Combining PAC and HAOPs with Microgranular Adsorptive Filtration to Enhance Water Treatment
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Immense effort has been made over the past few decades to address the challenge of sustainable drinking water production. As a result of this endeavor, low- and high-pressure membrane filtration have been developed as a reliable and efficient water treatment technology. However, application of membranes is restricted due to fouling, which is accumulation of contaminants in the feed on the membrane surface or within membrane pores during filtration. Fouling severely deteriorates the process efficiency by increasing trans-membrane pressure (TMP) and lowering membrane permeability. In drinking water treatment natural organic matter (NOM) is usually the main membrane foulant, causing fouling by restricting or blocking the pores and/or forming a gel layer on the membrane surface. NOM is also the cause of several other problems in drinking water treatment such as affecting taste and odor, formation of harmful disinfection by-products (DBPs), and increasing the required dose of coagulant and adsorbent. Although conventional NOM pre-treatment processes such as coagulation with metal-based coagulants or adsorption onto powdered activated carbon (PAC), can capture NOM to some extent, there is need for more efficient and economical methods that remove NOM and mitigate membrane fouling. In the past few years, a novel pretreatment technology, called microgranular adsorptive filtration (μGAF), has been developed by Benjamin’s group at the University of Washington. This process integrates adsorption and granular media filtration. It is reported that μGAF with heated aluminum oxide particles (HAOPs) can substantially remove NOM and mitigate the downstream membrane fouling. However, a previous effort for application of PAC in μGAF failed partly because the PAC did not have a comparable NOM removal efficiency. The research presented in this dissertation studied if any PAC can present the advantages that HAOPs offer in the μGAF process. Three commercially available PACs were tested. PACs with different manufacturing conditions had distinct NOM removal efficiency and adsorption kinetics and when used in μGAF, they had different efficiencies for capturing membrane foulants. Among the tested PACs, SA SUPER possessed a higher NOM removal efficiency and rate of adsorption. It effectively adsorbed high molecular weight (HMW) NOM molecules such as biopolymer fraction and humic substances, resulting in significant mitigation of the fouling of the downstream membrane. Overall, at low doses, it outperformed the other two PACs, performing comparable to HAOPs. μGAF substantially enhanced the performance of HAOPs and SA SUPER compared to batch adsorption. The enhancement, however, was more significant for HAOPs than SA SUPER. Size exclusion chromatography confirmed the increase in the removal efficiency of the HMW biopolymer fraction and humic material when adsorbents were used in μGAF. Utilization of the mixture of HAOPs and SA SUPER, both in batch adsorption and μGAF, led to a significant increase in the total NOM removal efficiency and consequently a dramatic decrease in the DPB formation potential of the treated water. SA SUPER was more effective than HAOPs in adsorbing fluorescent NOM both in batch and μGAF. However, despite the reports in the recent years, no rational correlation was found between the removal of fluorescent NOM and mitigation of the downstream membrane fouling. Effect of process parameters on μGAF performance was also investigated for both HAOPs and PAC SA SUPER. It was reported that surface of the HAOPs layer is more effective than its depth in removing large humic substances. However, this effect was limited to HAOPs and the surface of the SA SUPER layer did not have the similar capability. On the other hand, increasing the depth of the SA SUPER layer at a fixed effective adsorbent dose, enhanced the removal of membrane foulants, whereas for HAOPs, it resulted in a slight decrease in the removal of humic substances due to the decrease in the ratio of the adsorbent surface layer to total volume of water treated. For both adsorbent, increasing the flux to the μGAF unit, did not have a considerable effect on the process performance.
- Civil engineering