Environ Sci Pollut Res DOI 10.1007/s11356-016-6156-9

RESEARCH ARTICLE

Treatment of real industrial wastewater using the combined approach of advanced oxidation followed by aerobic oxidation Lokeshkumar P. Ramteke 1 & Parag R. Gogate 1

Received: 12 October 2015 / Accepted: 20 January 2016 # Springer-Verlag Berlin Heidelberg 2016

Abstract Fenton oxidation and ultrasound-based pretreatment have been applied to improve the treatment of real industrial wastewater based on the use of biological oxidation. The effect of operating parameters such as Fe2+ loading, contact time, initial pH, and hydrogen peroxide loading on the extent of chemical oxygen demand (COD) reduction and change in biochemical oxygen demand (BOD5)/COD ratio has been investigated. The optimum operating conditions established for the pretreatment were initial pH of 3.0, Fe2+ loading of 2.0, and 2.5 g L−1 for the US/Fenton/stirring and Fenton approach, respectively, and temperature of 25 °C with initial H 2 O 2 loading of 1.5 g L−1. The use of pretreatment resulted in a significant increase in the BOD5/COD ratio confirming the production of easily digestible intermediates. The effect of the type of sludge in the aerobic biodegradation was also investigated based on the use of primary activated sludge (PAS), modified activated sludge (MAS), and activated sludge (AS). Enhanced removal of the pollutants as well as higher biomass yield was observed for MAS as compared to PAS and AS. The use of US/Fenton/stirring pretreatment under the optimized conditions followed by biological oxidation using MAS resulted in maximum COD removal at 97.9 %. The required hydraulic retention time

Responsible editor: Gerald Thouand * Parag R. Gogate [email protected] Lokeshkumar P. Ramteke [email protected]

1

Chemical Engineering Department, Institute of Chemical Technology (formerly UDCT), Mumbai 400019, India

for the combined oxidation system was also significantly lower as compared to only biological oxidation operation. Kinetic studies revealed that the reduction in the COD followed a first-order kinetic model for advanced oxidation and pseudo first-order model for biodegradation. The study clearly established the utility of the combined technology for the effective treatment of real industrial wastewater. Keywords Biodegradation . Ultrasound . Fenton oxidation . Modified activated sludge . Hydraulic retention time

Introduction The conventional approach for the treatment of industrial wastewater is generally based on physicochemical and mechanical methods with subsequent biological treatment. In an actual industrial effluent, different types of organic pollutants are present, which are not readily degraded by conventional treatments especially in the case of biorefractory compounds. It is imperative to develop combined techniques where pretreatment can improve biodegradability giving subsequent better utilization of the intermediates in the actual biological oxidation (Mandal et al. 2010; Rivas et al. 2003; Beltran et al. 2000; Garcia-Montano et al. 2006; Ghaly et al. 2009). There are several approaches being proposed for pretreatment including sonication, ozonation, solvent extraction, Fenton chemistry, electrochemical oxidation, and photocatalytic degradation (Oller et al. 2011; Karthikeyan et al. 2011; Rathi 2002; Cortez et al. 2010; Stasinakis 2008; Beltran et al. 2000; San Sebastian et al. 2003). Chemical or advanced oxidation approaches seem to be more favorable, as even a partial oxidation of pollutants

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using free radicals might significantly increase their biodegradability (Oller et al. 2011). Subsequent biological oxidation process would be more effective for the treatment of such pretreated wastewater having high biochemical oxygen demand (BOD 5)/chemical oxygen demand (COD) ratio (Metcalf and Eddy 1979). In the case of biological treatment, microbes generally need long residence times to oxidize pollutants due to significant mass transfer resistances and possible toxic effects of the pollutants on the growth of the microorganisms. It is very important to develop combined processes that can intensify the biological oxidation based on pretreatment using advanced oxidation processes. Advanced oxidation processes (AOPs) work on the principle of generating OH radicals (oxidation potential 2.8 V) in sufficient quantities that are capable of oxidizing organic pollutants present in the effluent (Gogate and Pandit 2004; Stasinakis 2008) either completely or partially. Among the different types of AOP, Fenton oxidation has been used in the present work either operated individually or in combination with ultrasound. Fenton oxidation is based on the use of iron salts (Fe2+) in combination with H2O2 which generates hydroxyl radicals (OH•) with enhanced efficacy under acidic conditions (Mandal et al. 2010). In Fenton oxidation, typically no energy input is required for the dissociation of H 2O2, and hence, this approach offers a cost-effective method for the generation of hydroxyl radicals as compared to other AOPs based on the use of UV irradiations or ultrasound (Zazo et al. 2005). Fenton oxidation has been reported to be successful against different toxic effluents giving high reduction in COD (San Sebastian et al. 2003; Rivas et al. 2003; Karthikeyan et al. 2011). Bautista et al. (2007) reported that Fenton oxidation can be used to treat different industrial wastewaters including textile, paper, pharmaceutical, dyes, cork processing, olive oil, and petroleum industry wastewaters. The application of ultrasound is effective in wastewater treatment because of induced cavitational effects that can result in the destruction of organic pollutants contained in wastewaters (Gogate 2008; Elbeshbishy and Nakhla 2011). Ultrasonic reactors may not result into sufficient levels of degradation especially in the case of real effluents and hence may not be economically viable due to the requirement of much larger treatment times and energy for complete mineralization of the pollutant. The efficiency of ultrasound for water treatment might be enhanced by combining ultrasound with an advanced or biological oxidation process (Gogate and Pandit 2004). For instance, the degradation of recalcitrant components could be increased by combination with Fenton’s reagent due to the enhanced production of hydroxyl radicals based on the breakage of ferryl ion complex (Eskelinen et al. 2010).

