International Journal of Environment Science and Technology Center for Environment and Energy Research and Studies (CEERS) ISSN: 1735-1472 EISSN: 1735-2630 Vol. 5, Num. 1, 2008, pp. 11-16

International Journal of Enviornmental Science and Technology, Vol. 5, No. 1, Winter 2008, pp. 11-16

Pollution reduction and biodegradability index improvement of tannery effluents

¹*M. A. Aboulhassan; ¹S. Souabi; ²A. Yaacoubi

¹Laboratoire de Génie de l’Eau et de l’Environnement, Faculté des Sciences et Techniques, BP 146 Mohammedia, Maroc
²Laboratoire de Chimie Organique Appliquée, Equipe Environnement et Méthodologie, Faculté des Sciences Semlalia, BP 2390, Marrakech, Maroc
*Corresponding Author Email: a.aboulhassan@gmail.com Tel.: 212 67 51 78 17Fax: 212 23 31 53 53

Received 29 May 2007; revised 20 June 2007; accepted 14 October 2007; available online 26 December 2007

Code Number: st08002

ABSTRACT

Al₂ (SO₄)₃, 18H₂O, FeCl₃ and Ca (OH)₂ were used for the treatment of tannery wastewaters. The influences of pH and coagulant dosages were studied. Conditions were optimised according to the pollutant removal efficiencies, the volume of decanted sludge and the biodegradability index improvement. The results indicate that 6771% of total COD, 76-92% of color and 79-97% of Cr can be removed using the optimum coagulant dosages at the optimum pH range. Al₂ (SO₄)₃, 18H₂O and Ca (OH)₂ produced better results than FeCl₃ in terms of COD, color and Cr removal as well as in terms of biodegradability improvement. Moreover, Al₂ (SO₄)₃, 18H₂O and FeCl₃ produced the least amount of sludges for a given amounts of COD, color and Cr removed in comparison with Ca (OH)₂. Al₂ (SO₄)₃, 18H₂O seems to be suitable for yielding high pollutant removals and corresponding low volumes of decanted sludges in addition to improving wastewaters biodegradability index.

Key words: Tannery wastewater, coagulation, sludges production, biodegradability index

INTRODUCTION

The tanning of hides and skins to convert them into leather has been an important activity since antiquity. Approximately 30-40 m³ of water are usedper t of hide processed (Suthanthararajan, et al., 2004). With the present annual global processing capacity of 9 x 10⁹ Kg hides and skins, it is estimated that 30-40 x 10¹⁰ litres of liquid effluent is generated (Thanikaivelan, et al., 2004). This gives rise to two major problems for the leather industry: the availability of good quality water and the treatment of such large quantities of effluent. The tannery wastewater is a mixture of biogenic matter of hides and a large variety of organic and inorganic chemicals. Wastewater from tanneries usually contains high levels of salinity, organic loading, inorganic matter, color matter, dissolved and suspended solids, ammonia, organic nitrogen and specific pollutants (sulphide, chromium and other toxic metal salt residues), (Ros and Gantar, 1998). The potential environmental impact of the chemicals used in tannery operations has been widely acknowledged. Cr containing effluents findtheir way in the environment at disposal sites where Cr undergoes oxidation reactions and forms Cr (Vl) (Bartlett and James, 1979).AsCr (VI) is readilysoluble in water, it leaches down in the soil profile and could contaminate groundwater (Sumathi, et al., 2005). Furthermore, the discharge of such colored wastewater into the environment is not only aesthetically displeasing, but also impedes light penetration, damages the quality of the receiving streams and may be toxic to the treatment processes, to food chain organisms and to aquatic life (Mahdavi Talarposhti, et al., 2001). Thus, leather tanning generates many complex and high loaded effluents that require treatment before being discharged into the environment. Various physico-chemical techniques have been studied for their applicabilitytothe treatment of tannery wastewater (Orhon, et al., 1998;Amokrane, et al., 1997).Among thesearecoagulation, flocculation, ozonation, reverse osmosis, ion exchange and adsorption (Arvanitoyamis, et al., 1989). Coagulation flocculation is one of the important treatments given to the industrial effluent before discharging them into receiving waters to remove toxic waste. Many researchers have investigated the coagulation of tannery wastewater. However, few of them attempted to fully investigate the optimisation of coagulation for color reduction or in conjunction with subsequent biological treatability of coagulated tannery wastewater. Indeed, the coagulation process is not always perfect and may result in treated wastewaters of which the characteristics did not meet the proposed effluent standards. Consequently, a further treatment is often necessary. This paper describes experimental studies that were conducted on tannery wastewaters in order to evaluate coagulation precipitation process efficiency for the treatment of tannery effluents, especiallyin terms of organic matter, color and Cr removal as well as sludges production. In addition, the effect of coagulation flocculation process on biodegradability index improvement is also discussed.

