INTRODUCTION:

The pulp and paper industries use huge quantities of freshwater and lingo-cellulosic materials for various production steps of paper manufacture, which results in the generation of large quantities of polluted effluents. The paper mill effluents are dark in color with unpleasant odor. The dark color appears due to the presence of various organic materials such as resins, tannins, wood extractives, lignin, synthetic dyes and their degradation products [1, 2]. The pulp and paper mill effluents also contain harmful substances including chlorinated phenols, which are generated during the bleaching stages of pulp.

The chlorinated phenols are highly toxic in nature and affect the aquatic life severely. The paper mill effluents are characterized by showing extreme fluctuations in terms of pH, organic content, bio-chemical oxygen demand (BOD) and chemical oxygen demand (COD)[3]. The discharge of untreated industry effluents into water bodies is thus a major environmental concern. The presence of dark color in effluents reduces the amount of sunlight entering the fresh water bodies and thus interferes in the photosynthetic process of aquatic biota and also causes toxic effects in them [4, 5].

Color is thus the first contaminant to be treated in wastewater and it must be removed before wastewater is discharged into fresh water bodies. Major pollutants in pulp and paper mill effluents include high suspended solids, adsorbable organic halides (AOX), biologically oxidizable and non-oxidizable components, color, pH, lingo-cellulosic materials and various other soluble substances [6]. Bio-treatment methods employed conventionally are not effective enough for the treatment of most of synthetic dyestuffs due to the recalcitrant nature and complex poly-aromatic structure of dyes. The metabolites of reductive cleavage of the azo bond under anaerobic conditions have been found to be more toxic than the intact dye molecules [7]. Efficiency of any physico-chemical or biological treatment process largely depends on the quality, nature and concentration of organic compounds. Besides, the physico-chemical characterization of effluents is important for its safe reuse. Thus, due to high levels of contamination in industrial effluents, the recycling of wastewater has been recommended [8, 9].

MATERIALS AND METHODS:

The effluent samples were collected from the outlets of the paper industries in Sitapura Jaipur. All effluent samples were black colored and turbid. The samples were analyzed for various physicochemical parameters as per standard analytical methods[10]. The pH of the effluent samples was measured using Elico pH meter and the color was estimated spectrophotometrically.The biochemical oxygen demand (BOD) of the effluents was measured as per standard procedure. The dilute effluent samples were taken in two sets of 300 mL BOD bottles. One of the BOD bottles was incubated for 5 days in BOD incubator at 20^oC. The dissolved oxygen of the sample was found by Winkler’s method. BOD was calculated as:

BOD (mg/L^-1) = (D_{0 -}D₅)

Where, D₀= dissolved oxygen of sample as measured on first and fifth day of incubation respectively. The COD was found by open reflux method. Total suspended solids (TSS) and toal dissolved solids (TDS) were determined from the weight of residue left by evaporating exactly 250 ml of sample completely on water bath. The total phenol content was estimated by colorimetric method by using 4-amino antipyrene.

RESULTS AND DISCUSSIONS:

All effluent samples used in present study were black in color and highly turbid with huge amounts of solid suspended matter present in it. The physico-chemical characteristics of the effluents of paper industry studied are given in Table 1.

Table 1: Physico-chemical characteristics of the effluents of paper industry.

Parameter	Parameter Value	MPL*
Color (CU)	4531 ± 13	100
pH	8.78 ± 1.1	6.5-8.5
TSS (mg/L)	2138 ± 36	100
TDS (mg/L)	937 ± 31	3500
Phenols (mg/L)	76 ± 1.3	1-5.0
BOD (mg/L)	872 ± 21	100
COD (mg/L)	3121 ± 23	250

± represents the standard error for the mean values of triplicates

* Maximum permissible values as per CPCB

All parameters studied were above the permissible limits. The analysis of paper industry effluents indicated high BOD and COD in it. The effluents were also rich in organic solids with a high level of TSS and TDS. The effluents were alkaline and showed high total phenol content. A variety of oxidation and condensation reactions occur during chlorination or extraction stages and pulp cooking. The lignin degradation products and the structures with chromophoric groups are responsible for color absorption in the visible spectrum [11]. Around 70% of color has been reported to originate during the extraction stage of bleaching operations [12]. It has been established that the color causing compounds in paper mill effluents are usually colloidal in nature, carrying negative charge and this forms the basis for developing the color removal methods. The compounds with an average molecular weight of 5600 comprise the majority of color [13].

