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Treatment of Tannery Effluent by Locally Available Commercial Grade Lime

M. A. Sabur
Department of Chemistry, Jahangirnagar University, Bangladesh

M. M. Rahman
Department of Chemistry, Jahangirnagar University, Bangladesh

S. Safiullah
Department of Chemistry, Jahangirnagar University, Bangladesh

Abstract/ Executive Summary:

The treatment of tannery effluent was carried out based on the locally available commercial grade lime. Collected tannery effluent was acidic (pH 3.94) in nature with total chromium content, chemical oxygen demand (COD), total suspended solids (TSS), total dissolved solids (TDS) and the conductivity at 30°C were 987 mg/L, 3175.32 mg O2/L, 22915 mg/L, 13360 mg/L and 60.7 mS/cm, respectively. The effluent was treated by chemical precipitation method and the dependency on pH in the treatment was investigated by altering the pH from 6.0 to 11.0 by the gradual addition of lime slurry. The treated effluents were characterized primarily in terms of COD and conductivity and we obtained lowest COD (345 mg O2/L) and conductivity (21.4 mS/cm at 30°C) values at pH 9. Then a large scale treatment was carried out with 20.0 L of the effluent in the same experimental condition at pH 9 and the removal of chromium, COD, TSS, TDS and conductivity (at 30°C) were achieved up to 99.79, 60.99, 97.22, 98.27 and 41.85%, respectively. Chloride concentration was also measured before and after treatment but no significant change was observed.

Keywords: Tanning; Effluent; ETP; Chemical precipitation; COD; TSS; TDS.

Location: Department of Chemistry, Jahangirnagar University

Start Date:

End Date:

Program Area: Risk Management in Agriculture

Commodity: Efluent

Objectives:

The objectives of the present work is to characterize the wastewater discharged from tannery industries and their treatment for the removal of chromium and other undesirable materials by using low cost locally available commercial grade lime by chemical precipitation method and to make it up to standard for the discharge to the neighboring water bodies or to the environment.

Methodology:

Around 50.0 mL effluent sample was collected in a plastic container (previously washed with dilute HNO₃) from the Chowdhury Tannery, Hazaribag, Dhaka, when it was in operational condition. A few mL of conc. HNO₃ was added to the effluent to prevent the growth of microbial and was sealed to avoid possible air oxidation. The raw effluent was then analyzed for some physico-chemical parameters such as pH, total chromium, chemical oxygen demand (COD), total suspended solids (TSS), total dissolved solids (TDS), chloride concentration etc. within 72 h of the sample collection. Initially batch experiments were carried out with 200 mL effluents in five 500 mL beakers and pHs of each solution were adjusted to 6, 7, 8, 9, 10 and 11 using lime slurry prepared by mixing solid lime (dried at 110°C) with water. With the addition of lime slurry, blue coloured precipitation was noticed, settle down rapidly at the bottom of the beakers. The whole systems were allowed to stand for around 6 h and then the supernatants were separated from the deposited sludge. The COD and conductivity of the treated effluents were analyzed and based on these results, dosages and other operational conditions were optimized initially. From the results, it was observed that the quality of the treated effluents was better at pH 9 compared to the trials at other pHs. Then further treatment to a greater extent was carried out with 20.0 L effluent and lime slurry was added gradually until pH of the system reached 9.0. pH was measured at different stages of our study by a digital pH meter (CG-818, Schott Gerate), TSS was determined by filtering a well-mixed sample through a weighed standard glass-fiber filter and the residue retained on the filter was dried to a constant weight at 103-105°C, the increase in weight of the filter represents the TSS. The filtrate obtained from the TSS determination was evaporated at 180°C and the TDS was determined gravimetrically. Conductivity was recorded using a digital conductivity meter (RM6 LAUDA) and the COD was measured by open reflux method. Extraction of chromium from the sludge was carried out by digesting the dried sludge using concentrated H₂SO₄ on a sand bath at 100ºC for 3 h and the leachate obtained was diluted with distilled water to the desired level for the determination of chromium. Flame atomic absorption spectrophometric method was used to quantify chromium. Chloride was estimated by treating specific amount of effluent with excess standard AgNO₃ solution followed by back titration with a standard solution of ammonium thiocyanate. All the experiments were carried out in the Department of Cchemistry and in Wazed Miah Science & Research Centre, Jahangirnagar University.

Source of Information: J. Sci. Res. 5 (1), 143-150 (2013)

Article Link (Online Journal):

Funding Source:

Total Budget (Tk.) :

Achievement/Findings:

The government of Bangladesh has set the characteristics of effluent at discharge point for industrial units and projects [28] in which total chromium should not exceed 0.5 mg/L for in-land surface water. But for large processing units, the value is allowable up to 2.0 mg/L. According to this discharge quality standard to inland surface water, the maximum permissible limit for pH, COD, TSS, TDS, electrical conductivity and chloride content are 6-9, 200 mg O2/L, 150 mg/L, 2100 mg/L, 1.200 mS/cm and 600 mg/L, respectively. Higher values are sometimes allowable for the discharge to public sewer or to irrigated land. Thus, from the results of our treatment, the treated effluent can be considered suitable for its discharge practically to the nearest water bodies. The result from the leaching experiments indicates that the extracted chromium can be reused for further tanning. However, it should be mentioned that chromium liquor produced in such a manner may contain dirt and other undesirable materials which should be removed before it is subjected to further tanning.

Knowledge Management: Journal