Fundamental mechanisms of fouling and cleaning of forward osmosis membranes mimicking seawater desalination and waste water reclamation

The process of forward osmosis (FO) has been reported to be a promising potential alternative to current pressure driven membrane processes such as nanofiltration and reverse osmosis. However, the lack of full understanding on essential aspects of the process such as membrane fouling and the inherent phenomena of concentration polarization is still a major drawback impeding its progress towards real world applications. The coupled effects of membrane fouling and concentration polarization have catastrophic impact on permeate flux. Therefore, this research work was aimed at investigating the key mechanisms that lead to membrane fouling in a forward osmosis process used for brackish/seawater desalination as well as wastewater reclamation. Model organic compounds representative of various common organic foulants (polysaccharides, proteins, humic substances and fatty acids) were used to prepare different synthetic feed streams representative of seawater and wastewater. These feed water types were then used to test the fouling behaviour of forward osmosis membranes such as the much studied traditional cellulose triacetate memmbrane as well as thin film composite membranes. Severe flux loss was observed in feed streams containing alginate, bovine serum albumin and humic acid; this was attributed to the presence negatively charged functional groups (i.e. -COOH) on organic foulants that resulted in specific reaction with cations particularly calcium ions leading to complexed moieties that were easily deposited onto the membrane surface under significant permeation drag. Protein molecules were found to severely foul the polyamide thin film composite membrane through multiple layer adsorptions. Membrane structural surface properties (porous structure of support layer), had a dominant role in foulant deposition and in organic gel layer accumulation. Foulants got deposited on the “valley” of the rough surface of the support layer hindering permeation and back-diffusion of solutes resulting in severe internal concentration polarization. The key foulant-foulant interactions that influence membrane fouling behaviour during combined fouling were investigated by conducting series of systematic experiments using various combinations of organic and colloidal foulants. Foulant-foulant interactions were identified through sequential fouling experiments; where the membrane was fouled with the two foulants in alternating sequences. The quartz crystal microbalance (QCM-D) with dissipation monitor was also used to investigate surface interactions between colloidal particles and organic foulants. It was revealed that when two foulants co-exist in a FO process, fouling is usually dominated a single foulant. This was evident when alginate co-existed with silica colloids in the same feed solution; cake layer formation was dominated by alginate aggregates. The alginate had a primary effect on the cake layer formation. The new approach of sequential fouling experiments and QCM-D analysis provided proof of the adsorption of alginate onto the surface of silica particles surfaces which led to altered colloid-colloid interactions such that the resulting cake layer had the same hydraulic characteristics as that of the alginate fouling layer. A similar observation was observed when both alginate and bovine serum albumin were present in the same feed solution. However, eventually more severe permeate flux loss was observed compared to that caused by either foulant suggesting that Bovine serum albumin molecules initiated foulant deposition, thereafter synergistic effects between the two foulants through electrostatic and hydrophobic interactions lead to severe flux loss. Characterization analyses using advanced contact angle measurements were used to gain insight onto foulant-membrane interactions at contact as well foulant feed water stability. Given the virtual absence of electrostatic interactions at seawater level ionic strength (the increase in feed ionic strength was shown to reduce the Debyle length and electric double layer to such an extent that normal electrostatic interactions effectively suppressed), interaction of the membrane and foulant at contact was found to be better explained by the short-range acid-base interactions from the two interacting surfaces. The surface free energies correlated strongly with the rates of membrane fouling and predicted membrane fouling by alginate. However, due to the nature of the resulting fouling layer from combined fouling (dominance of one foulant) the prediction of the fouling behavior due to multi-component feed solutions was not as accurate as those for single foulants. A strategy to clean fouled forward osmosis membranes was developed by gaining insight into the key physical cleaning mechanisms involved in a simple osmotic backwash process during the cleaning of FO membranes fouled under seawater conditions. The osmotic backwash process was found to effectively restore membrane flux (> 80%) of cellulose triacetate membrane fouled with alginate during seawater desalination. The fouling layer (alginate gel) removal was shown to be a function of both permeate flux rate induced by the cleaning draw solution and the shear force generated by increased cross-flow velocities of bulk solution. It was also revealed that the high permeate rate responsible for faster membrane fouling (increased foulant deposition rate) was a requirement for the effective fouling layer removal during osmotic backwash. Therefore, this implies that the ideal operational conditions for a long FO run include moderate permeate fluxes with high feed cross-flow velocity