The phenolic compounds in paper mill effluents arise during chlorine or chlorine dioxide kraft pulp bleaching process. Phenolic compounds present in paper mill effluents exhibit huge toxic effects on humans, aquatic life and the environment. An extremely disagreeable taste and odor is imparted to water by phenolic compounds [14, 15]. The adverse effects produced due to dark color and high phenolic concentration in paper mill effluent is a serious public concern. Thus, the treatment of effluent to reduce its color and phenol content to permissible levels is an urgent need. Physicochemical processes including chemical precipitation techniques are commonly used tertiary treatments to remove color, residual suspended solids, biologically recalcitrant COD and toxicity that still remains in the effluents after secondary treatment [18].

The use of salts of aluminum and iron as precipitating agents to remove color, COD and toxicity due to mechanical pulping has been reported earlier [16, 17]. In order to reduce the pollution load in the effluents samples analyzed, three chemical precipitation treatment methods were employed. The metal salts used for chemical precipitation treatments are aluminum chloride, ferric chloride and barium chloride.

CHEMICAL PRECIPITATION TREATMENT

The chemical precipitation treatment was carried to precipitate out the color causing organic dyes and phenolic compounds from the paper effluent.

Precipitation of paper effluents using aluminum chloride

The chemical precipitation treatment was carried out by mixing 0-6 gL^-1 of aluminum chloride with 100mL paper mill effluent. On adding the metal salt, a light brown floc was formed which sedimented down as sludge in nearly 2 to 3 hrs. The supernatant formed was almost clear. The pH of effluent was reduced from 8.78 to 4.6. On varying the pH and metal salt dosage, the optimum pH and salt dose required for the effective decolorization of effluent observed was 4 gL^-1and 6.0 respectively. The maximum decolorization observed at pH 6.0 was 99.60%. Also, the reduction percentages of phenols, TSS and COD observed were 51.31%, 96% and 59% respectively under same treatment conditions. Table 2 shows the results obtained by chemical precipitation using aluminum chloride salt dose 5.0 gL^-1and pH 6.0

Table 2: Characteristics of paper mill effluent obtained after chemical precipitation treatment with 5.0 gL^-1 aluminum chloride at pH 6.0

Parameter	Values of Raw Effluent	Treated Effluent	MPL (CPCB)
Color (CU)	4531 ± 13	18.0 ± 10	100
pH	8.78 ± 1.1	5.6 ± 0.2	6.5-8.5
TSS (mg/L)	2138 ± 36	145 ± 12	100
COD (mg/L)	3121 ± 23	1534 ± 23	250
Phenols (mg/L)	76 ± 1.3	37 ± 1.4	1-5.0

± represents the standard error for the mean values of triplicates

The chemical precipitation of paper effluents using aluminum chloride was not effective enough in terms of pollution reduction. Therefore, an alternative treatment technique or further treatment was used to address the pollution problems for safe reuse of effluent.

Treatment of paper effluents using ferric chloride:

The chemical precipitation of paper effluents was carried out by treating 0-5.0 gL^-1 ferric chloride salt dose to 100mL of the raw effluent, which resulted in the formation of dark brown floc, and later settled down as sludge. The supernatant obtained after centrifugation was almost clear with light color. It was observed that the addition of ferric chloride reduced the pH of effluents from 8.78 to 4.0. On varying the pH and metal salt dosage, the optimum pH and salt dose required for the effective decolorization of effluent observed was 3.0 gL^-1and 5.0 respectively. The maximum decolorization observed at pH 6.0 was 99.24%. Also, the reduction percentage of phenols, TSS and COD observed was 46.05%, 96.44% and 67.28% respectively under same treatment conditions. Table 4.5 shows the results obtained by chemical precipitation using ferric chloride dose of 3.0 gL^-1and pH 5.0

