Investigating if Osmotic Effects Explain the Influence of Sodium Chloride Upon the Filtration Characteristics of Polyacrylic Acid
Author
Pedersen, Jakob Aagaard Lükensmejer
Term
4. Term
Education
Publication year
2016
Submitted on
2016-06-10
Pages
80
Abstract
This thesis examines whether osmotic effects explain how sodium chloride alters the filtration characteristics of polyacrylic acid (PAA), used as a model for polyanionic filter cakes relevant to fouling in membrane bioreactors. Dead-end filtration experiments (piston-based and continuous setups) formed PAA cake layers while varying salt concentration, using PAA of different molecular weights, and a simple Donnan-equilibrium model was applied to estimate the gel’s osmotic pressure. The experiments showed that increasing NaCl reduced the filtration resistance of the PAA cakes, even though the cake is expected to be more mechanically compressed at higher salt. The cake was compressible at low salt concentrations, but its compressibility decreased as salt increased. Because the model predicts that gel osmotic pressure declines with increasing salt, osmotic pressure within the cake is a plausible cause: reduced osmotic pressure can drive deswelling (lower water uptake), which may lower hydraulic resistance and make the cake less prone to further compression. The study thus suggests that osmotic effects in polyelectrolyte layers are important drivers of filtration behavior in saline conditions.
Dette speciale undersøger, om osmotiske effekter kan forklare, hvordan natriumklorid påvirker filtrationskarakteristika for polyakrylsyre (PAA), brugt som model for polyanioniske filterkager relateret til fouling i membranbioreaktorer. Med dødfiltreringseksperimenter (pistonbaseret og kontinuerligt setup) blev PAA-filterkager dannet, mens saltkoncentrationen blev varieret, og der blev brugt PAA med forskellige molekylvægte. En enkel model baseret på Donnan-ligevægt blev anvendt til at vurdere gelens osmotiske tryk. Resultaterne viste, at filtrationsmodstanden faldt ved højere NaCl-koncentrationer, selvom filterkagen samtidig forventes at være mere mekanisk komprimeret. Filterkagen var kompressibel ved lave saltkoncentrationer, men kompressibiliteten aftog ved højere saltkoncentrationer. Da modellen forudsagde, at gelens osmotiske tryk falder med stigende saltkoncentration, fremstår osmotisk tryk i filterkagen som en plausibel forklaring: lavere osmotisk tryk kan forårsage desvulmning (mindre vandoptag) af kagen, hvilket kan reducere den hydrauliske modstand og gøre kagen mindre tilbøjelig til yderligere komprimering. Studiet peger dermed på, at osmotiske effekter i polyelektrolytiske lag kan være afgørende for filtrationsadfærd i salte miljøer.
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