Treatment of paper and pulp mill effluent by coagulation

Pulp and paper mill effluent is highly polluting and is a subject of great environmental concern. In the present research we studied the removal of chemical oxygen demand (COD) and colour from paper mill effluent, using the coagulation process. A batch coagulation study was conducted using various coagulants such as aluminium chloride (AlCl3), polyaluminium chloride (PAC) and copper sulphate (CuSO4·5H20). The initial pH of the effluent had a tremendous effect on the COD and colour removal. The PAC reduced COD by 83% and reduced colour by 92% at an optimum pH of 5.0 and a coagulant dose of 8 mL L−1. With AlCl3, at an optimum pH of 4.0 and a coagulant dose of 5 g L−1, 72% COD removal and 84% colour removal were observed. At an optimum pH of 6.0 and a mass loading of 5 g L−1, 76% COD reduction and 78% colour reduction were obtained with copper sulphate. It was also observed that, after addition of coagulant, the pH of the effluent decreased. The decrease in the pH was highest with AlCl3, followed by PAC and then CuSO4·5H20.


Introduction
Integrated pulp and paper mills generate wastewater that has very high biochemical oxygen demand (BOD) and chemical oxygen demand (COD), high pH, turbidity, high temperature and intense colour, and contains toxic substances and recalcitrant organics. The colouring body present in the wastewater from a pulp and paper mill is organic in nature and is comprised of wood extractives, tannin resins, synthetic dyes, lignin and lignin-degradation products formed by the action of chlorine on lignin [1]. The paper industry consumes large amounts of water (about 250-300 m 3 per ton of paper) and generates an equal amount of wastewater [2]. Black liquor (originating from the chemical pulping stage) contains lignin, carbohydrates, organic acids, sulphur compounds, phenolic compounds, terpenes, resin, etc.
Pulp and paper mill effluents contain a number of compounds that are harmful to receiving waters and are inhibitory or recalcitrant to biological treatment. Conventional treatment processes like chemical pretreatment [3], lagooning [4] and activated sludge treatment [5,6] are not very efficient; hence, they are not adequate to fulfil the regulatory discharge standards for surface waters (COD < 0.1 kg m − 3 , BOD < 0.03 kg m − 3 ) and sewers (COD < 0.3 kg m − 3 , BOD < 0.1 kg m − 3 ), as applicable in India. Therefore, the pulp and paper industry has to use a tertiary polishing stage to meet the effluent discharge standards.
The various processes for treatment of pulp and paper mill effluent have been reviewed by Pokhrel and Viraraghavan [7]. Chemical precipitation (and/or coagulation) using alum, ferric chloride, lime, ferrous sulphate and polyaluminium chloride (PAC) have been studied extensively [8][9][10][11][12]. Although economical, in comparison with other methods, the precipitation method has other associated drawbacks like the need for dewatering and disposal of the generated sludge. An effective settling process followed by concentration of the sludge by filtration is one of the remedies for this problem. After filtration the residues may be dried and incinerated, which will produce energy [13]. After incineration the remaining ash contains micronutrients (including Cu/Al), which can be ground and mixed with organic manure to be further used in agriculture and horticulture [14].
Most of the reported research work used aluminium, ferrous salts and aluminium polyelectrolyte for the coagulation-flocculation process, to remove the toxic organic materials from the wastewater, making it amenable for secondary treatment such as wet air oxidation or biological treatment. The present paper deals with COD and colour removal from black liquor using aluminium salts, like AlCl 3 and PAC, and also the new *Corresponding author. Email: pkchaudhari@rediffmail.com coagulant CuSO 4 ·5H 2 0. It has been found that pH has a tremendous effect on the coagulation process. To find the optimum pH at constant mass loadings of the coagulants (CuSO 4 ·5H 2 O = 5 g L − 1 , AlCl 3 = 5 g L − 1 , PAC = 5 mL L − 1 ), after adding coagulant the initial pH was varied using acid and alkali. By this method we were able to minimize the coagulant dose, whereas the present practice is to vary the coagulant dose at a fixed value of pH. The optimum pH is that at which the maximum COD reduction is obtained.

Effluent
Black liquor was obtained from a local integrated kraft pulp and paper mill. This liquor had a COD of about 7 × 10 5 mg L − 1 . Since the black liquor was obtained after subjecting it to evaporation through a multiple effect evaporator, dilution of the sample was required. Synthetic wastewater was prepared by diluting this black liquor with distilled water to obtain a COD value of about 7000 mg L − 1 . The average characteristics of the synthetic wastewater are presented in Table 1.

