Treatment of tannery wastewater using activated sludge process

Abstract

Leather sector is the third largest foreign currency earning sector in Bangladesh. Out of total 170 tanneries in Bangladesh, 150 of them are located at Hazaribagh in Dhaka. About 21,600 m3 of highly polluted tannery wastewater is generated daily by leather manufacturing process in this area and crude disposal of this waste are creating serious hazard to the environment. An attempt has been made to analyze the present status of Hazaribagh in terms of tannery wastewater treatment. Now it is prerequisite to develop an appropriate treatment system of tannery wastewater. For this, it is obvious to manage each fraction of the treatment system in a cost effective and environment friendly way. Experimental results showed that tannery wastewater contains very high concentration of organic matter, solids, sulfates, sulfide and chromium in tannery effluent. These characteristics were not suitable for using a single treatment system- physical, chemical or biological. For this reason, combined system of treatment (physical-chemicalbiological) was used. As a part of physical treatment system, primary sedimentation was run out. And this primary sedimentation removed a significant portion of color, COD and BOD5 and most of the chromium. The study was carried out by using different proportion of acclimatized sludge to combined raw sample (tannery wastewater + domestic sewage) in three different bench scale reactors with three separated F/M ratios, extended aeration was carried out in bench scale reactors for 03 days at room temperature and the pH was maintained between 6.5 and 7.0. Color, COD and BOD5 were determined for every reactor and results showed increasing pattern of color value and decreasing residual value of COD and BOD5 with less decline rate. These phenomena indicated that if color value was wanted to be reduced, then chemical treatment option such as alum coagulation could be adopt. Here a suitable F/M ratio was also selected on the basis of the capacity of parameter reduction. Apart extended aeration, the reactors were subjected to aeration-anoxic cycle with pre and post alum coagulation to fulfill the aim of developing appropriate technology for reduction of wastewater parameters. The performance was analyzed on the basis of comparative analysis of removal percentages of Color, COD and BOD5 reduction achieved in Case-1 with extended aeration were 33%, 75% and 82% respectively whereas for Case-2 (aeration-anoxic cycle with post alum coagulation) removal percentages were 85%, 89% and 96% respectively and for Case-3 (aeration-anoxic cycle with pre alum coagulation) removal percentages were 75%, 78% and 89% respectively.

COD reduction curve for Case-1 and Case-2 followed irregular pattern, hence it can be said that the biodegradation rate is seed-specific and various operational processes. COD reduction in Case-2 is much higher than case-1. Because Case-2 contained anoxic period which contributed in more NH4 reduction and due to this incidence, bacteria mainly oxidized organic matter after anoxic period which leads to higher COD reduction. Yield coefficient, Y was gotten 0.57 for Case-1 that means growth of microorganism was occurred here though decay rate of micro-organism was higher. Yield coefficient, Y was gotten 0.60 for Case-2 that means growth of microorganism was higher whereas decay rate of micro-organism was lower. Presence of ammonia creates problems in oxidization of organic matter. Ammonia reduced gradually for Case-1 and reduced drastically for Case-2. Anoxic period that means oxygen limiting period converts nitrate into nitrogen gas through denitrification. Hence Case-1 performance in removing ammonia was poor for not existence of anoxic unit. From the performance of three cases, it was evident that F/M ratio of 0.22 is highly potential ratio for biological treatment of tannery waste and it is also found that Case-2, where aeration-anoxic cycle with post alum coagulation was continued up to 30 hour detention time with 0.22 F/M ratio, showed the optimum results in terms of residual color (126 Pt-Co unit, 85% removal), residual COD (232 mg/l, 89.49% removal) and residual BOD5 (46 mg/l, 96% removal). Due to this reason, the standards of Case-2 were chosen as optimum condition for tannery wastewater treatment. The criteria of Case-2 can be used for further study or design purpose. Based on the principals of Case-2, the economic feasibility of tannery wastewater treatment plant can also be studied further.