Study on the Leather Industrial Effluents: Chemical and Microbial Approaches to Control Environmental Pollution

Abstract

Effluents from raw hide processing tanneries, which produce wet-blue, crust leather and finished leather contain high levels of inorganic compounds including trivalent chromium (Cr) and sulphides in most cases. Organic and other ingredients are responsible for high BOD5 (Biological Oxygen Demand in 5 days) and COD (Chemical Oxygen Demand) values and represent an immense pollution load, causing technical problems, sophisticated technologies and high costs in concern with effluent treatment. In Bangladesh, most of the leather industries discharge the effluent without any treatment into a nearby water body and cause significant pollution. The present study was conducted aiming to characterized the tannery effluents and evaluate the performances of an innovative tannery wastewater process based on a combined physico-chemical and biological degradation. In this study, effluents were collected from different manufacturing units (MU) in the three selected leather industries and also collected composite effluents from three monitoring points around the effluents dumping area situated at Hazaribagh industrial zone in Dhaka, Bangladesh during the year 2008-20 I 0. The effluents were stored and analyzed for various physic-chemical parameters. The study also evaluated the performance of treatment processes combining of physico-chemical and microbial approaches to reduce toxic elements from the tannery effluents. The analysis results illustrate that the collected tannery effluents at different manufacturing units (MU) in the three selected leather industries contained extremely high values of TSS, TDS, TS, BOD5, COD, so/·, Na, Ca, Cr, As, Cd and Pb. The results show that the highest value of so/· and As were found in the liming and un-heiring effluents, the highest values of TDS, er, Cr, Cd and Pb were found in the pickling and chrome tanning effluents and the highest value of COD and BOD5 were found in the retaining and dyeing effluents. These values were far above the standard permissible limits for ISW in BDS, ISI-2000 and USEPA-2000. The mean concentration of TSS, TDS, so/·, er, BODs, COD, Na, Cr, As, Cd and Pb in the collected composite tannery effluents in all monitoring points were also exceeded the standard disposal limits. The concentration of metal ions in the composite tannery effluents were observed in the order Na> Mg> Ca> Cu> Zn> Cr> Pb> Ni> Cd> Mn> As. The pollution levels at the three monitoring points was in the order P1 > P2> P3. The study also reveals that the concentration of various metal ions in the sludge contained very high values, and in particular, the concentration of Cr and Na ions were about 10000 and 6000 mg/Kg, respectively. The concentration of ions in the sludge was in the order: Cr> Na> Ca> S> Mg> P> Cu> TN> Zn> Organic compound> Pb> As> Cd. The analysis results of physico-chemical parameters suggest that the untreated leather industrial effluents contained very high level of toxic pollutants and could not be discharged into surface water and land, which pose potential threats to human health, aquatic life and the entire environment. This study was also conducted a series of treatment processes consisted of physical, chemical and biological in order to reduce the toxic pollutants level. The collected composite effluent was allowed to settle for 1 day and filtered through different filtered media namely (i) sand-stone (ii) saw-dust and (iii) sand-stone combined with saw dust. The analysis results of the filtered effluents suggest that sand-stone filtered media could be used to reduce pollution to certain levies, but the efficiencies were not enough to consider its alone use. The study also evaluated the performance of removal efficiencies of selected coagulants namely, FeC'3, Fe2O3 and FeSO4• The study optimized the coagulant dose and pH for each coagulant. The coagulants FeC'3 and FeSO4 showed maximum removal efficiencies for majority of the parameters at an optimum dose of 150 mg/L and neutral ph. The other coagulant Fe2O3 showed the maximum efficiency at a dose of 100 mg/L and pH around 9. The analyses results reveal that removal efficiency of color and TSS achieved about 100% for all coagulants. The coagulant Fe2O3 removed Cu and Cr almost 100%. The mean removal efficiency for all parameters were 91 %, 89% and 85%, for the coagulants Fe2O3, FeC'3 and FeSO4, respectively, indicating that Fe2O3 showed the best removal performance among the coagulants. It may be mentioned that Fe2O3 showed lower removal efficiency only for NH4-N and TN compared to the coagulant FeC'3. The overall removal performance of the coagulants was in the order Fe2O3> FeC'3> FeSO4• The analysis results of the chemical treated effluents illustrated that most of the physical and chemical parameters were found well below the prescribed limits. The study also carried out microbial treatment with bacterial species for the synthetic and composite untreated tannery effluents to remove heavy metal ions. The isolated bacteria showed resourceful tolerance against some selected heavy metal ions, i.e., Cr, Cd and Pb without producing any byproduct. The isolated metal ions tolerance bacteria were applied to synthetic and composite tannery effluents. The study results revealed that Cr concentration in the treated synthetic effluent was found almost "zero" (0) mg/L and the removal efficiency was about 100% using isolated bacteria at a culture dose of 3ml (OD-1) at pH 7.2 and 37°C for an inoculation period of 10-15 days but the Cr removal efficiency showed lower, i.e., 84%, when treated the composite tannery effluent with the same bacterial isolates under the same inoculation conditions. The strain shows comparatively lower efficiencies for degrading metal ions of Cd and Pb. The bacterial isolate was identified as Enterobacter species by 16S rRNA sequencing and biochemical studies. The study observed that the microbial strains (Enterobacter species) were successfully grown and enrichment in the presence of heavy metal ions and thus could be used in degrading heavy metal ions such as Cr, Cd and Pb from the discharged tannery effluents. Finally, the study concludes that a combine treatment approaches, i.e., physical, chemical and microbial processes could be applied for tannery effluents to reduce environmental pollution and save the water bodies from further deterioration.