Font Size: a A A

Research On Biologically Pretreated Coal Gasification Wastewater Treatment By Two Stage Membrane Bioreactor

Posted on:2017-01-03Degree:DoctorType:Dissertation
Country:ChinaCandidate:S Y GuFull Text:PDF
GTID:1221330503469915Subject:Municipal engineering
Abstract/Summary:PDF Full Text Request
Coal gasification process played basical and supporting role in the development of coal chemical industry, however, the coal gasification wastewater has been becoming the determining or limiting factor for the sustainable development of coal chemical industry, due to its high pollution intensity which is difficult to be treated. In order to achieve the win-win situation of industry development and environment protection, complete reuse of wastewater with feasible treatment was required. As known that membrane bioreactor(MBR) has now played more and more important role in the wastewater treatment, owing to its highly efficient and stable performance. The present work firstly investigated the influence of coal gasification wastewater loading on the MBR performance, then the two stage MBR process was studied to treat biologically pretreated coal gasification wastewater, in which, the operation parameters, influence factors and mechanism were discussed, in addition, the development and characteristics of membrane fouling were represented and the fouling mechanism was also investigated.The influence of coal gasification wastewater loading on the MBR performance was discussed. The results showed that increasing influent loading could significantly improve the adaptability of the activated sludge to the coal gasification wastewater. In the starting period, huge foam expansion in the MBR was observed, causing the microbes exposure to the air and death which would lead to the fluctuation to the performance, but this issue could be solved by methanol addition, which provided carbon source and energy as co-substrate. MBR hybrid powdered activated carbon(MBR-PAC) represented more efficient and stable performance than conventional MBR, mainly owing to enhanced biodegradability by PAC. When the COD and ammonia loading rates increased to 4.33 kg m-3 d-1 and 0.31 kg m-3 d-1, the removal efficiencies of COD and ammonia in MBR-PAC reached 93.6% and 61.6%, respectively. In addition, PAC could efficiently control the foam expansion and mitigate the serious membrane fouling. Contrast to the conventional MBR, the transmembrane pressure(TMP) jump in MBR-PAC delayed for 15 d and the TMP decreased by 34.2% after 50 d operation. Scanning electron microscopy showed remarkable morphology diversity among the activated sludge, surface fouling and cake layer, additionally, the results showed compaction process along the permeation direction of the membrane, representing the deteriorated pore blocking fouling.The performance and influence factor of biologically pretreated coal gasification wastewater treatment by the first stage MBR were discussed. It was known that this wastewater exhibited low biodegradability and series enhancements have been investigated to improve the first stage MBR performance. The results showed that the methanol addition with 39 mg COD/L, hydraulic retention time of 24 h and granular activated carbon dosage of 5 g/L could significantly promote COD, total phenol and ammonia removal, giving the efficiencies of 73.1%, 41.5% and 40.1%, respectively. The ratio of inorganic carbon(sodium bicarbonate) and influent ammonia of 3:1(mol:mol) could remarkablely improve the ammonia reduction, giving the removal efficiency of 77.2%. Meanwhile, serious foam expansion was not observed in the first stage MBR, which played positive effect on the operation stability. According to the transmembrane pressure variation in the first stage MBR, the membrane fouling of cake layer and pore blocking were exacerbated with the increasing methanol dosage, while the increasing granular activated carbon dosage might mitigate the cake layer and pore blocking fouling, and prolonging hydraulic retention time only showed significant effect on the cake layer fouling control. Additionally, the results showed membrane fouling could be efficiently removed by the hydraulic cleaning with the chemical cleaning(submerged in oxalic acid solution(0.5%) for 1 h and sodium hypochlorite solution(0.5%) for 1 h).The influence of low intensity ultrasound on second stage MBR treating first stage MBR effluent was discussed. With the ultrasound parameters of intensity of 0.4 W/cm2, frequece of 40 k Hz and irradiation time of 35 min, the activity of the microorganisms in activated sludge was significantly improved, mainly owing to increased permeability of the microbe cells and improved enzyme ability induced by cavitation and mechanical effect, and continuous experiments showed that this improvement would not immediately stop when the ultrasound stopped.The influence of ozonation on second stage MBR treating first stage MBR effluent was discussed. The results represented that the ozone dosage, reaction time, p H, suspended solids concentration, ultrasound and addition of catalyst(activated sludge based activated carbon which supported manganese oxide) played important role in the ozonation performance. Increasing ozone dosage and prolonging reaction time both improved the ozonation performance, high p H in the coal gasification wastewater would also improve the ozonation performance while the high suspended solids concentration represented the adverse effect. The ultrasound not only enhanced the ozone molecule transfer rate in the liquid but also improved the ozone molecule decomposition rate, while the catalyst accelerated the ozone chain reaction by the abundant superficial hydroxyl and basic group, resulting in improved oxidation effect by the hydroxyl radical. It was thought that in the advanced oxidation hybrid biological treatment process, the former should be paid more attention on both the COD removal and biodegradability improvement. The results showed the ability of improving biodegradability followed as: catalyst>ultrasound>increasing p H. The results confirmed that catalytic ozonation hybrid second stage MBR could efficiently remove COD with the concentrations lower than 60 mg/L, under the condition of controlling the membrane fouling, which met the requirements of wastewater reuse system for further treatment and this played positive effect on the zero liquid discharge of the coal gasification wastewater.
Keywords/Search Tags:coal gasification wastewater, membrane bioreactor, biodegradation, catalytic ozonation, membrane fouling
PDF Full Text Request
Related items