, MSc. degree in 2002 from University of Santa Catarina, Brazil and Ph.D. in 2005 from the University of Taubaté. Dr. Guimaraes teaches disciplines related to mathematics and computer programming, also working on research related to advanced oxidation processes, mathematical modeling and neural networks at the School of Engineering of Lorena, University of São Paulo. E-mail: [email protected]

Authors’ information Carla Loures was born in Juiz de Fora, Brazil, Master of Science in Chemical Engineering (University of São Paulo) in environmental areas (wastewater treatment and characterization of solid and liquid effluents)/2011. Doctor of Science in Production Engineering area by the State University Paulista Júlio de Mesquita Filho. Mrs Loures operates on the following subjects: Mathematical Methods and Statistical Engineering, with emphasis in Advanced Oxidation Processes. Tel.: (55)12-3159-5076 E-mail: [email protected] Hélcio Izário Filho. Doctor of Science in Chemistry from by the State University of Campinas. He is currently a professor in the School of Engineering of Lorena Universidade de São Paulo. Escola de Engenharia de Lorena, University of São Paulo. Dr. Izario Filho has experience in chemistry with emphasis in analytical instrumentation, specifically absorption spectrometry and atomic emission spectrophotometry, acting on the following themes: characterization of the inorganic environment (analytical characterization of solid and liquid waste and Advanced Oxidation Processes). E-mail: [email protected] Gisella Lamas Samanamud. Masters in Chemical Engineering by the University of São Paulo/2011. She is currently a researcher / student at the University of Texas at San Antonio (UTSA), TX, USA. MSc Lamas Samanamud currently works in the Laboratory of Environmental Engineering on the development of wastewater treatment, Advanced Oxidation Processes, photochemical processes and molecular biology. E-mail: [email protected] André Luiz de Souza. MSc in Chemical Engineering from the University of São Paulo, 2011. He is lately working as professor and High School Coordinator. E-mail: [email protected]



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