Desalination 250 (2010) 87–94

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Advanced oxidation process and biotreatment: Their roles in combinedindustrial wastewater treatment

Tamal Mandal a,⁎, Sudakshina Maity a, Dalia Dasgupta b, Siddhartha Datta c,⁎a Department of Chemical Engineering, NIT, Durgapur, Indiab Department of Biotechnology, BCET, Durgapur-12, Indiac Department of Chemical Engineering, Jadavpur University, Kolkata-32, India

Abbreviations: SAIL, Steel authority of India; DPL, DDurgapur Chemicals Limited.⁎ Corresponding authors. Mandal is to be contacted at

neering, NIT, Durgapur, Mahatma Gandhi Avenue, DurgapTel.: +91 9474533097 (Mobile); fax: +91 343 2547375.Engineering, Jadavpur University, Raja S.C.Mallik Road9830108902 (Mobile), +91 33 2335 9345; fax: +91 33 2

E-mail addresses: tamal_mandal@yahoo.com (T. Ma(S. Datta).

0011-9164/$ – see front matter © 2009 Published by Edoi:10.1016/j.desal.2009.04.012

a b s t r a c t

a r t i c l e i n f o

Article history:Accepted 21 April 2009Available online 14 October 2009

Keywords:WastewaterFenton's reagentsFenton's oxidation and coagulationH2O2/FeSo4 ratioBiochemical treatment (T. ferrooxidans)Combined treatment processSynergistic effect

The use of Fenton's reagents in destruction of waste material present in Tambla Tributory (Durgapur,India)industrial wastewater has been investigated. Significant drop in COD removal has been observed. Optimisationof process parameters like pH, temperature, H2O2 and FeSO4 has been done. Temperature and pH played a keyrole in this treatment process, in addition the process initially liberated heat due to reaction between FeSO4 andH2O2. From the experimental results it has been observed that with increasing FeSO4 and H2O2 concentrationthe degradation of waste increases. At an optimum concentration of FeSO4 (6 gm/l) and H2O2 44.40 gm/lreduced 60% COD, whereas 220gm/l H2O2 was required for 95% COD removal. To reduce cost and the H2O2

concentration formaximumwaste degradation, Fenton's oxidation process followed by biochemical treatmentwas tried at same experimental condition. The treatment enhanced the overall removal efficiency of COD, BOD,salinity and colour significantly. The microbial treatment by Thiobacillus ferrooxidans, following Fenton'sreagents treatment, showed that the COD reduction has reached to about 97% compared to 60% with Fenton'sreagents and 17% with T. ferrooxidans alone in 24 h, showing the synergistic effect. Thus the combinedtreatment results indicate the possibility to minimize the Fenton's reagents without compromising theefficiency of the process but ultimately reducing the overall treatment cost. This study seems to be very muchimportant and economical by reducing the required H2O2 amount to about five times using a suitable micro-organism. This hybrid treatment system showed 97% COD reduction can be achieved within two days.

urgapur projects Limited; DCL,

Department of Chemical Engi-ur, West Bengal-713209, India.Datta, Department of Chemical, Kolkata-32, India. Tel.: +91413 7121.ndal), provc@admin.jdvu.ac.in

lsevier B.V.

© 2009 Published by Elsevier B.V.

1. Introduction

The effluents having contaminants suchas synthetic chemicals, dyes,organic matters, refractory organic waste, heavy metals etc aredischarged to the nearestwater bodieswith orwithout any preliminarytreatments. This causes serious damage to the DO level and ecologicalbalance of the ecosystem of the nearby receiving water bodies [1,2].Thus numerous studies are going on for finding a suitable technology tothe wastewater treatment. Within that advanced oxidation processes(AOPs) have led theway in the treatment of aqueouswaste. It is rapidlybecoming the chosen technology for its many applications such asorganic pollutant destruction in the form of toxicity reduction, Bio-degradability improvement BOD/COD removal as well as odour and