Precipitation of paper effluents using barium chloride:

The chemical precipitation of pollutants was carried out by mixing of 0-5.0 gL^-1 barium chloride with 100mL of raw paper mill effluent, which resulted in the formation of a fine grey precipitate and them changed into coarse aggregates. The precipitate formed settled quickly at bottom of the reaction vessel. The supernatant was clear and lighter in color. A slight decrease in pH of effluents was observed after the treatment. On varying the pH and metal salt dosage, the optimum pH and salt dose required for the effective decolorization of effluent observed was 2.0 gL^-1and pH 10.0 respectively.

Table 3: Characteristics of paper mill effluent after chemical precipitation treatment with 3.0 gL^-1ferric chloride at pH 5.0

Parameter	Values of Raw Effluent	Treated Effluent	MPL (CPCB)
Color (CU)	4531 ± 31	34 ± 23	100
pH	8.78 ± 1.1	5.0 ± 0.1	6.5-8.5
TSS (mg/L)	2138 ± 36	76 ± 13	100
COD (mg/L)	3121 ± 23	1021 ± 35	250
Total Phenols (mg/L)	76 ± 1.3	41 ± 1.4	1-5.0

± represents the standard error for the mean values of triplicates

Table 4: Shows the results obtained by chemical precipitation using barium chloride dose of 2.0 gL^-1 and pH 10.0.

Parameter	Raw Effluent	Treated Effluent	MPL(CPCB)
Color (CU)	4531 ± 13	232 ± 15	100
pH	8.78 ± 1.1	7.21 ± 0.3	6.5-8.5
TSS (mg/L)	2138 ± 36	161 ± 18	100
COD (mg/L)	3121 ± 23	1211 ± 13	250
Phenols (mg/L)	76 ± 1.3	12 ± 1.7	1-5.0

± represents the standard error for the mean values of triplicates

The maximum decolorization observed at pH 10.0 was 94.87%. Also, the reduction percentage of phenols, TSS and COD observed was 53.89%, 92.46% and 61.19% respectively under same treatment conditions. The treatment of paper industry effluents with barium chloride is not efficient enough in terms of COD removal. The phenols were also removed partially. The excessive dose of barium chloride leaves the residual concentration of Ba²⁺ ions in the solution, which may be harmful. In order to address these problems, wastewater needs to be treated further for safe reuse.

Comparison of metal salt precipitation treatments of paper mill effluents:

Tables 5 summarizes the results obtained for the color, pH, TSS, COD and phenols removal by the metal salts used for chemical precipitation of pollutants. The results indicate that the conditions required for the precipitation of color are different from those which favor the precipitation of phenols. Hence, it is evident that a given precipitation treatment cannot be effective for the removal of both color and phenols at the same time. Thus, for effective treatment of effluents, a sequential treatment method may be followed.

CONCLUSION:

The chemical precipitation of paper mill effluents is a promising treatment technique especially for removing color. It has been observed that a given precipitation treatment cannot be effective enough for the complete removal of all pollutants the same time. The limited efficiency of most of the existing physicochemical color removal methods is due to varying molecular weights of color causing compounds. This is one of the reasons for lack of a single method that can remove color from all types of effluents. Thus, for effective treatment of effluents, a sequential treatment method including adsorption by activated carbon or the more advanced treatment techniques like advanced oxidation processes may be followed.

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Received on 14.05.2017 Modified on 28.05.2017

Asian J. Research Chem. 2017; 10(4):597-600.

DOI:10.5958/0974-4150.2017.00100.6

Metal salt used	pH range	Effective salt dose (g/L)	Effective pH	Color removal (%)	Phenols removal (%)	COD removal (%)	Type of sludge and supernatant
AlCl₃	2.0-7.0	5.0	6.0	99.60	51.31	59.0	Brown floc. and clear yellowish supernatant
FeCl₃	2.0-7.0	3.0	5.0	99.24	46.05	67.28	Deep brown floc. and clear supernatant
BaCl₂	8.0-12.0	2.0	10.0	94.87	53.89	61.19	Greyish precipitate and clear supernatant