electrochemical degradation of pulp and paper mill wastewater. part 2. characterization and analysis...

Electrochemical Degradation of Pulp and Paper Mill Wastewater. Part 2.Characterization and Analysis of Sludge

S. Mahesh, B. Prasad, I. D. Mall, and I. M. Mishra*

Department of Chemical Engineering, Indian Institute of Technology, Roorkee,Roorkee - 247 667, Uttaranchal, India

In this paper, we report the characteristics and the management of the sludge generated by the batchelectrochemical (EC) treatment of the black liquor (BL) of a small paper mill based on agricultural waste asraw material. The study shows that the sludges obtained from the EC treatment of the BL at its natural pHwithout any additive and with 625 mg dm-3 sodium chloride (NaCl) or 10 mg dm-3 polyacrylamide (PAA)during the EC process had very good settling characteristics. PAA addition hastens the EC process, and thesludge settling rate improves considerably. The settling velocity of the sludge obtained from the EC treatmentwithout any additive could be satisfactorily correlated with the model of Richardson and Zaki. The results ofthe gravity filtration of the treated BL showed that the addition of NaCl (625 mg dm-3) improved the filtrationcharacteristics and reduced the values of the specific cake resistance (R). The values of the specific cakeresistance (R) and the resistance of the filter medium (Rm) were in the range of 3.25-4.67× 1011 m kg-1 and2.37-2.98× 109 m-1, respectively. Prolonged use of iron anodes produces a large number of dents on theirsurface. SEM images of the electrodes show that the dents formed because of the anode dissolution duringthe EC process. The sludge has lower ash content and fixed carbon and higher volatile matter than that ofIndian coal. Thermal analysis showed good combustion characteristics and complete oxidation of the ECprocess sludge at about 400°C, with a heating value of 11.33 MJ kg-1. The sludge can be dewatered, dried,and used in the furnace/incinerators for its heat recovery, and the ash may either be blended with organicmanure for use in agriculture/horticulture or may be blended with clay/coal fly ash to make bricks/ceramictiles for the building industry.

Introduction

Pulp and paper mills in India using agribased raw materialsgenerally do not have chemical recovery units as also inadequatetreatment facilities to meet the prescribed effluent standards fordischarge. The wastewater emanating from such units have highCOD (∼2000 mg dm-3), BOD (∼650 mg dm-3), color (∼1750platinum cobalt units (PCU)), and total solids (∼2100 mg dm-3).Thermochemical treatment (catalytic thermolysis)1 and coagula-tion/flocculation followed by adsorption2 have been recom-mended as very good pretreatment options for such wastewaters.Recent interest in electrochemical treatment led us to investigatethe efficacy of the electrochemical degradation of pulp and papermill wastewaters with respect to COD and color.3

The black liquor (BL) of an agribased pulp and paper millwas treated in a 2 dm3 batch electrochemical reactor using DCpower supply and iron plate electrodes arranged in parallel.3

The anodic dissolution during EC treatment formed metallichydroxides which were found in the sludge. The electrodisso-lution of iron and its conversion into hydroxides of differentvalencies mimics the process of coagulation and flocculation,with the capture of colloidal/dissolved solids in the wastewater.The iron hydroxides form the nuclei of the colloidal particlesaround which an adsorption layer of cations and anions getsorganized carrying over it a positive charge. Gases releasedduring electrolysis (and electrooxidation) include chlorine,oxygen, and hydrogen at the cathode. The hydrogen producedat the cathode as a result of the redox reaction removes dissolvedorganics or any suspended and colloidal material by flotation.Chlorine and oxygen act as oxidants to remove oxidizable

organics as CO2 and water. The microflocs that form duringelectrochemical (EC) treatment agglomerate and precipitate atopthe reactor; adhere to the edges of the electrode as a coating (atalkaline pH, not otherwise), and settle at the bottom of thereactor after EC treatment. The characteristics of the sludge,viz., its volume, sludge volume index (SVI), settling charac-teristics, filterability, and solids flux are very important param-eters in the overall evaluation of the effectiveness of electro-chemical treatment and the design of the filtration unit/settlingtank. The treatment and disposal of such electrochemicallyproduced sludge is perhaps the most important environmentalproblem.

