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Journal of Hazardous Materials 178 (2010) 1055–1064

Contents lists available at ScienceDirect

Journal of Hazardous Materials

journa l homepage: www.e lsev ier .com/ locate / jhazmat

retreatment of petrochemical wastewater by coagulation and flocculation andhe sludge characteristics

hilpi Verma, Basheshwar Prasad ∗, Indra Mani Mishraepartment of Chemical Engineering, Indian Institute of Technology, Roorkee 247667, India

r t i c l e i n f o

rticle history:eceived 12 November 2009eceived in revised form 12 February 2010ccepted 14 February 2010vailable online 20 February 2010

a b s t r a c t

In the present study, coagulation–flocculation was investigated as a pretreatment process for thetreatment of purified terephthalic acid (PTA) wastewater. The effect of various inorganic and organiccoagulants on the treatment of wastewater collected from flow equalization tank of an effluent treat-ment plant was studied. The settling and filtration characteristics of the sludge were also studied. Thejar tests revealed that the wastewater was best treated when 3000 mg l−1 of ferric chloride was dosed at

eywords:oagulation–flocculationludge volume indexettlingilterabilityTA

pH 5.6. At optimum conditions, COD of the wastewater was reduced by 75.5%. The results of the gravityfiltration of the treated wastewater showed that the addition of cationic polyacrylamide (175 mg l−1) toferric chloride coagulation improved the filtration characteristics and reduced the specific cake resis-tance. Scanning electron microscopy and energy dispersive spectroscopy studies were also conductedto know the sludge structure and composition, respectively. Thermal analysis of the sludge showed thatthe oxidation of the sludges in the present study is a three step process. However, with the addition ofC-PAA to ferric chloride coagulation system, the oxidation was found to be a two step process.

. Introduction

Purified terephthalic acid (PTA) and dimethyl terephthalateDMT) are the most widely used xylene-based chemicals in petro-hemical industry [1]. The most well known application of PTA isn the manufacture of polyethylene terephthalate bottles that are

idely used for carbonated drinks. Other applications include tex-ile fibers and polyester films used in audiovisual, photographicnd packaging fields [1,2]. World PTA consumption capacity is esti-ated to grow at the rate of 7% per year during the years 2008–2013

3].During the manufacturing of PTA, various pollutants are emit-

ed in environment and a major portion is released in the waternvironment. The wastewater is generated in the catalyst recoveryection, during cleaning and washing; PTA scrubbing and from theistillate which produces acid water [4].

Major aromatic compounds found in PTA wastewater are p-oluic acid, benzoic acid, 4-carboxybenzaldehyde, phthalic acidPA) and terephthalic acid (TA) with minor concentrations of

-formylbenzoic acid, methyl acetate and p-xylene [5–9]. Thehthalate forms are very toxic in nature, particularly because ofheir endocrine disrupting ability of phthalates. They also adverselympact on reproduction capacity and development of humans [10].

∗ Corresponding author. Tel.: +91 1332 285323.E-mail address: basehfch@iitr.ernet.in (B. Prasad).

304-3894/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.jhazmat.2010.02.047

© 2010 Elsevier B.V. All rights reserved.

TA is also found to have carcinogenic effects [11]. Based on theirtoxicity, US EPA has listed phthalate, its esters and degradationproducts as priority pollutants.

No specific discharge quality standards for phthalates or toluicacids, etc. have been prescribed by the regulating agencies inIndia. The pollution potential is generally estimated in terms ofchemical oxygen demand (COD) and total acidity. Both being veryhigh in PTA wastewaters (COD ∼ 2500–3000 mg l−1; total acid-ity ∼850–100 mg l−1 as acetic acid), the regulating agencies haveprescribed a permissible limit of COD ≤ 125 mg l−1 for petrochem-ical wastewaters for release into surface waters [12]. ThereforePTA wastewater has to be treated extensively to meet the abovestipulated discharge quality standard, before being allowed to bedischarged into surface waters.

Numerous treatment methods are suggested in literature for thetreatment of toxic wastewaters. Several investigators have studiedthe possibility of using various biological anaerobic [5,7,8,13–20]and aerobic [4,9,21] treatment technologies for the treatment ofPTA wastewater. Physico-chemical treatment technologies exploitthe physico-chemical characteristics of substances present inwastewater. The technologies which have the potential for use inthe treatment of PTA wastewater include adsorption [22–25], coag-

ulation and flocculation [21,26] and oxidation processes [27,28].Coagulation and flocculation are used extensively in drinking watertreatment world-over. These methods are also used extensivelyin industrial processes for the removal/separation of organ-ics/inorganics dissolved/suspended as colloids in water/solution.

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056 S. Verma et al. / Journal of Hazar

ecently, Garg et al. [29], Srivastava et al. [30] and Chaudhray et al.31] have shown the efficacy of thermochemical precipitation andoagulation and flocculation on the removal of organics/mineralsresent in wastewaters of different units of agri-based pulp andaper mill, distillery wastewater and black liquor from an inte-rated pulp and paper mill for the removal of COD and color.nfortunately very little work on coagulation and flocculation haseen found in the literature for the treatment of PTA wastewa-ers. The colloidal particles in wastewater are typically in the rangef 0.01–1 �m size and bear negative charge. The attractive forcesetween the particles are considerably less than the repelling forcesf electric charge. Under these conditions, the Brownian motioneeps the particles in stable suspension. Since, colloidal particlesannot be removed by sedimentation process in a reasonable periodf time, chemical coagulants and flocculants are used to assist inhe removal of particles. The commonly used coagulants are ferrichloride, ferric sulfate, ferrous sulfate, alum, polyaluminum chlo-ide and lime [32]. Natural gums are also being used as coagulants:uar gum as a flocculant aid in food processing and potable waterreatment [33]; Cassia javahikai seed gum has been used as a coag-lant aid for the decolorization of textile dye solutions [34]; use ofkra gum for the treatment of tannery effluent [35] and chitosan toreat wastewater from milk processing plant [36].

