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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 batch electrochemical (EC) treatment of the black liquor (BL) of a small paper mill based on agricultural waste as raw material. The study shows that the sludges obtained from the EC treatment of the BL at its natural pH without 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 the sludge settling rate improves considerably. The settling velocity of the sludge obtained from the EC treatment without any additive could be satisfactorily correlated with the model of Richardson and Zaki. The results of the gravity filtration of the treated BL showed that the addition of NaCl (625 mg dm-3) improved the filtration characteristics and reduced the values of the specific cake resistance (R). The values of the specific cake resistance (R) and the resistance of the filter medium (Rm) were in the range of 3.25-4.67 × 1011 m kg-1 and 2.37-2.98 × 109 m-1, respectively. Prolonged use of iron anodes produces a large number of dents on their surface. SEM images of the electrodes show that the dents formed because of the anode dissolution during the EC process. The sludge has lower ash content and fixed carbon and higher volatile matter than that of Indian coal. Thermal analysis showed good combustion characteristics and complete oxidation of the EC process 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 organic manure for use in agriculture/horticulture or may be blended with clay/coal fly ash to make bricks/ceramic tiles for the building industry. Introduction Pulp and paper mills in India using agribased raw materials generally do not have chemical recovery units as also inadequate treatment facilities to meet the prescribed effluent standards for discharge. The wastewater emanating from such units have high COD (∼2000 mg dm-3), BOD (∼650 mg dm-3), color (∼1750 platinum cobalt units (PCU)), and total solids (∼2100 mg dm-3). Thermochemical treatment (catalytic thermolysis)1 and coagulation/flocculation followed by adsorption2 have been recommended as very good pretreatment options for such wastewaters. Recent interest in electrochemical treatment led us to investigate the efficacy of the electrochemical degradation of pulp and paper mill wastewaters with respect to COD and color.3 The black liquor (BL) of an agribased pulp and paper mill was treated in a 2 dm3 batch electrochemical reactor using DC power supply and iron plate electrodes arranged in parallel.3 The anodic dissolution during EC treatment formed metallic hydroxides which were found in the sludge. The electrodissolution of iron and its conversion into hydroxides of different valencies 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 particles around which an adsorption layer of cations and anions gets organized carrying over it a positive charge. Gases released during electrolysis (and electrooxidation) include chlorine, oxygen, and hydrogen at the cathode. The hydrogen produced at the cathode as a result of the redox reaction removes dissolved organics 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 during electrochemical (EC) treatment agglomerate and precipitate atop the reactor; adhere to the edges of the electrode as a coating (at alkaline pH, not otherwise), and settle at the bottom of the reactor after EC treatment. The characteristics of the sludge, viz., its volume, sludge volume index (SVI), settling characteristics, filterability, and solids flux are very important parameters in the overall evaluation of the effectiveness of electrochemical treatment and the design of the filtration unit/settling tank. The treatment and disposal of such electrochemically produced sludge is perhaps the most important environmental problem. The EC treatment for COD and color removal have been reported in the first part of the paper.3 This paper focuses on the physicochemical characterization and disposal management aspects of the sludge generated from the EC treatment of BL generated from the cooking-washing section of an agribased small paper mill manufacturing kraft paper. The sludge from the EC process as well as the sludge obtained from the posttreatment of the supernatant of the EC treated BL have been used in the present study. Besides the settling characteristics of the precipitated sludge at the optimal conditions, the filterability of the EC treated effluent was also studied. The physical characterization of the electrodes (anodesprior to and after the EC process) was done using a scanning electron microscope. The elemental composition of the sludge was determined using a CHNS analyzer. The thermal analysis of the untreated BL and EC treated sludge (top scum and settled sludge) and posttreated sludge were carried out by using a thermal analyzer. Experimental and Analytical Methods
* To whom correspondence should be addressed. Tel.: 91-1332285715. Fax: 91-1332-276535, 273560. E-mail:
[email protected].