The EC treatment for COD and color removal have beenreported in the first part of the paper.3 This paper focuses onthe physicochemical characterization and disposal managementaspects of the sludge generated from the EC treatment of BLgenerated from the cooking-washing section of an agribasedsmall paper mill manufacturing kraft paper. The sludge fromthe EC process as well as the sludge obtained from the post-treatment of the supernatant of the EC treated BL have beenused in the present study. Besides the settling characteristics ofthe precipitated sludge at the optimal conditions, the filterabilityof the EC treated effluent was also studied. The physicalcharacterization of the electrodes (anodesprior to and after theEC process) was done using a scanning electron microscope.The elemental composition of the sludge was determined usinga CHNS analyzer. The thermal analysis of the untreated BLand EC treated sludge (top scum and settled sludge) and post-treated sludge were carried out by using a thermal analyzer.

Experimental and Analytical Methods

Materials. The BL used in EC treatment had the followingcharacteristics: BOD∼615-670 mg dm-3, COD ∼2000 mg

* To whom correspondence should be addressed. Tel.: 91-1332-285715. Fax: 91-1332-276535, 273560. E-mail: [email protected].

5766 Ind. Eng. Chem. Res.2006,45, 5766-5774

10.1021/ie0603969 CCC: $33.50 © 2006 American Chemical SocietyPublished on Web 07/04/2006

dm-3, chlorides∼48-62 mg dm-3, total solids∼2100 mgdm-3, pH ∼6.86-7.12, total alkalinity∼380-410 mg dm-3,conductivity ∼0.746-0.791 mmho cm-1, and color∼1750platinum cobalt unit (PCU). The effect of 4, 6, and 8 plateconfigurations, applied current density, initial pH (pH0), andthe addition of NaCl, polyacrylamide (PAA), and polyaluminumchloride (PAC) on the process performance of the EC reactorhad been studied earlier.3 An 80% COD removal was obtainedat the optimum current density of 55.56 A m-2 and at a cellvoltage of 2.5-3 V achieving an electrolysis time (ET) of 60min with a 6 plate EC reactor. The sludge (top scum and settledsludge) generated during EC degradation process were collected,filtered, sun-dried, and stored in glass containers. These werekept in a desiccator prior to their analysis for physicochemicaland thermal characteristics.

Settling and Filterability. The mixture of liquid-solidsuspensions from the EC process were mixed, and the resultantslurry was used to study the settling and filterability character-istics of the sludge. The sludge sedimentation tests wereperformed using a 1 dm3 graduated glass cylinder (28 cm highand 5.85 cm i.d.). No stirring was done during the tests. Thewell mixed slurry was homogenized before pouring it into theglass cylinder and was allowed to remain under quiescentconditions. The position of the upper interface was measuredas a function of time. The frequency of the measurement of theinterface height was chosen in conformity with the settling rate.Each sedimentation run lasted for about 40-45 min. Thefilterability of the sludge was tested using a gravimetric filterhaving a pore size of ca. 11µm (grade 1) supported over aceramic Buechner funnel of 93 mm internal diameter (filter area) 6.793× 10-3 m2; the slurry was filled up to 60% volume ofthe funnel). The volume of filtrate collected in the graduatedvertical cylinder was recorded at regular time intervals neglect-ing the filtrate volume obtained in the first 2 min.4 The end ofthe filtration phase is attained when the points in the∆t/∆VversusV plot deviate from the initial linear plot.5 These datawere used to calculate the specific cake resistance. From themoment at which the liquid disappeared from the top surfaceof the cake formed, a certain desired dewatering time wasallowed, and then the cake was carefully removed, weighed,and dried at 105°C until it attained a constant weight. Theresidue was expressed as mass of solids in the slurry.

Physicochemical Characterization.SEM images of ironplates before and after EC treatment were obtained using thescanning electron microscope (LEO 435VP, England) operatingwith SE1 detector. The proximate analysis of the EC generatedsludge was determined as per Indian Standards.6 The heatingvalue of the sludge was estimated by using a standard adiabaticbomb calorimeter7 equipped with a digital firing unit (Toshniwal,Bombay). The C, H, N, and S elemental analysis of the sludgewas carried out using an Elementar Vario EL III (ElementarAnalyzensysteme GmbH, Germany).