MATERIALS AND METHODS

The samples were collected from a tannery located at Mohammedia city in Morocco. Different wastewater streams are generated at different times and as a result, the effluent characteristics in the main drain vary significantly (Table 1). Since no equalisation tank is provided, the hourly samples collected over one production cycle (8 h.) are thoroughly mixed in a drum to make a representative sample. Al₂(SO₄)₃,18H₂O, FeCl₃ and Ca(OH)₂ are used as chemical coagulants.

Jar test experiments were conducted under controlled laboratory conditions using a standard jar test apparatus. Four equal-volume polyethylene beakers were used to examine the four different dosages of coagulant or initial pH values in each run. Sample bottles were thoroughly shaken for the resuspension of possibly settling solids and the appropriate volume of sample was transferred to the corresponding jar test beakers. The experimental process consists of three subsequent stages: Initial rapid mixing stage at 160 rpm took place for 5 min. and it was followed by a slow mixing stage for 20 min. at 30 rpm; the final settling step lasts for another 1 h. To evaluate the efficiency of coagulants on tannery wastewater treatment, the following parameters were determined: turbidity, chemical oxygen demand (COD), biological oxygen demand (BOD), color, chromium content and the amount of the sludges produced.

Color measurement: Prior to color measurement, the sample was filtered through a 0.45 µm Millipore membrane filter to prevent turbidity. Color measurements were carried out with a spectrophotometer. Since the wastewater contains different kinds of dyes (depending on the production), the traditional methodof applying themaximum absorbance was not utilized. Color is determined using a UV-visible spectrophotometer (Model 7800 UV/VIS) by measuring the absorbance at three wavelengths (436,525, and 620 nm) and taking the sum of these three measurements (Olthof and Eckenfelder, 1976; Aysegül and Enis, 2002). Chromium: the concentration of chromium in the liquid phase was determined by graphite furnace atomic absorption spectrometry (SCHIMADZU AA-6800).

Turbidity: the turbidity was determined by turbidity meter (HI 93703 Microprocessor turbidity meter). COD, BOD and other physicochemical parameters analyzed were determined according to the standard methods (AFNOR, 1999).

Volume of sludges: At the end of the slow mixing stage, the beaker contents were transferred into Imhoff cones and allowed to settle for one h. The volume of the settled sludges in the cone was recorded according to the volumetric method (Eaton, et al., 1995).