Materials. The BL used in EC treatment had the following characteristics: BOD ∼615-670 mg dm-3, COD ∼2000 mg
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dm-3, chlorides ∼48-62 mg dm-3, total solids ∼2100 mg dm-3, pH ∼6.86-7.12, total alkalinity ∼380-410 mg dm-3, conductivity ∼0.746-0.791 mmho cm-1, and color ∼1750 platinum cobalt unit (PCU). The effect of 4, 6, and 8 plate configurations, applied current density, initial pH (pH0), and the addition of NaCl, polyacrylamide (PAA), and polyaluminum chloride (PAC) on the process performance of the EC reactor had been studied earlier.3 An 80% COD removal was obtained at the optimum current density of 55.56 A m-2 and at a cell voltage of 2.5-3 V achieving an electrolysis time (ET) of 60 min with a 6 plate EC reactor. The sludge (top scum and settled sludge) generated during EC degradation process were collected, filtered, sun-dried, and stored in glass containers. These were kept in a desiccator prior to their analysis for physicochemical and thermal characteristics. Settling and Filterability. The mixture of liquid-solid suspensions from the EC process were mixed, and the resultant slurry was used to study the settling and filterability characteristics of the sludge. The sludge sedimentation tests were performed using a 1 dm3 graduated glass cylinder (28 cm high and 5.85 cm i.d.). No stirring was done during the tests. The well mixed slurry was homogenized before pouring it into the glass cylinder and was allowed to remain under quiescent conditions. The position of the upper interface was measured as a function of time. The frequency of the measurement of the interface height was chosen in conformity with the settling rate. Each sedimentation run lasted for about 40-45 min. The filterability of the sludge was tested using a gravimetric filter having a pore size of ca. 11 µm (grade 1) supported over a ceramic Buechner funnel of 93 mm internal diameter (filter area ) 6.793 × 10-3 m2; the slurry was filled up to 60% volume of the funnel). The volume of filtrate collected in the graduated vertical cylinder was recorded at regular time intervals neglecting the filtrate volume obtained in the first 2 min.4 The end of the filtration phase is attained when the points in the ∆t/∆V versus V plot deviate from the initial linear plot.5 These data were used to calculate the specific cake resistance. From the moment at which the liquid disappeared from the top surface of the cake formed, a certain desired dewatering time was allowed, and then the cake was carefully removed, weighed, and dried at 105 °C until it attained a constant weight. The residue was expressed as mass of solids in the slurry. Physicochemical Characterization. SEM images of iron plates before and after EC treatment were obtained using the scanning electron microscope (LEO 435VP, England) operating with SE1 detector. The proximate analysis of the EC generated sludge was determined as per Indian Standards.6 The heating value of the sludge was estimated by using a standard adiabatic bomb calorimeter7 equipped with a digital firing unit (Toshniwal, Bombay). The C, H, N, and S elemental analysis of the sludge was carried out using an Elementar Vario EL III (Elementar Analyzensysteme GmbH, Germany). Thermal Analysis. Thermal analysis of the sludge was carried out by using a thermal analysis (TA) instrument (PerkinElmer Pyris Diamond). Thermogravimetric (TG) differential thermogravimetric (DTG) and the derivative thermal (DTA) analyses were carried out from the data and plots obtained from the instrument. This instrument operates with the following specifications: weight of the sample, 10-15 mg (max. 100 mg); temperature range, ambient to 1500 °C; TG measurement range (sensitivity), 200 mg (0.2 µg); DTG measurement range, 0.5-1 mg min-1; DTA measurement range (sensitivity), (1000 mV (0.06 µV); balance type - horizontal differential; thermocouple,
Figure 1. Sludge density and sludge volume as a function of initial system pH COD: 2000 mg dm-3, number of plates: 6, CD: 55.56 A m-2, temperature: 31 °C -[- density, -0- volume.