Thermal Analysis. Thermal analysis of the sludge wascarried out by using a thermal analysis (TA) instrument (Perkin-Elmer Pyris Diamond). Thermogravimetric (TG) differentialthermogravimetric (DTG) and the derivative thermal (DTA)analyses were carried out from the data and plots obtained fromthe instrument. This instrument operates with the followingspecifications: weight of the sample, 10-15 mg (max. 100 mg);temperature range, ambient to 1500°C; TG measurement range(sensitivity), 200 mg (0.2µg); DTG measurement range, 0.5-1mg min-1; DTA measurement range (sensitivity),(1000 mV(0.06µV); balance type- horizontal differential; thermocouple,

Pt-Pt Rh (13%); heating rate, 0.01-100 K min-1; atmosphere,air and inert nitrogen gas.

The thermoanalytical curves of the solid samples wereobtained from this instrument under air and nitrogen atmo-spheres with a flow rate of 0.4 dm3 min-1. Approximately 10-11.5 mg of the sample was heated in an alumina crucible in adynamic atmosphere from the ambient temperature to 1000°Cat the heating rate of 10 K min-1 using calcinedR-Al2O3 asthe reference material.

Results and Discussion

Sludge Characteristics. (a) Density, Volume, and SVI.Sludge production is an important parameter in characterizingand estimating the cost-effectiveness of the EC process. Thesystem pH during the EC treatment of the wastewater has aprofound impact on the sludge characteristics. Figure 1 showsthe volume of the sludge produced per unit dm3 of the BL afterEC treatment and the sludge density at the optimum currentdensity of 55.56 Am-2 and 60 min ET. It is found that the sludgevolume formed is not a function of anode consumption.Maximum sludge volume is formed at about neutral pH0. Thismeans that the EC treatment of the raw BL without any pHadjustment produces a maximum amount of sludge (∼33.5%of the BL volume) at 30 min settling time and∼26% onovernight settling. The final pH (pHf) of the EC treated BL wasobserved to be always around 11.65 over the pH0 rangeinvestigated. For the pH0 > 9, the increase in pH is marginalbecause of the formation of ferric hydroxide species togetherwith the attack on the cathode by the hydroxyl ions which leadsto a very small increase in pH. The curve in Figure 1 shows aparabolic nature with a maxima and minima of the sludgevolume formation at lower and higher pH0 (pH0 ∼5 and 11,respectively). The sludge density, however, increases with anincrease in pH0 from 5 to 7 and then more or less steadies upto pH0 ∼11. A low amount of Fe(OH)3 formation at pH0 < 7and pH0 > 9 has resulted in a lower sludge volume and sludgedensity as well.

The most common parameter, sludge volume index (SVI),was used to quantify the settling characteristics of the ECgenerated sludge. SVI is defined as the volume (in dm3 × 10-3)occupied by 1 g of thesludge after 30 min of the settling. The

Figure 1. Sludge density and sludge volume as a function of initial systempH COD: 2000 mg dm-3, number of plates: 6, CD: 55.56 A m-2,temperature: 31°C -[- density,-0- volume.

Ind. Eng. Chem. Res., Vol. 45, No. 16, 20065767

SVI was calculated from the following relation

whereH30 is the height of sludge after 30 min of settling,H0 isthe initial height of the slurry, andX0 is the initial solidsconcentration in the slurry. SVI values for the initial systempH0 5, 7, 9, and 11 were found to be∼ 269, 312, 154, and 133,respectively. Lower SVI values at acidic and alkaline pH0 arebecause of the formation of a smaller sludge volume at 30 minsettling time. The interstitial water, i.e., water trapped betweensludge particles, leads to a lower solids content in the sludgecake. SVI values at optimal PAA (10 mg dm-3) and NaCl (625mg dm-3) dosages were found to be∼165 and 183; showing areduction in SVI by 47% and 41% as compared to the SVIvalues of the sludge obtained from the EC treatment at pH0 ∼7 without any aid. The reduction in SVI value from 312 to 183is probably due to the chloride ions released during ECtreatment.8