RESULTS AND DISCUSSION

The effect of pH on turbidity removal from jar tests for coagulation of tannery wastewater is shown in Fig. 1. Fig. 2, 3 and 4 show the effect of coagulant addition on the reduction of COD, color and chromium at the optimum coagulation pH for each coagulant (pH 5 for FeCl₃ and pH 8 in the case of Al₂(SO₄)₃,18H₂O). The ratios between the amount of sludges produced and the amount of COD, color and chromium removed are presented in Fig. 5. Fig. 6 shows the biodegradability index of wastewaters for the optimal doses of coagulants. Finally, the optimal coagulant doses and also the cost of the different products used are reported in Table 2. The solution pH is an important factor in the coagulation process (Duan and Gregory, 2003). The use of coagulant at its optimum pH displays maximum pollutant removal. In addition, with such optimum pH conditions, the soluble residual aluminium and iron content in the wastewater will be lower than 0.5 mg/L and 2 mg/L, respectively (Amokrane, et al., 1997; Letterman and Driscoll, 1988). It can be seen that turbidity removal is most effective at a pH range between 7 and 8 for Al₂(SO₄)₃, 18H₂O and between 5 and 7 for FeCl₃ (Fig. 1). In case of both aluminium and iron, the hydroxide is of very low solubility and an amorphous precipitate Me (OH)₃ (Me: Metal) can form at intermediate pH values. This is of enormous practical significance in the reaction of these materials as coagulants. The total amount of soluble species in equilibrium with the amorphoussolid is effectively the solubility of the metal and it can be seen that in each case, there is a minimum solubility at a certain pH value. The pH range of 6.5-7.5 for Al₂(SO₄)₃, 18H₂O and 6.5 8.5 for FeCl₃ were determined as the optimal pH ranges of the removal of COD from tannery wastewaters (Song et al., 2004). On the basisof an initial COD concentration of 3442 mg/L, the addition of 100 mg/L of coagulant decreases COD by 2, 18 and 34% using Al₂(SO₄)₃, 18H₂O, Ca(OH)₂ and FeCl₃, respectively (Fig. 2). The results indicate that a maximum COD removal of 71% can be achieved through using both Al₂(SO₄)₃, 18H₂O and Ca(OH)₂ at 600 mg/L and 1000 mg/L, respectively.