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 were obtained from this instrument under air and nitrogen atmospheres with a flow rate of 0.4 dm3 min-1. Approximately 1011.5 mg of the sample was heated in an alumina crucible in a dynamic atmosphere from the ambient temperature to 1000 °C at the heating rate of 10 K min-1 using calcined R-Al2O3 as the reference material. Results and Discussion Sludge Characteristics. (a) Density, Volume, and SVI. Sludge production is an important parameter in characterizing and estimating the cost-effectiveness of the EC process. The system pH during the EC treatment of the wastewater has a profound impact on the sludge characteristics. Figure 1 shows the volume of the sludge produced per unit dm3 of the BL after EC treatment and the sludge density at the optimum current density of 55.56 Am-2 and 60 min ET. It is found that the sludge volume formed is not a function of anode consumption. Maximum sludge volume is formed at about neutral pH0. This means that the EC treatment of the raw BL without any pH adjustment produces a maximum amount of sludge (∼33.5% of the BL volume) at 30 min settling time and ∼26% on overnight settling. The final pH (pHf) of the EC treated BL was observed to be always around 11.65 over the pH0 range investigated. For the pH0 > 9, the increase in pH is marginal because of the formation of ferric hydroxide species together with the attack on the cathode by the hydroxyl ions which leads to a very small increase in pH. The curve in Figure 1 shows a parabolic nature with a maxima and minima of the sludge volume formation at lower and higher pH0 (pH0 ∼5 and 11, respectively). The sludge density, however, increases with an increase in pH0 from 5 to 7 and then more or less steadies up to pH0 ∼11. A low amount of Fe(OH)3 formation at pH0 < 7 and pH0 > 9 has resulted in a lower sludge volume and sludge density as well. The most common parameter, sludge volume index (SVI), was used to quantify the settling characteristics of the EC generated sludge. SVI is defined as the volume (in dm3 × 10-3) occupied by 1 g of the sludge after 30 min of the settling. The
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Figure 2. Time course of sludge settling as a function of initial system pH. 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 was calculated from the following relation
SVI )
1000H30 H0X0
Figure 3. Effect of NaCl addition to the EC reactor on sludge settling COD: 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.
(1)
where H30 is the height of sludge after 30 min of settling, H0 is the initial height of the slurry, and X0 is the initial solids concentration in the slurry. SVI values for the initial system pH0 5, 7, 9, and 11 were found to be ∼ 269, 312, 154, and 133, respectively. Lower SVI values at acidic and alkaline pH0 are because of the formation of a smaller sludge volume at 30 min settling time. The interstitial water, i.e., water trapped between sludge particles, leads to a lower solids content in the sludge cake. SVI values at optimal PAA (10 mg dm-3) and NaCl (625 mg dm-3) dosages were found to be ∼165 and 183; showing a reduction in SVI by 47% and 41% as compared to the SVI values of the sludge obtained from the EC treatment at pH0 ∼ 7 without any aid. The reduction in SVI value from 312 to 183 is probably due to the chloride ions released during EC treatment.8 (b) Settling. The slurry obtained from EC treatment was subjected to sedimentation tests in a 1 dm3 graduated glass cylinder. Figure 2 shows the time-course of the settling of sludge in terms of a dimensionless height of the solid-liquid interface (H/H0) as a function of settling time at different initial system pH0. At the beginning, a very short period of relatively slow sludge settling is seen primarily because of the Brownian motion of the particles. This is followed by a steady-state decrease in the height of the solid/liquid interface, exhibiting the regime of zone settling. Thereafter, the transition settling period ensues. Finally, a steady-state compression settling takes place with a much smaller rate of decrease in the height of the sludge supernatant interface. The interface between the supernatant and the sludge is identifiable for the EC treated BL at pH0 5 and 7. However, as the pH increases, the supernatant turns murkier and cloudy, although the interface is still identifiable. Floc flakes deposit on the anode surfaces lowering the COD removal efficiency as also the sludge volume. Figure 3 shows the effect of the addition of NaCl to the EC reactor on the settling characteristics of the sludge formed. As the NaCl dosage increases, the sludge settling rate deteriorates, and the sludge
Figure 4. Effect of PAA addition to the EC reactor on sludge settling COD: 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.