(b) Settling. The slurry obtained from EC treatment wassubjected to sedimentation tests in a 1 dm3 graduated glasscylinder. Figure 2 shows the time-course of the settling of sludgein terms of a dimensionless height of the solid-liquid interface(H/H0) as a function of settling time at different initial systempH0. At the beginning, a very short period of relatively slowsludge settling is seen primarily because of the Brownian motionof the particles. This is followed by a steady-state decrease inthe height of the solid/liquid interface, exhibiting the regime ofzone settling. Thereafter, the transition settling period ensues.Finally, a steady-state compression settling takes place with amuch smaller rate of decrease in the height of the sludgesupernatant interface. The interface between the supernatant andthe sludge is identifiable for the EC treated BL at pH0 5 and 7.However, as the pH increases, the supernatant turns murkierand cloudy, although the interface is still identifiable. Floc flakesdeposit on the anode surfaces lowering the COD removalefficiency as also the sludge volume. Figure 3 shows the effectof the addition of NaCl to the EC reactor on the settlingcharacteristics of the sludge formed. As the NaCl dosageincreases, the sludge settling rate deteriorates, and the sludge

volume increases. Since NaCl addition resulted in an increasein the anode consumption, the sludge mass and volumeformation is also large. Figure 4 shows the effect of the additionof PAA (0.0005-0.004%) to the EC reactor on the settlingcharacteristics of the sludge. A very small amount of PAA (g10mg dm-3) addition during EC treatment effects considerableimprovement in the settling characteristics of the sludge withabout the same amount of COD and color removal as thatwithout any additive. At an addition of 5 mg dm-3, the settlingrate is poorer than that without PAA. As the PAA dosage tothe reactor increases from 10 to 40 mg dm-3, the settlingcharacteristics undergo a phenomenal change. At PAA dosage> 30 mg dm-3, the three-stage settling turns into a two-stage,i.e., zone and compression settling and the transition zone isobliterated. At a PAA dosage of 40 mg dm-3, the sludge

Figure 2. Time course of sludge settling as a function of initial systempH. COD: 2000 mg dm-3, number of plates: 6, CD: 55.56 A m-2,temperature: 31°C -]- pH0 5, -9- pH0 7, -4- pH0 9, -/- pH0 11.

SVI )1000H30

H0X0(1)

Figure 3. Effect of NaCl addition to the EC reactor on sludge settlingCOD: 2000 mg dm-3, pH0 7.08, number of plates: 6, CD: 55.56 A m-2,temperature: 31°C; NaCl, mg dm-3: -[- 5000,-/- 2500,-2- 1250,-O- 625, -9- 0.

Figure 4. Effect of PAA addition to the EC reactor on sludge settlingCOD: 2000 mg dm-3, pH0 7.08, number of plates: 6, CD: 55.56 A m-2,temperature: 31°C PAA, mg dm-3: -4- 40, -0- 30, -2- 20, -/-10, -O- 5, -9- 0.

5768 Ind. Eng. Chem. Res., Vol. 45, No. 16, 2006

clarification takes place in 10 min compared to 45 min requiredwith PAA dosage< 30 mg dm-3.

(c) Modeling of Settling Velocity. Several experimental runswere conducted to obtain settling velocity at various initial solidsconcentration in the treated BL. For this purpose, the BL treatedwithout any additive was either diluted with the supernatant orconcentrated by removing the supernatant from the settlingchamber to make the sludge concentration in the range of 2231-390 g dm-3. In this way, the solution chemistry is maintainedconsistent. The slurry with different sludge concentration wasused to observe the descent of the sludge-supernatant interfacewith time in a cylindrical settling column. Figure 5 shows theplot of (H/H0) versus time with initial sludge concentration (X)as the parameter. At the minimum solids concentration of 390mg dm-3, the settling curve shows all the three settling regimes:zone settling initially, followed by transition zone settling, andultimately compression settling. With an increase inX, the zonesettling period gets extended. AtX g 1844 mg dm-3, the settlingis very poor; the sludge particles remain in discrete suspensionwith no identification of interface between the supernatant andthe sludge.

The concentration of sludge at a timet was determined byusing the following expression

The sludge settling velocity is calculated as the slope of thesludge settling curve within the zone settling regime (Figure5). Plots were drawn for the variation of the sludge settlingvelocity as a function of the initial sludge concentration in theslurry for the observed experimental data. A large number ofempirical and semiempirical models have been tested to fit withthe experimental data.9 Most of the models fit the experimentaldata adequately and satisfactorily. The experimental data fittedwell with the Richardson and Zaki model10

whereV is the settling velocity (m s-1), X is the initial solids

concentration (mg dm-3), K is the free falling velocity ofindividual particles as obtained from the fitting of the experi-mental data, andn is the factor which converts the initial massconcentration into fractional volumetric concentration. Thevalues of K and n were found to be 2.408 and 0.365,respectively. A plot of the solids flux versus solids concentrationis shown in Figure 6. This curve can be employed for the sizingthe sludge settling tank.