Due to the higher treatment costs and limited applicability associated with US and/or Fenton’s reagent, the combination of these methods with biological treatment process is a more promising approach. Pretreatment using Fenton and/or ultrasound can be used to enhance the BOD5/COD ratio for any given wastewater above 0.4, which is considered good for aerobic oxidation (Metcalf and Eddy 1979). The minimum value of the BOD5/COD ratio for the application of biological treatment is considered as 0.4 (Symons 1960) though in reality values around 0.6 to 0.65 would mean much higher ease for biological oxidation. Also, the combination approach can result in a significant decrease in the required treatment time for the biological wastewater treatment process where one of the major drawbacks is the requirement of higher treatment time due to toxicity of contaminants toward microorganisms (Chan et al. 2009). Pretreatment can definitely decrease toxicity enhancing the aerobic oxidation rate (Ramteke and Gogate 2015b, c; San Sebastian et al. 2003). Depending on the characteristics of wastewater, the efficacy of Fenton’s reagent, US, and US/Fenton as pretreatment will be decided (Bautista et al. 2007; Elbeshbishy and Nakhla 2011; Ramteke and Gogete 2015b, c). Hence, it is very essential to establish the optimum pretreatment conditions as well as evaluate the efficacy of the combination treatment approach especially for real industrial effluents. Thus, the novelty of the present work dealing with optimization of combined treatment for real industrial effluent is justified. The work also focused on reducing the hydraulic retention time of aerobic biological activated sludge treatment for real industrial wastewater using the pretreatment based on Fenton, US, or US/Fenton. In addition, three types of sludge, namely primary activated sludge (PAS), activated sludge (AS), and modified activated sludge (MAS), have been used in the biological treatment process (Ramteke and Gogate 2015a) so as to establish the best type of sludge in the activated sludge process. In order to check the efficiency and the most appropriate conditions of the pretreatment, the variation in the biodegradability index was monitored with time. The effect of the operating variables such as pH, iron, and hydrogen peroxide loading has also been investigated in order to establish the optimum conditions for the most efficient application of the different pretreatment strategies involving US and Fenton. In addition, kinetic models for the overall approach based on the COD reduction have been established (Grau et al. 1975; Lafi et al. 2009). Overall, the present work aims at intensifying aerobic oxidation using pretreatment technology which can convert pollutants into easily digestible intermediates. The novelty of the work lies in the fact that the work considers the use of real industrial effluent obtained from a specialty chemical industry and such studies are not

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commonly observed in the literature and most of the studies deal with the use of synthetically prepared solutions with model constituents.

Materials and methods Materials Real industry wastewater samples were collected from a specialty chemical manufacturing unit (company name not disclosed due to confidentiality issues). Five liters of effluent was obtained in plastic bottle containers. The effluent was transferred to the laboratory, preserved, and stored at 4 °C for further study in terms of physiochemical analysis and treatment experiments. Analytical grade (AR) chemicals, namely hydrogen peroxide solution, sodium tripolyphosphate, ferrous sulfate (FeSO4⋅7H2O), sulfuric acid, sodium hydroxide, potassium dihydrogen phosphate, dipotassium hydrogen orthophosphate, sodium monohydrogen phosphate heptahydrate, ferric chloride, magnesium sulfate, manganese sulfate, calcium chloride, phosphoric acid, peptone casein agar (PCA), glucose, peptone, beef extract for bacto (bacterial growth), and ammonium chloride, were obtained from S.D. Fine Chemical Pvt. Ltd Mumbai (India). Peptone casein agar was used to identify potential bacteria. The laboratory methods used in this study were either standard methods (APHA, AWWA 1998) or specified by the manufacturer of the instrument. COD and BOD were analyzed following the standard method with potassium dichromate and sodium thiosulfate, respectively. For BOD characterization, polyseed and glucose-glutamic acid (GGA) procured from Environmental Express, USA, were used for making seed control and GGA standards respectively. pH was measured using a pH meter (Equiptronics, India) under aseptic conditions. Characterization of wastewater used in the study was performed before the experimental studies, and the obtained results are given in Table 1. In this work, two samples of wastewater were used which have significantly different values of COD (11,866.7 and 23,733.4 mg L−1). The corresponding BOD5/COD ratio was 0.496 and 0.454, respectively, indicative of relatively better biodegradability.

Table 1

Characterization of real industrial effluent

Analyzer parameter

Industrial effluent

Chemical oxygen demand, COD (mg L−1) D.F. 10−2 Biochemical oxygen demand, BOD (mg L−1) D.F. 10−2 Total dissolved solids, TDS (mg L−1) Total suspended solids, TSS (mg L−1)

11,866.7 5884.1

pH BOD5/COD Color

27,580 2100 5.67 0.496 Dark brown

MAS used in this study was prepared under aerobic conditions using different industrial sludge and municipal sewage procured from CETP, Mumbai (Maharashtra) and using the biomanure collected from Sandoz Pvt. Ltd. Mahad (Maharashtra). The detailed method for the preparation of the MAS, which is mainly waste biomass containing bacteria and various organic compounds, has been reported in a previous work (Ramteke and Gogate 2015a). Experimental methodology Fenton pretreatment Fenton oxidation studies were performed in batch mode operation using 500 mL working volume in a reactor equipped with a stirrer. Fenton oxidation was based on adding ferrous sulfate (FeSO4⋅7H2O) initially followed by the addition of the required amount of 30 % (w/w) H2O2 with stirring. For every batch, 500 mL of effluent was used and pH adjustments were performed using HCl solution. The reaction temperature was maintained constant at 24(±1) °C for all the experiments. The effect of pH in Fenton reaction was investigated by varying pH values in the range of 2–5. For all Fenton experiments, the effect of Fe2+ dose was investigated by varying the Fe2+ concentration in the range of 0.5–3.0 g L−1, and the effect of H2O2 was also investigated by using different concentrations (1.0, 1.5, and 2.0 g L−1) at a constant Fe2+ concentration of 2.5 g L−1. US pretreatment

Identification of sludge The aerobic biological treatment was carried out using three different types of sludge, namely PAS, AS, and MAS. The primary activated sludge was obtained from the CETP, Mumbai (India). The AS was prepared by using inoculant culture F1 (Pseudomonas putida F1) obtained from NCL, Pune and stored on nutrient agar slants (7 mL) at 4 °C. The

Ultrasound pretreatment experiments were performed in a reactor with 1.0-L capacity using an ultrasonic horn procured from M/s Dakshin, India. The ultrasonic horn operates at a fixed frequency of 22 kHz and a power dissipation of 120 W. The on time set was 10 s, whereas the off time was 5 s. Ultrasound pretreatment was performed using a batch volume of 1.0 L at a reaction temperature of 25 (±1) °C.

