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Research Progress on the Treatment of Dyeing and Printing Wastewater Using Membrane Bioreactors Liu Hao, Zhou Yuan, Wang Jun, Xi Danli, Gao Pin (School of Environmental Science and Engineering, Donghua University, Shanghai 201620) Abstract: Dyeing and printing wastewater is a type of industrial wastewater characterized by high organic content, high color intensity, poor biodegradability, and difficulty in degradation. It is difficult to meet discharge standards using traditional wastewater treatment processes; as a result, there is increasing interest in researching new and efficient treatment methods. The membrane bioreactor is a new type of water treatment technology that possesses advantages unmatched by traditional wastewater treatment processes. The printing and dyeing wastewater treated using this technology can even meet the standards for reused water. Based on this, this paper provides an overview of the improvement techniques for membrane bioreactors that are currently under extensive research, and identifies the issues that still need to be resolved, offering a reference for future developments. Keywords: membrane bioreactor; printing and dyeing wastewater; improvements to membrane bioreactors. Chinese Library Classification Number: X703.1 Document Code: A. Printing and dyeing wastewater is extremely difficult to treat due to its low visibility, toxicity, and even the presence of low concentrations of dyes. Common treatment methods include adsorption, coagulation, chemical oxidation, and biological methods. The regeneration or replacement of adsorbents in the adsorption method is rather troublesome; the coagulation method generates a large amount of chemical sludge, while the chemical oxidation method is costly. Biological treatment has become the most widely used technology for treating dye wastewater at present, due to its low treatment costs and stable operation. Single aerobic or anaerobic biological treatment can only remove a portion of the organic matter in wastewater, and it cannot address the issue of color. To effectively remove the refractory organic pollutants and color in wastewater, an anaerobic-aerobic combined treatment process has been developed. Generally, the hydraulic retention time in the anaerobic tank is 4–12 hours, during which only hydrolysis and acidification occur. Difficult-to-degrade dyes are hydrolyzed and acidified under the catalysis of decolorizing bacteria in anaerobic reactors, breaking down into small organic molecules that are further utilized by aerobic bacteria in the subsequent aerobic reactors, thereby achieving the degradation of the dyes. Studies show that the anaerobic- aerobic process achieves removal rates of 83%~97% for COD and 80%~87% for color intensity. With the increase in the variety of synthetic fibers in recent years, as well as the use of chemical pulps such as PVA, the COD level in printing and dyeing wastewater can reach 2000–3000 mg/L, making it very difficult for wastewater treated by traditional biological methods to meet the discharge standards. To improve the treatment efficiency of biological treatment systems for printing and dyeing wastewater, researchers have conducted various studies on them, and membrane bioreactors are one of such approaches. 1 Membrane bioreactor: A membrane bioreactor is a treatment process that combines membrane separation technology with biological treatment technology. The efficient retention capability of the membrane components increases the concentration of the mixture within the bioreactor, allowing for the separation of hydraulic retention time from sludge retention time. This prolongs the sludge age, enabling the concentration of numerous microorganisms that are less capable of surviving, thereby improving the ability to treat recalcitrant organic substances. Therefore, in the treatment of printing and dyeing wastewater, membrane bioreactors possess advantages that are unmatched by other processes. 1.1 System performance The main challenges in treating printing and dyeing wastewater are the removal of chroma and refractory organic substances. Cai Huiru and others conducted a comparison between membrane bioreactors and the SBR method for treating dye wastewater. The results showed that, under the same water quality conditions, the membrane bioreactor achieved 5–10% higher COD removal rates and decolorization efficiency compared to the SBR method. Z. Badani et al. used a separate membrane bioreactor to treat printing and dyeing wastewater, achieving removal rates of over 97% for COD and over 70% for color intensity. Tong Zhi et al. used A/O membrane bioreactors to treat printing and dyeing wastewater; when the hydraulic retention time was 9–10 hours and the inlet COD and colority were 1500–2300 mg/L and 800–1200 times respectively, the removal rates for COD and colority reached 95% and 90% respectively. The research