Study On The Performance Of Immersed Ultrafiltration For Algal-Rich Water Treatment And Control Of Membrane Fouling

With the quick development of industry, the pollution of the drinking water sources has been getting more and more serious. Especially the eutrophication phenomenon which frequently happens makes the waterworks with conventional processes facing austere challenges. It is a key problem to remove the algal cells and their extracellular organic matter (EOM) in eutrophic water to insure the safety of drinking water.It is a new technical approach to remove the algal cells and their EOM by ultrafiltration (UF) membrane. To find out intrinsic rules and characteristics of the growth and secretion of the algal cells and the EOM will contribute to find out the practicable water treatment technology and the rational UF membrane application. Microcystis aeruginosa (M. aeruginosa) cultured in the lab was used as the main object in this research. This dissertation consisted of the following contents: 1. To find out the rules and characteristics of the algal cell concentration and the EOM with the time development by qualitative and quantitative analyses; 2. To find out the efficiencies and mechanisms of algal-rich water treatment by UF and PAC/UF bioreactor; 3. To find out the effect of running parameters of the membrane reactor and influent properties on the algal-rich water treatment by polyvinylchloride (PVC) UF membrane; 4. To study the efficiency of polluted source water treatment by immersed UF membrane; 5. To investigate the cleaning efficiencies and mechanisms of different chemical reagents on the fouled membrane by algal-rich water.By investigating M. aeruginosa cells cultured in the lab and their extracellular organic matter (EOM), it was found that the cell concentration increased with time and reached maximum at stationary phase, then decreased. The isoelectric point of the algal cells was at pH4⁵ during both exponential and stationary phases. With pH increase, the electronegativity of algal cells strengthened. Meanwhile, the EOM in the different phases showed high hydrophilicity and high molecular weight (MW, more than 30 kDa). By analyzing the important components of EOM (carbohydrates, proteins and MC-LR_eq), it was found that their concentrations were gradually increasing with the growth of algal cells. The ratio of carbohydrates to dissolve organic carbon (DOC) decreased with the time development, while both the ratios of proteins to DOC and that of proteins to carbohydrates increased.By comparing the efficiency of algal-rich water treatment by UF and PAC/UF bioreactor, it was found that UF achieved an absolute removal of M. aeruginosa cells, but a poor removal of algogenic organic matter (AOM) released into water, with severe membrane fouling. Compared with UF process, PAC/UF bioreactor significantly reduced the development velocity of TMP and enhanced the removal of DOC (by 10.9±1.7%), UV₂₅₄ (by 27.1±1.7%), and MC-LR_eq (by 40.8±4.2%). However, PAC had little effect on the rejection of high MW AOM such as carbohydrates and proteins. The mechanisms of organic matter removal in UF reactor included membrane sieving, biological oxidation and strengthening sieving by cake layer. The biological oxidation effect in PAC/UF reactor was much more obvious than that in UF reactor. It was also identified that PAC reduced the concentrations of carbohydrates and proteins in the reactor due to decreased light intensity; as well as the MC-LR_eq concentration by PAC adsorption when there was little effect of lower light intensity on MC-LR_eq secretion.The influences of the running parameters (including flux, aeration, backwashing) on the trans-membrane pressure (TMP) development were also investigated. The influence of flux on membrane fouling enhanced with the algal cell concentration increasing. Increasing the ratio of air to liquid could alleviate membrane fouling phenomenon. However, it was considered that the best ratio of air to liquid was 12:1 under the test condition in this dissertation . It was also found that increasing pH value enhanced the electronegativity of the pollutant and membrane surface, and promoted the increasing of the ratio of high MW EOM, which led to the reduction of the TMP increasing velocity. Meanwhile, it was revealed that with the comparison of K⁺ and Al³⁺, Ca²⁺ was more efficient to alleviate membrane fouling. It was deduced that Ca²⁺ contributed to the formation of high MW materials. The algal cells and humic acid had synthetic effect on the membrane fouling, which could cause more serious membrane fouling.The efficiency of polluted source water treatment by immersed UF membrane with the pretreatment of coagulation and sedimentation was studied in pilot experiment for practical reconstruction. Without being influenced by raw water quality, turbidity in UF effluent was absolutely lower than 0.1NTU. The algal cells could not be found in UF effluent. Due to the pretreatment of coagulation and sedimentation, the contents of organic matters in UF effluent basically reached Chinese sanitary standard for drinking water. During running phase (for 40 days), membrane flux always kept a high value (not less than 40 L/(m²Â·h)). The recovery of membrane flux could reach up to 93.6% after cleaning by 300 mg/L NaClO at the end of the experiment.To further understand the membrane fouling caused by algal cells and their EOM, the cleaning efficiencies of hollow-fibre PVC membrane with different chemical reagents (NaOH, HCl, EDTA, and NaClO) after ultrafiltration of algal-rich water were also investigated. The result showed that 100 mg/L NaClO exhibited the best performance in terms of removing the irreversible fouling resistance (88.4±1.1%). This might be attributed to the fact that NaClO could eliminate almost all the major foulants such as carbohydrate-like and protein-like materials on the membrane surface, as suggested by attenuated total reflection fourier transform infrared spectroscopy analysis. However, negligible irreversible resistance (1.5±1.0%) was obtained when the membrane was cleaning by 500 mg/L NaOH, although the NaOH solution could also desorb a portion of the major foulants from the fouled PVC membrane. Scanning electron microscope and atomic force microscope analyses demonstrated that NaOH could change the structure of the residual foulants on the membrane, making them more tightly attached to the membrane surface or change the characteristics of membrane surface. This phenomenon might be responsible for the negligible membrane permeability restoration after NaOH cleaning.