colour removal. Literature reveals that a lot of effluents like carpetdyeing wastewater [3], trihalomethanes [4], cork cooking wastewater[5], synthetic dye Orange II [6], Acid dyebath effluent [7], textilesecondary effluents [8], dyewastewater [9] are effectively being treatedby Fenton's reagents. Fenton's treatment also improves the biodegrad-ability of the wastewater [10]. The major drawback of Fenton'streatment appears to be the requirement of large concentrations ofH2O2 and FeSO4 in the treatment process. It is also supported by severalstudies that the H2O2/Fe2+ ratio are the key to improve the efficiency ofthe Fenton's treatment. Tang and Tassos, Kochany and Lugowski [4,11]have pointed out that optimal H2O2/Fe2+ ratio has to be maintained toachieve the maximal degradation efficiency. The optimum reactionconditions like temperature, pH, H2O2 and FeSO4 have to be optimizedto achieve maximum waste degradation by Fenton's reagents. Thoughmost of the literature reported [6–8] that 30 °C is the optimumtemperature for Fenton's oxidation, there are studies, suggesting thatthis may vary with the type of effluents [6,7]. Nowadays bioremedia-tion/biotreatment has also proved to be a new technology forwastewater treatment and people are finding the significant role ofmicro-organisms in reducing the COD level of different types ofwaste inindustrial effluents [12–15]. Among the various micro-organismsstudied for wastewater treatment process, chemolithotropic bacteria

Table 1The effluent (wastewater) characteristics in Tamla nalah: in general.

Temp. (°C) 30–40pH 6.5–11.5Colour Slight Brownish–dark BrownishTDS (mg/L) at 105 °C 200–440TSS (mg/L) at 105 °C 320–450BOD (mg/L) 900–1100COD (mg/L) 2700–4000Phenol (mg/L) 4–12Cyanide (mg/L) 0.4–2.5Free Ammonia (mg/L) 45–65Fixed Ammonia (mg/L) 1100–1800Oil & Grease (mg/L) 20–40Sulphide (mg/L) 10–12

Except these some other unwanted chemicals are available in the Tambla water likePhenolic compound, cyanide as CN, Ca2, Cr, Cu, Ni, Zn, Mg2+, Fe ion etc.

88 T. Mandal et al. / Desalination 250 (2010) 87–94

have shown to have potential role in wastewater treatment underaerobic condition [16–20]. Thiobaccilus ferrooxidans is a member ofchemolithotropic group and it grows in a FeSO4 containing inorganicmedia (9 K) at pH 2.5–3.5. Advanced oxidation process reduces thetoxicity level of the wastewater and allows the micro-organisms togrows Thus enhances the biodegradability [10]. The present study is

Pic. 1. Sampling site of Tamla tributary, Durga

therefore designed to see the effect of combination of Fenton's reagentsand suitable micro-organisms in wastewater treatment, so that aneconomical, time saving tools can be designed for effective wastewatertreatment system.

2. Materials and methods

2.1. Wastewater

Durgapur is called the rurh of Bengal (W.B, India). A number ofgiant manufacturing units like Durgapur steel plant (DSP), Alloy steelplant(ASP), Durgapur Chemicals (DCL), Durgapur projects LTD (DPL)and East India pharmaceuticals are located surrounding the city. Inaddition a number of power plants are there to cater the powerdemand of various industries and localities. The wastewater gener-ated are normally disposed off to the nearby channel (Tamlatributary) from these industries either with minor treatment orwithout treatment (Table 1). This channel finally finds its way into theriver Damodar, the only potable source of water. Day by day thequality of water is being degraded, which is ultimately affecting thehealth of human beings and the entire ecosystem of the aquatic life.Wastewater samples were collected from the different points ofTamla tributary (see Pic. 1), which is not the waste of any specific

pur, Dist: Burdwan, West Bengal (India).

Fig. 1. Changes in COD with time and rising of temperature by Fenton's treatment ofTamla wastewater. Conditions: H2O2 concentration=44.40–277.5 gm/l, pH=3.5,Initial Temp=30 °C (ambient), FeSO4 conc.=1.5–6gm/l, initial COD=2740 mg/l.

89T. Mandal et al. / Desalination 250 (2010) 87–94

industrial wastewater but it is a mixture of different industrial anddomestic wastewater.

2.2. Reagents

Hydrogen peroxide (30%, Density 1.11 kg/l, Merck, India), Ferroussulphate, Potassium chloride, Mercury sulphate, Silver sulphate,Ammonium biphosphate, Potassium dichromate, magnesium sul-phate, conc. Sulphuric acid and other chemicals (.Merck, India). Purityof all the chemicals is around 99%.

