The Research And Engineering Application Of Muili-stage Anoxic/Oxic Process Treating Taneery Wastewater

The tannery wastewater is characterized by complicated composition, highcontent of organic substances and ammonia（NH₄⁺-N）, as well containing poisonousand harmful substances, which resulting in difficulty in effective treatment. Afternecessary pre-treatment, Conventional Aerobic Sludge Process or Sequencing BatchReactor or Oxidation Ditch is the predominant techniques to treat tannerywastewater in China.Through the techniques mentioned above, the organicsubstances and poisonous substances in tannery wastewater has been removedhighly, but this was often not so for NH₄⁺-N, especially for nitrogen, which becomesan urgent common problem for tannery wastewater treatment. As the emissionstandard continuously improved, a new economical technique for tannerywastewater treatment with high removal efficiency of ammonia and nitrogen has anurgent need for research and development. Based on the characterization of highcontent of organic substances and ammonia, this paper introduces Multi-stageAnoxic/oxic Process （MAOP） to treat tannery wastewater. The feasibility of theprocess to treatment tannery wastewater has been stuied by bench-scale test, as wellthe key technical parameters been determined. Based on the results, the engineeringdesign of MAOP basing on a pratical tannery wastewater treatment project has beencompleted, as well as the economic cost-benefit analysis.The result of the bench-scale test in a four-stage MAOP with volume of1cubicmeter treating the effluent of first stage biological treatment showed that four-stageMAOP is effective against CODCr, whether or not it is enhanced by step-feeding,which is less than the secondary emission standards of GB8978-1996. The MAOPwith inflow distribution ratio of4:3:2:1and R of100%running at HRT of48h andSRT of18d achieves a ratio of NH₄⁺-N elimination up to97.7%. The effluent ofNH₄⁺-N is about3.6mg/L, which met the primary emission standards ofGB8978-1996. It proved that MAOP was feasible for the treatment of tannerywastewater. The design parameters determined by the test included SRT≥18d,MLSS=3500mg/L, R=50%-100%, the proportion of oxic volume in each A/O stagevolume>1/4. The distribution ratio of influent shloud be determined by nitrogenremoval and the BOD content of influent.Based on the study above and pre-treatment technique, the design of a tannerywastewater treatment project with scale of6500m3/d has been done. In the project,45%of effluent has been discharged and the other reused. The emission standardsare as follows: CODCr≤160mg/L, BOD₅≤30mg/L, TCr≤1.5mg/L andNH₄⁺-N≤35mg/L. The reuse standards are as follows: CODCr≤60mg/L, BOD₅≤10 mg/L and NH₄⁺-N≤10mg/L. By comparison, this paper selected a bonding processof "physicochemical pre-treatment-high concentration aerobic sludge process-MAOP" as the main process to treat tannery wastewater. This is, based on thesegregation of high and low concentration wastewater, first the chrome-bearingwastewater and sulfur-contaning wastewater was pre-treated, then its effluentmixing with other factory wastewater formed comprehensive wastewater whichtreated by the combined process of high concentration aerobic sludge process-MAOP, and finally appling the process integrated coagulating sedimentation, sandfiltration with biological activated carbons to treat the biological treatment effluent.After the complete of process design, structure design, architectural design andelectrical design, the economic cost-benefit analysis has been accomplished. Theresults showed that the total investment of the project with6500m3/d was28.14million Yuan RMB. The running cost of tannery wastewater treatment was5.30Yuan RMB per ton. The CODCr, BOD and NH₄⁺-N removal was21680,7800and439ton per year, respectively. At the same time reuse of make-up process water was1314ton per year, which reduced its spending on process water by2988Yuan RMBper day.