NERENBERG RESEARCH GROUP

HOLLOW-FIBER MEMBRANE BIOFILM REACTORS

For drinking-water treatment, biofilm processes offer many powerful advantages over a suspended-growth system. The most important advantage is the ability to provide high-density cell aggregates on reactor surfaces. Biomass retention through biofilms also is advantageous for slow-growing bacteria, such as nitrifiers and hydrogen-oxidizing autotrophs. Biofilms increase the removal capacity per unit volume of reactor and makes the process economical.

Hollow-fiber membranes can be used to deliver of gaseous substrates to a biofilm growing on their surface without “bubbling,” providing a safe and efficient treatment system. Hollow-fiber membranes can have been used to deliver hydrogen, oxygen, methane, and other gases for a variety of water, wastewater, and groundwater treatment applications. The fibers can be made from hydrophobic and hydrophilic materials. Figure 1 illustrates that microporous, hydrophobic membranes have dry pores, while hydrophilic membranes have liquid-filled pores. Since gas molecules diffuse much faster through a gas than through a liquid, hydrophobic membranes have much higher gas transfer efficiencies.

Figure 1. Gas transfer in hydrophobic and hydrophilic hollow-fiber membranes (following Mulder, 1998)

A key feature of hydrophobic hollow-fiber membranes is that they can be operated at high gas pressures without bubbling. When membrane pores are fairly large, such as with silicon membranes, bubbles begin to form when the gas pressure slightly exceeds the hydrostatic pressure of the liquid. In contrast, when the pores are small, the water surface tension on the pores can provide a significant resistance to the formation of bubbles, allowing higher applied pressures. Higher gas pressures improve mass transfer by providing a greater driving force. Images of microporous membranes are shown in Figure 2 and 3. We currently are studying MBfRs for bromate reduction, nitrification/denitrification, and microbial fuel cells

Figure 2. Image of a bundle of hollow fiber membranes, a cross section of a single membrane, and SEM image of a microporous fiber surface. The pore size is around 0.15 um x 1 um. (Nerenberg, 1993)

Figure 3. Bundle of 98 hollow fibers in a ¾” shell. The discoloration on the fibers is the biofilm.