2.3. Experiments

T. ferrooxidans (ATCC19859) strain was used in this study. Thisstrain was originally collected from Dept. of Microbiology, Universityof Helsinki, Finland and the strain was subcultured regularly in 9 Kmedium for reuse. The major operational condition like temperature,pH, H2O2 and FeSO4 doses were investigated for the Fenton's andbiochemical treatment of Tamla wastewater, Durgapur, India.

2.4. Treatment of wastewater

The experiments were conducted in batch reactors taking 100 mlof wastewater sample in 250 ml conical flasks. The pH of thewastewater was adjusted with 1 M concentrated sulphuric acid forFenton's treatment. The required amount of FeSO4 (1.5–9.0 gm/L) andH2O2 (44.40–262 gm/L) were added, mixed by stirring continuouslyand kept at a required temperature for different reaction timeparticularly at 30 min and 24 h. After each time point, the sampleswere allowed to stand for 30 min, supernatants were decanted inseparate conical flasks. The pH of the supernatants were raised toabove 7.0 for precipitation and then filtered for analysis of absorptionin UV/VIS range and COD, BOD, colour and salinity were measuredafter coagulation step at each experimental point. To prevent theinterference of residual H2O2 in the COD test [9,21,22], the pH of thesupernatant was raised to 7.0 and above and allowed the decompo-sition of H2O2 to O2 and H2O.

The following tests were performed to get an optimum experi-mental condition for maximum waste degradation by advancedoxidation process (Fenton's reagents).

1. The effect of Fenton's reagents on COD reduction with varyingamount of H2O2 44.40–262.6 gm/L and FeSO4 of 1.5–9 gm/L at30 min and 24 h were studied and compared with the respectivecontrol.

2. Optimization of process parameters like pH, temperature, time,H2O2, and FeSO4 dose were performed.

3. T. ferrooxidans was chosen, as the optimum pH for the Fenton'streatment and the growth of T. ferrooxidans is nearly the same inthe acidic range of 2.5–3.5.The acclimatized T. ferrooxidans wascultured in 9 K medium in a BOD incubator shaker at 30 °C and pH— 2.5. From this 20 ml of culture sample was taken and centrifugedat 30,000 rpm to precipitate the microbial cell. The residue wasadded to 100 ml of wastewater in a 250 ml conical flask along with10% of 9 K medium, incubated at 30 °C on rotary shaker with120 rpm for 24 h. The treated samples were collected at eachexperimental time point, allowed to stand for 10–15 min andcentrifuged. Supernatants were used for COD, BOD, colour andsalinity analysis.

4. Combined effects of Fenton's treatment and T. ferrooxidans werecarried out to develop an economic treatment process forwastewater (by reducing the amount of H2O2).Wastewater treatedwith Fenton's reagents [H2O2 — 44.40 gm/L, FeSO4 — 6 gm/L] at50 °C and pH 3.5 kept for 24 h were again treated with 10% ofT. ferrooxidans a bacterial culture for 24 h at 30 °C and at pH. 3.5.The COD and BOD levels were comparedwith proper control. It was

observed that, the microbial growth was tremendously effected bythe presence of residual H2O2. However it was seen that after24 hours, all residual H2O2 was gone due to self decomposition.Thus to eliminate the toxic effect of residual H2O2 on the T. fer-rooxidans growth, the 24 h Fenton's treated wastewater were usedin combination treatment instead of 30 min.

2.5. Analysis

The COD and BOD of untreated and treated samples weremeasured according to standard protocol of APHA [10] and reactordigestion method for a COD range of 0–1500 mg/l using automaticCOD analyser of LoviBond Germany. The following electrode was usedto analyse the different parameters: Orion four star Ion analyser (pHISE Bench top) Thermo electron corporations, USA (SN012820), forNH3 Orion 95-12 Ammonia electrode, for Cyanide Orion 94-06 andOrion 96-06 ionplus were used. For pH (pH meter WTW, Ino Lab PH/Ion-735, Germany), conductivity and salinity were measured by InoLabCond 720, with electrode TetraCon 325, WTW, Germany. The absor-bance of the samples was determined by UV/VIS spectrophotometer(Tech Comp, UV/VIS-2300, China). The colour was determined with themeasurement of absorbance of the sample.