(d) Filterability. The filtration characteristics of the ECtreated BL can be studied by using either a plate and framefilter or a rotary vacuum filter. Gravity filtration can also beused for generating experimental data. Gravity filtration isgenerally considered a constant pressure filtration by neglectingthe effect of the change in the hydrostatic head on the totalpressure. The force balance for the gravity filtration using afilter paper on a Buechner funnel can be written as a differentialequation11

where∆t is the time interval of filtration (s),∆V is the filtratevolume collected up to that time interval (m3), C is the solidsconcentration in the slurry (kg m-3), R is the specific cakeresistance (SCR), m kg-1, µ is the viscosity of the filtrate (Pa.s),∆P is the pressure drop across the filter (Pa),A is the area offiltration (m2), and Rm is the resistance of the filter medium(m-1). SCR is also called as the specific resistance to filtration(SRF).

The filterability of the BL treated without any additive andwith 625 mg dm-3 NaCl and 10 mg dm-3 PAA dosages wastested using a gravity filter. The change in the hydrostatic headwas assumed to be negligible, as the funnel was filled up to50-60% of its volume. The volume of the filtrate was observedas a function of time, and a plot was made between∆t/∆V andV. Figure 7 shows such plots for the BL treated without anyadditive at different pH0 and at 31°C. It can be seen that thefilterability deteriorates as the pH0 increases. At pH0 5 and 7,the sludge is sturdy giving a clear filtrate. However, as the pH0

increases, the filtrate turns murkier showing poor filtration. Theaddition of NaCl or PAA to the reactor during BL treatment at

Figure 5. Settling curves at various initial solids concentration. COD: 2000mg dm-3, pH0 7.08, number of plates: 6, CD: 55.56 A m-2, temperature:31 °C. X, mg dm-3: -[- 2231, -2- 2028, -/- 1844, -O- 1676,-4- 1514,-0- 1350,-9- 1230,-|- 1133,-s- 998, -]- 920,-b- 868, -×- 600, -‚- 390

C )C0 × total height

height of suspension after timet(2)

V ) K(1 - nX)4.65 (3)

Figure 6. Variation of solids flux versus solids concentration. COD: 2000mg dm-3, pH0 7.08, number of plates: 6, CD: 55.56 A m-2, temperature:31 °C.

∆t∆V

) µA∆P(RCV

A+ Rm) (4)


its natural pH further improves the filterability. This can be seenfrom the values ofR andRm as given in Table 1. The additionof NaCl/PAA during EC treatment results in the reduced valuesof R andRm. These values have been estimated from the plotsof (∆t/∆V) versusV (Figure 8) for BL treated at optimalconditions.

Typical values ofR for activated sludge given by Barnes etal.12 are (4-12) × 1013, (3-30) × 1013, (2-20) × 1011, and

(3-10)× 1011 m kg-1 for activated sludge, biodigester sludge,conditioned digested sludge, and conditioned primary sludge,respectively. Thus, the specific cake resistance for the EC treatedBL is less than that of the activated sludge indicating that thesludge from the EC treated BL has better filterability incomparison to municipal sludges. The values ofR and Rm

reported by Lele et al.13 during thermal treatment of alcoholdistillery effluent were 5.46× 10-10-9.36 × 10-10 m kg-1

and 4.4× 10-8-10.15× 10-8 m-1, respectively. Most recently,Chaudhari et al.14 during the catalytic thermolysis of a biodi-gester effluent of an alcohol distillery plant reportedR valuesof 0.46× 10-10-90.39 m kg-1 andRm values of 51.38× 10-8-657.12× 10-8 m-1, respectively, for the initial system pH0