When dissolved in water, these metal salts produce hydrolysisroducts. These products act on colloidal particles through threeistinct mechanisms: charge neutralization, adsorption and sweepocculation. Charge neutralization is effective in removing neg-tively charged colloidal and higher molecular weight dissolvedrganic compounds (DOC). This phenomenon occurs mainly in theegion of low pH, where hydrolyzed monomeric and polymericetal species are present abundantly. The colloidal particles get

estabilized by the adsorption of mono- and polynuclear hydrolysispecies. This causes precipitation and particle aggregation. How-ver, adsorption can continue beyond neutralization to reversehe charge at colloidal surfaces. In this way the repulsive energyarrier to coagulation is established again and colloids are re-tabilized. With the addition of more coagulant, metal hydroxideocs are formed through orthokinetic flocculation. Thereafter, theocs aggregate and settle. With the settling of theses flocs, the col-

oidal particles present in the suspension get trapped or enmeshed,iving rise to the phenomena of sweep flocculation [37–41]. Ferrichloride has been shown to be a better coagulant than aluminumhloride in terms of DOC removal from the secondary effluent [42].

Wen et al. [26] used a number of coagulants, viz. chloridesf aluminum, calcium, magnesium and iron for the removal ofA from high pH synthetic wastewater and found ferric chlo-ide to be the most effective coagulant in removing TA andOD. Karthik et al. [21] used such coagulants as ferrous sul-ate, ferric chloride, aluminum sulfate and polyaluminum chloridePAC) in combination with anionic polyelectrolyte to study theOD and BOD removal characteristics of combined PTA wastew-ter under alkaline conditions. PAC in combination with limend polyelectrolyte was found to remove ∼63% COD and ∼45%OD. Coagulation–flocculation process followed by bio-oxidationhowed very good improvement in COD and BOD removal efficien-ies (97.4% COD and 99.4% BOD). They found that the pretreatmenty coagulation–flocculation enhanced the biodegradability of PTAastewater in terms of BOD/COD ratio—from 0.45 in raw wastew-

ter to 0.67 after coagulation–flocculation.From the above, it is clear that the literature is scarce in the

reatment of PTA wastewaters by coagulation–flocculation. The

im of the present study is to evaluate the suitability of inor-anic and organic coagulants for coagulation–flocculation processor the pretreatment of PTA wastewater to reduce its COD with-ut any dilution. The effect of different parameters, such as initialH and chemical (coagulant) concentration on the COD removal

aterials 178 (2010) 1055–1064

were also studied. Besides, the settling characteristics of the precip-itated sludge and filterability of the effluent after the pretreatmentwere also studied. Thermogravimetric analysis (TGA) of the precip-itate formed after the coagulation was performed to understand itsthermal degradation characteristics.

2. Materials and methods

2.1. Chemicals

The inorganic chemical coagulants, viz. analytical grade ferricchloride, ferrous sulfate and alum were procured from M/S, RFCL,New Delhi and polyaluminum chloride (PACl) (analytical grade)was supplied by VAM organics, Gazraula, India. Guar gum, xanthangum, sodium alginate and carboxy methyl cellulose (CMC) wereprocured from HiMedia, Mumbai, and used as organic coagulants.Cationic polyacrylamide (C-PAA) (HiMedia, Mumbai) was used insettling studies.

2.2. Source of wastewater

The wastewater was collected from the flow equalization tankof the effluent treatment plant (ETP) of a PTA production unit innorthern India. The wastewater was stored at 4 ◦C in a deep freezerand used in the experiments without any dilution.

2.3. Analysis of physico-chemical parameters

The wastewater was analyzed for various physico-chemicalparameters such as pH, COD, BOD, total solids (TS), total alkalin-ity (TA), total acidity as acetic acid, p-toluic acid, benzoic acid,4-carboxybenzaldehyde and methyl acetate by using standardmethods [43]. The COD value was assayed with a COD ana-lyzer (Aqualytic, Germany). The standard dichromate closed refluxmethod [43] was used for COD determination. The concentration ofmetals (cobalt, manganese, chromium, iron, calcium, magnesium,sodium, bromine and aluminum) in the wastewater was measuredby an atomic absorption spectrometer (Avanta GBC, Australia).Experiments were carried out in duplicate under the same set ofconditions.

2.4. Chemical coagulation and flocculation

Coagulation studies were performed using a jar test apparatus(Swastika Scientific and Electrical Works, Ambala, India). The opti-mum concentration of various coagulants with respect to removalof COD was determined through extensive jar test studies. Thewastewater was treated in batch mode in 1 l glass jar at variouscoagulant dosages and analyzed. The experimental procedure con-sisted of three phases: a period of 1 min was allowed for the flashmixing of the coagulant, at 120 rpm. It was followed by a furtherslow mixing at 40 rpm for 30 min and in the final stage the flocswere allowed to settle for 90 min. The supernatant obtained after90 min of settling was subjected to COD analysis. Equal volumeglass beakers were used to examine different dosage of coagulantsin the range of 500–3500 mg l−1. For PACl, 4000 mg l−1 was alsoused. The samples taken in the beakers were thoroughly shaken forthe mixing of settled solids and appropriate volume of the same wasthen transferred to the corresponding jar test beakers. For coagu-lation experiments, pH was varied from 2 to 10 by addition of 1NH2SO4 for acidic region and 1N NaOH for alkaline range.

2.5. Sludge volume index, settling and filterability

The most common parameter, sludge volume index (SVI)was used to quantify the settling characteristics of the

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The characteristics of wastewater which was collected from anearby petrochemical plant as analyzed are presented in Table 1.The wastewater, comprising of both organics and inorganics, isslightly acidic in nature.

Table 1Characteristics of PTA industrial wastewater.

Parameters Concentration(mg l−1), except pH

pH 5.6COD 2776BOD 480Total Acidity as Acetic Acid 960p-Toluic Acid 407Benzoic Acid 2104-Carboxy benzaldehyde NDBromide 138Methyl acetate 200Normal propyl acetate NDCobalt 18Manganese 22Chromium 0.01

S. Verma et al. / Journal of Hazar

oagulation–flocculation generated sludge. SVI is defined ashe volume occupied by 1 g of the sludge after 30 min of theettling. The SVI was calculated from the following relation:

VI = 1000 H30

HoXo(1)

here H30 is the height of sludge after 30 min of settling, H0 is thenitial height of the slurry and X0 is the initial solids concentrationn the slurry.