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US/Fenton pretreatment US/Fenton pretreatment was carried out in a glass cylindrical reactor. The ultrasound horn at a fixed frequency of 22 kHz with a rated output power of 120 W was used in combination with Fenton oxidation in a batch mode using a 500-mL working volume of real industrial wastewater at constant reaction temperature of 24(±1) °C and at a constant pH of 3.0. In the first step, ferrous sulfate (FeSO4⋅7H2O) was added to the effluent and then a desired loading of 30 % (w/w) H2O2 was added with stirring. All the experiments were performed for 1 h treatment time, and samples were withdrawn regularly at periodic intervals for analysis. Aerobic oxidation Aerobic biological oxidation experiments were performed in a 3.0-L jacketed glass reactor equipped with a pitched blade type impeller using different types of activated sludge. The biological oxidation experiments were performed according to the method described in our earlier work (Ramteke and Gogate 2015a). After the advanced oxidation treatment, the removal of chlorine and other toxic compounds was achieved by treating samples using calcium oxide (CaO) with rapid stirring at 150 rpm for 5 min to increase pH to approximately 5. Subsequently, the pH was adjusted to 7–8 using NaOH solution, and a small amount of 0.2 % (w/w) polyacrylamide solution was added for flocculation. This was followed by 5 mg L−1 polyacrylamide addition with 50 rpm mixing

speed to promote the formation of flocs for 5 min. It is important to note that pH adjustment is necessary to maintain good conditions for biological oxidation. After precipitation, sodium tripolyphosphate at a loading of 4 mg L−1 was added into the treatment vessel to make sure that wastewater provides sufficient nutrition for the microbes in the subsequent biological oxidation. The supernatant was fed into the jacketed glass reactor for the subsequent biological treatment, where it was mixed with 100 g TSS L−1 sludge (per liter indicates the volume of wastewater used for the study). The pH of the pretreated and untreated samples as control was always brought to 6.0–7.0 for biological oxidation. The biological treatment was carried out for 80 h for the pretreated and untreated samples. Samples were withdrawn after regular intervals of time (10 h intervals) for the analysis of COD (S), BOD, and biomass (X). The experimental methodology used in the work along with the schematic diagram of the aerobic biological oxidation setup has been depicted in Fig. 1. All experiments were performed at 25 °C. Figure 2 represents the microbial isolates obtained during the growth phase of the sludge exposed to the industrial effluent environment as per the method described in our earlier work (Ramteke and Gogate 2015a). It can be established that mixed bacterial colony was observed in the plate as per theanalysis performed using the simple bacterial analysis method described by Abdulkadir and Waliyu (2012). In addition, isolation and characterization of the colony population of microbes in the case of MAS was performed in the presence of industrial wastewater as nutrient medium

Fig. 1 Experimental methodology for treatment of real industrial influent and schematic diagram of the aerobic biological oxidation setup

Environ Sci Pollut Res Fig. 2 Modified prepared activated sludge (MAS) and isolated bacteria using nutrient agar plating method

with plating on nutrient agar media (Abdulkadir and Waliyu 2012). Two levels of dilution at 50−1 and 100−1 were used and the standard APHA method was followed for analysis (APHA, AWWA 1998) (Fig. 2).

Result and discussion The influence of operating parameters on the efficacy of pretreatment and the biological oxidation has been investigated using the analysis of COD, BOD, and BOD5/COD ratio as the representative parameters to follow the response of the system. Advanced oxidation process as pretreatment Fenton pretreatment Figure 3 depicts the COD removal efficiency obtained at different pH values over the range of 2.0–5.0 at a fixed pretreatment time of 40 min and a constant temperature of 25 °C. The constant loading of H2O2 and Fe2+ used in these experiments was 1.5 and 2.5 g L−1, which means a Fe2+ to H2O2 weight ratio of 1.67. Figure 3 confirmed that the highest COD removal was obtained at a pH value of 3.0, and reduced treatment efficiency was observed at lower and higher pH values as compared to the optimum value of 3.0. At significantly lower pH, H2O2 is stabilized as H3O2+, and also, the reaction between OH• and H+ is significant as compared to the desired oxidation (Mandal et al. 2010; Jamil et al. 2011). Due to the precipitation of Fe3+ as Fe(OH)3 which hinders the reaction between Fe3+ and H2O2, the extent of oxidation decreases at higher pH values beyond 3.0. Moreover, the decomposition of H2O2 to O2 and H2O is catalyzed by Fe(OH)3 which decreases the production of OH•. It is also probable that the formation of highly stable Fe(II) complexes is obtained at higher pH condition (Benitez et al. 2001; Karthikeyan et al. 2011). The obtained results match well with the

literature reports confirming that the optimum pH was around 2.5–3.5 which is also a key parameter in deciding the efficiency of Fenton processes (Cravotto et al. 2005). It is also important to note here that after treatment using Fenton under acidic conditions, it is required to bring back the pH to neutral conditions for subsequent biological oxidation. The initial pH was kept constant at 3.0 for all the subsequent experiments of pretreatment based on this optimum consideration. Figure 4a and Table 2 depict the COD removal efficiency obtained at 25 °C using different H2O2 doses of 1.0, 1.5, and 2.0 g L−1. The initial Fe2+ dose of 2.5 g L−1 and the COD loading of 11,866.7 mg L−1 were kept constant in all the cases. It can be seen from the figure that the extent of COD removal increases with an increase in the H2O2 loading from 1.0 to 1.5 g L−1. The observed increase can be attributed to the fact that a higher production of hydroxyl radicals is obtained at higher loading of hydrogen peroxide enhancing the rate of COD removal. It is important to note that scavenging reaction of excess H2O2 is dominant at a higher concentration above

Fig. 3 Effect of initial pH on the COD removal after 40 min [Fe2+]0 = 2.5 g L−1 and [H2O2]0 = 1.5 g L−1 ([COD]0 = 11,866.7 mg L−1; [BI]0 = 0.496; temperature = 25 °C)

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1.5 g L−1 (Karthikeyan et al. 2011), and hence, no further improvement in the final COD removal efficiency is obtained at higher loading of H2O2 beyond 1.5 g L−1. It is important to adjust suitably the loading of H2O2 used as any excess H2O2 remaining in the solution after the treatment also causes toxicity to the subsequent biological oxidation process (Bautista et al. 2007; Jamil et al. 2011). Thus, the selection of optimum loading of H2O2 is a must to obtain efficient operation, and in the present work, it has been established as 1.5 g L−1. Figure 4b depicts the obtained results for the COD removal efficiency and variation in BOD5/COD ratio at 25 °C with a constant initial H2O2 concentration of 1.5 g L−1 giving a H2O2 to COD initial weight ratio of 0.13, and a Fe2+ loading of 2.5 g L−1. From Fig. 4b, it can be seen that significant changes in BOD5/COD ratio are obtained in a relatively short reaction time of about 40 min using 1.5 mg L−1 H2O2 dose. The time of Fig. 4 a Evolution of COD removal using different H2O2 doses and b evolution of COD and BOD5/COD ratio at constant 1.5 g L−1 H2O2 ([Fe2+]0 = 2.5 g L−1; [COD]0 = 11,866.7 mg L−1; [BI]0 = 0.496; temperature = 25 °C)

