Study On Removal Method Of Organic Complex Chromium In Tanning Dyeing Wastewater And Sludge

The Cr-containing wastewater in the tanning industry mainly comes from chrome tanning,retanning and dyeing process.In these processess,the addition of a large amount of organic chemicals riches,the speciation of Cr in the wasterwater,which makes it difficult to stabilize the concentration of Cr below 1.5 mg·L^-1 with pretreatment of coagulation and sedimentation,and even stays between 0.2-0.5mg·L^-1 after biochemical treatment.It is to clarify the speciation of Cr in Cr-containing wastewater and sludge has been being the primary task of pollution control of the tanning industry aiming at the reduction of Cr.To solve the challenges in Cr-containing wastewater,including of large amount of Cr mud,low efficiency of Cr remocal and high running cost,this study attempted to use the Fenton method and the“pre-biochemical+coagulation sedimentation”process to treat the retanning dyeing wastewater.The“microfiltration+ultrafiltration”separation and classification method combined with infrared spectroscopy and other analytical methods were adopted to compare the composition and de-Cr effect of the Cr-containing wastewater before and after treatment.And the“pre-biochemical+coagulation sedimentation”process was evaluated in terms of microbial ecology,while the effect of acid leaching of Cr in sludge was explored,which provided theoretical and technical basis for the establishment and operation of new innotive treatment process of Cr-containing wastewater.The main findings are as follows:（1）Cr（III）might be oxidized to Cr（VI）when the effluent of biochemical treatment was treated by Fenton oxidation.Therefore,the effect of Cr（VI）conversion under different oxidation conditions was investigated.The results showed that the optimal content ratio of Fenton reagent H₂O₂ and Fe²⁺was 6:1,under which the reduction of TOC and chromaticity reached 23.80%and 94.12%,respectively.The hydroxyl radical destroied the stability of Cr（III）,and a small amount of Cr（III）was oxidized to Cr（VI）.The Cr（VI）could be reduced to Cr（III）again by increasing the H₂O₂ content,so Fenton had its treatment limit on a complete removing of Cr in the wastewater which was stably aroud 0.5 mg·L^-1.（2）The original Cr-containing wastewater,effluents of 24 h biological treatment and coagulation sedimentation were classified by the“microfiltration+ultrafiltration”method.When the it was found that,after the determination of Cr contents from different components,Cr was mainly distributed in three particle sizing regions,that were more than 450 nm,2-3 nm,and less than 2 nm.By characterization of Cr in dyeing wastewater before and after treatment,we demonstrated that the biological treatment performed better on neutral and negative Cr removal,while the coagulation sedimentation method gave a opposite effect.The 3D-EEM and UV-vis were used to detect the 2-3 nm and less than 2nm areas of the original Cr containing wastewater.It was observed that both of them contained the structure of aromatic ring amino acids and the former had lower aromaticity than the latter.Distribution test indicated supporting by IR spectrum and XPS spectrum analysis for Cr and TN in raw water,that Cr in the raw water less than 2 nm was mainly complexed with carboxyl and amino groups.The dyeing Cr-containing wastewater could be stably controlled to 0.5 mg·L^-1 or less by the“pre-biochemical+coagulation sedimentation”process.（3）Based on 16S rDNA high-throughput sequencing technology,the“pre-biochemical+coagulation sedimentation”process was evaluated in the perspective of microbial community changes.The results showed that the entering of Cr-containing wastewater did not decrease the species abundance and distribution uniformity in the activated sludge system,which was even higher than original sludge.In the early stage of Cr-containing wastewater treatment,Proteobacteria was the dominant population in the sludge system with an increasing content along with the biochemical treatment time.The pollutants in the Cr-containing wastewater enhanced the permeability of the microbial cell membrane and substance exchange,although they weakened the metabolic activities of the microbes.（4）The Cr-containing sludge formed by the above-mentioned de-Cr process mainly contained 19 kinds of metals,mainly including Ca,Na,Fe,Cr and Al.The Cr in the sludge was mainly present as acid-soluble state.Four kinds of inorganic acids were used to rinse the Cr-containing sludge,and the optimal condition was under the solid-liquid ratio of 1:30,the concentration of H₂SO₄ and HCl of 0.5mol·L^-1,the concentration of H₃PO₄ and HNO₃ of 1 mol·L^-1,the elution time of H₂SO₄ and HCl of 24 h,and that of H₃PO₄ and HNO₃ of 8 h.The leaching effect of inorganic acid on Cr was arranged in perfomence order from the best to the worst:H₂SO₄>HCl≈H₃PO₄>HNO₃.