Environmental Impact Evaluation Of Typical Processes In Leather Manufacture Based On Wastewater Treatability

Leather processing plays an important role to promote the economy prosperity in many developing countries. At the same time, it is an important industrial activity in terms of environment protection, since it is characterized by the emission of large amount of wastewater and solid waste. To reduce this pollutant emission, many new leather-making processes were developed on the basis of leather quality maintenance. This situation is even involved in each unit process of leather processing. Then, to make a preferred selection among these processes for the purpose of minimizing pollution emission from leather manufacture, the environmental impact evaluation of the processes is necessary. On the other hand, environmental impact evaluation is also required to promote the adoption of cleaner technology by tanneries. At present, such an evaluation is becoming increasingly essential for tanneries due to strict pollution and discharge legislations.There are many studies concerning the relation of environmental pollution and leather-making processes. Most of them aim at discussing the increase or decrease of solid wastes and wastewater in quantity, but few is about their quality, particularly, the difference of treating difficulty. To meet the discharging specification, different quality of wastewater from leather-making processes exhibits different treating difficulty, namely different treatability. This has been recognized even in the case that the wastewaters contain equal BOD5 or COD concentration. So, it is of limitation to characterize the pollution property of leather-making processes only based on theanalysis of waste quantity. Therefore, the treatability of wastewater is supposed to be the basis for environmental impact evaluation of leather-making processes. Such a basis is rational and acceptable not only in terms of pollution characterization, but also economics.Traditionally, beamhouse and tanning procedures are the operations responsible for the major part of the pollutants from a tannery. The objective of this study is to evaluate the environmental impact of beamhouse and tanning processes in leather manufacture by exploring the treatability of wastewaters, including settling and biodegrading properties. It is typified in the beamhouse procedure by comparing enzyme assisted soaking and unhairing (ESU) processes with traditional soaking and unhairing (CSU) processes, and in tanning procedure by comparing chrome and vegetable tanning processes. As for the beamhouse procedure, wastewater was collected from soaking and unhairing operations, and was employed for evaluation of pollution characteristics. As for the tanning procedure, the wastewaters generated by chrome and vegetable tanning operations were collected, respectively. The settling and biodegrading properties of all the collected wastewaters were studied for evaluating the environmental impact of relevant leather-making processes.The discharge of CSU and ESU wastewaters, for processing 100 kg wet salted pigskins, was 276L and 218L, respectively. The contents of suspended solids (SS), total solids (TS), biological oxygen demand (BOD5), chemical oxygen demand (COD) and sulfide of CSU wastewater were 4400mg/L, 24000mg/L, 2680mg/L, 6733mg/L and 1020 mg/L, and the contents of ESU wastewater were 4000mg/L, 9200mg/L, 1984mg/L, 4500mg/L and 207 mg/L, respectively. In comparison with CSU wastewater, ESU wastewater had smaller volume and lower load of pollutants. These partially reflect the environmental friendliness of enzyme assisted soaking and unhairing processes in beamhouse procedure in terms of quantity of wastewater.The sludge percentage of ESU wastewater decreased to 16.5% after settling for 1 h, while the percentage of CSU wastewater kept at 18.0% even after 3 h of settling. The removals of SS, TS, COD and BOD5 of ESU wastewater were 80.9%, 42.0%,46.4% and 50.9%, and the settling efficiencies of the pollutants reached 67.6%, 35.1%, 38.7% and 42.5% in 1 h, respectively. But after 18 h of settling, the removals of the pollutants of CSU wastewater were 79.1%, 30.2%, 43.1% and 31.6%, and the relevant settling efficiencies were 66.8%, 25.5%, 36.4% and 26.7%, respectively. It was showed that ESU wastewater required lower dose of coagulant but with smaller sludge volume and higher settling velocity, as well as higher pollutant removals and settling efficiencies than CSU wastewater.The biodegradation of ESU and CSU wastewaters could naturally happen, and 58.2% and 37.7% of the total oxygen-consuming matters could be degraded within 20 days, respectively. In the case of CSU wastewater, a longer lag period