volume increases. Since NaCl addition resulted in an increase in the anode consumption, the sludge mass and volume formation is also large. Figure 4 shows the effect of the addition of PAA (0.0005-0.004%) to the EC reactor on the settling characteristics of the sludge. A very small amount of PAA (g10 mg dm-3) addition during EC treatment effects considerable improvement in the settling characteristics of the sludge with about the same amount of COD and color removal as that without any additive. At an addition of 5 mg dm-3, the settling rate is poorer than that without PAA. As the PAA dosage to the reactor increases from 10 to 40 mg dm-3, the settling characteristics 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 is obliterated. At a PAA dosage of 40 mg dm-3, the sludge
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Figure 5. Settling curves at various initial solids concentration. COD: 2000 mg 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
clarification takes place in 10 min compared to 45 min required with PAA dosage < 30 mg dm-3. (c) Modeling of Settling Velocity. Several experimental runs were conducted to obtain settling velocity at various initial solids concentration in the treated BL. For this purpose, the BL treated without any additive was either diluted with the supernatant or concentrated by removing the supernatant from the settling chamber to make the sludge concentration in the range of 2231390 g dm-3. In this way, the solution chemistry is maintained consistent. The slurry with different sludge concentration was used to observe the descent of the sludge-supernatant interface with time in a cylindrical settling column. Figure 5 shows the plot of (H/H0) versus time with initial sludge concentration (X) as the parameter. At the minimum solids concentration of 390 mg dm-3, the settling curve shows all the three settling regimes: zone settling initially, followed by transition zone settling, and ultimately compression settling. With an increase in X, the zone settling period gets extended. At X g 1844 mg dm-3, the settling is very poor; the sludge particles remain in discrete suspension with no identification of interface between the supernatant and the sludge. The concentration of sludge at a time t was determined by using the following expression
C)
C0 × total height height of suspension after time t
(2)
The sludge settling velocity is calculated as the slope of the sludge settling curve within the zone settling regime (Figure 5). Plots were drawn for the variation of the sludge settling velocity as a function of the initial sludge concentration in the slurry for the observed experimental data. A large number of empirical and semiempirical models have been tested to fit with the experimental data.9 Most of the models fit the experimental data adequately and satisfactorily. The experimental data fitted well with the Richardson and Zaki model10
V ) K(1 - nX)4.65
(3)
where V is the settling velocity (m s-1), X is the initial solids
Figure 6. Variation of solids flux versus solids concentration. COD: 2000 mg dm-3, pH0 7.08, number of plates: 6, CD: 55.56 A m-2, temperature: 31 °C.
concentration (mg dm-3), K is the free falling velocity of individual particles as obtained from the fitting of the experimental data, and n is the factor which converts the initial mass concentration into fractional volumetric concentration. The values of K and n were found to be 2.408 and 0.365, respectively. A plot of the solids flux versus solids concentration is shown in Figure 6. This curve can be employed for the sizing the sludge settling tank. (d) Filterability. The filtration characteristics of the EC treated BL can be studied by using either a plate and frame filter or a rotary vacuum filter. Gravity filtration can also be used for generating experimental data. Gravity filtration is generally considered a constant pressure filtration by neglecting the effect of the change in the hydrostatic head on the total pressure. The force balance for the gravity filtration using a filter paper on a Buechner funnel can be written as a differential equation11
µ RCV ∆t ) + Rm ∆V A∆P A
(
)
(4)
where ∆t is the time interval of filtration (s), ∆V is the filtrate volume collected up to that time interval (m3), C is the solids concentration in the slurry (kg m-3), R is the specific cake resistance (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 of filtration (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 and with 625 mg dm-3 NaCl and 10 mg dm-3 PAA dosages was tested using a gravity filter. The change in the hydrostatic head was assumed to be negligible, as the funnel was filled up to 50-60% of its volume. The volume of the filtrate was observed as a function of time, and a plot was made between ∆t/∆V and V. Figure 7 shows such plots for the BL treated without any additive at different pH0 and at 31 °C. It can be seen that the filterability 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. The addition of NaCl or PAA to the reactor during BL treatment at
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Figure 7. ∆t/∆V as a function of filtrate volume for the treated BL without any 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, -4pH0 9, -/- pH0 11. Table 1. Specific Cake Resistance (R) and Resistance of the Filter Medium (Rm) EC treatment
R (m kg-1)
Rm (m-1)
no additive NaCl (625 mg dm-3) PAA (10 mg dm-3)
6.891285 × 3.246416 × 1011 4.669278 × 1011
8.9107 × 108 2.3747 × 109 2.9800 × 109
1011
Figure 8. ∆t/∆V as a function of filtrate volume for the treated BL at optimal 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; -O625 mg dm-3 NaCl; -9- no additive.
