Evaluation and kinetic study on enzyme supplementation to biological treatment of vegetable tanning process wastewater

Vegetable tannins bind to the collagen of raw hides/skins and transform them into leather. The polyphenolic nature of tannin makes it inhibitory to the microorganisms and hinders the biodegradation process. Studies were carried out by external supplementation of tannase enzyme to enhance the biodegradability of tannins present in vegetable tanning process wastewater (VTW). Aerobic treatment studies were carried out for (1) VTW alone and (2) by external supplementation of tannase enzyme to VTW. The performance of the treatment process was assessed for biochemical oxygen demand (BOD5), chemical oxygen demand (COD) and tannin removal efficiencies, separately. The results indicated that the removal efficiencies of COD, BOD5 and tannin were 27%, 39% and 34.1%, respectively, for VTW alone and 51.4%, 63% and 57%, in case of tannase enzyme-supplemented VTW, for the same residence time of 24 h. Pyrogallol, which is a breakdown product of tannic acid, was found to be 324 mg/L in enzyme-supplemented reactor and 97 mg/L in reactor without tannase enzyme supplementation. Based on the experimental studies, biodegradation kinetic coefficients were derived. In the aerobic treatment process for the tannase enzyme-supplemented VTW, the values derived for maximum specific growth rate (mu(m)), yield coefficient (Y), decay coefficient (K-d), Monod half-saturation constant (K-s) and substrate inhibition constant (K-i) were 0.068 h(-1), 0.436, 0.015 h(-1), 345 mg/L and 46 mg/L, respectively. The role of tannase enzyme supplementation in enhanced removal of tannin content during the aerobic treatment process was confirmed by FTIR and GC-MS analysis.