Membranes for bioreactors
Fraunhofer Institute for Interfacial Engineering and Biotechnology
- Fig. 1: BioTopp sewage treatment plant for on-site applications (Ökoservice GmbH).
Sewage treatment is an ongoing topic in the broad field of water purification technology. Conventional domestic wastewater treatment plants consist of collection, treatment, and disposal which cause high energy consumption and high costs. In low populated areas or remote locations, decentralized wastewater treatment plants could be more advantageous. However, it is not allowed to utilize the sludge, which is obtained after the first mechanical pretreatment (the fecal sludge), in the agricultural sector. The issue of cost-intensive removal, sludge transport to and post-treatment in conventional sewage treatment installations can be avoided in the BioTopp sewage plant (Fig. 1) with an integrated membrane process. In this system, aerobic stabilized sludge is generated which can be dried and sanitized on-site and can be used for agricultural purposes. Significant boundary conditions concerning these systems are low costs, ease of use, sustainability, and low maintenance.
The main challenge for the implementation of these systems is the membrane performance. The filtration aspects of the current commercial membranes are not satisfying and the maintenance of filtration performance over long periods of time requires lots of process technological effort. Insufficient control of the pore size, pore size distribution and interfacial effects result in disadvantageous permeate flow mainly caused by membrane fouling. Two approaches respective to membrane development and property tailoring have been followed to reduce fouling:
- Mixed matrix systems, consisting of nanoparticles embedded in the polymer matrix.
- Coatings prepared by Plasma Enhanced Chemical Vapour Deposition (PECVD), whereby very thin functional layers are deposited.
- Fig. 3: Pore size and Nitrogen flow of two membranes with the same composition but prepared via two different processes.
- Fig. 4: Scanning electron microscope picture of a membrane prepared by the phase inversion process.
Mixed matrix microfiltration membranes containing poly(ethersulfone) (PES) and titania nanoparticles are prepared by the phase inversion process. Besides titania, other materials like silica and carbon nanotubes are used as nanostructured materials. The water permeability coefficients of the mixed matrix membranes with various amounts of titania nanoparticles are presented in Fig. 2. The membranes which had a humidity treatment before the phase inversion process are significantly higher than the membranes prepared by direct phase inversion. Larger pores are confirmed by porometry (Fig. 3). A typical morphology of a membrane prepared by phase inversion is presented in Fig. 4.
PECVD is performed in low pressure plasmas at room temperature in a parallel plate reactor, whereby mainly the treatment time and composition of the gas phase are varied. Characterization of the surface treated mixed matrix membranes includes contact angle measurement (static and dynamic), scanning electron microscopy, X-ray photoelectron spectroscopy, porometry, pure water flux, molecular weight cut-off and fouling tests.
The membrane type, loaded with 9% titania and prepared by direct phase inversion, was coated with various kinds of PECVD to deposit nanolayers with the desired functionality. In Figure 2, the contact angle measurements of the functionalized porous membranes are presented. The contact angle of the hydrophilic coatings, SiOx and acrylic acid (AAc), was zero resulting in a fast liquid intrusion into the porous structure. Hexamethyldisiloxane (HMDSO) and trifluormethane (CHF3) based nanloayers were hydrophobic. The chemical composition within the penetration depth of approximately 10 nm was measured with XPS and the results confirmed the presence of the deposited layers.
The influence of particle selection, membrane preparation, membrane composition of the mixed matrix membranes and the effect of various types of nanocoatings (hydrophilic, hydrophobic) are studied with respect to fouling behaviour. Therefore, different methods are implemented at Fraunhofer IGB, like bovine serum albumin (BSA) adsorption, apple juice filtration and activated sludge filtration. In addition, the PECVD coated mixed matrix membranes are tested with respect to their material and interfacial properties for the performance in membrane bioreactors.