results of Zheng Xiang et al. showed that when the HRT was 7 hours and the inlet COD and BOD5 were 179–358 mg/L and 44.8–206 mg/L respectively, the average removal rates for COD, BOD5, colority, and turbidity were 92.1%, 98.4%, 60.7%, and 98.9% respectively, with the effluent meeting the standards for miscellaneous water use. Due to the wide variety of pollutants in printing and dyeing wastewater, its complex composition, and the significant variations in the composition and volume of the wastewater as it is discharged in an intermittent manner, high shock loads are generated, placing high demands on the treatment systems. Studies by M. Brik et al. show that when the COD of the influent water ranges from 1380 to 6033 mg/L and the pH is between 6.36 and 9.67, the COD removal rate is 60% to 90%, while the color intensity is reduced by more than 87%. Xiong Xiaojing et al. studied the effect of the influent pH value on the performance of A/O membrane bioreactors in treating printing and dyeing wastewater. When the influent pH value varied within the range of 5.0–9.0, the pH value in the anaerobic tank remained stable around 6.0, which allowed the pH values in the aerobic tank and in the effluent of the system to remain around 7.0 and 7.5 respectively. This ensured that the entire system operated at its optimal capacity for degrading dyes and COD. Studies by Qing Chunxia and others have shown that when the COD level in the influent water ranges from 208.54 to 2592.00 mg/L and its color intensity is between 52 and 256 times, the hydrolysis-acidification membrane bioreactor system maintains removal rates of 80%–90% for COD and 87.5% for color intensity. These removal rates are significantly better than those achieved by the on-site wastewater treatment plants during the same period. Based on the above research results, it can be seen that membrane bioreactors possess high efficiency in removing COD and color; at the same time, they have strong shock resistance, which enables them to handle printing and dyeing wastewater effectively. However, membrane fouling and high energy consumption pose constraints on the application and development of membrane bioreactors. Therefore, in order to reduce energy consumption, mitigate membrane fouling, and further improve removal efficiency, researchers have modified membrane bioreactors when treating printing and dyeing wastewater. 1.2 Improvements in membrane bioreactors 1.2.1 Activated carbon——membrane bioreactors When powdered activated carbon (PAC) is added to membrane bioreactors, the adsorptive properties of PAC and the affinity of microorganisms enable a large number of biological flocs, dispersed colloids, and some soluble substances in the mixture to rapidly surround the PAC particles, forming larger flocs. This reduces the concentration of soluble pollutants. As the number of microorganisms in these flocs increases, more extracellular polymers are secreted. When other flocs or free bacteria come into contact, their respective extracellular polymers entangle with each other, thereby further connecting the flocs to form larger flocs with PAC particles as their backbone. Compared to sludge flocs without PAC, the addition of PAC results in larger floc particles and an increased porosity, which helps to improve the critical membrane flux and enhance the treatment efficiency. At the same time, due to the skeletal role played by PAC in sludge flocs, the compressive strength of these flocs is increased, which reduces filtration resistance, improves filtration performance, and slows down membrane fouling. PAC-membrane bioreactors can achieve a removal rate of over 95% for COD and color in dye wastewater, and compared to conventional membrane bioreactors, they reduce membrane fouling. 1.2.2 Coagulation-membrane bioreactor: By adding a coagulant to the membrane bioreactor, positively charged particles are generated, which undergo adsorption, electro-neutralization, and bridging with the negatively charged colloids and soluble organic substances present in the activated sludge. This process removes the colloids and soluble organic substances, increases the size of the activated sludge flocs, and improves the properties of the mixed liquid. Zou Haiyan et al. introduced the coagulant Fe(OH)3 into the membrane bioreactor to create a bio-iron method-membrane bioreactor system. Parallel comparison tests were conducted with conventional membrane bioreactors when treating printing and dyeing wastewater, and the results showed that the bio-iron method-membrane bioreactor system had significant advantages in terms of improving treatment efficiency, reducing membrane fouling, and enhancing sludge properties. With a volumetric load 25% higher than that of conventional membrane bioreactors, the bioiron-based membrane bioreactor achieved higher removal efficiencies for COD, dyes, and NO-3N by 1.0%, 9.5%, and 5.2%, respectively, compared to conventional membrane bioreactors. When the membrane flux was 25% higher than that of conventional membrane bioreactors, the pressure drop was only 36% of the latter’s value; under conditions without sludge discharge, the stable operation duration of the bioiron-based membrane bioreactor was 2.5 times that of the conventional ones. The sludge flocs in the bioiron-based membrane bioreactor were loose and porous, granular in structure, with larger particles – whose average diameter was 3.10 times that of those in conventional membrane bioreactors. A large number of protozoa and metazoans were present in this system, and the specific oxygen uptake rate of the sludge (SOUR) was 1.23 times that of conventional membrane bioreactors. 