its natural pH further improves the filterability. This can be seen from the values of R and Rm as given in Table 1. The addition of NaCl/PAA during EC treatment results in the reduced values of R and Rm. These values have been estimated from the plots of (∆t/∆V) versus V (Figure 8) for BL treated at optimal conditions. Typical values of R for activated sludge given by Barnes et al.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 treated BL is less than that of the activated sludge indicating that the sludge from the EC treated BL has better filterability in comparison to municipal sludges. The values of R and Rm reported by Lele et al.13 during thermal treatment of alcohol distillery 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 biodigester effluent of an alcohol distillery plant reported R values of 0.46 × 10-10-90.39 m kg-1 and Rm values of 51.38 × 10-8657.12 × 10-8 m-1, respectively, for the initial system pH0 1-10. The variation in the R and Rm values reported may be ascribed to several factors, viz., the nature of the effluent, treatment conditions, floc characteristics of the sludge, and the source of the effluent. Plots were drawn for fractional cumulative filtrate volume (V/V0) as a function of filtration time. It was seen that the sludge generated at the natural pH (pH0 7.08) of the BL had the best filtration characteristics. The sludge generated at pH0 9 exhibited poor filtration, and the sludge generated at pH0 11 showed the worst filtration property. The fractional filtrate volume, V/V0, is found to have drastically reduced from ∼93% at pH0 7.08 to ∼ 22% at pH0 11 after 45 min of filtration; these figures demonstrate the difficulty in the filtration of the sludge obtained by treating BL at a pH other than its natural pH0. Poor filtration is probably the manifestation of the wide size distribution of the sludge particles. The smaller size particles remain dispersed in the treated BL and are difficult to be destabilized to aggregate into larger sized particles. The particles of smaller sizes get entrapped into the interstices of the filter medium and the cake and block the pores inhibiting filtrate migration. It may, however, be noted that the addition of NaCl enhances the cell conductivity and anode dissolution and deteriorates the sludge settling, whereas the addition of PAA during EC treatment improves the sludge settling characteristics. The values of R and Rm given in Table 1, however, indicate that the addition of NaCl (625 mg dm-3) shows better filtration than the addition of PAA (10 mg dm-3). The lower the values of R, the better is the filtration characteristics. SEM Images. SEM images of iron electrodes (anode) before and after EC treatment were obtained to compare their surface texture. Figure 9(a) shows the original iron anode plate surface prior to its use in EC experiments. The surface of the anode is uniform with nanosized crystals. Figure 9(b) shows the SEM of the same anode after several cycles of its use in EC experiments for a total duration of 5 h. The anode surface is now found to be rough, with a number of dents of about 100200 µm in width and depth. These dents are formed around the nucleus of the active sites where the anode dissolution occurs producing iron hydroxides. A magnified view of one such dent on the electrode surface is shown in Figure 9(c). The formation of a large number of dents may be attributed to the anode material consumption at active sites due to the generation of dioxygen at its surface. These dents end up as deep holes with sharp edges, which tend to entrap degradation byproducts such as microflocs and sludge particles. Thus, the active surface in the dent is blocked for further participation in the degradation of COD and color. Adsorbed species from the solution onto the active sites of the dents and onto the microflocs and the sludge particles also show higher resistance to degradation. Thus, the EC activity gets retarded on the exposure of the anode surface for longer duration during EC treatment. After repeated
Ind. Eng. Chem. Res., Vol. 45, No. 16, 2006 5771 Table 2. Characteristics of EC Generated Sludge under Optimal Conditions and that of Indian Coal analysis 1. proximate analysis inherent moisture (%) volatile matter (%) ash (%) fixed carbon (%) 2. heating value (MJ kg-1) 3. sludge particle density (kg m-3) 4. elemental analysis top scum settled sludge
Figure 9. (a) Original iron anode plate surface prior to use in EC experiments, (b) anode plate surface used in the EC experiments five times, and (c) dent on the anode surface.
cycles of EC runs, these dents increase in size leaving behind an eroded surface. The post-treatment of the electrochemically treated wastewater by chemical coagulation using alum along with PAA (ratio 18:1) further captures the solid particles and generates a small amount of sludge (
