Study of the fouling of flat sheet membranes used in membrane bioreactiors caused by different foulant compounds

Abstract

Consulta en la Biblioteca ETSI Industriales (Riunet)

Membrane bioreactors (MBR) are one of the most advanced technologies used nowadays for water treatment, combining biological treatment with membrane technology. This results in a high effluent quality and small space requirements. The most important drawbacks of this technology are high energy consumption related to pumping and aeration (for cleaning the membranes), fouling and lifetime of the membranes. In order to understand some of the main challenges for MBR technology, this project focuses on the study of how different foulant compounds present in wastewater affect the performance of the membranes. Physical cleaning methods were also used with the purpose of evaluating their effectiveness when heavy fouling is present. Seven membranes were initially tested. Its properties (contact angle, scanning electron microscopy (SEM), pure water flux and molecular weight cut-off (MWCO)) were studied in order to select 3 of them for further research on fouling based on the results obtained. UP150 (Nadir, PES), PA350 (Nanostone, PAN) and LY100 (Synder, PVDF) were chosen, each of them from a different providers and active layer materials. The foulants selected for the study were sodium alginate (NaAlg), and mixtures of NaAlg, CaCl2 and bovine serum albumin (BSA). The experiments were carried out in an aeration cell that was designed to resemble a real MBR. A pump extracted the permeate at a selected flux, and the transmembrane pressure (TMP) and air flow measurements were constantly registered. A flux-step method was developed to study the TMP evolution at different fluxes. From the gathered information it can be concluded that the mixture of NaAlg + CaCl2 caused the highest fouling due to the crosslinking of these elements which formed a resistant gel layer on the three membranes. NaAlg caused internal fouling that later developed also as a less critical gel layer. This could be determined from the fouling curves obtained. NaAlg + BSA caused a higher but less critical fouling for the membranes BY100 and PA350. However, for the UP150, the TMP raised significantly causing the highest fouling. From these experiments, the permeability loss (regarding the pure water permeability) was calculated for the three membranes and solutions, showing that the BY100 had the best behavior (18 % loss for NaAlg, 45 % loss for NaAlg + CaCl2 and 47 % loss for NaAlg + BSA) and the UP150 the worst. The critical fluxes were also calculated for the three membranes and solutions. To study the recoverability of the TMP for the different solutions, the double-step method was developed, where rising steps of flux were alternated with a flux of the previous step. It was found that for the NaAlg and NaAlg + BSA solutions, the TMP reached in the recoverability state was between 2-6 % higher than the first one, while for NaAlg + CaCl2, this value was between 15-30 % higher. This shows that when the gel layer is developed on the membrane’s surface when using NaAlg + CaCl2, the recoverability of TMP is significantly reduced In order to evaluate the different physical cleaning methods the following sequence was performed with a NaAlg + CaCl2 mixture: relaxation, relaxation + high aeration, backwash, and backwash + high aeration. The results showed that these methods were not effective for the membranes UP150 and PA350, where the TMP kept increasing as the foulants deposited on the membrane’s surface independently of the physical cleaning method performed. In the membrane BY100 a significant drop of TMP was observed, however, the gel layer was not removed.