resulting from existence of sulfide was observed. Furthermore, the model L1 = L0(1 - e-k{t-t0)) was developed to evaluate the biodegrading property of tannery wastewater by activated sludge, which describes the biodegrading lag time (to), oxygen uptaking constant, ultimate oxygen consumption (L0) and ultimate biodegradation extent (L0/COD). The results showed that the to, k and L0/COD values of ESU wastewater were 1.4h, 0.984d-1 and 65.5%, while those of CSU wastewater were 3.05h, 0.768d-1 and 58.2%, respectively. ESU wastewater showed better biodegrading property.All the results above indicated that, in comparison with conventional soaking and unhainng processes in beamhouse procedure, enzyme assisted soaking and unhairing processes has better environmental friendliness not only in terms of wastewater volume and loads, but in consideration of settling and biodegrading treatabilities of wastewater.In tanning procedure, the discharge of chrome tanning wastewater (CRT wastewater) for processing 100kg wet salted pigskins was 203 L, and it was 167L for vegetable tanning wastewater (VET wastewater). The SS, TS, BOD5, COD and chrome contents of CRT wastewater were 4200mg/L, 23900mg/L, 434mg/L, 4040 mg/Land 5390mg/L, and the SS, TS, BOD5 and COD contents of VET wastewater were 3250mg/L, 11440mg/L, 2100mg/L and 8900 mg/L, respectively. Comparedwith VET wastewater, CRT wastewater could be settled with faster velocity and smaller sludge volume. The sludge percentage of CRT wastewater was only 26% after 400 min of settling, while the percentage of VET wastewater maintained at 27% even after 24h of settling. The removals of TS, SS, BOD5, COD and Cr of CRT wastewater reached 78.9%, 84.3%, 54.1%, 75.6% and 90.1% within 18 h, and the settling efficiencies of the pollutants were 64.1%, 68.8%, 43.9%, 61.4% and 73.1%, respectively. The TS, SS, BOD5 and COD removals of VET wastewater were 44.1%, 83.2%, 45.1%, and 41.3% after 24h of settling, and the settling efficiencies were 32.2%, 60.7%, 32.9% and 30.1%, respectively. In a shorter settling period, CRT wastewater achieved higher pollutant removals and settling efficiencies, exhibiting better treatability in terms of settling property.The BOD5/COD values of CRT and VET wastewaters were 0.107 and 0.236, and their degree of biodegradation were 18.0% and 34.7% after biodegrading by activated sludge for 14 days, respectively. Both cases indicated low biodegradability because of the present of high content of chrome and tannins in CRT and VET wastewaters, respectively. The effect of dilution on the biodegradability of CRT and VET wastewaters were further investigated. The results showed that 10.2% and 47.5% of the total oxygen-consuming matters in the diluted CRT and VET wastewaters (1:9, v:v) were biodegradable ones, respectively. It was also proved that dilution had no substantial effect on the biodegradability of CRT wastewater, but it could largely improve the biodegradability of VET wastewater whose optimal dilution extent for biodegrading was at 1:14 (v/v). Furthermore, the changes of COD and tannin concentrations during the degrading process of VET wastewater were researched. It was indicated that the decrease of COD of VET wastewater resulted mainly from the utilization of non-tannin compositions by activated sludge, but not due to the degrading of tannins. The supematants of CRT and VET wastewaters after settling treatment were further employed for biodegrading tests. The results showed that the biodegradability of CRT wastewater was largely improved when chrome was efficiently removed from the supernatant by settling treatment. 36.9% of the totaloxygen-consuming matters, of the settled CRT wastewater could be degraded by active sludge. Comparatively, the biodegradability of the settled VET wastewater maintained at a higher level, and 56.5% of its total oxygen-consuming matters were found to be biodegradable ones.Generally, CRT wastewater is easier to be settled but with lower biodegradability. So, settling treatment is necessary for CRT wastewater to improve its treatability and to meet the discharging requirement. Contrarily, VET wastewater exhibited lower treatability in terms of settling, but it shows better biodegradability, especially when it was diluted. Usually, tanning wastewater is heavily diluted by other wastewater in tanneries or wastewater treatment plants, and the total treating load has different distribution in settling and biodegrading operations. Considering this situation together with the treating difficulty of chrome-containing sludge from the settling of chrome tanning wastewater, the conclusion could be drawn that vegetable tanning process is the cleaner one, in comparison with chrome tanning process.