外文原文-水中膜微滤技术在玻璃生产废水回用中的应用：试验性规模的测试与膜去除技术.pdf

Desalination 189 2006 170 180 0011 9164 06 See front matter 2006 Elsevier B V All rights reserved Corresponding author Use of submerged microfiltration membranes for glass industry wastewater reclamation pilot scale testing and membrane cleaning Suck Ki Kang Kwang Ho Choo Department of Environmental Engineering Kyungpook National University 1370 Sankyeok Dong Buk Gu Daegu 702 701 Korea Tel 82 53 950 7585 Fax 82 53 950 6579 email chookh knu ac kr Received 12 January 2005 accepted 27 June 2005 Abstract Pilot scale microfiltration MF tests were carried out to investigate the feasibility of submerged MF systems for the reclamation and reuse of glass industry wastewater To compare the effect of membrane modular types on flux plate and tubular modules were immersed in a single reactor simultaneously and their performances were evaluated in terms of water quality and membrane permeability The quality of treated water membrane permeate met the required water reuse standards with regard to turbidity and conductivity When continuous and intermittent MF operations were conducted at a constant flux mode the effectiveness of air scouring in controlling fouling was influenced by the module geometry and operating conditions The plate module gave a better performance because air bubbles entered its feed side while dislodging the accumulated particles To clean the fouled membranes various methods using different chemicals such as acidic caustic hypochlorite and chelating agents were compared in conjunction with or without ultrasound It was found that sonication in a caustic solution achieved maximal flux recovery of more than 95 This approach was further optimized to reduce the volume of the cleaning solution and the time for chemical rinsing Based on cost analyses it was demonstrated that MF systems for wastewater reclamation would have a reasonable payback time on investment should the current system be replaced by the proposed system Keywords Submerged microfiltration Glass industry Wastewater reuse Membrane cleaning doi 10 1016 j desal 2005 06 029 S K Kang K H Choo Desalination 189 2006 170 180171 1 Introduction Wastewater reclamation and reuse is of great interest and a viable option for many industrial sectors and countries which are suffering from water scarcity problems 1 7 Existing conven tional wastewater treatment processes which are composed of coagulation sedimentation filtra tion etc are often insufficient to meet the water quality required for reuse Thus membrane tech nologies are being considered as a reliable alter native for wastewater reclamation applications since they could play a key role in the removal of various physical chemical and microbiological contaminants in wastewater 5 8 12 A wide range of membrane processes from microfiltration MF to reverse osmosis RO filtration are cur rently available for wastewater reclamation de pending on the target compounds to be removed Dense membrane processes such as nano and RO filtration can remove even dissolved ions but at the cost of high energy consumption 12 14 Thus industrial plants are reluctant to adopt such energy intensive processes In this context MF or ultrafiltration UF appear to be more attractive only if the separation of particulate contaminants is concerned because they promise higher fluxes at relatively low pres sures 15 17 MF UF membranes have been widely applied as a pretreatment step to desalina tion or organic removal in place of conventional clarification or sand filtration processes 18 20 However in certain cases MF UF alone or in combination with physico chemical or biological treatment such as coagulation adsorption micelle formation and activated sludge processes 16 21 25 have provided reclaimed wastewaters that were able to meet the established wastewater reclamation criteria For instance membrane bio reactors MBRs which normally consist of an activated sludge tank plus a membrane have be come very popular for removing organic matter from municipal and industrial wastewater 5 11 24 26 Membranes had successfully replaced the settling clarifier and thereby it was possible to operate the system without any concern of biomass settling leading to a significant improve ment in treatment efficiency in terms of the removal of organics and colloidal particles In Korea water shortages are expected by the year 2006 because of increased water demand so currently the water management rule mandates that any plants or buildings consuming more than 1 500 m3 d of water must have a wastewater reuse facility Moreover the charge of process water supplied for companies located in the area of the Nakdong River was raised by one third since a special rule recently became effective To over come these issues a glass related industrial plant which generates 15 000 m3 d of wastewater containing particles from the grinding process of glass are considering to retrofit or replace the existing coagulation and fluidized sand filtration by MF 27 Since the current processes cause some