Ceramic Ultra- and Nanofiltration for Municipal Wastewater Reuse

Abstract

During the last decade, water reuse has been widely recognized in many regions of the world. Fouling of ceramic membranes, especially hydraulically irreversible fouling, is a critical aspect affecting the operational cost and energy consumption in water treatment plants. In addition, the reverse osmosis (RO) membranes, that are often used for water reuse plant, frequently face the problem of bio-fouling. The main objective of this thesis is to develop innovative applications of the ceramic ultra- and nanofiltration membranes for water reuse purpose. Improving RO pre-treatment using tight ceramic UF or ceramic NF To prevent the biofouling often relies on pre-treatment technology, since frequent chemical cleaning does not only increase the operational cost but also destructs the polyamide skin layer of RO membrane. A good RO pre-treatment should aim at both particulate removal to release clogging in the module, and organic carbon or nutrient limitation to release biofouling in the RO. Intense pre-treatment has been applied to alleviate the biofouling in RO membranes during wastewater reclamation. Whereas, current filtration-based pre-treatment processes cannot adequately prevent biofouling due to their poor removal of nutrients and organic carbon from feed water. The tight ceramic UF and ceramic NF membranes can potentially be deployed for RO pre-treatment in order to constrain the biofouling in RO by removing the organic carbon and phosphate (as nutrient). Theoretical research on the phosphate rejection by the charged tight ceramic UF was conducted, and the results have emphasized the importance of electrostatic interactions between the negatively charged membrane and the phosphate. The results indicate that the rejection of phosphate is dependent on the pH of the solutions and the results can be interpreted by Donnan exclusion and formation of an electrical double layer in the membrane pores. The greater phosphate rejection due to electrostatic repulsion results from a stronger overlapping of the electrical double layers in membrane pores. A Debye ratio (ratio of the Debye length to the pore radius) can be used to evaluate double layer overlapping in tight UF membranes. However, the membrane fouling caused by organic matter and cations can potentially influence the phosphate rejection by the tight ceramic UF. The phosphate rejection appeared to be linearly correlated to the surface charge of the organics in the feed waters. In addition, the biopolymers in WWTP effluent water organic matter (EfOM) can adsorb phosphate with the bridging of multivalent cations, which leads to higher phosphate rejection by the EfOM-fouled membranes than humics-fouled membranes. Sewer mining using ceramic nanofiltration Ceramic NF can be used for direct municipal sewage filtration aiming at energy, fresh water and nutrient recovery, so called sewer mining. Over 80% of organic carbon substrates and 90% of dissolved phosphate were rejected by a ceramic NF membrane (450 Da), but only 10% for ammonia. Concentration of the municipal sewage using ceramic NF increases the efficiency in the energy and nutrient recovery during the anaerobic digestion. The fouling on the ceramic NF membranes during sewage filtration can be suppressed by chemical cleaning with NaClO (0.1%) and HCl (0.1 mol L-1), while the cleaning of polymeric NF membranes is reportedly far more difficult. As such, sewer mining with ceramic NF is believed as an innovative and viable technology for energy, fresh water and nutrient recovery.