The suspensions from the coagulation and flocculation processere mixed, and the resultant slurry was used to study the settling

nd filterability characteristics of the sludge. The sludge sedimenta-ion tests were performed using a 100 ml graduated glass cylinder143 mm high). No stirring was done during the tests. The well-

ixed slurry was homogenized before pouring it into the glassylinder and was allowed to remain under quiescent conditions.he position of the upper interface between clear supernatant andhe settling solids was taken as a function of time. The frequency ofhe measurement of the interface height was chosen in accordanceith the settling rate. The method proposed by Richardson et al.

44] was used to design a continuous thickener based on a singleatch sedimentation test. This method gives a conservative esti-ate with an inherent high safety limit due to the changing nature

f the flocs and their settling and compression characteristics [29].uch parameters as sedimentation velocity (uc), concentration ofolids at time t (Ct), the concentration of the solids required in thenderflow (Cu) and the sedimentation flux were calculated. The uc

as found from the slope of the tangent at a given solid concen-ration at any time. The concentration of the sludge at a time t wasetermined by using the following formula:

= C0total height

height of suspension after time t(2)

Through the use of above values, the area of the sedimenta-ion tank for any effluent flow rate can thus be calculated usingollowing formula [29]:

= vfC0(1/C) − (1/Cu)

uc(3)

here vf is the volumetric flow rate of the effluent (m3 s−1) and C0s the initial solid concentration (kg m−3).

The filterability of the sludge was tested using a gravimetric fil-er (Whatman grade no. 1) having a pore size of ca. 11 �m supportedver a ceramic Buechner funnel of 93 mm internal diameter (fil-er area 6.793 × 10−3 m2; the slurry was filled up to 60% volumef the funnel). The force balance for the gravity filtration using alter paper on a Buechner funnel can be written in the form of aifferential equation [45]:

�t

�V= �

Af�P

(˛cCV

Af+ Rm

)(4)

here �t is the time interval of filtration (s), �V the volume of theltrate collected during �t (m3), V the cumulative average volumef the filtrate collected up to that time interval (m3), C the con-entration of the solids in the wastewater (kg m−3), ˛ the specificake resistance (m kg−1), � the viscosity of the filtrate (Pa s), �Phe pressure drop across the filter (Pa), A the area of filtration (m2)nd Rm the resistance of the filter medium (m−1) [46].

The volume of the filtrate collected in the graduated verticalylinder was recorded at regular time intervals. The end of the fil-ration phase is assumed when the experimental data in the �t/�V

ersus V plot deviate from the initial linear plot. The linear plot wassed to calculate the specific cake resistance. From the momentt which the liquid disappeared from the top surface of the cakeormed, a certain desired dewatering time was allowed, and thenhe cake was carefully removed, weighed and dried at 105 ◦C in an

aterials 178 (2010) 1055–1064 1057

oven until it attained a constant weight. The residue was expressedas dry mass of solids in the slurry. C-PAA was added to the FeCl3coagulation system to test its effect on SVI and the settling time ofthe suspension.

2.6. Physico-chemical characterization of sludge

Scanning electron microscopy (SEM) images of the sludge pro-duced after coagulation and flocculation treatment were obtainedusing the scanning electron microscope (LEO 435VP, England) oper-ating with SE1 detector. Electron dispersive spectroscopy (EDAX)analysis was done to determine elemental composition of the dif-ferent sludges using the FE-SEM (QUANTA 200 FEG). To identifythe various functional groups present in the sludge, FTIR analysis ofthe sludge was carried out using a FTIR spectrophotometer (NicoletAvtar 370, USA).

2.7. Thermal analysis

Thermogravimetric (TG) analysis of the sludge was carriedout by using a thermal analysis (TA) instrument (Perkin-ElmerPyris Diamond). This instrument operates with the followingspecifications: weight of the sample: 10–15 mg (max. 200 mg);temperature range: ambient to 1400 ◦C; TG measurement range:200 mg; differential thermogravimetric (DTG) measurement range:0.5–1 mg min−1; derivative thermal analysis (DTA) measurementrange (sensitivity): ±1000 mV (0.06 �V); balance type: horizon-tal differential; thermocouple: Pt Pt–Rh (13%); heating rate,0.01–100 K min−1; atmosphere: air and nitrogen. The TG, DTG andDTA traces of the solid samples were obtained from this instru-ment under air atmosphere with air flow rate of 200 ml min−1.Approximately 10 mg of the sample was heated in an alumina cru-cible in a dynamic atmosphere from the ambient temperature to1400 ◦C at the heating rate of 10 K min−1 using calcined—Al2O3 asthe reference material.

3. Results and discussion

3.1. Characteristics of PTA wastewater

Iron 0.74Calcium 11.6Magnesium 1.9Sodium 620Aluminum 0.23

1058 S. Verma et al. / Journal of Hazardous M

3

lptr

ac(taMtdtasgwmfaTcmaobflrgtnPtCcbCwosrs

Fig. 1. COD removal by inorganic coagulants at various pH (dosage-1500 mg l−1).

.2. Effect of pH on COD removal by various coagulants

Fig. 1 shows the effect of solution pH on COD removal by coagu-ants viz. ferric chloride, ferrous sulfate, alum and PACl. Pronouncedrecipitation was observed between pH 2 and 4, in comparison tohat at higher pH values. Fig. 1 reveals that the optimum pH foreduction of COD is different for different coagulants.

