Membrane-based intensification of wastewater treatment processes (WP3)
The integration of membrane processes in waste water treatment has the potential to considerably enhance overall process efficiency, to increase process safety and to lower environmental impact, as the membrane unit can be easily combined with other treatment steps and allows to selectively remove priority and hazardous priority pollutants. Both of these benefits will be followed within one component of the proposed project by developing two different innovative membrane processes: a membrane contactor (MC) having silicone coated membranes and a membrane chemical reactor (MCR). Themembrane contactor focuses on the selective removal of phenolic compounds from industrial wastewater (e.g. from polycarbonate or bisphenol-A production).This treatment solution allows to recycle raw materials back into the reaction process and/or offers to selectively remove organic pollutants which might even cause limitations in possible further treatment steps (such as electrolysis or biological degradation). Membrane contactors are a novel kind of apparatus for contacting processes such as absorption, desorption or liquid-liquid extraction. Both fluids, being contacted for mass transfer purposes, pass each other separated by a (typically porous) membrane. This results in a non-dispersive interface generation, the most important characteristic of membrane contactors, giving rise to a number of potential advantages (such as independent fluid flow rates, no formation of stable emulsions or foam, well defined and constant contact area, extremely large specific interfacial area) (Gabelman and Hwang, 1999; Melin and Rautenbach, 2004).Depending on the fluids and membranes applied, a broad field of applications can be covered. The versatility of membrane contactors can even further be extended by using composite membranes, such as a dense, very thin, selective coating on a porous support structure (Gabelman and Hwang, 1999; Reed et al., 1995; Klaassen et al., 2005; Yun et al., 1993). Within the project the liquid-liquid extraction of phenolic compounds from wastewater will be investigated and developed using membrane contactors with coated membranes. Phenolic compounds such as phenol, bisphenol A and nonylphenol can be found in aqueous streams from bisphenol A, polycarbonate, epoxy resin or phenolic resin production (Leisewitz and Schwarz, 1997). Taking into account the treatment problems of conventional technology for separating phenol from aqueous streams (liquid-liquid extraction – comparatively polar nature of phenol, stripping – very low vapour pressure, nanofiltration – too small molecular mass, activated carbon adsorption, wet oxidation or biological degradation – no recovery possible), the membrane contactor solution is rather attractive. The dense silicone layer on one side of the membranes provides the contactor with an additional selectivity, allows to use an aqueous sodium hydroxide solution as solvent phase (reactive extraction!) and prevents the system from phase mixing problems which are known for conventional, technically operated membrane contactors. The objectives are: -synthesis of coated membranes and design and construction of experimental test membrane modules; -research and technological development by experimental and numerical studies; -characterisation of system performance in terms of contaminant removal, selectivity, recoverability of components, treated volume per unit time and membrane area, etc.; -optimisation of membrane module design;-process development for applications specified by end-users: -technical realisation (scale-up) and system integration. The MCR-system combines a membrane filtration unit with an advanced oxidation process. The membranes are submerged into fluidised slurry of adsorbent/catalyst particles (TiO2) in waste water and retain the particulate matter, which adsorbs the pollutants and catalyses their destruction by chemical oxidation using UV-light within the regeneration stage. Current state of the art in advanced wastewater treatment is represented by (a) membrane bioreactors (MBRs) for intensive biotreatment, and (b) advanced oxidation processes (AOP) for intensive chemical treatment. MBR technologies are based on coupling of suspended biomass biotreatment with membrane separation. The rapid market penetration of the MBR technology in its submerged membrane configuration, quickly displacing the sidestream configured process, since the first commercial installation 15 years is striking. The efficacy of the technology is none-the-less constrained by the biodegradability and, ultimately, toxicity of the effluent: the technology is unable to treat toxic or recalcitrant organic matter. It is against this background that the merits of the novel submerged membrane chemical reactor (MCR) should be assessed.The MCR operates, as with the submerged MBR, with fluidised slurry retained by a membrane. However, the reaction is chemical rather than biochemical, proceeding via adsorption of the pollutants onto the titanium dioxide (TiO2) particles followed by their destruction by hydroxyl radicals promoted in-situ by UV irradiation. Whilst the use of UV/TiO2 is a known AOP, its combination with a membrane for retaining the TiO2 is a much more recent development. Thus far only sidestream MCR systems have been developed, and their penetration of the market is limited. The proposed submerged MCR process configuration is completely novel, representing innovation in technology transfer from the more familiar MBR. It is clearly of interest to establish the economic and technical viability of a treatment process which combines some of the most desirable facets of an intensive biotreatment process with that of an AOP. As with the MBR, the MCR produces a highly-clarified effluent, involves no hazardous chemicals other than for cleaning, and generates low quantities of waste. The key comparators, therefore, are the specific energy demand (energy expended per unit volume of treated effluent) and the degree of purification attained, i.e. the percentage of organics removal. A study determining the ranges of values of these key normalised parameters would constitute a significant addition to knowledge and will be carried out in this project. The objectives are the optimisation of the: -adsorption stage on TiO2 particles (e.g. catalyst dosing, residence time); -MCR process configuration in terms of overall set-up; -reactor coupling; -regeneration stage (e.g. operation conditions of UV system); -filtration stage (e.g. suitability of membranes and operation conditions); -process integration with up-/down-stream processes.
Literature
A list of the literature referenced in this text can be found here
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