application of pretreatment can be decided on the basis of the required changes in biodegradability and recommended design parameters (hydraulic retention time, HRT) for biological oxidation. In the present work, a cutoff biodegradability of 0.6 has been used to decide the optimum pretreatment parameters. From Fig. 4a, b and Table 2, it can be seen that COD removal efficiency was approximately 30.7 % with the corresponding COD value of 8225 mg L−1 and BOD5/COD value of 0.611 at 40 min as the pretreatment time. Figure 5 depicts the variation in the COD removal efficiency with pretreatment time at 25 °C using different doses of Fe2+ at a fixed initial amount of H2O2. From the data given in Fig. 5 and Table 2, it can be seen that the COD removal efficiency increases with an increase in the Fe2+ dose at a constant H2O2 dose of 1.0 g L−1. A careful analysis reveals that increasing the Fe2+ dose from 0.5 to 2.5 g L−1 results in an

Environ Sci Pollut Res Table 2 Effect of initial Fe2+ concentration with different H2O2 loadings for Fenton treatment on the BOD removal, COD removal, and BOD5/COD ratio at constant treatment time of 40 min

Fe2+ (g L−1)

1.0 g L−1 H2O2

1.5 g L−1 H2O2

2.0 g L−1 H2O2

COD (%)

BOD (%)

BI:BOD5/ COD

COD (%)

BOD (%)

BI:BOD5/ COD

COD (%)

BOD (%)

BI:BOD5/ COD

0.0

0.0

0.0

0.496

0.0

0.0

0.496

0.0

0.0

0.496

0.5

5.1

1.1

0.517

9.8

2.7

0.535

14.2

9.4

0.524

1.0 1.5

12.1 16.5

6.4 9.8

0.528 0.536

18.6 22.4

9.0 11.1

0.555 0.568

29.8 43.2

16.7 37.7

0.588 0.544

2.0 2.5

21.5 30.4

11.9 15.2

0.556 0.604

28.2 30.7

13.9 14.6

0.594 0.611

50.3 59.6

43.0 51.3

0.569 0.597

3.0

34.2

25.5

0.561

40.3

28.9

0.591

66.0

59.9

0.584

increase in the extent of COD removal from about 8 % to about 30 %, but a subsequent increase in the Fe2+ dose to 3 g L−1 only gives marginal improvement. The figure also shows the comparison of two loadings of H2O2 as 1 and 1.5 g L−1 at a constant dose of Fe2+ of 2.5 g L−1 confirming that a significant increase in the COD removal efficiency is obtained by increasing the loadings of H2O2. It has been also reported in the literature that Fenton pretreatment is effective for wastewater treatment and some studies have established that increasing the dose of Fe2+ ions is more favorable than increasing the dose of H2O2 (San Sebastian et al. 2003; Mandal et al. 2010). On the other hand, due to the higher cost of the reagent and the requirement of an additional treatment to remove unreacted iron, it is not practical to use excessive loading of ferrous iron. Thus, it is important to optimize the loadings of both the constituents, and the guidelines presented in this work would be useful. Table 2 depicts the improvement in the BOD5/COD (biodegradability index, BI) obtained at different initial Fe2+ concentrations after 40 min of treatment corresponding to three different initial H2O2 doses of 1.0, 1.5,

Fig. 5 Evolution of COD removal for different Fe 2 + doses ([COD]0 = 11,866.7 mg L−1; [H2O2]0 = 1.0 g L−1; temperature = 25 °C)

and 2.0 g L−1. The value of BOD5/COD (BI) varies with an increase in the Fe2+ concentration at different initial H2O2 doses of 1.0, 1.5, and 2.0 g L−1, and a maximum value of 0.611 was observed at 2.5 g L−1 Fe2+ and 1.5 g L−1 H2O2. From Table 2, it can be established that 2.5 g L−1 Fe2+ dosage would be enough in most cases for desired changes in biodegradability. Overall, the obtained results establish that 2.5 g L−1 of Fe2+ and 1.5 g L−1 of H2O2 loading are the best optimum conditions established for the Fenton pretreatment. US pretreatment The obtained results for the treatment of effluent using ultrasound with different power in combination with optimum loading of H2O2 have been shown in Fig. 6. It can be observed that after 60 min of ultrasonic irradiation, 1.1, 8.9, and 12.3 % of COD removal was obtained when the ultrasonic output power was 80, 100, and 120 W, respectively, in the absence of stirring. In the presence of stirring, 9.6, 14.6, and 16.9 % of COD removal was observed after 60 min of ultrasonic irradiation at similar power dissipation levels of 80, 100, and 120 W, respectively. The results confirmed that enhanced power dissipation levels increase the extent of degradation which is attributed to the enhanced cavitational effects. Rehorek et al. (2004) studied the ultrasonic decomposition of dyes and observed that the concentration of radicals increased exponentially with the increase in ultrasonic power. Based on the obtained results, ultrasound power dissipation of 120 W has been used for further experiments. The application of ultrasound (fixed power of 120 W and time of 40 min) in the presence of H2O2 was also examined at different loadings of H2O2 (with and without stirring) for the pretreatment of wastewater. Figure 7 gives the obtained data and it can be seen that COD removal efficiency increases with an increase in the dose of H2O2 over the range of 0.0– 2.0 g L−1. The maximum value of 9.7 and 8.9 % of COD removal was obtained at a 2.0 g L−1 loading of H2O2 with and without stirring, respectively, after 40 min pretreatment. The corresponding BOD5/COD value was 0.511 for both

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Fig. 6 Evolution of COD removal using ultrasound irradiation with different output power ([COD]0 = 11,866.7 mg L−1; [H2O2]0 = 1.5 g L−1; [BI]0 = 0.496; temperature = 25 °C)

treatment conditions. It is interesting to note that the combined treatment resulted in a higher extent of COD reduction as compared to only ultrasound which can be attributed to dissociation of hydrogen peroxide giving hydroxyl radicals. The combination of ultrasound and hydrogen peroxide does not seem to be an efficient treatment approach as the extent of COD removal was less than 10 %. The subsequent combination of ultrasound with Fenton chemistry was investigated with the objective of intensifying the treatment and also possibly reducing the oxidant loading. US/Fenton pretreatment Figure 8 gives the obtained data for COD removal obtained for the combined treatment approach using different loadings of Fe2+ at a constant loading of H2O2. The experiments were