1.2.3 High-efficiency microbe-membrane bioreactor: In a membrane bioreactor, the direct addition of microorganisms with specific degradation capabilities for dyes and slurries can improve treatment efficiency and reduce membrane fouling. FaisalIbneyHai introduced efficient bacteria into the membrane bioreactor, resulting in removal rates of 99% for chromaticity and 97% for total carbon. In comparative tests with a membrane bioreactor that did not contain such bacteria, under the same pressure, the membrane flux of the conventional membrane bioreactor declined significantly after just one day of operation, whereas the membrane flux of the membrane bioreactor with efficient bacteria showed a significant decline only after 45 days of operation. 1.2.4 Composite Membrane Bioreactor In a membrane bioreactor, the membrane separation system primarily serves to retain substances, while the bioreactor plays a decisive role in removing organic pollutants from water. Mi-AeYun conducted parallel comparative tests on the treatment of textile dyeing wastewater using an anoxic membrane bioreactor and an aerobic membrane bioreactor. The average removal rates for COD and dyes by the anoxic membrane bioreactor were 27% and 86.6%, respectively, while those for the aerobic membrane bioreactor were 86.6% and 72.9%, respectively. The literature examines the effectiveness of biofilms, upflow contact oxidation columns, and contact oxidation tanks combined with membrane systems to treat printing and dyeing wastewater, which lays a foundation for identifying the type of bioreactor that is suitable for membrane systems as well as the optimal operating conditions. Tak-Hyun Kim used ultrafiltration membranes and reverse osmosis membranes to construct membrane bioreactors, and conducted comparative tests on the treatment of reactive dyes. The latter achieved significantly higher removal rates for both color intensity and TOC, sometimes even reaching 100%; however, under the same pressure, the membrane flux of the former was five times that of the latter. This provides a basis for selecting the type of membrane that matches the biological system and for determining the treatment efficiency of membrane bioreactors. 1.2.5 The new type of cross-flow membrane bioreactor: Knops found in his research that reducing the length of the membrane fibers could increase water permeability; this led to the concept of cross-flow membrane modules, which involve the use of hollow fiber membranes or tubular membranes with a cortex on the outside, where the liquid flows perpendicularly to the fibers. This approach enhances the mass transfer process in the boundary layer, combining the advantages of cross-flow and plug-flow membrane bioreactors. Xiaolong and Zhu studied the treatment of printing and dyeing wastewater using cross-flow membrane modules. The results showed that the removal rates for turbidity and COD were 99% and 89%, respectively, while the energy consumption was one-tenth that of conventional counter-flow membrane bioreactors. 1.3 Key Research Issues in Membrane Bioreactors 1.3.1 Membrane Fouling Membrane fouling refers to the adsorption and deposition on the membrane surface or within its pores of particles, colloidal particles, and large solute molecules present in the wastewater. This occurs due to physical, chemical, or mechanical interactions with the membrane, resulting in a reduction in the pore size of the membrane or even blockage of those pores, which in turn affects the membrane’s flux and separation capabilities. British scholars believe that there are mainly three factors affecting membrane fouling: the properties of the membrane itself, the properties of the activated sludge, and the operating conditions of the MBR, with these three factors influencing one another. The properties of a membrane refer mainly to the physicochemical characteristics of the membrane material, such as the material composition of the membrane, the surface properties of the membrane, and the pore size of the membrane. Studies on anaerobic membrane bioreactors by K. H. Choo and others have shown that changes in membrane flux are closely related to the membrane material and pore size. The surface charge of the membrane