Preparation Of Dynamic Membrane Support Materials With Nanofiber As Backbones And Its Filtration Behaviors In Bioreactors

Dynamic membrane bioreactor(DMBR)relies on in-situ formed biofilms and retained solids on support materials with large apertures to separate the solids in the bioreactor.The layers of biofilms and/or retained solids,which act as an effective secondary membrane,are referred to as dynamic membranes(DMs).The DMBR can achieve low pressure and high throughput filtration at a very low cost,but the inconsistent quality of the effluent limits its widespread use.Previous studies found that the inhabitation of filamentous bacteria on the support materials with large apertures in dynamic membrane bioreactor(DMBR)could stabilize the DM structure and resulted in stable effluent quality.However,it is hard to cultivate the filamentous bacteria on support materials in DMBR intentionally.In present study,polyvinyl alcohol(PVA)nanofibers were electrospun onto a 180μm nylon mesh to form a sparse nanofiber spun layer to simulate attached filamentous bacteria.The effect of spinning time on the porosity and pore size distribution of the nanofiber spun layer was investigated and optimized.In addition,the effect of the nanofiber spun layer on DM formation,filtration behaviors and structure in DMBR was assessed by short-term and long-term tests.Finally,a method based on suspension filtration was explored to characterize the pore size and distribution of DM.The main findings of this study are as follows.(1)PVA nanofibers were electrospun on a 180μm nylon mesh by electrospinning technology to obtain a nanofiber spun-layer mesh.The experimental study compared the effect of different spinning times on the porosity and pore size distribution of the nanofiber spun-layer mesh.The results showed that the nanofiber spun-layer mesh with a spinning time of 1 hour(PVA-1h)had a similar porosity(14.9%)to the commonly used DMBR support material(i.e.25μm nylon mesh),but the main pore size was smaller,approximately 1.0-2.5μm.(2)Short-term screening tests showed that the sparse nanofiber-coated screen(PVA-1h)had a low initial effluent turbidity(5.3 NTU),could form DM within 5 minutes,and could maintain a high membrane flux operation.However,the dense nanofiber-coated mesh(PVA-2h)is prone to’delamination’during filtration,which can affect the long-term operation of the membrane module.(3)Long-term tests in bioreactors have shown that PVA-1h has better and more consistent effluent quality(0.71±0.31 NTU),comparable operating flux(30-50 30-50L/(m~2·h))and≤2 cm head loss compared to the widely used 25μm nylon mesh(1.04±0.60 NTU).Therefore,the formation of DMs with an appropriate density of nanofibers as a backbone will result in a faster and more stable formation of DMs,resulting in a better and more stable effluent quality from the DMBR without affecting the operational flow rate.(4)The DM formed in the support material consists mainly of biofilm or trapped biosolids,which are difficult to characterize in terms of pore size and distribution in the wetted state.In this study,a new method for measuring the pore size and porosity of DMs for suspension filtration was developed using the difference in particle filtration capacity of different DMs.The results show that the porosity of two DMs with significantly different surface morphology and bio-attachment were 10%-12%and 3%-5%,respectively,and the maximum pore size was about 10μm and 6μm,respectively.