Fig. 7 Evolution of COD removal at different H2O2 doses combined with US ([COD]0 = 11,866.7 mg L−1; [BI]0 = 0.496; temperature = 25 °C)

performed at two different values of initial COD as represented in Fig. 8a, b. It can be seen that a significant improvement in the rate and COD removal extent is obtained by increasing the Fe2+ loading. The extent of increase in the COD removal due to the presence of Fe2+ seems to depend on the initial COD and initial hydrogen peroxide present in the system. It can be seen from Fig. 8a that increasing Fe2+ loading gives significant benefits over the entire range of 1.0–2.0 g L−1 for an initial COD of 11,866.7 mg L −1 and H 2 O2 dose of 1.0 g L−1. Figure 8b represents that at an initial COD of 23, 733.4 mg L−1 and H2O2 loading of 2.0 g L−1, a higher Fe2+ dose above 1.0 g L−1 does not show any significant improvement. In the case of the given H2O2 to COD ratio (Fig. 8b), a higher COD means obviously a higher H2O2 requirement to produce hydroxyl radicals for a given Fe2+ dose. In addition, a higher concentration of organic matter favors the regeneration of Fe2+ from the reaction of Fe 3+ and organic radicals (Bautista et al. 2007): R• þ Fe3þ →Fe2þ þ Rþ

ð1Þ

On the other hand, higher Fe2+ concentrations favor the occurrence of the scavenging reaction as depicted below: Fe2þ þ OH• → Fe3þ þ OH−

ð2Þ

The dominance of scavenging reaction reduces the degree of enhancement in the oxidation process at a significantly higher loading of Fe2+. The reported results in Fig. 8 establish that 1.0–1.5 g L−1 Fe2+ loading would be appropriate for the combined approach of ultrasound and Fenton chemistry especially at higher initial COD loading. The required reaction time to reach satisfactory COD removal using pretreatment will be higher at lower oxidant loading. The oxidant loading and pretreatment time need to be optimized as the oxidant loading is also dependent on the type of pollutant present in the effluent as also demonstrated by Bautista et al. (2007). It can be seen from Table 3 that the maximum removal of COD and BOD was 43.2 and 20.4 %, respectively, at 2.0 g L−1 dose of Fe2+ with 1.5 g L−1 dose of H2O2 after 40 min treatment using ultrasound with stirring. The obtained BOD5/COD values were 0.653 and 0.695, which can be considered ideal for aerobic biodegradation (Ghaly et al. 2009). In the case of only Fenton, a similar increase in the biodegradability index was obtained at a Fe2+ loading of 2.5 g L−1, which confirmed that the combination with ultrasound allows a reduction in the required oxidant loading to obtain similar effects. In addition, the COD removal efficiency was dependent on the dose of H2O2, which might be due to the autodecomposition of H2O2 to oxygen and also water as well as the recombination of OH• radicals (Gogate and Pandit 2004; Mandal et al. 2010). The excess H2O2 may also react with OH• (Lodha and Chaudhari 2007) giving a scavenging action. Comparing the results obtained in the combined treatment of ultrasound (US)

Environ Sci Pollut Res Fig. 8 COD removal for different Fe2+ doses at US combined with Fenton at 25 °C: a COD0 = 11,866.7 mg L−1; [H2O2]0 = 1.0 g L−1; and b COD0 = 23,733.4 mg L−1; [H2O2]0 = 2.0 g L−1)

with H2O2 as well as Fenton, it has been established that maximum conversion of organic load (COD) is obtained using the combined treatment of ultrasound with the conventional Fenton process. In this process, a higher quantum of active hydroxyl radicals is generated due to the effective recycle of Fe2+, which enhances the conversion of organic load (COD) by the use of Fenton chemistry (Jamil et al. 2011). The intermediate complex (Fe-O2H2+) generated from the chemical reaction of Fe3+ and H2O2 is dissociated into Fe2+ and HO2• under the effect of ultrasound. Thus, the combined US process with Fenton chemistry (US/Fe2+/H2O2) gives a significant enhancement in the removal efficiency of organic load compared to US and Fenton treatments operating individually which is also confirmed from some

of the studies reported in the literature, although for model pollutants (Elbeshbishy and Nakhla 2011; Ramteke and Gogate 2015b, c). Comparison of pretreatment processes Figure 9a depicts the obtained data for COD removal for different processes and Table 4 depicts the comparison of the different pretreatment processes in terms of COD and BOD removal as well as BOD5/COD ratio based on the use of optimized loading of H2O2 and Fe2+ ion and 40 min of pretreatment. It can be observed that the maximum extent of removal of COD and BOD was 30.7 and 14.6 %, respectively, for Fenton chemistry at 2.5 g L−1 Fe2+ dose and 1.5 g L−1

Environ Sci Pollut Res Table 3 Effect of initial H2O2 and Fe2+ concentration for ultrasound treatment without and with stirring on the BOD removal, COD removal, and BOD5/COD ratio at constant treatment time of 40 min

Fig. 9 a Influence of different pretreatment processes on evolution of COD removal and b determination of first-order rate constants (K) ([H2O2]0 = 1.5 g L−1)

Fe2+ (g L−1)

H2O2 (g L−1)

Without stirring

With stirring

COD (%)

BOD (%)

BI:BOD5/ COD

COD (%)

BOD (%)

BI:BOD5/ COD

0.0

0.0

1.7

1.3

0.498

2.1

1.8

0.497

0.0

0.5

2.6

1.4

0.502

4.0

1.6

0.508

0.0 0.0

1.0 1.5

3.8 5.8

2.3 5.1

0.504 0.500

6.4 8.8

3.0 7.1

0.514 0.506

0.0 1.0

2.0 1.5

8.9 18.9

6.1 7.7

0.511 0.564

9.7 24.7

6.7 10.7

0.512 0.588

1.5

1.5

28.6

11.8

0.612

35.6

19.7

0.619

2.0 2.5

1.5 1.5

36.2 51.8

16.0 39.8

0.653 0.619

43.2 53.5

20.4 41.8

0.695 0.620

3.0

1.5

55.4

47.9

0.579

60.8

53.2

0.592

Environ Sci Pollut Res Table 4

Summary of experimental results for different pretreatment processes followed by aerobic biological oxidation process ([H2O2]0 = 1.5 g L−1)

Pretreatment effluent

Aerobic biological oxidation

Pretreatment process

Irradiation time (min)

Influent COD (mg L−1)

Influent BOD (mg L−1)

Influent BOD5/COD

HRT (h)