3. Results and Discussion

3.1. COD reduction by Fenton's reagent with time

For the study of the effect of Fenton's reagents, on wastewaterdegradation, the amount of H2O2 and FeSO4 was varied from 44.4–266.40 gm/L to 1.5–9.0 gm/L respectively. The required amount ofH2O2 and FeSO4 was added to each 100 ml of wastewater sample, keptat 50 °C, pH of 3.5 for 30 min and 24 h in a rotary BOD incubatorshaker. The COD tests after Fenton coagulation were performed. Theresults are represented in Fig. 1. It has been observed that Fenton'streatment has tremendous potential to degrade this mixed industrialwaste present in the samples. With increasing the amount of H2O2,FeSO4 though the ratio is fixed, the %COD reduction increased. Themaximum %COD reduction achieved was about 90% and 95%respectively for 30 min and 24 h treatment time, when the amountof H2O2 is 222.0 gm/L, FeSO4—7.5 gm/L was used. The results obtainedcan be explained on the basis of the fact that, with increasing theamount of H2O2 and FeSO4 in Fenton's reagents, the generation of OH•

(Hydroxyl) ion increases, which effectively decrease the COD from the

Fig. 2. Removal of COD with changing initial pH by Fenton's treatment of Tamlawastewater. Conditions, Temp=50 °C, H2O2 concentration=44.40 and 111 gm/l,FeSO4=6.0 gm/l, initial COD=2740 mg/l and time of reaction=24 h.

Fig. 3. Removal of COD with changing reaction temperature by Fenton's treatment ofTamla wastewater. Conditions, pH=3.5, H2O2 concentration=111 gm/l, FeSO4=6.0 -gm/l, initial COD=2740 mg/l and time of reaction=24 h.

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wastewater. The reaction mechanism for Fenton's reagent can bepresented by the following equations: [8,9]. Fenton's reagents is amixture of H2O2 and FeSO4 and RH is the organic waste materialpresent in the reaction system which is oxidized by the hydroxylradical produced by this reagents.

H2O2 þ Fe2þ→Fe

3þ þ OH− þ OH•; k ¼ 70M

−1s−1 ð1Þ

OH• þ RH→H2O þ R•; k ¼ 109−10

10M

−1s−1 ð2Þ

R• þ Fe3þ→R

þ þ Fe2þ ð3Þ

Rþ þ H2O→ROH þ H

þ: ð4Þ

The other side reactions are also important for the fate of con-centration of total H2O2 and FeSO4 in Fenton's treatment [24]

OH• þ OH•→H2O2 ð5Þ

OH• þ H2O2→H2O þ HO•2; k ¼ 3:3 � 10

7M

−1s−1 ð6Þ

The peroxide radicals (HO2• ) produced from reaction (6) are

capable to further oxidize other species including Fe2+ present in thereaction medium [25]

HO•2 þ Fe

2þ→O2 þ Fe3þ þ H

þ; k ¼ 1:26� 10

6M

−1s−1 ð7Þ

There is a possibility to be auto regenerated of Fe+2 in this systemand act as catalyst

Feþ3 þ H2O2→HO•

2 þ Fe2þ þ H

þ ð8Þ

In addition it was observed that with time, temperature wasincreased in both experimental and control set if the experiments areconducted in ambient temperature (around 30 °C). The reactionbetween FeSO4 and H2O2 for the generation of the OH• radical, theeffective component for the waste degradation appears to beexothermic. It was observed from experimental results presented inthe Fig. 1 that, the temperature was increased to about 40 °C from30 °C (10 °C rise) after 10–15 min of reaction time, when 44.40 gm/LH2O2 and 1.5gm/L FeSO4 were used and it increased with increasingthe amount of H2O2 and FeSO4 (Fig. 1). The magnitude of temperaturerise was same in both the experimental and control set. This signifiesthat heat is generated due to the reaction between H2O2 and FeSO4

only (Eq. (1)), and this heat may help to gear up the reaction betweenOH• and the waste material present in the water. The temperature risewas also found to be dependent on the pH of the reaction. At theoptimum pH of 3.5 the rise of temperature maximum up to 57 °Cwithin 15 min. Whereas temperature rise was 42 °C and 40 °C for thereaction pH of 2.0 and 8.0 in 45 min and 55 min respectively. This alsoindicates that the reaction rate is very much pH dependent. Rise oftemperature is directly proportional to the reaction rate i,e OH•

generation from H2O2 to FeSO4 (Eq. (1)). It may also conclude thatreaction between H2O2 and FeSO4 (Eq. (1)) is slower below and abovethe pH 3.5, which may also effect on overall COD reduction.