However, the maximum percentage of COD removal that FeCl₃ could remove is 67% at the coagulant dose of 400 mg/L. Residual CODconcentration is of 996 mg/Lfor both Al₂(SO₄)₃, 18H₂O and Ca(OH)₂ and of 1122 mg/L for FeCl₃. This may be explained by the solubility of a part of COD. Consequently, it cannot be removed by decantation. Color is considered in this work as it may affect the feasibility of a subsequent biological treatment. Dyes present in wastewaters cause significant problems at treatment plant, since those compounds are hard to degrade through biological means. The effect of coagulation flocculation on color removal shows that this process is effective on color reduction; the maximum percentage of color removed are 92%, 81% and 77% using Ca(OH)₂, Al₂(SO₄)₃, 18H₂O and FeCl₃, respectively (Fig. 3). Substances producing color consist either of colloidal metallic hydroxides (e.g., iron hydroxides) or of organic compounds (e.g., dyestuff), which have a much smaller particle size. These substances can be removed by coagulation, which serves to agglomerate the very small particles into sizes that can be settled or can be removed by filters or absorption. Coagulation using FeCl₃ appeared to be less effective than Al₂(SO₄)₃, 18H₂O and Ca(OH)₂ in removing color.This maybeexplained assuming the fact that coagulants containing Fe produce color problems in effluents including sulphide or vegetable tannins (Bousher, et al., 1997). Table 1 indicates that an effective chromium reduction. This may be explained the tannery wastewaters contain appreciable amounts by the association of coagulant and pH effects. The of sulphide. Moreover, the company, in addition to results indicate that the residual chromium chrome leather production, uses also the vegetable concentrations using FeCl₃, Al₂(SO₄)₃, 18H₂O and tanning process. For an initial wastewater chromium Ca(OH)₂ are 9.7, 4.8 and 1.2 mg/L, respectively. When concentration of 47 mg/L, the chromium removal physico-chemical treatment is applied to waste water reaches 97,89and 79%using Ca(OH)₂, Al₂(SO₄)₃, 18H₂O by coagulation-flocculation,a large amountof sludges and FeCl₃,respectively (Fig. 4).Ros and Gantar, (1998) is generated. Sludges production may affect the investigated the effectof pH on chromiumremoval and economic feasibility of the proposed method. concluded that coagulation should be operated at an Therefore, when choosing a coagulant, one aspect to alkaline range to achieve maximum chromium removal. be considered is how much of sludges will be produced However, the optimal coagulation pH (Fig. 1) is acidic (James and O’melia, 1982). In order to compare the in case of FeCl₃ and slightly alkaline for Al₂(SO₄)₃, results obtained using such coagulants, the ratio 18H₂O. In addition, the acidic characters of Fe³⁺ and between the amount of sludges produced and the A1³⁺ (Lewis acids) decreased the pH of the medium. amount of COD, color and chromium removed has been Then, the percentage of chromium removed using FeCl₃ estimated (Aboulhassan, et al., 2005; Aguilar, et al., (only 79%) can be explained by the acidic pH of the 2002). Fig. 5 shows that Al₂(SO₄)₃, 18H₂O and FeCl₃ medium. On the contrary, the addition of Ca(OH)₂ produced the least amount of sludges for a given increases the pH of the wastewaters (pH 9.1) and allows amount of COD, color and chromium removed in comparison with Ca(OH)₂. Indeed, Ca(OH)₂ is very effective in the removal of COD, color and chromium. However, it produces more sludges. Considering the results obtained, if a small volume of sludges is to be treated, the Al₂(SO₄)₃, 18H₂O and FeCl₃ are suitable. However, Al₂(SO₄)₃, 18H₂O is more effective in the removal of COD, color and chromium than FeCl₃. Consequently, Al₂(SO₄)₃, 18H₂O is suitable. A significant proportion of the soluble matter in the effluent (e.g. soluble COD) is not removed by the physico-chemical treatment. In order to achieve a higher quality of treated water, further treatments will be needed prior to discharge. It is likely that a biological treatment, as an inexpensive process, would need to be incorporated for maximum treatment. Many authors use the BOD₅/COD ratio as biodegradability index. Wastewater can be considered readily biodegradable if it has a ratio value between 0.4 and 0.8 (Metcalf and Eddy, 1985;Al-Momani, et al., 2002). As shown in Fig. 6, Al₂(SO₄)₃, 18H₂O and Ca(OH)₂ lead to noticeable improvement in the biodegradability index. However, the BOD₅/COD ratio of treated wastewaters exceeds 0.8 using FeCl₃. This may be explained by the fact that treated wastewaters using FeCl₃ are Cr and color riches, compared to these obtained using Al₂(SO₄)₃, 18H₂O and Ca(OH)₂. These pollutants were known to be toxic to biological treatment processes (Mahdavi Talarposhti, et al., 2001; Florence and Bately, 1980; Sumathi, et al., 2005). In order to reduce the soluble pollution, treated wastewaters using Al₂(SO₄)₃, 18H₂O or Ca(OH)₂ can be subject to a further treatment using biological process. These coagulants reduce Cr wastewaters content and enhance the biodegradability index more than FeCl₃. The proper determination of coagulant and flocculant types and dosages will not only improve the resulting water characteristics, but also decreases the cost of treatment. Table 2 shows the optimal coagulant doses and also the cost of the different used products. The use of Al₂(SO₄)₃, 18H₂O affects the cost of treatment. However, it is more efficient than Fe³⁺, Cl^- and Pb to a noticeable reduction of the decanted sludges compared with Ca(OH)₂. Al₂(SO₄)₃, 18H₂O seems to be suitable in the treatment of tannery wastewaters for yielding a high pollutant removal and a corresponding low volume of decanted sludge as well as biodegradability index improvement for the subsequent biological treatment.

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AUTHOR (S) BIOSKETCHES

Aboulhassan, M. A., P.hD., Faculty of Sciences and Technologies, Mohammedia, Morocco.Email: a.aboulhassan@gmail.com

Souabi, S., P.hD., Professor, Faculty of Sciences and Technologies, Mohammedia, Morocco.Email: s_souabi@yahoo.fr

This article should be referenced as follows:

Aboulhassan, M. A.; Souabi, S.; Yaacoubi,A., (2008). Pollution reduction and biodegradability index improvement of tannery effluents. Int. J. Environ. Sci. Tech., 5(1), 11-16.

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