Hydrolyzing metal salts of iron and aluminum are widely useds primary coagulants in wastewater treatment to reduce the con-entration of particulate matter and dissolved organic compoundsDOC). When the aluminum and iron salts are dissolved in water,he metal (M) ion hydrates and is hydrolyzed to form monomericnd polymeric species and solid precipitates: MOH2+, M(OH)2

+,2(OH)2

4+, M(OH)45+, M(OH)3

0 (s) and M(OH)4− [47]. The reduc-

ion of dissolved organics during coagulation with metal salts atifferent pH follows two different mechanisms, commonly referredo as charge neutralization and adsorption. At very low pH, bothluminum and iron species are present as A13+ and Fe3+ in aqueousolution. The key chemical properties, in addition to the functionalroups which provide negative charge, are the size or moleculareight and the hydrophobicity [48]. At low pH, the anionic organicolecules present in the effluent co-ordinate with metal cations to

orm insoluble charge-neutral products. The neutralized organicsre able to form bigger flocs by Van der Waal forces of attraction.hese bigger flocs then settle down. In the alkaline range, metalations form insoluble metal complexes. The metal hydroxide poly-ers are positively charged, hydrophobic and have a large surface

rea and an amorphous structure. These polymers get adsorbednto the surfaces of organic anionic particles and become insolu-le [47,49]. The organics generally get adsorbed on to preformedocs of metal hydroxides and thereafter get precipitated. The netesult is the removal of dissolved organics with different functionalroups at different solution pH. The maximum COD removal mayhus occur at a pH where the combined effect of both the mecha-isms is maximum. The optimum pH was 5.6 for ferric chloride andACl with COD removal efficiency of 54.29% and 17.27%, respec-ively. For ferrous sulfate and alum, the optimum pH was 8.5 withOD removal efficiency of 20.75% and 18.74%, respectively. Organicoagulants viz. guar gum, xanthan gum, sodium alginate and car-oxy methyl cellulose were not found effective in COD removal. TheOD removal at 500 mg l−1 dosage of guar gum and xanthan gum

as in the range of 15.7–18.6% and 2.4–9%, respectively At a dosage

f 100 mg l−1, the COD removal was 10.4–14.29% and 6.2–7.2% withodium alginate and carboxy methyl cellulose, respectively. As theesults with these organic coagulants were not encouraging, furthertudies with them were discontinued.

aterials 178 (2010) 1055–1064

The pH of minimum solubility for Fe(III) is near pH 8.8. However,the optimum pH in our study was 5.6, which is the original pH of thewastewater. Therefore, the major colloids present are negativelycharged at pH 5.6. The first layer of cations attracted to the nega-tively charged surface are “bound” to the colloids and travel withthem if the colloids get displaced in the suspension. The aquometal-lic ion of iron, thus formed is soluble in water. It becomes part ofthe ionic cloud surrounding the colloid and, because of its greataffinity for surfaces, gets adsorbed onto the surface of the colloidswhere it neutralizes the surface charge. Once the surface charge hasbeen neutralized, the ionic cloud dissipates and the electrostaticpotential disappears, inducing free contact between the colloidalparticles and the ionic cloud [39]. Under acidic pH conditions, themonomeric iron species dominate and the charge neutralizationand precipitation occur. With an increase in hydroxyl ion con-centration, polymeric species are formed and Fe(OH)3 becomespredominant, facilitating adsorption. At pH 5.6, which is the opti-mum pH for ferric chloride, Fe(OH)3 concentration is maximumin comparison to other iron species. Therefore, precipitation andadsorption of organics onto preformed ferric hydroxide flocs are theprincipal phenomena for COD removal by ferric chloride [50]. In thecase of polyaluminum chloride (PACl), for which the optimum pH isalso 5.6, charge neutralization is considered to be the main drivingforce for COD removal. As the amounts of Al3+, Al(OH)2+, Al(OH)2

+

are more than Al(OH)3, it is clear that the PACl, having multivalentaluminum ions, neutralized the particle charges and the hydrolyzedaluminum flocs enmeshed the colloids and drove them to settle[29,30]. For ferrous sulfate, the optimum pH for COD removal wasfound to be 8.5. At this pH, the presence of Fe2+ and Fe (OH) + is max-imum and the COD removal occurs predominantly by charge neu-tralization. For alum, the optimum pH for COD removal is 8.5. At thispH, the maximum particle removal occurs by sweep flocculation.

3.3. Determination of optimum dosage for COD removal

The optimal dosages of the coagulants were determined by vary-ing the coagulant dose at the optimal pH. The study range of thedosage was 500–4000 mg l−1. The optimum dosage for ferric chlo-ride and ferrous sulfate was 3000 and 2000 mg l−1, respectively. Theoptimum dosage for alum and PACl was obtained as 3500 mg l−1.At optimum conditions of pH and coagulant dosage, the maximumCOD removal efficiency was 75.57% with ferric chloride. It may beattributed to the following reasons: (1) the increase of adsorptivesurface due to increase of ferric hydroxide precipitates and (2) theformation of flocs and acid anion complexes by electrostatic inter-action. The complexes led to an enhancement of removal of colloids.The maximum COD removal efficiency with ferrous sulfate, alumand PACl was, respectively, 20.7%, 33.1% and 49.7% (Fig. 2). Whenthe dosage of ferric chloride was increased beyond 3000 mg l−1, theremoval efficiency decreased. With the addition of larger dosageof the coagulant, the surface charge of the particles gets reverseddue to continued adsorption of mono- and polynuclear hydrolysisspecies of ferric chloride. As the colloidal particles become posi-tively charged, they cannot be removed by perikinetic flocculation.

It may be pointed out that the BOD5/COD ratio was 0.17 forthe original wastewater, which after treatment with ferric chlo-ride at optimum conditions improved to 0.38. This shows that thebiodegradability of PTA wastewater gets improved and the pre-treated PTA wastewater can be subjected to biological treatment.

3.4. Sludge volume index, settling and filtration

3.4.1. Sludge volume indexSVI value for the FeCl3 coagulation system at optimum condi-

tions was 284.4 ml g−1. When C-PAA was added at different dosagesof 100, 150, 175 and 200 mg l−1 to FeCl3 coagulation system. The

S. Verma et al. / Journal of Hazardous Materials 178 (2010) 1055–1064 1059

Fig. 2. COD removal by inorganic coagulants at various doses.

Fa

Stbta

3

t

TC

ig. 3. Settling characteristics of sludge in the treated effluent with ferric chloridelone and with C-PAA added with FeCl3.