3.1.1. Effect of pH on waste degradation by Fenton's reagentsFrom the initial study it is understood that, role of pH on the

reaction system is very important, since pH plays an important role inOH• production [10,11,26]. So the wastewater was treated withFenton's reagents, at different pH ranging from 2–6 keeping all otherexperimental conditions constant. Thedegradation pattern at differentpH from 2.0 to 6.0 is shown in Fig. 2. It has been observed that theoptimum pH is 3.5 at which maximum COD reduction has beenachieved. It has been also observed that, for pH greater than 4.0 thedegradation is reduced. The reasons could be the reduced rate of

generation of hydroxyl radical (OH•) because of the formation of theferric hydroxo complexes, which subsequently form {Fe(OH)4} athigher pH [11]. The results also indicate that at pH below 3.0 thedegradation of the waste is also less. This may be due to the formationof complex species {Fe(H2O)6}2+, which reacts more slowly withperoxide compared to that of {Fe(OH)(H2O)5}2+. In addition, theperoxide gets solvated in presence of high concentration of H+ ion, toform stable oxonium ion {H3O2}+. An oxonium ion makes peroxideelectrophilic to enhance its stability and presumably reduces substan-tially the reactivity with Fe2+ ion [27]. This trend also satisfies thetrend of heat generation and overall COD reduction presented in Fig. 1.Thus pH 3.5 was considered as optimum pH for further studies.

3.2. The effect of temperature on COD removal by Fenton's treatments

The effect of temperature on COD removal of wastewater treatmenthas been investigated for the temperature range of about 30 °C to 60 °Cwith two different dose of Fenton's reagents. The results presented inFig. 3 show that, the degradation of waste increases gradually with thetemperature and at around 50 °C the degradation is nearly maximum.Above 50 °C rate of degradation is not increased insignificantly. In thisstudy, it was also observed that there is about 2.5 times increase in

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percentage reduction of CODwhen temperature is increased from 30 °Cto 50 °C and then it remains same. Previous studies have indicated therole of temperature on waste degradation [6,7,10]. The results obtainedin this study have been found to follow the same trend as that ofprevious researchers, though some literatures [5,6,28] reported that30 °C is the optimum temperature for Fenton's treatment. But thepresent study shows that 50 °C is optimum and which correlates withthe work of Alaton and Teksoy [7] where the 50 °C is the optimum forthe treatment of acid dye bath effluent. Ramirez et al. [6] reported thatthe temperature of 29 and50 °C asoptimum for colour andTOC removalfor the degradation of the synthetic dye orange II. Few studies [6,28]indicated that hydrogen peroxide decomposes into oxygen andwater atabove 50 °C and can effect greatly in overall COD reduction,which is notobserved in the present study, as there is no decrease in COD reductionabove 50 °C.

3.3. Effect of FeSO4 Concentration

The effect of FeSO4 concentration in Fenton's reagents on CODremoval was investigated, by changing the FeSO4 concentrationbetween 1.5 to 9.0 g/l, while keeping the concentration of H2O2, pHand temperature constant at 44.44 mg/l, 3.5 and 50 °C respectively. Theresults (Fig. 4) depict that COD reduction efficiency has been increasedfrom 25 to 65%with increasing FeSO4 from1.5gm/l to 6gm/l. So, there isabout 2.8 times enhanced COD reductionwith 4 times increase in FeSO4

concentration. Hence it may be stated that higher ferrous concentrationcauses generation of more OH• radicals and accelerate the redoxreaction. Another important thing is that Fe2+ converts into Fe3+ asshown in Eqs. (1) and (7), which act as coagulant resulting in improvedCOD reduction. But more than 6.0gm/L FeSO4 did not enhanced CODreduction significantly rather slightly reduced COD reduction withhigher concentration of FeSO4, which indicate that OH• generation ishindered in the presence of extra FeSO4 concentration. In addition theFe3+ formed can react with H2O2 (Eq. (8)) to generate Fe2+ andhydroperoxyl radicals [HO2

• ] in the reaction medium. The oxidationcapacity of HO2

• is less compare to OH•, which effect the overall CODreduction. Thus the optimum Fe2+ concentration with maximum CODreduction in this experiment is 6.0.