1-10. The variation in theR andRm values reported may beascribed to several factors, viz., the nature of the effluent,treatment conditions, floc characteristics of the sludge, and thesource of the effluent. Plots were drawn for fractional cumulativefiltrate volume (V/V0) as a function of filtration time. It wasseen that the sludge generated at the natural pH (pH0 7.08) ofthe BL had the best filtration characteristics. The sludgegenerated at pH0 9 exhibited poor filtration, and the sludgegenerated at pH0 11 showed the worst filtration property. Thefractional filtrate volume,V/V0, is found to have drasticallyreduced from∼93% at pH0 7.08 to∼ 22% at pH0 11 after 45min of filtration; these figures demonstrate the difficulty in thefiltration of the sludge obtained by treating BL at a pH otherthan its natural pH0. Poor filtration is probably the manifestationof the wide size distribution of the sludge particles. The smallersize particles remain dispersed in the treated BL and are difficultto be destabilized to aggregate into larger sized particles. Theparticles of smaller sizes get entrapped into the interstices ofthe filter medium and the cake and block the pores inhibitingfiltrate migration.

It may, however, be noted that the addition of NaCl enhancesthe cell conductivity and anode dissolution and deteriorates thesludge settling, whereas the addition of PAA during ECtreatment improves the sludge settling characteristics. The valuesof R andRm given in Table 1, however, indicate that the additionof NaCl (625 mg dm-3) shows better filtration than the additionof PAA (10 mg dm-3). The lower the values ofR, the better isthe filtration characteristics.

SEM Images.SEM images of iron electrodes (anode) beforeand after EC treatment were obtained to compare their surfacetexture. Figure 9(a) shows the original iron anode plate surfaceprior to its use in EC experiments. The surface of the anode isuniform with nanosized crystals. Figure 9(b) shows the SEMof the same anode after several cycles of its use in ECexperiments for a total duration of 5 h. The anode surface isnow found to be rough, with a number of dents of about 100-200µm in width and depth. These dents are formed around thenucleus of the active sites where the anode dissolution occursproducing iron hydroxides. A magnified view of one such denton the electrode surface is shown in Figure 9(c). The formationof a large number of dents may be attributed to the anodematerial consumption at active sites due to the generation ofdioxygen at its surface. These dents end up as deep holes withsharp edges, which tend to entrap degradation byproducts suchas microflocs and sludge particles. Thus, the active surface inthe dent is blocked for further participation in the degradationof COD and color. Adsorbed species from the solution ontothe active sites of the dents and onto the microflocs and thesludge particles also show higher resistance to degradation. Thus,the EC activity gets retarded on the exposure of the anodesurface for longer duration during EC treatment. After repeated

Figure 7. ∆t/∆V as a function of filtrate volume for the treated BL withoutany additive at different pH0 COD: 2000 mg dm-3, number of plates: 6,CD: 55.56 A m-2, temperature: 31°C -]- pH0 5, -9- pH0 7, -4-pH0 9, -/- pH0 11.

Table 1. Specific Cake Resistance (R) and Resistance of the FilterMedium (Rm)

EC treatment R (m kg-1) Rm (m-1)

no additive 6.891285× 1011 8.9107× 108

NaCl (625 mg dm-3) 3.246416× 1011 2.3747× 109

PAA (10 mg dm-3) 4.669278× 1011 2.9800× 109

Figure 8. ∆t/∆V as a function of filtrate volume for the treated BL atoptimal EC treatment conditions COD: 2000 mg dm-3, number of plates:6, CD: 55.56 A m-2, temperature: 31°C -/- 10 mg dm-3 PAA; -O-625 mg dm-3 NaCl; -9- no additive.


cycles of EC runs, these dents increase in size leaving behindan eroded surface. The post-treatment of the electrochemicallytreated wastewater by chemical coagulation using alum alongwith PAA (ratio 18:1) further captures the solid particles andgenerates a small amount of sludge (<1.5% v/v) resulting in100% color removal.

Physicochemical and Elemental Characterization of ECSludge.The proximate analysis and the heating value of thesludge from EC treatment of BL is shown in Table 2 andcompared with those of Indian coal.

The proximate analysis shows a considerably lower ashcontent in the EC generated sludge than that of coal. The volatilecontent of the sludge is∼2 times that of the Indian coal, andthe heating value of the EC generated sludge is about 55% thatof coal. The elemental analysis shows a decrease in the CHNScomposition of the settled sludge than that of the top scumgenerated during the EC treatment.