VI values decreased to 274, 226, 177 and 185 ml g−1. SVI value athe optimal C-PAA (175 mg l−1) dosage showed a reduction in SVIy 37% as compared to the SVI values of the sludge obtained fromhe coagulation treatment at optimal conditions with FeCl3 withoutny aid.

.4.2. SettlingFig. 3 shows the settling characteristics of the sludge in the

reated effluent with ferric chloride, and with ferric chloride and

able 2omparative picture of the values of specific cake resistance (˛) and resistance (Rm) for d

S. no. Type of sludge

1. Domestic wastewater sludge(a) Activated sludge(b) Biodigester sludge(c) Conditioned digested sludge(d) Conditioned primary sludge

2. Thermochemical precipitation sludge (CuSO4 usedas catalyst, pulp and paper mill effluent)

3. Electrochemical sludge (iron used as electrodes,pulp and paper mill effluent)(a) Without any additive(b) With NaCl as additive(c) With PAA as additive

Fig. 4. �t/�V as a function of filtrate volume for treated effluent with and withoutadditive.

C-PAA. Addition of C-PAA (175 mg l−1) at optimum dosage duringcoagulation treatment considerably improves the settling charac-teristics of the sludge with about the same amount of COD removalas that without C-PAA.

3.4.3. FilterabilityThe sludge obtained from the coagulation treatment has to be

dewatered before its disposal so as to reduce its volume and themoisture content. The dewatering can be carried out by filtrationusing either a plate and frame filter or a rotary vacuum filter. Gravityfiltration is generally used for generating and gathering experimen-tal data in the laboratory.

The filterability of the treated wastewater with only FeCl3 andwith the addition of 175 mg l−1 C-PAA dosage was tested by usinga gravimetric filter. Assuming the change in the hydrostatic headto be negligible, and considering the gravity filtration as a con-stant pressure filtration process, Eq. (4) gives a linear plot between�t/�V and V for the effluent treated. The values of ˛ and Rm canbe obtained from the slope and intercept of the plot in Fig. 4.as 2.49 × 1011 m kg−1 and 1.23 × 105 m−1. After the addition ofC-PAA (175 mg l−1) along with FeCl3, the filtration characteris-tics got improved and the values for ˛ and Rm were found to be1.14 × 1010 m kg−1 and 3.80 × 105 m−1, respectively. Smaller val-ues of ˛ show an improvement in the filtration characteristics. The

specific cake resistance for the coagulation treated wastewater isfound to be lower than that of the electrochemical sludge obtainedfrom black liquor treatment, thermochemical precipitation sludgeand activated sludge (Table 2). This indicates that the sludge fromthe coagulation treated wastewater has better filterability in com-

ifferent kinds of sludges.

˛ (m kg−1) Rm (m−1) Reference

Barnes et al. [51](4–12) × 1013 –(3–30) × 1013 –(2–20) × 1011 –(3–10) × 1011 –

(102.8–272.2) (0.69–4.84) × 105 Garg et al. [29]

Mahesh et al. [46]

6.8 × 1011 8.9 × 108

3.2 × 1011 2.3 × 109

4.6 × 1011 2.9 × 109

1060 S. Verma et al. / Journal of Hazardous M

Fa

pa

3

tpualppcd�aosoovt

ig. 5. FTIR spectra of flocs obtained by using (a) FeCl3, (b) FeSO4, (c) Alum, (d) PAClnd (e) wastewater.

arison to municipal sludges, thermochemical precipitation sludgend electrochemical sludge [29,46,51,52].

.5. FTIR spectral analysis

The presence of various functional groups in the sludge showshat in the electrostatic interaction between flocs and cationslayed an important role in the removal of colloids during coag-lation process. The surface chemistry of the sludge obtainedfter coagulation and flocculation treatment by various coagu-ants was studied using the FTIR spectroscopy to determine theresence of functional groups. The FTIR spectra exhibit similareak locations, but the relative intensity of the transmittancehanges with different coagulants (Fig. 5). The FTIR spectra of theried PTA wastewater exhibits a broad band at 3438 cm−1 due to(OH) of either water or hydroxyl group present in the wastew-ter. The presence of aromatic ring is exhibited by the presencef bands in the regions of 1590–1450 cm−1 depicting the ring

tretching. The C–H bending bands have appeared in the rangesf 1275–1000 cm−1 (in-plane bending) and 900–690 cm−1 (out-f-plane bending) [53]. The bands of the out-of-plane bendingibrations of aromatic compounds are strong and characteristic ofhe number of hydrogen atoms in the ring. In the sludges obtained

Fig. 6. SEM images of (a) FeCl3 flocs, (b) FeSO

aterials 178 (2010) 1055–1064

after treatment with FeCl3, FeSO4, alum and PACl, a broad bandbetween 3150 and 3650 cm−1 is found. This shows the presenceof both the free and hydrogen bonded OH groups and the O–Hstretching of the polymeric compounds on the sludge interface.There are multiple sharp and medium peaks in the region of1600–1400 cm−1depicting the aromatic C C stretching. The pat-tern of peaks varies depending upon the substitution pattern. Thepeaks in this region are quite sharp for ferric chloride sludgein comparison to sludges obtained from different coagulants. At1685 cm−1, there is a sharp peak in ferric chloride treated sludgeshowing the aromatic aldehyde C O stretching. Wen et al. [26]reported that the carbonyl stretching band of the carbonyl groupof terephthalic acid was centered at 1687 cm−1 and was promi-nent in Fe(OH)3 flocs. The band in the region of 1310–1250 cm−1

shows the presence of esters (aromatic C–O stretching). In allthe sludges, the characteristic infrared bands of organic halogencompounds are also found in the region of 800–400 cm−1 [53].It may be inferred from FTIR analysis that there are some simi-larity in peaks observed in dried PTA wastewater and the treateddried sludge with some shifting of peak positions. It confirms thatthe components of PTA wastewater get attached to coagulantsthrough some inherent complex chemical phenomenon, whichresults in the removal of COD of PTA wastewater by these inorganiccoagulants.