Residual COD (mg L−1)

% COD removal

Overall % COD removal

Untreated

0

11,866.7

5884.1

0.496

30

3210.0

72.9

72.9

40

1774.9

85.0

85.0

686.4 2762.5

94.2 76.3

94.2 76.7

Only US

US/H2O2

20

11,857.7

5879.3

0.496

60 30

40

11,659.8

5879.0

0.498

40

1709.4

85.3

85.6

60 30

654.5 2382.1

94.4 78.7

94.5 79.9

40 60 30

1589.3 648.1 2022.4

89.3 94.2 81.3

86.6 94.5 83.0

40 60 30 40 60 30 40 60 30

1156.4 378.0 548.6 369.4 84.7 978.6 482.8 232.9 377.5

89.3 96.5 92.8 95.1 98.9 88.1 94.1 97.2 94.4

90.3 96.8 95.4 96.9 99.3 91.8 95.9 98.0 96.8

40 60

246.5 40.7

96.3 99.4

97.9 99.7

20

11,725.1

5817.5

0.496

40

11,615.3

5584.0

0.500

20

11,629.5

5781.6

0.497

40

10,818.6

5469.0

0.506

US/Fenton (Fe2+ = 2.0 g L−1)

20 40

9654.0 7576.4

5627.1 4944.4

0.583 0.653

Only Fenton (Fe2+ = 2.5 g L−1)

20 40

10,271.4 8224.6

5384.8 5025.5

0.524 0.611

US/Fenton/stirring (Fe2+ = 2.0 g L−1)

20

9168.3

5441.4

0.593

40

6738.7

4682.5

0.695

US/H2O2/stirring

H2O2 dose after 40 min of treatment. The maximum removal of COD and BOD was 36.2 and 16.0 %, respectively, for US/Fenton and 44.2 and 20.4 %, respectively, for US/Fenton with stirring at optimized loading of oxidants with 2.0 g L−1 of Fe2+ and 1.5 g L−1 of H2O2. Table 4 also depicts that the BI values were 0.653, 0.611, and 0.695 after treatment using US/Fenton, Fenton, and US/Fenton/stirring, respectively. On the basis of the highest value of BI which was obtained for the US/Fenton/stirring process, this approach can be selected as the most efficient pretreatment process. It can be also seen from the Table 4, that the untreated sample gave a minimum rate of degradation of contaminants due to a high toxicity level (more biorefractory compounds) in wastewater, and the Fenton or US/Fenton pretreated samples showed a higher rate of removal of contaminants because of the presence of intermediates formed during Fenton pretreatment oxidation, which are easily digestible. Different well-known kinetic models (Lafi et al. 2009) were also used to establish the kinetic performance of advanced oxidation in terms of the variation in COD with pretreatment time. For this kinetic study, first-order and secondorder kinetic models were used. From the results presented in Fig. 9b, it can be seen that the first-order kinetic model gives a straight line fitting with an acceptable correlation coefficient

(range of R2 = 0.93–0.99) for advanced oxidation of industrial wastewater by only Fenton, US, US/H2O2/stirring, US/ Fenton, and US/Fenton/stirring. On the other hand, the experimental data fitting using the second-order kinetic model have lower values of correlation coefficients which establish that the first-order kinetic model is the best suitable model to describe the advanced oxidation of industrial wastewater. The first-order rate constant, K, obtained for different pretreatment schemes was 0.0085, 0.0004, 0.0028, 0.0109, and 0.0135 min−1 for only Fenton, US, US/H2O2/stirring, US/ Fenton, and US/Fenton/stirring, respectively. The maximum value of K obtained for the process of US/Fenton/stirring also clearly establishes the utility of this pretreatment scheme. To study the changes in molecular and structural characteristics of real industrial effluent as a result of oxidation via US, US/H2O2, Fenton, and US/Fenton processes, representative UV-visible spectra for the treated solution as a function of reaction time was obtained as depicted in Fig. 10. As observed from these spectra, before the oxidation, the absorption spectrum of wastewater was characterized by one main band in the visible region, with absorption peaks at 435, 460, 553, and 607 nm, and by another band in the UV region located at 373, 376, 381, and 387 nm. These peaks were associated with the

Environ Sci Pollut Res

Fig. 10 UV-vis spectral changes of wastewater after pretreatment

contaminants present in the wastewater. The disappearance of the visible band over time was due to the destruction of the structures by oxidation (Silverstein et al. 1991). During US/Fenton/stirring degradation, the absorbance values diminish all over the spectral window and no more specific peak remains after 40 min of reaction. In addition to this changes in the absorbance over the visible region, the decay of the absorbance at the UV-visible band was also considered as evidence of degradation of the effluent molecule and its intermediates. Effect of pretreatment on biological oxidation The pretreated real industrial effluent using advanced oxidation process was further subjected to aerobic biological oxidation. The rates of oxidation as well as the HRT for the pretreated effluents were compared with the rates obtained when the pretreatment was not used and only aerobic oxidation was applied directly to the wastewater. The obtained results for the different combinations of pretreatment and aerobic oxidation have been given in Table 4 and Fig. 11. It can be seen from Fig. 11 and Table 4 that the maximum COD removal efficiency of about 99.7 % was obtained for the combined process of US/Fenton/stirring followed by aerobic biological oxidation. In general, the experimental data confirmed that a higher extent of COD removal using aerobic biological oxidation is obtained for the pretreated samples as compared to the untreated samples. The residual COD values after 40 h HRT for the aerobic biological oxidation were 482.8, 369.4, and 246.5 mg L−1 for different samples subjected to Fenton, US/Fenton, and US/Fenton/stirring, respectively. The values further decreased to 232.9, 84.7, and 40.7 mg L−1 in the same order of pretreatment after 60 h HRT. Similarly, the residual COD of 1774.9 and 686.4 mg L−1 was obtained after 40 and 60 h HRT, respectively, for the untreated sample. Thus, it can be established that the