Thermal Analysis of Sludge.Figure 10 (a-d) shows thethermogravimetric analysis (TGA), differential thermogravi-

metric analysis (DTGA), and derivative thermal analysis (DTA)curves for the precipitated sludge (top jelly scum and settledsludge) after EC treatment as well as the sludge obtained afterthe post-treatment of the EC treated supernatant. The thermalcharacteristics were observed both in the oxidizing (air) as wellas inert (nitrogen) atmospheres at the heating rate of 10 K min-1

and air/nitrogen flushing rate of 0.4 dm3 min-1.Untreated BL. Figure 10(a) shows the TG/DTG and DTA

behavior of the solid residue of the black liquor under oxidizingenvironment. The TG trace shows a gradual decrease in theresidual sample mass up to a temperature of 512°C sheddingabout 55% of the initial sample mass. The weight-loss rate isfound to be extremely slow, and up to 653°C (over atemperature span of 141°C), the weight-loss is only 3.4%. Thismeans that the black liquor sample loses moisture at an almoststeady rate along with volatalization of light volatiles up to 512°C, and, thereafter, the sample becomes dry and stable. At 653°C, the oxidation of the dry sample started, and the sample lostweight quickly (∼12.7%) over a temperature range of 653-672 °C (a temperature span of 19°C). The maximum weight-loss rate was 1.35 mg min-1 at Tmax of 672 °C (see DTGAtrace). The peak temperature for the exothermic reaction asexemplified by the DTA curve was atTp ) 678 °C with heatrelease of 1008 mJ mg-1. Beyond 672°C, the weight-loss issteady but very slow, giving off∼15.1% mass from 672 to 987°C (over a temperature increase of 315°C).

Top Scum and Settled Sludge after EC Treatment.Figure10(b,c) shows TGA, DTGA, and DTA curves of the jellyprecipitated top scum. The nature of the TGA trace showsdehydration and volatalization (removal of volatiles) of thesample and the degradation of the lignin and other componentsup to a temperature of 230 and 202°C losing 12.7 and 11.8%of its initial mass for the top scum and the settled sludge,respectively. Between 202 and 396°C, a temperature span of194 °C, the precipitate oxidizes, losing about 32% of its mass.The peak rate of weight loss of 1.86 mg min-1 is at atemperatureTmax ) 239 °C as well as a subsidiary weight lossof 0.27 mg min-1 is seen at a temperatureT ) 295 °C. Theoxidation of top scum is found to be exothermic, with a heatevolution of 1279 mJ mg-1, the peak of the exotherm being ata temperature ofTp ) 262 °C. A small peak at 302°C with aheat evolution of 670 mJ mg-1 is also seen. Similarly, theoxidation of settled sludge is also found to be exothermic witha total heat evolution of 3248 mJ mg-1 with two exotherm peaksat 262°C and 303°C. The oxidation seems to be complete at382 °C for top scum and 396°C for settled sludge, and thetraces of TG, DTG, and DTA reflect this fact. Garg et al.1 haveshown that the temperature of complete oxidation of theprecipitated sludge for the thermochemical treatment of the BLfrom an integrated pulp and paper mill was 435°C. It is foundthat the organics of the precipitate get oxidized leaving behind

Figure 9. (a) Original iron anode plate surface prior to use in ECexperiments, (b) anode plate surface used in the EC experiments five times,and (c) dent on the anode surface.

Table 2. Characteristics of EC Generated Sludge under OptimalConditions and that of Indian Coal

analysisEC settled

sludge Indian coal

1. proximate analysisinherent moisture (%) 12.87 9-11volatile matter (%) 42.39 18-25ash (%) 17.69 25-37fixed carbon (%) 27.11 27-48

2. heating value (MJ kg-1) 11.33 20.683. sludge particle density

(kg m-3)1067 2350

4. elemental analysis C(%) H(%) N(%) S(%)top scum 25.82 3.275 0.817 0.424settled sludge 17.40 2.938 0.610 0.212


the ash fraction (∼55.1% in the case of the top scum and 54%in the case of the settled sludge).