3.4. Role of H2O2 and reaction time in waste treatment

The effects of H2O2 doses were checked in %COD reduction. For thisdifferent amount of H2O2 in the range of 44.40– 277.50 gm/lwere used

Fig. 4. Removal of COD with changing FeSO4 by Fenton's treatment of Tamlawastewater. Conditions: H2O2 concentration=44.40 g/l, pH=3.5, Temp=30 °C, initialCOD=2740 mg/l and time of reaction=24 h.

at 6.0gm/l of FeSO4 concentration. To treat the wastewater it was keptat pH 3.5 and temperature 50 °C for 30 min and 24h. The results areshown in Fig. 5. It reveals from the results, shown in Fig. 3 that %CODreduction increases with increasing H2O2 concentration for each timepoint [4,11]. At 44.40gm/l H2O2, COD removal was 60.32% and itincreased to 95.37% when H2O2 was used 277.50gm/l for 24 h. Thisincrease in COD removal is similar to the findings presented byGulkaya et al. and Kang and Hwang [3,29] This may be due to the factthat increased amount of H2O2 reacts with more FeSO4 and producesmore amount of hydroxyl radical leading to more waste degradation.Another point could be the production of Ferric sulphate, which act ascoagulant and help in enhanced COD reduction. Bae et al. [25] havereported that maximum CODwas reduced by ferric coagulation ratherthan Fenton oxidation in their textile wastewater. The results arecorrelated with the observation of Rameej et al. and Tang and Tassosand Kochany and Lugowski [4,6,11]. In the present study when thetreatment timewas extended to 24 h from30 min the %COD reductionreached to 95.37%, which is not verymuch significant when comparedwith the 30 min treatment time (90%), at the highest H2O2

concentration of 277.50gm/l, which follow the equilibrium time isreported in the range of 10–30 min for Fenton's oxidation reaction.[9,26,30,31]. In this study it was observed that for 48% COD reductionthe amount of H2O2 used is 44.40gm/L, whereas for 95% CODreduction, the amount of H2O2 required is 277.50gm/L, indicatingthat about six times more H2O2 is required to achieve just double CODreduction which makes this process very costly.

3.5. Analysis of removal of organic compounds by Fenton's reagent usingUV spectophotometer

The degradation of organic compound was monitored, by thestudy of the absorbance of the untreated and treated (after oxidationand coagulation) sample of wastewater in UV absorbance range from190–600 nm. The original wastewater shows five peaks at 266,270,350,400 and 470 nm. Among these the absorbance is maximumat350 nm. After Fenton's oxidation, the peaks have been found todisappear and a continuous decrease in absorbance has beenobserved, it signifies that the Fenton's oxidation can remove theorganics present in the wastewater samples. When Fenton's treatedwastewater were further passed through coagulation stage it isobserved that the absorbance at those wavelength is less compared tothe absorbance after oxidation stage, which signify that the remainingorganics are also being removed by coagulation.

Fig. 5. Removal of COD with changing treatment time and H2O2 concentration byFenton's treatment of Tamla wastewater. Conditions: pH=3.5, Temp=50 °C, initialCOD=2740 mg/l and FeSO4=6.0 gm/l.

Fig. 7. Removal of COD with time by Thiobaccilus ferrooxidans of Tamla wastewater.Conditions, pH=3.0, Temp=30 °C, initial COD=2740 mg/l.

92 T. Mandal et al. / Desalination 250 (2010) 87–94

3.6. Effect of H2O2/Fe2+ ratio

The molar ratio of H2O2/Fe2+ plays an important role in COD removalfromwastewater sample. Gulkaya et al. showed that the ratio effective for95% COD removal from carpet dyeing wastewater is 95 to 296. Belowand above this ratio the treatment is inefficient. Some other researcheralso showed the different ratio for the treatment of specific wastewater.Like Casero et al. [42] reported that the required molar ratio for theoxidation of amine around 5–40. For the removal of azo dye activeyellow lightfast 2KT required a wide range of molar ratio (5.8–340)[9,26]. Tang and Huang shows that the effective ratio 5–11(w/w) fortreatment of chlorinated aliphatic organicswhereas Tang and Tossos [4]suggested molar ratio 2:1–5:1 for bromoform oxidation. So H2O2/Fe2+

molar ratio in wide variation plays a key role in waste degradationdepending on the types of waste, as well as their loads.