Coagulants and chemicals
Analytical reagent grade chemicals were used for the analysis of the parameters during the coagulation-flocculation studies. Laboratory reagent grade AlCl 3 and analytical grade CuSO 4 ·5H 2 O were obtained from S.D. Fine Chemicals Ltd, Mumbai (India), and a commercial grade aqueous solution of PAC was obtained from M/s Jubilant Organosys Ltd, Gajraula, India. The characteristic properties of the coagulants are listed in Table 2.

Experimental method
Diluted black liquor of 0.20 dm 3 was placed in a 0.50 dm 3 glass beaker. The pH of the effluent was noted. A known amount of the coagulant was added to the effluent, and the initial pH (pH 0 ) was adjusted by adding aqueous NaOH (1 M) or H 2 SO 4 (1 M) solution, with a deviation in accuracy of 0.05 pH. Then the liquor was rapidly mixed with a stirrer for five minutes at 40 rpm and, thereafter, slowly mixed for 15 minutes at 20 rpm, at room temperature (25-32 ° C). The effluent sample was then kept quiescent for six hours. About 10 mL of supernatant liquor was placed in a centrifuge tube and centrifuged (Model R 24, Remi Instruments, Mumbai, India) for 20 minutes at 4000 rpm and then analysed for its COD and colour value. Centrifugation was performed to eliminate the effect of inorganics in colour determination. After centrifugation the COD value reduced slightly. These steps were repeated at different dosages of the coagulant. The experiments were planned with the objectives of: (i) optimizing pH for maximum COD removal, (ii) obtaining the optimum coagulant dose at the optimum pH for maximum COD removal.
The experimental detail of the coagulation study is given in Figure 1.

Analytical methods
The COD of the treated and centrifuged black liquor was determined by the dichromate open reflux method as per Standard Methods [15]. The BOD of a liquid sample was determined by incubating the seeded sample for five days at 20 ° C [15]. The pH of the samples was determined using a digital pH meter (Toshniwal Instrument Pvt. Ltd, Ajmer, India). The suspended solid was determined by filtering the 100 mL sample through Whatman 42 filter paper and drying the remaining solid at 105 ° C. The mass of this solid was noted and recorded as the concentration of suspended solids in mg L − 1 . The total solid was determined by evaporating a 100 mL wastewater sample at 105 ° C in a dish. When changes in the mass of solids had ceased, it was weighed and the reading was noted as the concentration of total solids in mg L − 1 . The dissolved solid was determined by deducting the suspended solids from the total solids. The conductivity of the sample was deter- The metal hydroxide polymers have an amorphous structure, very large surface area, and possess positive charge [18]. These hydroxides are hydrophobic, causing them to adsorb/neutralize the organic anionic particle surface and become insoluble [16,18].
Copper and aluminium cations tend to associate and complex with a number of functional groups and ligands, especially with polar molecules and with oxygen-containing functional groups like hydroxyl, phenolic and carboxylic groups. The local negative charge of these groups is neutralized by the Al and Cu cations, resulting in colloid destabilization and precipi-  Figures 2 and 3 show the effect of pH on the COD reduction and colour removal, respectively, of the wastewater having an initial COD value of 7000 mg L − 1 at ambient temperature (25-32 ° C) using different coagulants. The coagulant mass loading was kept uniform at 5 g L − 1 for AlCl 3 and CuSO 4 ·5H 2 O and 5 mL L − 1 for PAC. The runs were taken at different initial pH (pH 0 ) values, i.e. at 2.0, 4.0, 5.0, 6.0, 7.0 and 8.0. The treated effluent was then centrifuged and its COD was determined. For AlCl 3 the COD reduction was found to be considerable at pH 0 < 5.0 and maximum at pH 4.0 resulting in a COD reduction of 74%. At pH 0 > 5, the COD reduction was found to decrease. For the coagulation by PAC, the maximum COD reduction (70%) was noted at pH 0 = 5.0. After increasing or decreasing this initial pH, the COD reduction was found to decrease in both cases. For 5 g L − 1 of CuSO 4 ·5H 2 O, the COD reduction was increased as the pH 0 was increased from 2.0 to 6.0, and then decreased as pH 0 was increased from 6.0 to 8.0. The maximum 80% COD reduction was obtained using this coagulant at pH 0 6.0 CuSO 4 ·5H 2 O) than that of AlCl 3 (37.50 mM ≈ 5 g L −1