Post-Treatment of the Supernatant.The post-treatment ofEC treated supernatant was carried out with alum and PAAgiving a 100% color removal from the BL.3 Figure 10(d) showsthe thermal degradation behavior of the sludge (1.0-1.5% v/v)obtained after the post-treatment of the supernatant. The TGtrace shows a gradual decrease in the residual sample mass upto a temperature of 523°C shedding about 57.1% of the initialsample mass (masking of oxidation zone occurs). Therefore,the sample becomes dry and stable. Several peak rates of weight-loss of 0.13 mg min-1 at 68.3°C, 0.14 mg min-1 at 217°C,0.20 mg min-1 at 336°C, and 0.15 mg min-1 at 461°C wereobserved. The oxidation is found to be exothermic with a heatevolution of 2493 mJ mg-1 in the temperature range of 343-465°C. The oxidation is complete at 523°C leaving behind anash fraction of 41%.

Management Aspects of EC Generated Sludge.Sludgecharacterization has been a prerequisite in any sludge disposalprogram as it provides a correct interpretation of the leachingand sludge-soil interaction mechanism.15 The analytical methodselaborated above provide sufficient information on the composi-tion (elemental and proximate) and the heating value of the ECsludge (top scum and settled sludge) using iron electrodes. Itwas noted that the sludge in the slurry form does not have anystickiness to the walls of the reactor/container except for thegreenish jelly appearance. The sludge has∼42% of volatilematter and a relatively low ash content (∼18%). The heatingvalue is not too large but is∼55% that of the Indian coal. Thecarbon content in the top scum is∼26%. Addition of NaCl (625mg dm-3) during EC treatment enhances the filtration charac-teristics. After filtration, the sludge can be utilized in makingfuel briquettes which could be fired in the boilers/incineratorsto recover its energy value. The bottom ash obtained afterincineration/combustion may be used for blending with organicmanure for use in agricultural/horticultural fields. The bottomash may also be blended with clay/coal fly ash to make bricks/tiles for the building industry.

Conclusion

This study reveals that the EC treatment of the BL at nearneutral pH without any additive produces sludge with goodsettling properties. The addition of NaCl (625 mg dm-3) andPAA (10 mg dm-3) to the EC reactor reduces EC time (ET)considerably and produces sludge with good settling charac-teristics. PAA addition enhances the settling rate appreciablysthe three-stage settling characteristic turns into a two-stagesettling with the transition settling getting obliterated. SVI ofthe sludge obtained from the treatment at optimal conditionswithout any additive and with the addition of PAA (10 mgdm-3) and NaCl (625 mg dm-3) were found to be 312, 165,and 183, respectively. The settling velocity of the sludgeobtained from EC treatment without any additive could besatisfactorily correlated with the Richardson and Zaki model.10

Addition of NaCl (625 mg dm-3) and PAA (10 mg dm-3)improved the filtration characteristics considerably withR andRm values for gravity filtration being in the range of 3.25-4.67× 1011 m kg-1 and 2.37-2.98 × 109 m-1, respectively. Thevalues ofR andRm for the EC generated BL sludge are muchbetter than those reported for the municipal sludges and thesludges obtained from the thermolysis of biodigester effluentfrom an alcohol distillery. Filterability of the sludge shows amarked change with the pH0 of the EC treatment. The naturalpH (∼7) of BL produces sludge with best filtration character-istics. The SEM images showed the changes in the anode platestructure with dents formed after repeated EC cycles. The ECtreatment efficiency shows deterioration because of the forma-tion of dents on the anode surface with prolonged use of theEC reactor. The thermal analysis of the sludge showed thecompletion of the oxidation process at 382, 396, and 523°Cfor the top scum, settled sludge, and the sludge obtained afterthe post-treatment of the supernatant with alum and PAA. Theheating value of the sludge was found to be 11.33 MJ kg-1.

The filtered sludge can be dried and fired in the furnaces/incinerators for its heat recovery. The bottom ash can be usedfor blending with organic manure for its use in agriculture/horticulture or can be blended with clay/coal flyash for use inmaking bricks/ceramic tiles for the building industry.

Figure 10. DTA-DTG-TG plots of EC generated sludge at various conditions in air atmosphere: (a) black liquor, (b) top scum, (c) settled sludge, and (d)post-treatment.


Acknowledgment

The authors thank the reviewers of the manuscript for theiruseful comments. S.M. gratefully acknowledges the Presidentof the JSS Mahavidyapeetha, Suttur, Mysore, Karnataka, for agrant of leave to undertake research under the Quality Improve-ment Program of the Government of India.

Literature Cited

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ReceiVed for reView March 29, 2006ReVised manuscript receiVed May 19, 2006

AcceptedMay 22, 2006

IE0603969


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