Fig. 11 COD removal efficiency in combined AOP and biological oxidation process (HRT in bioreactor = 40 h; [H2O2]0 = 1.5 g L−1)

residual COD decreased by more than 10 times for the approach of ultrasound combined with Fenton pretreatment in the presence of stirring as compared to untreated wastewater. In other words, the applied pretreatment has effectively increased biodegradability increasing the rate of biodegradation and hence minimizing the HRT required for similar levels of treatment as compared to untreated wastewater. The obtained differences for various pretreatments can be attributed to the fact that overall aerobic oxidation efficiency mainly depends on the nature of intermediates formed during each pretreatment, which would be different. It is established that US/Fenton with and without stirring (oxidant dose of 2.0 g L−1 Fe2+ and 1.5 g L−1 H2O2) at 40 min treatment was the most promising pretreatment condition for the combined operation giving 96.3 and 95.1 % COD removal after 40 h HRT in aerobic biodegradation, respectively. The other pretreatment processes such as ultrasound with and without H2O2 as well as US/H2O2/stirring are not able to produce sufficient biodegradable intermediates after 40 min of treatment at a constant dose of 1.5 g L−1 H2O2 for all the cases. The maximum extent of COD removal obtained using only biological oxidation of the untreated sample was 85.0 and 94.2 % after 40 and 60 h HRT, respectively (Table 4). Thus, it can be said that an HRT of 40 h for the pretreated samples using US/Fenton with stirring approach is giving better COD removal as compared to the 60-h HRT in the case of the untreated sample being subjected directly to biological oxidation. Overall, it has been conclusively established that the combined AOP and biological oxidation process is more effective as compared to single stage oxidation either by AOP or aerobic oxidation.

Environ Sci Pollut Res

Kinetics of biological oxidation of treated and untreated samples Biological oxidation experiments for establishing the kinetic data for untreated and US/Fenton/stirring pretreated samples were conducted using MAS sludge and also two different types of sludge (PAS and AS) to compare the activity of different types of sludge at different HRTs. The pretreatment time was fixed at 40 min for US/ Fenton/stirring treatment, and constant H 2 O 2 dose of 1.5 g L−1 and Fe2+ dose of 2.0 g L−1 were used which resulted in a residual COD (mg L−1) of 6738.7 mg L−1 and BOD5/COD of 0.695. Table 4 gives the obtained data for each of the untreated and pretreated samples subjected to biological treatment. It can be seen that COD removal observed for US/Fenton/stirring pretreated samples was higher as compared to the untreated samples, with the corresponding aerobic oxidation time (HRT) decreasing to about 40 h which is significantly lower as compared to the untreated condition (HRT of 60 h). In other words, for a similar extent of removal using the pretreated samples, significantly lower HRT is required as compared to the untreated samples. The observed results were attributed to the fact that the pretreatment process converts organic compounds into easily biodegradable products, enhancing the efficiency of the subsequent biological steps. Figure 12a, b depicts the variation in the residual COD and kinetic analysis for biological oxidation with different pretreated effluents. From Fig. 12a, it is confirmed that maximum COD removal of 94.4 % at 30 h treatment time was achieved for the US/Fenton/stirring treated sample. On the other hand, maximum COD removal of 94.2 % at the end of 60 h treatment time was obtained for the untreated sample. The results confirmed that the initial rates of aerobic oxidation were significantly enhanced, giving the best results using the combined treatment process. In addition, the final COD for the industrial effluent obtained was 1308.4 and 900.8 mg L−1 for US/Fenton without and with stirring, respectively, which is significantly lower as compared to the COD value of 6369.3 mg L−1 obtained for the untreated sample after 30 h of treatment. The kinetic equations describing the removal of COD in the biological process would be defined by the following model proposed by Contreras et al. (2003). − ¼

1 dðCODÞ COD ¼ k COD X dt k SðCODÞ þ COD k COD COD ¼ K COD COD k SðCODÞ

ð3Þ ð4Þ

where X is the biomass concentration, kCOD is the maximum COD removal rate constant, kS(COD) is the variable depending on the total number of components in a multicomponent

substrate, and KCOD is a function of the biomass activity depending on the physiological state of microorganisms. As per the results given in Fig. 12b, it can be established that the experimental data fits well into the first-order kinetic model. The values of the kinetic parameter KCOD for the untreated and pretreated samples were established from the fitting. The kinetic constants were observed to be around 0.047 h−1 for the untreated and US-treated samples with a slightly higher value for the US/H2O2/stirring of about 0.055 h−1. In addition, aerobic oxidation kinetic constant of 0.084, 0.075, and 0.068 h−1 was obtained for the samples treated using US/Fenton/stirring, US/Fenton, and Fenton, respectively. The higher oxidation rate constants for the treated samples may be attributed to the fact that easily digestible intermediates are formed in the pretreatment, which provide a higher rate of aerobic oxidation (Walling 1998; Chan et al. 2009). The low value of rate constant for the ultrasound-treated sample indicates that no drastic change in the combination of intermediates is obtained which is attributed to the limited oxidation capacity of ultrasound. During the biological oxidation process, samples were withdrawn at regular intervals to analyze the levels of mixed liquor suspended solid (MLSS) and COD which is the biodegradable substrate for all the three types of sludge. The dense mixed culture colony population of MAS sludge was observed in 100−1 dilution samples because of favorable environment to the growth of the microbes. Aerobic biodegradation was performed at varying initial substrate concentrations (COD) of 11,866.2 and 6738.7 mg L−1 and fixed initial biomass (X0) of 1.5 g L−1. Figure 13a, b represents the variation in the substrate and biomass concentrations with time for the untreated and US/Fenton/stirring pretreated samples using three different types of sludge. It is observed that COD removal obtained using PAS, MAS, and AS is about 64.8, 96.3, and 89.5 %, respectively, in the US/Fenton/stirring pretreated sample. The obtained removal is higher than that obtained for the untreated sample (about 35.4, 85.0, and 69.4 %, respectively) after 40 h of aerobic oxidation time. Comparison of the results for the different types of sludge revealed that maximum COD removal is observed for the case of MAS sludge as compared to PAS and AS. From Fig. 13a, it can also be established that biodegradation is rapidly taking place within the first 40 h, and beyond this time, a marginal increase is obtained. Figure 13b represents the evolution of biomass concentration with aerobic oxidation time in the case of the untreated and pretreated effluents using three different types of sludge. It can be observed from Fig. 13b that biomass concentration increases up to a certain time and then decreases, which confirms that a certain maximum value of biomass concentration exists (Aiba et al. 1973). Also, the value of maximum biomass obtained is dependent on the initial substrate concentration (COD). In this situation, it can be established that an optimum aerobic oxidation time (HRT) exists and the value of optimum is between 30 and 40 h in which maximum evolution of biomass