Keeping the cost of H2O2 inmind, the optimumH2O2/Fe2+ ratio wastried to evaluate in the present study, which canminimize the requiredamount of Fenton's reagents [H2O2 and Fe2+]. In the present study ratio(wt/wt) of H2O2/Fe2 was used keeping H2O2 concentrations at aconstant value of 111gm/L for the treatment of Tamla channelwastewater. The results in Fig. 6 indicate that % COD removal increasedto 90% as the ratio reached to 16 and remained constant in between theratio of 16 to 50. The optimum ratio obtained from the best fitting curveand regression analysis was 20.00, in between the value range of thework of Casero et al. [42] for the amine oxidation. In fact thewastewaterused in the present study (collected from Tamla channel Durgapur,India) is a mixed wastewater of different industries may consist ofamine compounds in thewaste stream. At the ratio ofmore than 50, theCOD removal decreased and it continued for higher ratio. The probablereason behind this may be the sufficient OH• radical was not produced(Eq. (1)) due to lack of FeSO4 concentration in the reactionmedium. Onthe other hand when the ratio of H2O2/Fe2+ was low, at around 13, thetotal COD removalwas insignificant. This is probably due to the fact that,the FeSO4 concentration in excess can help to produce hydroperoxylradical [HO2

• ] (Eq. (8)), which has less oxidation capacity than hydroxylradical. Thus the overall COD reduction has reduced. Keeping the ratioconstant at 20 some other experiments were designed to find out theconcentrations of H2O2 and FeSO4 for more that 90% COD reduction.According to the results the COD removal remains constant for H2O2

concentration of 100–320 gm/l and FeSO4 concentration of 5–18gm/l.For doses lower than these ranges, COD removal decreased remarkablyalthough the ratio was kept constant. This result reveals that to get asatisfactory COD removal, not only the required H2O2/Fe2+ ratio isimportant but the sufficient amount of H2O2 and FeSO4 is needed toproduce adequate amount of OH•. Keeping the optimum ratio of H2O2

and FeSO4 at 20 the exact amount of H2O2 and FeSO4 needed formaximum COD removal can be calculated following Gulkaya et al. [3].

Fig. 6. Removal of CODwith changing ratio H2O2/FeSO4, by Fenton's treatment of Tamlawastewater. Conditions, pH=3.5, Temp=50 °C, initial COD=2740 mg/l and time ofreaction=24 h.

3.7. The combination of biochemical and Fenton's reagents for thetreatment of mixed industrial wastewater

From the results, it is proved that certain ratio ofH2O2 and FeSO4, playssignificant role in COD reduction of the mixed wastewater sample in thepresent study. It has been found thatwith high concentration of H2O2 andFeSO4 the%CODreduction increases. It is also observed that atfixed FeSO4

concentrations of 6gm/l, 44.40gm/l H2O2 reduces about 60% of COD and277.70 gm/l H2O2 reduces about 95% COD when treated for 24 h. Thatmeans for getting 35% more COD reduction at least six times more H2O2

was needed. This is tremendously affecting the treatment cost. Thus nextattemptwasmade, to reduce the H2O2 requirement. To reduce the cost ofthe wastewater treatment process by Fenton's reagents. So microbialtreatment process was used along with Fenton's reagent in this study.Here T. ferrooxidans was selected for the treatment. T. ferrooxidans needsFeSO4 to obtain energy for its cellular growth by oxidizing Fe+2 to Fe+3

and pH 2.5 to 3.5 for its healthy growth, which is verymuch similar to theexperimental conditions for the wastewater treatment by Fenton'sreagents. The attributes of the present study is that the microbes T. fer-rooxidans can degrade waste effectively with time (Fig. 7). Fig. 7 depictsthat initially the rate of degradation of waste is very high and it graduallydecreases with time. It is observed that the microbes showed theirmaximum activity at pH 2.9±0.5 and temperature at 30±2 °C. Theexperimental results are presented in the Figs. 8 and 9, for temperatureand pH respectively which indicate that stringent process control isrequired for better waste oxidation through biological pathway.

Fig. 8. Removal of COD by Thiobaccilus ferrooxidans of Tamla wastewater with changingtemperature. Conditions, pH=3.0, Temp=30 °C, initial COD=2740 mg/l.

Fig. 9. Removal of COD by Thiobaccilus ferrooxidans of Tamla wastewater with changingpH. Conditions, pH=3.0, temp=30 °C, initial COD=2740 mg/l, time 7 days.

Fig. 10. Removal of COD, BOD, Colour and Salinity by Chemical Treatment (CT, Fenton'streatment), Biochemical Treatment (BT, Thiobaccilus Ferrooxidans) and combinedtreatment of Tamla wastewater.

