Study on the Treatment of Wastewater from Polyester Fabric Alkali-peeling Process by Hydrolysis-acidification/Hybrid Membrane Bioreactor with Suspended Carriers
|Keywords||PAP-wastewater Membrane bioreactor (MBR) Terephthalic acid (TA) Membrane fouling Carrier Hydrolysis-acidification Biosorption Resources reclaim|
Now the polyester fabric alkali-peeling process wastewater （PAP-wastewater） is one kind of the popular textile industrial wastewater in China due to the high concentration of terephthalic acid （TA） and the poorly biodegradable polyester oligomer and chemical promoters. The treatment of PAP-wastewater by hydrolysis-acidification/ hybrid membrane bioreactor （HMBR）, as well as the membrane fouling control and operational performance of HMBR, was investigated in this dissertation. The following results have been obtained:The biodegradation of TA on aerobic and anoxic condition, as well as the impact of ethylene glycol （EG） on the biodegradation of TA, was researched. It was found that TA was not toxic to the TA degrading cultures on the aerobic condition, whether the concentration of TA was high or low. There was a lag phase prior to degradation of TA and EG when TA and EG were fed as sole carbon source. Furthermore, the lag period of EG was longer than TA. However, once the cultures were acclimated, TA and EG started to be degraded rapidly. Although EG didn’t inhibit the aerobic primary biodegradation of TA, it inhibited the aerobic ultimate biodegradation of TA. EG inhibited the anoxic biodegradation of TA, but TA was consumed at the same rate as in the experiments performed with TA as sole carbon source after complete consumption of EG It can be concluded that TA is easily degradable on aerobic condition and more difficult to be degraded on anaerobic condition and hardly degradable on anoxic condition, while EG is easily degraded on aerobic, anoxic and anaerobic condition.The result showed that the biosorption and desorption of TA by unadapted aerobic and facultative sludge could be characterized by Freundlich isotherm. Two Freundlich equations, q=8.6170Ce0.4207 and q=4.0764Ce0.5405, were established to describe the biosorption of TA by unadapted aerobic and facultative sludge, respectively. So q=9.4723Ce03271 and q= 4.2353 Ce03688 for desorption of aerobic and facultative sludge, respectively. The maximum biosorptive uptakes of TA by aerobic and facultative sludge were 39.06, 31.45mgTA.gSS"1, respectively. The difference between biosortion and desortion Freudlich constants suggested that biosorption of TA was an partly irreversible process. In bioreactor the removal of TA by activated sludge was due to biodegradation rather than biosorption.The experiment demonstrated that the hydrolysis-acidification/ HMBR process was an economically attractive alternative for the treatment of PAP-wastewater. The effluent COD average 55.9mg/L and the following results were obtained at HRT =（9.0+7.2）h: t)Cod=96% and tita>99% for the total system, r|CoD<10% and TyrA<5% for hydrolysis-acidification bioreactor, t|cod>94% and t|Ta>98% for HMBR. The hydrolysis-acidification pretreatment improved the biodegradability of PAP-wastewater. The impact of volumetric loading rate （Uv） was slightly on the performance of hydrolysis-acidification bioreactor, but largely on HMBR. The effluent COD was below 80 mg/L and ticod averaged 95% for HMBR when Uv of HMBR was below egCOD.I/’.d"1.The observations revealed that suspended carriers played an important role in governing the filtration conditions and decreasing fouling resistance. The results indicated that the cake resistance of HMBR decreased by 86%, the critical flux increased by about 20% and the rate of membrane fouling decreased by 70% in comparison with MBR. The effects of the membrane flux （J）, sludge concentration （X） and carriers volume （C） on the increasing rate of the transmembrane pressure were in decreasing order: C<X<J. The addition of suspended carriers changed the mechanism of membrane fouling in HMBR. The cake resistance was the main contributor to the total resistance in MBR process, while the pore blocking resistance was the major contributor in HMBR. Furthermore, the addition of carriers increased the species of the microbial community in HMBR.Comparison between membrane bioreactor （MBR） and hybrid membrane bioreactor （HMBR） was carried out under similar operational conditions. It was indicated that a long membrane-washing period could be achieved only when the membrane flux was below the critical flux. During the long-term running, the transmembrane pressure （TMP） of HMBR increased slower than MBR. In the experiment, the sludge concentrations of MBR andHMBR stabilized at about 14gMLSS/L. No obvious trend of MLVSS/MLSS was observed, but it was found that the particle size of sludge floes decreased with operation time. Filtration resistance of mixed liquor of MBR increased slower than HMBR with operation time. The results revealed that the colloids in the supernatant were the main contributors to the rapid increasing of filtration resistance of mixed liquor. The soluble microbial products （SMP） could accumulate in bioreactor, but also its components could change. The sludge dehydrogenase activity was measured in the long-term running and it was found that it decreased with operation time.On the condition of no sludge wasted, the theoretical yield factors （Y） and the biomass decay constants （Kj） for bioreactor in MBR and HMBR were below that in the traditional activated sludge process: Y（MBR）= 0.315, Y（HMBR）= 0.301mgVSS/mgCOD, Kd（MBR）= 0.03 Id"1, Kd（HMBR）= 0.035 d"1. On the condition of SRT=50d, Y and Kd for bioreactor in MBR and HMBR corresponded to that in the traditional activated sludge process: Y（MBR）= 0.375, Y（HMBR）= 0.353mgVSS/mgCOD, Kd（MBR）= 0.071 d"1, Kd（HMBR）= 0.067 d"1. The substrate degradation rate constant （K） of TA artificial wastewater, （TA+EG） artificial wastewater and PAP-wastewater were 0.001, 0.00078 and 0.00072d"\ respectively. Compared with K （0.004-0.03） in the traditional activated sludge process, the K value in HMBR （or MBR） was very low.The acid precipitation pretreatment was adopted before the biodegradation of PAP-wastewater in terms of the resource reclaim and reuse of wasted resource. The parameters of acid precipitation process were obtained as follows: pH3.5-4.0, normal temperature, stirring at reaction for 1-2 minutes with 300rpm and at flocculation for 10 minutes with 60-100rpm, sedimentation for 12h. The particle size of TA floes reclaimed by acid precipitation averaged 11.56um. The recovery rate of TA was 77% and the removal rate of COD arrived at 50.5%. The B/C value of PAP-wastewater before and after acid precipitation were 0.37 — 0.40 and 0.27-0.31 respectively, which indicated that TA improved the biodegradability of PAP-wastewater. There was 78% effluent COD below 50mg/L and the following results were obtained: ticod=97% and t|ta>99% for the total system. It should be noted that the relative contributions of hydrolysis-acidificationbioreactor and HMBR to the total removal rate were 29.8*^ 67%.