Effect of pH on COD and colour removals
AlCl 3 ). This may be due to the presence of greater unfilled orbital in Cu than Al. The anionic compounds of wastewater act as a good complexing agent and electron donor to the Cu.  The order of variation in the colour reduction was found to be similar to the order of COD reduction. (Figure 3). The decolorization is expressed as the per cent decrease in the absorbance of the biodigester effluent sample compared with the untreated sample at wavelength (λ) = 263 nm. The optimum colour reduction was found to be 86%, 78% and 82% for AlCl 3 , CuSO 4 .5H 2 O and PAC treated effluent at their optimum pH 0 : pH 0 = 4.0 for AlCl 3 , pH 0 = 5.0 for PAC and pH 0 = 6.0 for CuSO 4 ·5H 2 O. For the coagulation with CuSO 4 ·5H 2 O, the COD reduction is highest in comparison with AlCl 3 and PAC ( Figure 2); however, the colour removal is lower than for these coagulants, which may be due to the colour of CuSO 4 ·5H 2 O itself. Nevertheless, the colour reduction was found to be very good (78%) at optimum pH 6.0 and mass loading of 5 g L −l .
The carboxylic and phenolic groups coordinate with metal cations at low pH as compared with hydroxyl and aliphatic hydroxyl groups. However, coagulation-flocculation at a particular pH will depend on the amount of the particular functional groups taking part in the coordination and complexation with metal cations. The removal of dissolved organics during coagulation and precipitation with metal salts at different pH values follows two distinct mechanisms. At low pH, the effluent containing anionic organic molecules coordinate with metal cations and form insoluble metal complexes. At higher pH and elevated coagulant doses, the organics adsorb preformed flocs of metal hydroxides and get precipitated. The net result of the two mechanisms is that the removal of dissolved organic compounds with different functional groups can occur over a wide pH range, and a maximum COD and colour removal may occur at a pH where the combined effect of both the mechanisms is maximum. Figures 2 and 3 also reveal that the optimum pH 0 for COD and colour removal for all the coagulants falls in the acidic range and that the optimum pH 0 is specific to each coagulant. Thus, it may be concluded that the COD and colour reduction of black liquor is strongly dependent on the pH 0 value. At the mentioned temperature range (25-32 °C) without coagulants, no settling was observed in spite of variation in pH.

Effect of mass loading on COD and colour removal
The addition of different coagulants in the effluent and its flash mixing creates proper coagulation conditions. Gentle mixing thereafter initiates floc formation, complexation and adsorption of the organics resulting in the precipitation and settling of the insoluble solids.
For different coagulants, the effect of mass loading on the COD reduction of the synthetic wastewater (COD 0 = 7000 mg L −1 ) was studied at the ambient temperature of 25 °C at their optimum pH 0 (Figure 4). The coagulant mass loading was varied from 2 to 9 g L −1 for AlCl 3 and CuSO 4 ·5H 2 O, and 2 to 9 mL L −1 for PAC. It was observed that, as PAC coagulant dose was increased, the COD reduction increased up to a dose of 8 mL L −1 , after which the COD reduction was almost unchanged. Thus, the 8 mL L −1 of PAC dose is optimum, resulting in 84% COD reduction. For CuSO 4 ·5H 2 O coagulant, the COD  reduction was found to increase up to the 5 g L −1 dose, giving a maximum COD reduction of 76%. After a further increase in coagulant loading, the COD reduction decreased, indicating that 5 g L −1 is the optimum dose of CuSO 4 ·5H 2 O. The decrease in COD reduction at a higher dose of copper sulphate is due to restabilization of neutralized organic anions [20]. For AlCl 3 coagulant the COD reduction increased considerably up to the loading of 5 g L −1 , giving ∼ 74% COD reduction, After an increase in coagulant loading to 7 g L −1 , the COD reduction increased marginally, and thereafter decreased with further increases in coagulant dose. The colour reduction at the optimum pH for coagulation for different coagulants is presented in Figure 5. For optimum mass loading of CuSO 4 ·5H 2 O (5 g L −1 ), AlCl 3 (5 g L −1 ) and PAC (8 mL L −1 ) at optimum pH 0 , the colour reductions are found to be 75%, 88% and 92%, respectively. The colour compounds present in the paper mill effluent are negatively charged and are neutralized by the positively charged coagulant, thus removing the colour from the wastewater. The colour reduction increased up to a breakpoint, after which it started decreasing because of the increase in the coagulant concentration. The addition of metal ions to wastewater decreases its pH. The decrease in pH is proportional to the amount of metal ions. Figure 6 shows the variation in pH with coagulant dose. The decrease in pH is less for the dose of 8 mL PAC in comparison with dose of 5 g AlCl 3 . Since the decrease in pH depends on the amount of metal ion, it may be concluded that the aluminium content in PAC is less than in AlCl 3 . Having a low amount of aluminium in PAC, in comparison with AlCl 3 , gives a high COD and colour reduction. This may be due to the presence of polymeric Al silicates in PAC. The X-ray diffraction analysis of PAC identified that the major components were AlCl 3 , Al 2 O 3 , Al 2 SiO 5 and 3Al 2 O 3 ·2SiO 2 [10]. The gel structure of PAC also enmeshes the organics present in the wastewater. Thus, the complexation (and consequent precipitation) and the capture of the organics in the gel are responsible for the higher COD and colour reductions by PAC, as compared with those obtained with AlCl 3 and CuSO 4 ·5H 2 O.  Figure 6 shows the variation in pH after addition of different doses of coagulants. The decrease in pH after the addition of coagulant may be due to several hydrolytic reactions that are taking place during coagulation, forming multivalent charged hydrous oxide species and generating the H 3 O + ion during each step, thus reducing the pH value. Chaudhari et al. [13] also reported that the coagulant addition depresses pH to highly acidic levels, as the coagulant dose is highly correlated with pH. Higher doses of coagulants without the need for optimization are reported to be due to two reasons [21]: firstly, the increment in the rate of aggregate formation   and secondly the enmeshing of particulates/organic compounds into large aggregates.