is the same as that of the mixture, which helps to reduce contamination on the membrane surface. Hydrophobic membranes have less adsorption of proteins compared to hydrophilic membranes, allowing for a relatively higher membrane flux. An increase in the membrane pore size leads to an increase in membrane flux, but excessively large pore sizes can facilitate the growth of microorganisms. The roughness of the membrane surface can either increase the likelihood of pollutant adsorption or enhance the turbulence on the membrane surface, thereby hindering the adsorption of pollutants there. The changes in the properties of the mixture and the optimization of operating conditions have been discussed earlier, so they are not repeated here. 1.3.2 Energy Consumption – Reasons for the high energy consumption of membrane bioreactors (1) During the operation of an MBR, a certain membrane driving pressure must be maintained. (2) The active sludge concentration in the reactor is high; generally, increasing the aeration volume is used to improve the oxygen mass transfer in the water. (3) Pollution causes the membrane flux to decline rapidly; it is necessary to increase the flow rate to scour the membrane surface and reduce membrane fouling in order to maintain the required membrane flux. When treating printing and dyeing wastewater, Xiangzheng et al. utilized gravity-driven water discharge, which significantly reduced energy consumption. Although the energy usage was still slightly higher than that of traditional biological treatment methods, the quality of the treated water was good and stable, meeting the \"Quality Standards for Urban Non-potable Water\" and satisfying the requirements for reuse. This approach helped to alleviate water resource shortages. Additionally, since membrane bioreactors produce almost no excess sludge, it solved the problem of difficult sludge disposal. 1.3.3 Membrane cleaning Different cleaning methods can be selected depending on the type of membrane contamination. Backwashing refers to the process of flushing from inside to outside the membrane. After backwashing, the membrane filtration pressure difference temporarily decreases, but it rises again quickly once operation resumes; the flow rate of the treated water decreases. Moreover, excessive backwashing volume and pressure may cause damage to the membrane structure. Air sparging involves stopping the inflow and outflow of water and increasing the intensity of aeration in order to remove the sediment accumulated on the membrane surface. After one session of air sparging, the membrane bioreactor can continue to operate for about two weeks. Chemical washing refers to the process of introducing a solution of a certain concentration into the membrane, allowing it to flow from one end to the other while coming into full contact with the inner surface of the membrane, thereby killing and oxidizing the microorganisms that grow on its surface. Studies have shown that for the membrane fouling caused by printing and dyeing wastewater, chemical cleaning can restore the membrane flux to over 90%, whereas physical cleaning (air aeration combined with backwashing) can only restore it to about 70%; the cleaning effect of NaOH is better than that of NaClO. 2 Conclusions: The quality and quantity of dyeing and printing wastewater vary greatly, and it contains a large amount of refractory organic pollutants. Such wastewater has high COD values, high color intensity, and poor biodegradability. For a long time, biological treatment processes have been the main method for treating this type of wastewater both domestically and internationally. Membrane bioreactors can be installed as an upgrade to existing biological treatment systems; they do not require additional space, offering flexibility and convenience. The efficient filtration capability of membrane components allows for a high sludge concentration within the bioreactor, thereby enhancing its ability to withstand shock loads. It can operate stably under conditions of high volumetric load and low sludge load, and its effectiveness in treating dyeing and printing wastewater is significantly better than that of other treatment processes. The textile and dyeing industry consumes a large amount of water, and in China’s large and medium-sized cities, the water supply covers less than half of the demand. Membrane bioreactors are effective and reliable in treating wastewater from this industry; the quality of the treated water generally meets the standards for urban non-potable use, thus helping to alleviate water shortages. Therefore, as further research is conducted on issues such as membrane fouling and energy consumption, membrane bioreactors are bound to gradually replace traditional biological treatment methods in the treatment of printing and dyeing wastewater, thus becoming the most valuable water treatment technology. Last edited by hesonchang214 on 2009-4-16 13:27.]