Study On The Improvement Of Biodegradability Of Papermaking Tobacco Sheet Wastewater By Electrochemical Techniques

Reconstituted tobacco sheet （RTS） is made via recomposing and processing by using tobacco stems, leaf scraps, tobacco dust and some parts of low-grade tobacco leaf. RTS is widely used as one of an important material of cigarettes and cigars production. It was estimated that about60to80tons of wastewater will be generated during papermaking process when one ton of reconstituted tobacco sheet is produced. Papermaking tobacco sheet wastewater was characterized by recalcitrant chemical oxygen demand （COD）, high content of fine suspended solids （SS）, intensive color and low biodegradability. Currently, the main treatment method of this wastewater is biological method, but nicotine, solanone and suspended solids existed in tobacco sheet wastewater are biorefractory and can inhibit the bacterial activity of pollutant degradation, resulting in the poor performance of treatment, significant color residue in the discharged effluent and lower adjustability of the water quantity. For this reason, exploring a simple and cost-effective pretreatment process to improve the biodegradibilty of wastewater is crucial for conventional biological treatment. Considering the characteristics of paper-making tobacco sheet wastewater, chemical coagulation, electrocoagulation, electrochemical oxidation, and electrocoagulation combined with electrochemical oxidation were applied to treatment the papermaking tobacco sheet wastewater. The operating parameters affecting the performance of wastewater were investigated and optimized. The treatment efficiency and improvement of biodegradatibilty under the optimized conditions were compared, according to the analysis results of gas chromatography-mass spectrometry （GC-MS）, the removal mechanisms of pollutants were also proposed. The following conclusions may be obtained:1. Treatment of tobacco sheet wastewater by chemical coagulation was investigated, polyaluminium chloride （PAC） and aluminum sulfate （Al₂（SO₄）3） were used as coagulants and polyacrylamide （PAM） was used as coagulant aid. The results showed that PAC was more suitable for treating this wastewater than A12（SO₄）3.64.1%of COD_t,6.6%of COD_s,96.5%of turbidity and84.0%of suspended solids （SS） could be removed under the following optimum conditions:1500mg/L of PAC,20mg/L of PAM and initial pH5. The effluent was yellow, the transmittance value and color of was74.8%and50times, respectively. Thus, coagulation could effectively remove the large amount of refractory suspended solids by neutralization and imcreased the BOD₅/COD ratio from0.067to0.312, however, some dissolved organic compounds are difficult to remove through coagulation process. The cost is7.19RMB for per ton wastewater and the corresponding sluge was0.322m³. 2. Electrocoagulation was applied to treat the papermaking tobacco sheet wastewater, the results showed that aluminum is more appropriate for anode than iron in terms of transimittance value of effluent and stainless steel net is more effective for turbidity and SS removal than aluminum as cathode.66.1%of COD_t,25.7%of COD_s,97.5%of SS,98.1%of turbidity could be removed by electrocoagulation using aluminum anode and stainless steel net cathode under40mA/cm2of current density,2.5g/L of NaCl,6min of reaction and without adjusting the pH of wastewater. The effluent was transparent, the transmittance value and color of was85.5%and25times, respectively. Therefore, electrocoagulation can remove not only SS through precipitation but also part of dissolved organic pollutants through adsorption of flocs, the BOD₅/COD ratio of wastewater increased from0.075to0.426,40.5%of nicotine,21.4%of β-nicotyrine and47.5%of solanone could be removed, whereas the organic acids such as oleic acid and palmitic acid could not be obviously eliminated. The cost including the consumption of anode and energy is4.35RMB for per ton wastewater and the corresponding sluge was0.201m3.3. Electrochemical of papermaking tobacco sheet wastewater was carried out using (3-PbO₂and Ti/RuO₂-IrO₂as anodes and stainless steel net as cathode. In the case of (3-PbO2anode,55.5%of COD_t,15.5%of COD_s,75.2%of turbidity and69.3%of SS could be removed under the following conditions:50mA/cm2of current density,1.5g/L of NaCl, initial pH5and30min of reaction. The effulent is no color and the transmittance value was71.7%. While Ti/RuO₂-IrO₂is used as anode,51.2%of COD_t,9.8%of CODS,81.2%of turbidity and75.3%of SS could be removed without adjusting the pH of wastewater,50mA/cm²of current density,1.5g/L of NaCl and30min of reaction. The effluent is light brown and the transmittance value was54.0%. Although electrochemical oxidation is less effective for the removal of turbidity and SS, but β-nicotyrine, solanone and other recalcitrant components can be converted into small molecule organic acids induced by the hydroxyl radical electrogenerated. As a result, the color and the toxicity of wastewater reduced, the BOD₅/COD ratio of wastewater increased from0.06to0.496（β-PbO₂as anode） and0.467（Ti/RuO₂-IrO₂as anode）. Nicotine is can be detected in the effluent of Ti/Ru₂2-IrO₂treated, this may be due to the lower oxidation ability. The cost mainly included the consumption energy per ton wastewater is14.47RMB and14.45RMB, the corresponding floated sludge were0.078m3and0.089m3, respectively.4. The papermaking tobacco sheet wastewater was firstly subjected to elcetrocoagulation process using aluminum anode and stainless steel net cathode under40mA/cm²of current density,2.5g/L of NaCl,6min of reaction and without adjusting the pH of wastewater. Then the effluent was withdrawn and treated by electrochemical oxidation. In the case of β-PbO₂anode, the final COD removal reached83.9%and the BOD₅/COD ratio increased from0.061to0.852under20mA/cm, pH5and30min of reaction. When Ti/RuO₂-IrO₂is used as anode, the final COD removal was82.8%and the BOD₅/COD ratio increased from0.061to0.804. Therefore, SS and recalcitrant components could be effectively removed by electrocoagulation combined with electrochemical oxidation process, resulting in the significantly improvement of biodegradability of wastewater. Additionally, the removal of SS during elctrocoagulation was benefical to reduce the resistance of mass transfer of electrochemical oxidation and reduce the consumption of energy. The cost for per wastewater was6.96RMB （β-PbO₂as anode） and9.34RMB （Ti/RuO₂-IrO₂as anode）.