problems related to the increased conduc tivity of treated water and the insufficient removal of particles MF would be effective for such water reclamation purposes as shown in Fig 1 Therefore in the current study a pilot scale MF system which includes two types of sub merged membranes such as tubular and plate mod ules was tested and its membrane performance was evaluated in terms of treated water quality and membrane permeability Additionally various membrane cleaning methods with acidic and caustic solutions chelating agents ultrasound or combinations thereof were applied and compared in terms of flux recovery for the selection and opti mization of effective membrane cleaning stra tegies 2 Materials and methods 2 1 Wastewater The glass industry wastewater used in the current investigation was generated by Samsung Corning Co Ltd Gumi Korea Fig 1 shows the existing treatment plant which is composed of coagulation sedimentation and sand filtration 172S K Kang K H Choo Desalination 189 2006 170 180 Since fine clay particles are added to polish the glass surface wastewater always contains a mix ture of clay and glass particles discharged during the grinding process in the production of CRT glass Raw wastewater was directly transferred through a pipeline to the pilot scale membrane unit but some of the wastewater effluent treated by the current treatment processes was recycled to the manufacturing process at the time of pilot scale testing The key quality characteristics of the glass industry wastewater are presented in Table 1 Wastewater samples collected were also transported to the laboratory and used for mem brane fouling and cleaning experiments 2 2 MF membranes and pilot scale system operation The pilot scale membrane system used in this study is shown in Fig 2 The system was com posed of a tank with total volume of 9 4 m3 and Fig 1 Flowchart of glass industry wastewater treatment involving a existing treatment processes and b a proposed submerged membrane system Al2 SO4 3 NaOH Polymer Wastewater Collection Tank Coagulation Sedimentation Sand Filtration Effluent Reservoir Discharge Permeate Reservoir Recycle MF UF Wastewater Collection Tank a b Table 1 Average quality of glass industry wastewater tested Item Value pH 8 6 Chemical oxygen demand mg L pH 10 Consequently it was concluded that submerged MF techniques with on site sonication are a promising option to make the MF system feasible for wastewater reclamation 3 4 Optimization of membrane cleaning The variation of cleaning efficiency was exa mined using a NaOH solution with ultrasound while repeatedly performing stirred cell MF and cleaning Fig 7 The cleaning efficiency decreas ed gradually with the repeated number of batches of MF but it was recovered by multiple cleanings more than 100 recovery indicated the removal of residual foulants that had not been rinsed away at the first conditioning step In addition it seemed that the volume of the cleaning solution was more responsible for flux recovery than the duration of cleaning applied Further optimization of the cleaning method is needed with respect to the time and solution volume required for clea ning Fig 8 shows the effect of different combina tions of the time and volume on membrane clean ing Conditions A and B were able to maintain nearly the same level as the initial recovery while Condition C was insufficient to recover the flux during the repeated MF Condition D that has the more number of cleaning with the smaller volume of cleaning solution was most effective in mem 0 50 100 150 200 0246810 Number of Filtration Run Flux Recovery by Cleaning A B C Fig 7 Flux recovery with sucessive MF under ultrasound using 1 mM NaOH with different volumes and duration A 50 mL and 30 min B 50 mL and 60 min C 100 mL and 30 min Data points in the vertical line indicate the flux recovery after additional cleaning without any MF of wastewater 0 50 100 150 0246810 Number of Filtration Run Flux Recovery by Cleaning A B C D Fig 8 Variation of flux recovery after chemical cleaning under ultrasound using a 1 mM NaOH solution when the respective volume duration repetition number for clean ing after each MF were as follows A 50 mL 30 min 2 times B 50 mL 15 min 2 times C 25 mL 30 min 2 times D 25 mL 30 min 3 times brane cleaning These results suggest the fact that the volume of the cleaning solution plays a more important role in cleaning than the duration of 178S K Kang K H Choo Desalination 189 2006 170 180 cleaning Additionally the cleaning efficiency can be satisfied as long as the membrane is rinsed multiple times even with a smaller volume of the solution The results help minimize the usage of cleaning solution and so make a better strategy for chemical cleaning Also the chemical dosage normalized per unit membrane area might be applied in pilot or full scale cleaning considering the base demands of the wastewater 3 5 Cost evaluation for the application of MF systems for wastewater reuse In order to compare the costs of wastewater reuse options pay off time POT which is also called