2830
Ind. Eng. Chem. Res. 2006, 45, 2830-2839
Electrochemical Degradation of Pulp and Paper Mill Wastewater. Part 1. COD and Color Removal 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
The electrochemical degradation of agri-based paper mill wastewater (black liquor) was investigated in a 2 dm3 electrolytic batch reactor using iron plate electrodes. Of the four-, six-, and eight-plate configurations, a current density of 55.56 A/m2 at neutral pH with a six-plate arrangement was found to be optimal, achieving a maximum chemical oxygen demand (COD) and color removal of 80% and 90% (175 platinum-cobalt units (PCU)), respectively. The chemical dissolution of iron was strongly influenced by pH0. Electrochemical treatment at higher pH0 (pH0 g 9) increases the dissolution of iron electrodes by an order of magnitude. At the optimal current density, the iron electrode consumed is 31.27 g/m2‚h, achieving maximum COD removal. An increase in salinity reduces the treatment time significantly, and the sludge settling characteristics also improve. The addition of polyacrylamide (10 mg/dm3) to the electrochemical reactor enhances the COD removal rate with a very short treatment time with excellent sludge settleability. Specific energy consumption (SEC) reduces from 6.64 to 5.73 kWh/kg of COD removed with the addition of NaCl (625 mg/dm3). The posttreatment of electrochemically treated wastewater by chemical coagulation using alum (360 mg/dm3) along with 20 mg/dm3 polyacrylamide (PAA) further reduced COD values to 9, the increase in pH is marginal (at pH0 ) 9, pHf - pH0 ) 2.36; and at pH0 ) 11, pHf - pH0 ) 1.22). This is attributed to the fact that the formation of ferric hydroxide species together with the attack on the cathode by the hydroxyl ions leads to a very smalle increase in pH. In other words, electrocoagulation serves to neutralize the pH of the solution. The electrochemical degradation process is found to be exothermic; the heat generation is a function of the initial pH of the cell solution. The higher the pH0, the higher is the end temperature of the solution after electrochemical treatment. It is, however, interesting to find that the initial rise (t < 20 min) in temperature is highest for pH0 7. This indicates a faster rate of degradation at about neutral pH0 during the initial period of treatment than at lower or higher pH0. The color of the cell solution also changes with pH0. At pH0 11, the supernatant of the cell effluent is reddish brown; it is yellowish green at pH0 9, a pleasing light green at pH0 5 just after EC treatment, and reddish yellow after 1 day of settling. Effect of the Addition of PAA and PAC. Anionic polyacrylamide (PAA) (CH2CHCONH2)n, a water-soluble polymer, was added in small amounts, to act as a coagulant aid, into the electrochemical reactor prior to the start of the electrochemical process to study its effect on the removal of COD and color. To check whether the PAA adds to the COD of the black liquor, the COD value of the wastewater was determined with and without PAA. It was found that PAA concentrations below 0.005% do not affect the COD value of the black liquor to be treated. The PAA concentrations in the present investigation were in the range of 0.0005-0.004% only. Figure 10 shows the COD removal as a function of EC time on PAA addition to the reactor. It is found that the addition of PAA at the concentration of 10 mg dm-3 enhances the COD removal by ∼13-14% more than that without PAA addition. At ∼60 min
EC time, 10 mg dm-3, PAA shows the best performance in terms of COD removal in comparison to other PAA dosages. However, the performance is poorer than that obtained without PAA. At a dosage of PAA >40 mg dm-3, early deposits (flakes) formed on the cathode reduced the COD removal efficiency. In fact, at a higher PAA dose, a current drop occurred from 5 to 4 A and there was an increase in the applied voltage from 9 to 12 V at ∼30 min. However, the voltage returned back to normal (9 V and 5 A) at ∼60 min time. In all cases, the electrochemical time was ∼30-40 min. After the electrochemical process, most of the precipitate/sludge tended to float (indicating flotation) because of microbubbles entrapped in the viscous, jelly-like sludge. It takes ∼5 min for the impellers to bring down the sludge for settling. Large-size flocs formed during the process are very stable, and their disintegration is not easy. The sludge, however, showed excellent settling characteristics. Color removal at 5 and 10 mg dm-3 PAA dosages is ∼95% (∼90 PCU). A change in pH from near neutral to just over 11.2 occurs after 15 min, after which pH remains constant at ∼11.71. A 40 mg dm-3 PAA dosage showed