Environ Sci Pollut Res Fig. 12 a Evolution of chemical oxygen demand (COD) and b kinetic study of the biodegradation using the MAS sludge ([H2O2]0 = 1.5 g L−1; [pH]0 = 7.0; temperature = 25 °C)

concentration is obtained. The results reveal that MAS sludge provides a good environment and higher specific activity for the growth of microbes which would have good ability to remove the pollutants as well as intermediates formed during the pretreatment process. It is also clearly established that the MAS sludge used for biological oxidation combined with pretreatment of US/Fenton/stirring offers an excellent approach for the treatment of real industrial effluent. The present study involved the use of a dimensionless number (q) to establish the kinetics of the biodegradation process, based on the substrate concentration (S) and biomass concentration (X) as given by the following expression: q¼

S=S 0 X =X 0

ð5Þ

where X0 is the initial biomass concentration and S0 is the initial substrate concentration (COD0). Figure 14 gives the

variation in the dimensionless number (q) with aerobic oxidation time (h), and it can be seen that exponential decay was observed. The mathematical relationship with the correlation coefficient of R = 0.91–0.99 was established using the leastsquare method for the exponential decay model based on three parameters: q ¼ 0:0552 þ 1:1089e−0:0539t

ð6Þ

q ¼ 0:0187 þ 1:0317e−0:0975t

ð7Þ

Equations (6) and (7) describe the fitting for the untreated and US/Fenton/stirring samples, respectively, obtained using MAS sludge (see Fig. 14). Equations (6) and (7) can be used further to obtain the expression for the reduction rate as follows:

Environ Sci Pollut Res Fig. 13 Evolution of a residual COD and b residual biomass concentration (X) during biodegradation of untreated and pretreated effluent by US/Fenton/ stirring using three different sludge ([Fe2+]0 = 2.0 g L−1 and [H2O2]0 = 1.5 g L−1 with pretreatment time 40 min)

−

dq ¼ 0:0539ðq−0:0552Þ dt

ð8Þ

−

dq ¼ 0:0975ðq−0:0187Þ dt

ð9Þ

Equations (8) and (9) have the general form of the pseudo first-order model. The obtained values of the model parameters for the untreated and US/Fenton/stirring pretreated approaches are as follows: rate constant of K = 0.0539 and 0.0975 h−1 and equilibrium q value of qe = 0.0552 and 0.0187, respectively. As per the results observed from Fig. 15, the pseudo first-order model (R2 = 0.99) fits the experimental data better than the power law model proposed by

Grau et al. (1975) which has R2 = 0.90. Substituting the value of q from Eq. (3) into Eqs. (8) and (9) gives dimensional rate equations (10) and (11) for the biodegradation of the untreated and pretreated samples, respectively, as follows:   dðS=X Þ S S0 ð10Þ ¼ 0:0539 −0:0552 − X0 dt X   dðS=X Þ S S0 ¼ 0:0975 −0:0187 − X0 dt X

ð11Þ

Coupled treatment based on AOPs and biological treatment is considered as a new approach (Rivas et al. 2003; Beltran

Environ Sci Pollut Res 1.2

-0.0457t

2

PAS; q=0.3689+ 0.7005e ; R = 0.91 -0.0539t 2 MAS; q=0.0552+1.1089e ; R = 0.95 -0.0382t 2 AS; q=0.0621+1.1317e ; R = 0.94 -0.0446t 2 PAS; q=0.0207+1.0213e ; R = 0.97 -0.0975t 2 MAS; q=0.0187+1.0317e ; R = 0.98 -0.0669t 2 AS; q=0.0317+1.0667e ; R = 0.97 Untreated Pretreaed

1.0

0.8

0.6

q [t]

Fig. 14 Determination of exponential decay constants to correlate dimensionless Bq^ with aerobic oxidation time for biodegradation of untreated and US/Fenton/stirring pretreated real industrial wastewater using three different sludge ([Fe2+]0 = 2.0 g L−1 and [H2O2]0 = 1.5 g L−1 with pretreatment time 40 min)

0.4

0.2

0.0 0

10

20

30

40

50

60

70

80

Aerobic oxidation time (h) et al. 2000). The main objective of pretreatment in this approach is to convert initial toxic and nonbiodegradable compounds into intermediates which can be easily digested by the sludge biomass. In the present study, coupling of biodegradation and AOP process has resulted in better efficacy as per data shown in Table 4. From Table 4, it has been established that the overall maximum COD removal efficiency of 96.9 and 97.9 % has been obtained at 40 h HRT using aerobic biological treatment of industrial effluent with pretreatment by Fenton and US/Fenton/stirring, respectively, for a 40-min time. The results confirm that the US/Fenton/stirring followed by aerobic biodegradation using MAS sludge is found to be the best approach for the treatment of real industrial effluents. Fig. 15 Determination of pseudo first-order and power law rate constants for biodegradation of untreated and US/Fenton/stirring pretreated real industrial wastewater using MAS sludge

Conclusions The performance of combined treatment of advanced oxidation (based on ultrasound and Fenton) followed by biodegradation process for the treatment of real industrial wastewater was investigated for the first time. The coupled treatment of advanced oxidation followed by the biodegradation process was established to give superior levels of COD reduction than any single stage treatment under similar operating conditions. US/Fenton/stirring as pretreatment was established to be the best pretreatment approach. The maximum efficacy was achieved using an initial pH of 3.0, a Fe2+ dose of 2.5 g L−1, and an initial H2O2/COD weight ratio corresponding to the

Environ Sci Pollut Res

theoretical stoichiometric value. Maximum COD and BOD removal efficiency of about 43.2 and 20.4 %, respectively, was obtained, with the corresponding BOD5/COD ratio increasing from 0.496 to 0.695 in 40 min of pretreatment which confirmed good degradability for aerobic oxidation. Overall maximum COD removal efficiency of 96.9 and 97.9 % has been obtained in 40 h HRT using aerobic biological treatment of industrial effluent which was previously treated for 40 min by Fenton and US/Fenton/stirring, respectively. The required HRT decreased for the combined approach and also the COD removal efficiency of these combined processes increased. The average effluent COD concentration of combined US/ Fenton/stirring with biodegradation using MAS sludge was reduced from an initial of 11,866.7 to 246.5 mg L−1 after 40 h HRT, which agrees well with the local standards. The average effluent COD concentration for the combination of Fenton with biodegradation using MAS sludge was obtained at 482.8 mg L−1 after 40 h HRT. Thus, depending on the requirement, the final selection of the pretreatment approach can be made. The result revealed that the combined process was an efficient method to treat real industrial wastewater. On the basis of the kinetic study, the combination of advanced oxidation process and biodegradation was found to be more efficient as compared to only biodegradation. Overall, the work has clearly demonstrated the approach for optimizing the combined treatment process for real industrial effluent based on the use of various advanced oxidation processes.

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