3.6. SEM micrographs and EDAX analysis

From the SEM micrographs (Fig. 6a–d), it is clear that the flocsproduced after the treatment of wastewater with ferric chlorideare hard and sturdy. Honeycomb like structure is present in thesludge generated from ferrous sulfate and alum treatment, withcolloidal particles entrapped in it, indicating the sweep floccula-tion for the colloidal particles in the wastewater mixture. The PACltreated wastewater flocs seemed to be small and fluffy. Chemical

composition of the sludge is given in Table 3. Among various ele-ments, carbon was found to be the dominant in all the sludge as theCOD removal indicates. Maximum carbon is found to be presentin the ferric chloride treated sludge. This is in consonance withmaximum COD removal by ferric chloride.

4 flocs, (c) Alum flocs and (d) PACl flocs.

S. Verma et al. / Journal of Hazardous Materials 178 (2010) 1055–1064 1061

Fig. 7. DTA–DTG–TG plots of coagulation–flocculation generated sludge at various conditions in air atmosphere: (a) FeCl3 sludge, (b) FeSO4 sludge, (c) Alum sludge, (d) PAClsludge and (e) FeCl3 and C-PAA sludge.

1062 S. Verma et al. / Journal of Hazardous Materials 178 (2010) 1055–1064

(Cont

3

e

TE

Fig. 7.

.7. TGA–DTGA–DTA of the sludge

Fig. 7a–e shows the thermogravimetric analysis (TGA), differ-ntial thermogravimetric analysis (DTGA) and derivative thermal

able 3DAX analysis of the sludge.

Element (wt%) FeCl3 FeSO4 Alum PACl

C 53.55 33.47 41.05 44.79N 00.00 00.00 01.96 02.57O 15.15 27.71 24.53 21.33Na 02.04 17.67 10.96 08.65Al 00.80 01.37 05.93 07.55P 00.13 00.00 00.50 00.27S 00.69 04.31 03.27 00.39Cl 03.34 00.11 00.37 13.52Cr 00.37 00.26 01.31 00.17Co 00.42 01.10 02.94 00.25

inued).

analysis (DTA) traces for the precipitated sludges. In Fig. 7a thetraces were obtained for the FeCl3 precipitated sludge at 10 K min−1

heating rate and 200 ml min−1 air flushing rate. The nature ofthe TGA trace shows dehydration and volatilization (removal ofvolatiles) of the sample up to a temperature of 349 ◦C losing about14.9% of its weight. Between short span of 349 and 394 ◦C theresidue oxidizes and loses about 15.2% of its weight. The peak rate ofweight loss at Tmax = 374 ◦C is 1.04 mg min−1. The oxidation is foundto be uniform and exothermic with a heat evolution of 2.54 MJ kg−1

with the peak of the exotherm being at a temperature, Tp = 393 ◦C.The oxidation rate of the residue slows down between 393 and1041 ◦C, with a marginal evolution of heat 0.260 MJ kg−1 at the

peak temperature of 1041 ◦C. The oxidation continued at very slowrate from 1041 to 1400 ◦C, leaving the ash fraction of 27.8% as theresidue.

In Fig. 7b, the thermal curves were obtained for the FeSO4 pre-cipitated sludge at 10 K min−1 heating rate and 200 ml min−1 air

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ushing rate. The TGA trace shows the three stages of weightoss: the first stage (25–93 ◦C) exhibits weight loss at a rate of.43 mg min−1, which corresponds to the first endotherm in theTA thermogram; the first endothermic peak, which initiated at

oom temperature, corresponds to the release of physisorbed waternd the pore liquid that were not removed during drying. This peakay also correspond to fusion or melting transition of the com-

ounds present in the wastewater. The second stage (301–601 ◦C)ncludes the main decomposition stage, which is exothermic with amax = 372 ◦C and has a weight loss of 24.8%. The peak was exother-ic due to carbon chain fragmentation or decomposition with early

xidation of different fragments. The third stage (901–1199 ◦C) isndothermic showing weight loss in the range of 20.3%. The secondndothermic peak (onset at 901 ◦C) may be due to the conden-ation of some functional groups that are usually removed in thehermal stabilization process. The removal of species at these tem-eratures is reflected in the TGA trace. The rate of weight loss showny DTG trace reaches a maximum in the second stage, where theombustion of volatiles takes place exothermically as indicated byTA.

In Fig. 7c and d, the thermal traces were obtained for the alumnd PACl precipitated sludges. The TG traces are showing a simi-ar three stage weight loss as observed for the FeSO4 precipitatedludge, but with different amounts of heat evolution and absorp-ion.

The traces in Fig. 7e obtained for FeCl3 precipitated sludge inhich C-PAA is also added as the settling agent are displaying a dif-

erent trend than the previous ones. The TG trace shows a gradualecrease in the residual sample mass up to a temperature of 350 ◦Chedding about 20.4% of the initial sample mass. This means that theludge loses moisture at an almost steady rate along with volatal-zation of light volatiles up to 350 ◦C. At 350 ◦C, the oxidation ofhe dry sample started, and the sample lost weight quickly (41.8%)ver a temperature range of 350–518 ◦C (a temperature span of68 ◦C). The maximum weight loss rate was 0.99 mg min−1 at Tmax

f 389 ◦C (see DTG trace). The peak temperature for the exother-ic reaction as exemplified by the DTA curve was at Tp = 415 and

76 ◦C with heat evolution of 5.637 MJ kg−1. Beyond 518 ◦C, theeight loss is steady but very slow, giving off 4% mass from 518

o 1400 ◦C.

.8. Conclusion

The present paper shows the results of treating PTA wastewa-er having a COD value of 2776 mg l−1 with various inorganic andrganic coagulants. Ferric chloride was found to be the most effec-ive coagulant with COD removal efficiency of 75.5%. The additionf C-PAA produces the sludge with good settling characteristics.he settling rate got improved and SVI of the sludge obtained fromhe treatment at optimal conditions with FeCl3 without any addi-ive and with the addition of C-PAA (175 mg l−1) was found to be84.4 and 177 ml g−1, respectively. Addition of C-PAA (175 mg l−1)

mproved the filtration characteristics considerably with ˛ and Rm

alues for gravity filtration being in the range of 1.14 × 1010 m kg−1

nd 3.80 × 105 m−1, respectively. The SEM images showed theoc structures for different sludges. EDAX and FTIR analyses ofhe sludges exhibit that the maximum removal of COD from PTAastewater was obtained with the use of FeCl3 as a coagulant.

eferences

[1] H.A. Wittcoff, B.G. Reuben, J.S. Plotkin, Industrial Organic Chemicals, 2nd ed.,Wiley-Interscience, 2004.