excellent settling characteristics (200 mL sludge in 5 min). Anode consumption tends to decrease with an increase in PAA dosage. At 40 mg dm-3 PAA dosage, the anode consumption is 1.235 g, while at 5 mg dm-3 PAA, the anode consumption is 1.498 g. COD removal is mainly by electroflotation when PAA is added to the reactor, while both sedimentation and flotation contribute to the COD removal in the absence of PAA. The COD and color removal at the optimum conditions (CD ) 55.56 A m-2, six-plate, pH0 7.02, and temperature ) 31 °C) with and without additives (NaCl and PAA) are shown in Figure 11. The color removal efficiency increases drastically on the use of additives (NaCl and PAA) during the initial 30 min of the electrolysis time. Some test runs were conducted to see the effect of polyaluminum chloride (PAC) (aluminum chlorohydrate, synonym [Al2Cl(OH)5]) addition to the reactor at optimal conditions of the EC process. At 2% PAC addition, the pH0 dropped down from 7.08 to 3.7 with large-scale foaming (yellowish-white foam) over the top of the reactor solution. A voltage drop also occurred from 9 to 6.5 V. Dark local patches appeared on the
Ind. Eng. Chem. Res., Vol. 45, No. 8, 2006 2837
Figure 12. Effect of electrical charge on COD removal: COD ) 2000 mg dm-3; pH ) 7.08; number of plates ) 6; current intensity ) 6 - 2; temp. ) 31 °C; EC time ) -4- 15 min, -O- 30 min, -9- 60 min, -[- 90 min.
cathode with a very small sludge volume having a cloudy appearance, exhibiting extremely poor settling characteristics. COD removal up to a maximum of 60% was possible with great difficulty in the filterability of the cell solution. Electrochemical degradation was very quick during the first 15 min from the commencement of the process. After 15 min, COD removal ceased although foaming continued. However, color removal was very poor, and the reactor liquid turned murky. Final pH was never above 4.3, even after 60 min time. It is concluded that PAC is not an effective aid in the process for black liquor treatment. Current Efficiency, Charge, and Energy Consumption. The current efficiency (CE) decreases with an increase in the electrolysis time. During the initial period of electrolysis (t < 20 min), the CE increases from ∼400% to ∼500% when NaCl (625 mg dm-3) is added to the reactor. The addition of PAA (10 mg dm-3) enhances the CE to ∼540%. However, after ∼30 min, the CE value for electrolysis without any additive is always marginally higher than that with the additives. Figure 12 shows the COD removal as a function of charge (Ah dm-3) at optimal conditions for 15, 30, 60, and 90 min electrolysis time. It is seen that the COD removal first increases with an increase in charge, attains a maximum, and then shows a decrease as the charge is increased. It is also seen that an increase in the EC time makes the change in the COD removal gradual, with the plot becoming flattest for the 90 min EC time. The specific energy consumption (SEC) is defined as the amount of energy consumed per unit mass of COD removed, expressed in kWh/kg of COD removed and is given as
energy consumption ) VIt/∆COD
(4)
where V is the voltage across the electrodes, I is the current in amperes, t is the time in hours, and ∆COD is the COD removal. Figure 13 shows SEC as a function of electrolysis time. It is found that SEC is highest when no additive is added to the solution. The addition of 10 mg/dm3 of PAA reduces the SEC, but only slightly. The addition of NaCl (625 mg dm-3) shows a significant decrease in SEC at all the EC process times. At the EC process time of 75 min, the SEC reduces from 6.64 to
Figure 13. SEC as a function of electrolysis time: COD ) 2000 mg dm-3; pH0 ) 7.08; number of plates ) 6; CD ) 55.56 A m-2; temp. ) 31 °C; -O- NaCl (625 mg dm-3), -/- PAA (10 mg dm-3), -9- without any additive.
Figure 14. Absorbance of wastewater as a function of wavelength: -2black liquor (COD 2000 mg dm-3), -9- black liquor after EC treatment, -O- posttreatment with alum and PAA.
5.73 kWh (kg of COD removed)-1. This means that the increase in conductivity due to salt addition helps in the SEC reduction. Posttreatment of the Electrochemically Treated Wastewater. The treated wastewater with a COD value of ∼490 mg dm-3 has a light yellow color measuring 300 PCU. This wastewater was mixed with a preoptimized dosage of alum (360 mg dm-3) along with a small amount (20 mg dm-3) of PAA. Addition of PAA up to 50 mg/dm3 does not change the COD value appreciably, as mentioned earlier. The intent of adding PAA was to ensure rapid settling of the sludge formed. It has been noted that the use of alum alone does not help much in the settling of EC sludge. The jar test lasted for