Variation in pH of effluent by adding coagulants
As reported earlier (Figure 2), pH 0 = 4.0, pH 0 = 6.0 and pH 0 = 5.0 are the optimum pH values for coagulation with AlCl 3 , CuSO 4 ·5H 2 O and PAC, respectively. After adding the optimum amount of coagulants to the wastewater, the pH of the solution changed and became close to the optimum pH 0 of the individual coagulant. Thus, further addition of alkali/acid is not needed to adjust the initial pH 0 of coagulation.

Comparison of results
Results obtained by other researchers treating paper mill effluent have been compared in Table 3, showing the percentage COD and percentage colour removals along with the effluent type, coagulant type and coagulant concentration. Stephenson and Duff [8] obtained 90% colour removal using a 10 g L −1 dose of aluminium chloride. This dose is quite high in comparison with the 86.3% colour removal at the 5 g L −1 dose in the present work. Copper sulphate was used by Garg et al. for thermal precipitation of black liquor [11], and 61.4% COD removal was obtained. The better result (76% COD removal) in the present work (using CuSO 4 coagulant) may be due to the different process used by Garg et al. They performed the thermal precipitation reaction at 95 °C. In the present work, the coagulation was performed at room temperature, ∼25 °C. Polyaluminium chloride gave better results (83% COD removal and 92% colour removal) than those obtained by Srivastava et al. [10].
The amount of COD present after the treatment with PAC, aluminium chloride and copper sulphate were respectively 1120, 1820 and 1680 mg L −1 and the colours were almost white/pale yellow. After further reduction in COD by the oxidation process, the effluent may be discharged to agricultural land. Traces of Cu/Al ion present in the effluent may be used as a catalyst. Furthermore, Cu may work as a micronutrient, fungicide and pesticide. However the concentrations of Al and Cu in the effluent were not evaluated.

Conclusions
The pH of the effluent was found to have a tremendous effect on COD and colour removal by the coagulation process. The pH value of 4.0, 5.0 and 6.0 were optimum values for AlCl 3 , CuSO 4 and PAC, respectively. The maximum COD and colour removal of 83% and 92%, respectively, were obtained using PAC (8 mL L −1 ) as compared with 76% and 78% with aluminium chloride (5 g L −1 ) and 74% and 76% with copper sulphate (5 g L −1 ) at their optimum pH 0 values.
After addition of coagulants, the pH of the effluent decreased. The decrease in pH was the maximum for AlCl 3 , which was followed by PAC and CuSO 4 . The addition of the catalyst followed by pH adjustment to its optimum value can minimize the coagulant dose considerably.
Polyaluminium chloride was found to be a better coagulant in comparison with the other two coagulants. However, copper sulphate may also be considered as a better prospective coagulant because the Cu content in the supernatant works as a good catalyst for wet oxidation in the next stage of treatment by wet oxidation.
Coagulation followed by settling and filtration of dense sludge solve the sludge disposal problem. The sludge can be dried and can be used as fuel owing to its organic nature.
The ash obtained after incineration can be used as a source of micronutrients as Al and Cu are micronutrients for crops/plants. The supernatant resulting from settling-filtration, containing Cu/Al, may also be used as a micronutrient source. The CuSO 4 content in treated effluent may work as a fungicide on crops.