Mixed-matrix membranes for wastewater treatment in a membrane bioreactor

BioTopp sewage treatment plant for decentralised applications (Ökoservice GmbH).
Figure 1: BioTopp sewage treatment plant for decentralised applications (Ökoservice GmbH).

Wastewater treatment is a very topical subject of developments in the field of water purification technology. In conventional municipal wastewater treatment plants, the processes of collection, treatment and disposal are usually carried out and these are associated with high energy consumption and high investment costs. In sparsely populated areas, decentralized wastewater treatment can be advantageous. However, the use in agriculture of the faecal sludge produced after the first mechanical pretreatment is prohibited. Cost-intensive disposal, sludge transport and post-treatment in a conventional wastewater treatment plant can be avoided in a wastewater plant with integrated membrane bioreactor by integrating a membrane process (Fig. 1). In such a system, aerobically stabilized sludge is produced, which can be dried, sanitized on site and then used in agriculture. The main advantages of such a membrane bioreactor system are low costs, ease of operation, sustainability and low maintenance.

Water permeability coefficients from membranes with different titanium dioxide loadings.
Figure 2: Water permeability coefficients of membranes with different titanium dioxide loadings.

The biggest challenge for the successful implementation of such systems is the performance of the membranes. Commercial membranes used so far do not show sufficient separation performance and to maintain this separation performance over a longer period of time requires a great deal of equipment. Insufficient control of pore size, pore size distribution and interfacial effects causes a reduction of permeate flow induced by membrane fouling. Two approaches are pursued to improve membrane performance and reduce membrane fouling.

 

  • Mixed-matrix systems, consisting of nanoparticles embedded in a polymer matrix
  • Coatings by means of Plasma Enhanced Chemical Vapour Deposition (PECVD), which allows the deposition of ultra-thin functional layers.

Membrane development

Pore size and nitrogen flow of two membranes.
Figure 3: Pore size and nitrogen flow of two membranes with the same composition but produced by different processes.
Scanning electron microscope image of a membrane.
Figure 4: Scanning electron microscope image of a membrane produced by the phase-inversion process.

Mixed-matrix microfiltration membranes consisting of poly(ether sulfones) (PES) and Titania nanoparticles are produced by a phase inversion process. In addition to Titania, other nanoparticles such as silica or carbon nanotubes (CNT) have been incorporated into the polymer matrix. The water permeability value as a function of the content of nanoparticles is shown in figure 2. If the membranes were treated in a climate chamber with defined humidity before the precipitation bath, they show significantly higher fluxes. These have larger pores, as shown by the porometer measurements (Figure 3). Typical morphologies of membranes produced by a phase inversion process are shown in figure 4.

PECVD coatings of these membranes were performed in a parallel plate reactor at room temperature in a low pressure plasma. Mainly different precursors were used and the coating time was varied. Thus, hydrophilic surfaces based on SiOx and acrylic acid on the one hand and hydrophobic surfaces based on hexamethyldisiloxane and trifluoromethane on the other hand were produced. Surface modified membranes were characterized by contact angle measurements (static and dynamic), scanning electron microscopy, XPS, porometry, water flow measurements, MWCO 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.

Outlook

The influence of particle selection (size, type), membrane manufacturing parameters, membrane composition and the effect of different plasma coatings on the fouling behaviour of the membranes is investigated. Different methods such as BSA adsorption, filtration of naturally cloudy apple juice and activated sludge have been established. The most suitable membranes will also be tested in a membrane bioreactor pilot plant.