93T. Mandal et al. / Desalination 250 (2010) 87–94

3.8. Reduction of H2O2 in combined treatment method

For reducing the use of H2O2 amount in Fenton's treatment, aparticular concentration of H2O2 and FeSO4was chosen atwhich about60% COD reduction took place. To achieve the high percentage of CODreduction using low amount of H2O2, the effect of microbial treatmentin combination with Fenton's reagent was studied. This combinedtreatment was compared with T. ferrooxidans treatment alone andFenton's treatment. In practice 100 ml wastewater was treated with10% adapted bacterial culture in 250 ml conical flask, which wasstirred continuously after plugging at 32 °C in a BOD incubator shaker(rpm—125) and kept for different periods of time. At different time oftreatment the COD was measured regularly [23]. From the result intreatment with T. ferrooxidans alone, it has been found that the %CODreduction is significant, which is about 17% for day one and 56% forseven days treatment. It seems that although microbial treatment isecofriendly and cost effective but it is time consuming. This is probablydue to the presence of toxic materials in the wastewater. Fenton'streatment has been found to reduce the COD level (95% in 24 hrstreatment with 222 gm/l of H2O2 and 6 gm/l of FeSO4 and more than60% with 44.40 gm/l H2O2 and 6 gm/l FeSO4) and the absorptionmaxima at different wavelengths like 270, 350 nm etc indicating thereduction of organic load of the wastewater. This eventually helps thegrowth of T. ferrooxidans in the reactionmedium. The Fenton's treatedwastewater sample was kept for 24 h to remove residual H2O2 andtemperature came down from 50 °C to 30 °C, as it can inhibit thebacterial growth and can ultimately effect the overall waste degrada-tion. There is no remarkable change in COD reduction for 24 h Fenton'streatment, compared to 30 min treatment (Fig. 1). This 24 h Fenton'streated wastewater sample was used for treatment with T. ferrooxi-dans. Combined treatment reduced the COD level much moresignificantly, which is about 97%. It also removes BOD, colour andsalinity remarkably (Fig. 10) compared to the treatment of Fenton'streatment (60%) and T. ferrooxidans alone (17%). The combinationtreatment showed synergistic effect in the %COD reduction as seenfrom the results (Fig. 10) by about 20%.This synergistic effect on CODreduction can be explained due to the elimination of the toxicchemicals from the wastewater by Fenton's reagents pre-treatment,(reduction in absorption maxima at different λ), which ultimatelyhelp themicrobes to grow and function properly, thus the reduction ofCOD, BOD, colour and salinity has increased significantly. Thiscombined treatment process therefore seems to be a much moreefficient technology, for this type of mixed wastewater. It is costeffective as it is reducing the H2O2 requirement more than five timesfor getting more than 97% COD reduction. From the present study,combined technology can be thought of as a powerful technique forwastewater treatment in coming years.

4. Conclusion

The COD removal of mixed industrial wastewater having phenoliccompounds, cyanides, organics etc have been studied. Effectiveeconomic and ecofriendly treatment process is developed usingFenton's reagents along with microbial process. Fenton's reagentsalone has shown effective role in COD reduction of this mixedindustrial wastewater which is about 95%± of total COD removal. Theprocess parameters like temperature, pH, FeSO4 concentration andH2O2 concentration have been optimized for getting maximum CODreduction. It has been found that the optimum temperature, pH, FeSO4

concentration and H2O2 concentration are 50 °C, 3.5, 6 gm/l and111 gm/l respectively. It was also observed that initially the processliberate heat due to reaction between H2O2 and FeSO4 i.e whenequation one (Eq. (1)) is predominant in the reaction system. For 95%COD reduction by Fenton's reagents, 222 g/l H2O2 and 6.0 gm/l FeSO4

were required. High H2O2 concentration is affecting the entire cost ofthe process. Thus microbial treatment process using T. ferrooxidanshas been tried along with Fenton's reagents to reduce the COD level aswell as to minimize the use of H2O2 concentration in Fenton reagents.Successfully the use of H2O2 has been reduced five times, incombination treatment while %COD reduction has been achieved at97%, which is the key success of this study. So the advanced oxidationprocess followed by specific microbial process can be an economicalecofriendly and safe method for the treatment of this combinedindustrial wastewater of Durgapur (India) zone.

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