[2] R. Kleerebezem, L.W. Hulshoff Pol, G. Lettinga, High rate treatment of tereph-thalic acid production wastewater in a two-stage anaerobic bioreactor,Biotechnol. Bioeng. 91 (2) (2005) 169–179.

[

[

aterials 178 (2010) 1055–1064 1063

[3] Anon., Tecnon Orbichem Consultants, U.K. as downloaded from the webpage (http://www.icis.com/v2/chemicals/9076062/paraxylene/uses.html), asaccessed on 12.08.09.

[4] G.R. Pophali, R. Khan, R.S. Dhodapkar, T. Nandy, S. Devotta, Anaerobic–aerobictreatment of purified terephthalic acid (PTA) effluent; a techno-economic alter-native to two-stage aerobic process, J. Environ. Manage. 85 (2007) 1024–1033.

[5] J.P. Guyot, H. Macarie, A. Noyola, Anaerobic digestion of a petrochemicalwastewater using the UASB process, Appl. Biochem. Biotechnol. 24/25 (1990)579–589.

[6] H. Macarie, J.-P. Guyot, Inhibition of the methanogenic fermentation of p-toluicacid (4-methylbenzoic acid) by acetate, Appl. Microbiol. Biotechnol. 38 (1992)398–402.

[7] J.C. Young, I.S. Kim, I.C. Page, D.R. Wilson, G.J. Brown, A.A. Cocci, Two-stageanaerobic treatment of purified terephthalic acid production wastewaters,Water Sci. Technol. 42 (5–6) (2000) 277–282.

[8] A. Noyola, W. Macarie, J.P. Guyot, Treatment of terephthalic acid plant wastew-ater with an anaerobic fixed film reactor, Environ. Technol. 11 (1990) 239–248.

[9] X.X. Zhang, S.P. Cheng, Y.Q. Wan, S.L. Sun, C.J. Zhu, D.Y. Zhao, W.Y. Pan, Degrad-ability of five aromatic compounds in a pilot wastewater treatment system,Inter. Biodeter. Biodegr. 58 (2006) 94–98.

10] M. Matsumoto, M. Hirata-Koizumi, M. Ema, Potential adverse effects of phthalicacid esters on human health: a review of recent studies on reproduction, Regul.Toxicol. Pharm. 50 (2008) 37–49.

11] S.T. Qi, X.R. Wang, X.K. Xu, Study on the bladder calculi and bladder cancerinduced by terephthalic acid in rats, J. Hyg. Res. 31 (1) (2002) 10–12.

12] Proposed Effluent and Emission Standards for Petroleum Oil Refineries,Central Pollution Control Board, 2005 (http://www.cpcb.nic.in/finalstandards-oilrefineries271205.doc) as accessed on 03.02.09.

13] T.J. Britz, L.C. Meyer, P.J. Botes, Anaerobic digestion of a petrochemical effluent,Biotechnol. Lett. 5 (2) (1983) 113–118.

14] L.H. Nel, J.D. Haast, T.J. Britz, Anaerobic digestion of petrochemical effluentusing an upflow anaerobic sludge blanket reactor, Biotechnol. Lett. 6 (11) (1984)741–746.

15] C.L. Chen, H. Macarie, I. Ramirez, A. Olmos, S.L. Ong, O. Monroy, W.T. Liu, Micro-bial community structure in a thermophilic anaerobic hybrid reactor degradingterephthalate, Microbiology 150 (2004) 3429–3440.

16] R. Kleerebezem, L.W. Hulshoff Pol, G. Lettinga, Anaerobic biodegradability ofphthalic acid isomers and related compounds, Biodegradation 10 (1) (1999)63–73.

17] R. Kleerebezem, L.W. Hulshoff Pol, G. Lettinga, The role of benzoate in anaer-obic degradation of terephthalate, Appl. Environ. Microbiol. 65 (3) (1999)1161–1167.

18] R. Kleerebezem, M. Ivalo, L.W. Hulshoff Pol, G. Lettinga, High-rate treatmentof terephthalate in anaerobic hybrid reactors, Biotechnol. Prog. 15 (3) (1999)347–357.

19] R. Kleerebezem, G. Lettinga, High-rate anaerobic treatment of purified tereph-thalic acid wastewater, Water Sci. Technol. 42 (5–6) (2000) 259–268.

20] C. Fajardo, J.P. Guyot, H. Macarie, O. Monroy, Inhibition of anaerobic digestionby terephthalic acid and its aromatic by products, Water Sci. Technol. 36 (6–7)(1997) 83–90.

21] M. Karthik, N. Dafale, P. Pathe, T. Nandy, Biodegradability enhancement ofpurified terephthalic acid wastewater by coagulation—flocculation process aspretreatment, J. Hazard. Mater. 154 (2008) 721–730.

22] E.L. Crepaldi, J. Tronto, L.P. Cardoso, J.B. Valim, Sorption of terephthalate anionsby calcined and uncalcined hydrotalcite-like compounds, Colloids Surf. A:Physicochem. Eng. Aspects 211 (2002) 103–114.

23] N. Adhoum, L. Monser, Removal of phthalate on modified activated carbon:application to the treatment of industrial wastewater, Sep. Purif. Technol. 38(2004) 233–239.

24] C.Y. Chen, Y.C. Chung, Removal of phthalate esters from aqueous solution bychitosan bead, J. Environ. Sci. Health A 41 (2006) 235–248.

25] C.Y. Chen, C.C. Chen, Y.C. Chung, Removal of phthalate esters by �-cyclodextrin-linked chitosan bead, Bioresour. Technol. 98 (2007) 2578–2583.