investment recovery time is defined as follows POT OC I S where I is the net investment needed for waste water reuse using a new process S is the net savings with the new process and OC is the net operating costs with the new process Table 4 compares the POT values of the two options when the existing current treatment process is rebuilt with small modifications and is completely re placed by a new system MF system The new MF system has higher costs in investment and electricity but the costs of cleaning chemicals only NaOH cleaning was considered in this analysis will be negligible because only small amounts of chemicals are used for membrane cleaning For example the price of 1 m3 of 12 5 N NaOH is US 128 so the total yearly cleaning cost is estimated to be US 100 assuming that approximately 130 L of 1 0 mN NaOH per 1 m2 of the membrane is used for one month which is considerably large in its usage for extensive cleaning With MF water savings are substantial assuming that 75 of treated water permeate is recycled Further recycling would increase the savings and thereby reduce the POT value Option 2 If the current system is substituted with Table 4 Comparison of cost analyses for installation of current and new systems Parameter Option 1 current system Option 2 new MF system Investment 6 000 000 10 200 000 Operating cost y Chemicals 189 312 100 Electricity 153 816 412 971 Membrane replacement y 0 150 000 Labor cost y 20 000 20 000 Water saving y 670 688 2 012 063 Pay off time y 20 3 75 the same equipment and process for wastewater reclamation Option 1 water recycling is limited maximum recycle cannot exceed 25 of treated water Therefore the POT value would be estimated to be extremely long 20 years which indicates that return on the investment is not possible at all with Option 1 However if the MF system is employed Option 2 the POT value could become much shorter approximately 3 75 years The investment recovery time of 3 75 years at 75 recycling is comparatively rea sonable while that at 100 recycling which is estimated to be 2 34 years could make the system more feasible Thus it is concluded that Option 2 which is considering MF systems would be much more attractive for wastewater reclamation in the glass industry than Option 1 as long as the current system needs to be replaced in any ways Although the advantage of Option 2 is obvious further work on the investment should be done before such a decision is made For instance more extended 6 months pilot tests using a MF system with a treatment capacity of more than a few hundreds tons of wastewater per day are needed along with in depth cost analyses If just part of the existing treatment plant is replaced or retrofitted with MF another cost evaluation for the modification might be also needed Overall however the conclusion made above would be valid though the detailed S K Kang K H Choo Desalination 189 2006 170 180179 results such as investment and POT can vary depending on the treatment capacity 4 Conclusions The reclamation of glass industry wastewater was carried out using submerged MF membranes in pilot scale plate and tubular modules The sys tem performances were evaluated in terms of treated water quality and membrane permeability during two month pilot scale operations The effect of various chemical cleaning methods on flux restoration was also explored with different combinations of chemicals and ultrasound in the laboratory scale experiments The pilot scale MF system was able to produce treated water per meate of high quality so it could satisfy the required water quality criteria for the manufac turing process MF should be operated at a flux of lower than 10 L m2 h to maintain the transmem brane pressure stably which is comparatively low but still reasonable Although extensive aeration was ineffective in removing adhesive particles once attached the plate module performed better than the tubular one because air bubbles can enter the feed side more easily An intermittent operation at a lower flux helped to prevent membrane foul ing depending on the modules tested For effective cleaning of the fouled membrane more than 90 flux recovery cleaning should be done using ultrasound under an alkaline solution pH 10 where silica particles can dissolve more readily The volume of solution and the number of repeti tion for chemical cleaning after each MF were more important than the duration of each cleaning The installation of MF systems for wastewater reclamation would return the investment reason ably and is worth considering if the current system should be replaced However further studies on employing larger scale MF systems at longer operation times are needed before a final decision is made from a practical point of view Acknowledgements Part of this work was supported by the Kyung pook National University Research Funds 2004 The authors thank Mr Ji Sun Ryu of Mirae Engi neering Co Ltd and Mr Jin Soo Park of Samsung Corning Co Ltd for their help in installing and operating the pilot scale MF system References 1 R Muje