Mitigation of membrane fouling in microfiltration & ultrafiltration of activated sludge effluent for water reuse

This research sought to obtain a more detailed understanding of the effects of various feed pre-treatment methods on membrane fouling in microfiltration (MF) and ultrafiltration (UF) of secondary effluent, using an activated sludge effluent in Victoria (Australia) as an example. The pre-treatments investigated included coagulation (without removal of the coagulated matter prior to MF or UF), ozonation, ozonation followed by biological activated carbon (BAC) filtration, and adsorption with an anion exchange resin (AER) and a powdered activated carbon (PAC).<br><br>The first part of the research dealt with dead-end filtration using polymeric membranes. The second part was dedicated to cross-flow filtration with ceramic (alumina and zirconia) membranes. <br><br>It was found that biopolymers, which contained polysaccharides and proteins (molecular weight (MW) >>20,000 Da), and humic substances (HS, MW=1,000–20,000 Da) in the effluent were the components responsible for fouling of all the membranes investigated. HS caused hydraulically irreversible fouling, whereas biopolymers contributed to both hydraulically reversible and irreversible fouling. <br><br>Coagulation with poly(aluminium) chlorohydrate and alum significantly improved the permeate flux and reduced hydraulically irreversible fouling in dead-end filtration. This was attributed to the coagulants removing the colloidal organic matter, including some biopolymers and HS responsible for fouling, from the raw effluent. Flux improvement by coagulation was also observed in cross-flow MF tests with the alumina membrane. <br><br>Ozonation led to significant flux improvement, which was attributed to the breakdown of some biopolymers and HS to compounds with lower MW and hydrophobicity. However, as it generated some low MW (<500 Da) compounds, this pre-treatment may accelerate membrane biofouling. BAC filtration of the ozonated effluent showed potential to reduce membrane biofouling since the filter removed the majority of the low MW compounds generated during ozonation. It also led to further flux improvement in MF and in UF with the 100 kDa membrane, but did not affect the flux of the tighter UF (30 kDa) membrane. The hydraulically irreversible fouling was reduced after ozonation whereas BAC filtration did not affect this type of fouling.<br><br>Pre-treatments with the AER and the PAC neither reduced nor worsened membrane fouling in dead-end filtration. This was due to the fact that although the AER and the PAC removed more than 50% of HS from the raw effluent, they did not remove biopolymers – the largest contributors to membrane fouling. Similarly, cross-flow filtration tests showed that while ozonation improved, pre-adsorption with the AER did not affect the flux performance of the ceramic membranes. <br><br>This work showed that only the pre-treatments which led to the removal (coagulation) or breakdown in the structures (ozonation) of biopolymers reduced membrane fouling. Therefore, pre-treatment for fouling reduction in MF and UF of secondary effluent should target the biopolymers. Furthermore, less hydrophobic MF membranes would likely to benefit more from feed pre-treatments for fouling reduction than more hydrophobic counterparts, as hydraulically irreversible fouling of the formers is likely to be less severe and more easily mitigated.<br>