26] Y.Z. Wen, S.P. Tong, K.F. Zheng, L.-L. Wang, J.Z. Lv, J. Lin, Removal of terephthalicacid in alkalized wastewater by ferric chloride, J. Hazard. Mater. B 138 (2006)169–172.

27] T.J. Park, J.S. Lima, Y.W. Lee, S.H. Kim, Catalytic supercritical water oxidation ofwastewater from terephthalic acid manufacturing process, J. Supercrit. Fluid26 (2003) 201–213.

28] R. Thiruvenkatachari, T.O. Kwon, J.C. Jun, S. Balaji, M. Matheswaran, I.S. Moon,Application of several advanced oxidation processes for the destruction ofterephthalic acid (TPA), J. Hazard. Mater. 142 (2007) 308–314.

29] A. Garg, I.M. Mishra, S. Chand, Thermochemical precipitation as a pretreatmentstep for the chemical oxygen demand and color removal from pulp and papermill effluent, Ind. Eng. Chem. Res. 44 (2005) 2016–2026.

30] V.C. Srivastava, I.D. Mall, I.M. Mishra, Treatment of pulp and paper mill wastew-aters with polyaluminium chloride and bagasse fly ash, Colloids Surf. A:Physicochem. Eng. Aspects 260 (2005) 17–28.

31] P.K. Choudhury, I.M. Mishra, S. Chand, Decolorization and removal of chemicaloxygen demand (COD) with energy recovery: treatment of biodigester effluent

of a molasses based alcohol distillery using inorganic coagulants, Colloids Surf.A: Physicochem. Eng. Aspects 296 (2007) 238–247.

32] Metcalf, Eddy, Wastewater Engineering: Treatment and Reuse, 4th ed., TataMcGraw-Hill Publication, New Delhi, 2003, pp. 478–479.

33] S. Gupta, J.E. Ako, Application of guar gum as a flocculant aid in food processingand potable water treatment, Eur. Food Res. Technol. 22 (1) (2005) 746–751.

1 dous M

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

064 S. Verma et al. / Journal of Hazar

34] R. Sanghi, B. Bhattacharya, V. Singh, Use of Cassia javahikai seed gum and gum-g-polyacrylamide as coagulant aid for the decolorization of textile dye solutions,Bioresour. Technol. 97 (2006) 1259–1264.

35] M. Agarwal, S. Rajani, A. Mishra, J.S.P. Rai, Utilization of okra gum for treatmentof tannery effluent, Int. J. Polym. Mater. 52 (2003) 1049–1057.

36] F.H. Chi, W.P. Cheng, Use of chitosan as coagulant to treat wastewater frommilk processing plant, J. Polym. Environ. 14 (2006) 411–417.

37] M. Rebhun, M. Lurie, Control of organic matter by coagulation and floc separa-tion, Water Sci. Technol. 23 (11) (1993) 1–20.

38] J.Q. Jiang, Development of coagulation theory and prepolymerized coagulantsfor water treatment, Sep. Purif. Rev. 30 (1) (2001) 127–141.

39] J. Duan, J. Gregory, Coagulation by hydrolysing metal salts, Adv. Colloid Inter-face 100–102 (2003) 475–502.

40] M. Yana, D. Wang, J. Ni, J. Qu, C.W.K. Chowc, H. Liu, Mechanism of natural organicmatter removal by polyaluminum chloride: effect of coagulant particle size andhydrolysis kinetics, Water Res. 42 (2008) 3361–3370.

41] F. Renault, B. Sancey, P.-M. Badot, G. Crini, Chitosan for coagulation/flocculationprocesses—an eco-friendly approach, Eur. Polym. J. 45 (2009) 1337–1348.

42] J. Haberkampa, A.S. Ruhl, M. Ernst, M. Jekel, Impact of coagulation and adsorp-tion on DOC fractions of secondary effluent and resulting fouling behaviour inultrafiltration, Water Res. 41 (2007) 3794–3802.

43] APHA (American Public Health Association), AWWA (American Water WorksAssociation), WPCF (Water PollutionControl Federation), Standard Methods forthe Examination of Water and Wastewater, APHA, Washington, DC, USA, 1995.

[

[

aterials 178 (2010) 1055–1064

44] J.F. Richardson, J.H. Harker, J.R. Backhurst, Coulson and Richardson’s Chem-ical Engineering, Particle Technology & Separation Processes, Vol.2, 5th ed.,Butterworth-Heinemann, Woburn, MA, 2003, pp. 254–260.

45] W.L. MaCabe, J.C. Smith, P. Harriot, Unit Operations of Chemical Engineering,6th ed., McGraw-Hill, New York, 2001, pp. 1008–1009.

46] S. Mahesh, B. Prasad, I.D. Mall, I.M. Mishra, Electrochemical degradation of pulpand paper mill wastewater. Part 2. Characterization and analysis of sludge, Ind.Eng. Chem. Res. 45 (2006) 5766–5774.

47] S.K. Dentel, J.M. Gossett, Mechanisms of coagulation with aluminum salts, J.Am. Water Works Assoc. 80 (1988) 187–198.

48] J.K. Edzwald, J.E. Tobiason, Enhanced coagulation: U.S. requirements and abroader view, Water Sci. Technol. 40 (9) (1999) 63–70.

49] H.W. Ching, T.S. Tanaka, M. Elimelech, Dynamics of coagulation of kaolin par-ticles with ferric chloride, Water Res. 28 (3) (1994) 559–569.

50] R.J. Stephenson, S.J.B. Duff, Coagulation and precipitation of a mechanical pulp-ing effluent. I. Removal of carbon, colour and turbidity, Water Res. 30 (4) (1996)781–792.

51] D. Barnes, P.J. Bliss, B.W. Gould, H.R. Vallentine, Water and Wastewater Systems,

Pitman Publishing Inc., New York, 1981.

52] A. Garg, I.M. Mishra, Shri Chand, Effectiveness of coagulation and acid precipi-tation processes for the pretreatment of diluted black liquor, J. Hazard. Mater.,under review.

53] B.H. Stuart, Infrared Spectroscopy: Fundamentals and Applications, John Wiley,2004, pp. 72–93.