An Efficient Approach for Enhanced Biodegradability and Metal Removal From Tannery Wastewaters

This study was conducted to evaluate treatment options for a tannery facility to upgrade their existing facilities to meet the following discharge limits: pH 5-12, COD <450 mg/L, BOD5 <200 mg/L, Cr <1.5 mg/L, sulfide <25 mg/L, and TSS <200 mg/L. Two waste streams (Stream #1 and Stream #2) were received from a tannery facility. Stream #1 had the following characteristics: pH 4-8, TSS 200 mg/L, COD 2,500 mg/L, BOD5 850 mg/L and total Cr <5 mg/L. Stream #2 had a high organic concentration (>15,000 mg COD/L) and high sulfide levels (>2,000 mg/L). A combination of physical-chemical treatment and biological treatment was employed for metal removal and enhanced biodegradability of the tannery wastewater.Physical-chemical treatment included jar tests conducted with and/or without pre-acidification followed by pH adjustment between 9 to10, and screening of different doses of coagulant to optimize pollutants removal. Respirometer tests were conducted to evaluate biodegradability of the organic fraction remaining after physical-chemical treatment. Impact of the oxidized sulfide stream on biodegradability of the organic fractions remaining after physical-chemical treatment was also evaluated. The mixing/blending ratio of the two streams was selected considering a rate of waste generation.Physical-chemical treatment at pH of 9-10 and coagulant (FeCl3) dose of 750 mg/L generated effluent characterized by total Chromium of 0.31±0.15 mg/L, COD of 999±71 mg/L and BOD5 of 468±41 mg/L. Pre-acidification with coagulation-flocculation removed >90% of the particulate organic matter and also improved biodegradability of the waste streams. Analytical results revealed that the BOD5/COD ratio of the physical-chemical effluent improved from 0.32 to 0.48.Biological treatment of physical-chemical treated effluent removed the biodegradable organic fraction and generated an effluent with the following characteristics: total COD 351±38 mg/L and total BOD5 30±5 mg/L. Even though the treatment options attempted in this study reduced sulfide from 272 mg/L to 56 mg/L, did not meet the discharge limit of <25 mg/L. In summary, a combination of physical-chemical and biological treatment was able to generate an effluent that could meet the discharge limit of COD <450 mg/L, BOD5 <200 mg/L, and Cr <1.5 mg/L.