Chemical Treatment Options for Tannery Wastewater

Abstract

Industrial pollution is a growing concern of the present days. Various industries produce different types of effluents, which contain pollutants in a varying range. Among the industries, tannery industry is one of the major concerns. Raw untreated tannery wastewater are directly discharged into the surface water bodies (Balu, Buriganga river) and thus pollute the environment heavily. Effluents from raw hides processing tanneries, which produce wet-blue, crust leather or finished leather, contain compounds of trivalent chromium and sulphides in most cases. Organic and other ingredients are responsible for high BOD (Biochemical Oxygen Demand) and COD (Chemical Oxygen Demand) values and represent an immense pollution load, causing difficulties in effluent treatment. Therefore, an attempt is undertaken to develop a simple and economic technique for treatment of tannery wastewater, especially to remove chromium that is applicable to and affordable by developing countries like Bangladesh. The main objective of this study was to find out an effective chemical treatment option for the tannery wastewater. This objective was accomplished through analysis of tannery wastewater parameters and several methods were applied for the treatment of wastewater, which had been found in the existing literature. The study was based on the use of magnesium salts contained in the liquid bittern (LB), which was produced as a by-product in the salt making industry, as a coagulating agent under alkaline condition. In addition, other chemicals like alum, NaOH and Ca(OHh were also used in treating the effluent. To conduct the study, wastewater samples called chrome liquor were collected from one tannery industry at Hazaribagh, Dhaka and analyzed prior to treatment. LB was collected from a common salt industry in Narayangonj. A series of jar tests was carried out to determine the effectiveness of LB in removing chromium. Similar jar tests were performed using alum as coagulant, and NaOH and Ca(OHh as precipitating agents. Removal of colour, COD, turbidity, iron, copper, manganese and VII lead were also studied for each jar test. The effect of settling time was also investigated in the treatment of tannery wastewater. Liquid bittern is found effective in removing chromium from tannery effluent. About 99% removal efficiency can be achieved using LB. The final concentration of chromium in treated wastewater satisfied the general Standard for Industrial Effluents depending on the initial concentrations, and in all cases satisfied standards set for tannery industry. The removal of colour, turbidity, iron, copper, lead, and manganese is also found effective in the LB chemical precipitation process. LB is not as much effective in removing COD from tannery effluent. NaOH and Ca(OH)2 are found effective in removing chromium by precipitation process. Therefore only using alkaline agent like NaOH to raise pH in the range of 8.5 to 10 can offer a low cost effective chemical treatment option for treating tannery wastewater. In the shorter settling time, NaOH is found a better alkaline agent than Ca(OH)2 in precipitation process. The performance of the two alkalis becomes almost same after 24 hour settling. In precipitation method, the removal efficiency of chromium, iron, lead, copper, iron, manganese, turbidity, and colour were found effective after 6 hour settling time. COD remains significantly high in all the treatment options used in this study. Alum coagulation is found not so effective in treating tannery effluent. LB and alkalis are similarly effective in treating tannery wastewater.