| The control of textile printing and dyeing industry discharges is of paramount importance, for it is the pillar of national industry. Main water regional urban sewage treatment plants discharge limits for major industries of Tai Lake in Jiangsu Province was issued on September 20th, 2007. Discharge limits of CODcr, NH4+-N, and Color were more stringent. In order to meet the new discharge standard, many of the sewage treatment plants should be upgraded. Before the upgrading of sewage treatment processes, it is necessary to optimize the technology as well as the entire process of simulation, in particular the benth BAF tests.In this paper, the hybrid device of A/O combined with BAF was used to treat synthetic printing and dyeing wastewater, and the items were explored as follows: the removal impact factors of CODcr and color of the operating parameters in BAF, the favorite conditions for biofilm formation, decolorizing process, and the kinetic parameters for CODcr removal in BAF and the treatment plant upgrading proposals on the basis of the experimental results obtained are as follows:1.Analysises of the section products in A/O device and the absorption spectrum of the ultraviolet-visible light and infrared spectra, showed the structure of the single azo reactive red X-3B became simpler with acid hydrolysis degradation in the hypoxic conditions was confirmed. At the same time, as the color group of the azo bond (-N = N-) was broken. The mother dye molecules through a series of redox reactions, ring cracked, end products of degradation may be the saturated fatty category of hydrocarbons, alcohols and amines, etc, which showed no maximum absorption peak in the range of ultraviolet - visible spectrum of 200-700 nm, which demonstrated that dye molecules had been degraded thoroughly. The destruction, degradation and absorption of the dye on the molecular structure in this phase performed the removal of color in the follow-up process of BAF, which ensured the discharge standards of final effluent.2. The control factors of BAF technology in the actual scene were explored. Gas water ratio and hydraulic load, the optimum operating parameters were determined: the optimum gas to water was 5:1 to 3:1, hydraulic load about 0.266m3/(m3·h), the flow of water 5.06L/h, HRT 1.18h. At this time the organic load was 0.080 kgCODcr/(m3·h), ammonia nitrogen load was 0.0087 [kgNH4+-N/(m3·h)], the average removal rate of CODcr and NH4+-N were 71% and 87.75%, respectively. CODcr and NH4+-N concentration in the effluent were maintained at 20.24-30.97mg/L and 1.09-1.94mg/L, respectively. The average removal rate of organic load and ammonia load were 72.25% and 87.36%. The average removal rate of turbidity and color were 71.84 % and 81%, respectively. Turbidity and color in the effluent were 4.2-6.2NTU and 2-4 times.3.The removal of organic matter(CODcr), NH4+-N , SS , turbidity , color mainly happened in height of <160cm, 100-160cm, <120 cm , less than 80 cm with more than 140cm, <100 cm of the filter. Judging from the water characteristics of sewage treatment plant secondary effluent in the actual printing and dyeing industry, increased the height (100-160cm) of the BAF was suggested, to enhance the effect and ensure water quality to meet emissions standards effectively.4. High intensity air scrubbing, slightly expansion water washing were used. The optimum wash way was set as first air scrubbing, second combined gas and water washing, and the last water washing. The appropriate intensity of gas is 10-14 L/(m2·s), which lasted for 5 mins in the first phase; the intensity of gas and water is 8-10 L/(m2·s) and 3 L/(m2·s) in the second phase, open the gas and water for washing for 10 min; the appropriate intensity of water is 4-5 L/(m2·s) for 12 mins in the last phase. The backwashing cycle determined is 3-5d, the proposed production of the backwashing control index is waste water turbidity reached 3-5 NTU.5. Through the establishment of the simplified model of BAF, and in accordance with the test results of CODcr changes along the height of the biofilter, the K values under different velocities were obtained by the use of graphic method, and the dynamics model of organic matter removal of BAF was final determined, whichis S=S0e. The practicality of the model was verified through a seriesof experiments, which could provide with a fundamental for actual design and operation.Finally, a retrofit program was developed for improving the water quality of Ni Jiaxiang Group. The separation of low and high contaminated flow, the pollution control of source and the deepening of terminal treatment strategy was selected. Through the choices of combination of filter media and optimization of operating parameters in BAF, the proper use of cinder from the boiler factory and the abandoned constructions as well as the maintenance of old equipment replacement, a sewage treatment plant upgrading program was completed successfully. After one-month regulation of the system and two-month steady operations of BAF with its effective removal of organic matter in the end of conventional treatment process of printing and dyeing waste water. CODcr was bellow 60mg/L and other items were in line with the new discharge standards. The results showed that the retrofit program was rational and feasible.
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