Biotechnology Letters 24: 315–319, 2002. © 2002 Kluwer Academic Publishers. Printed in the Netherlands. 315 Nitrification using polyvinyl alcohol-immobilized nitrifying biofilm on an O 2 -enriching membrane Yuan-Lynn Hsieh 1,∗ , Szu-Kung Tseng 1 & Yu-Jie Chang 2 1 Graduate Institute of Environmental Engineering, National Taiwan University, 71 Chou-ShanRd., Taipei 106, Taiwan, Republic of China 2 Department of Environmental Engineering, Tung Nan Institute of Technology, 92 Wan-Fu Hamlet, Shen-Kun Village, Taipei, Taiwan, Republic of China ∗ Author for correspondence (Fax: 886-2-23637854; E-mail: lynnhappily@yahoo.com.tw) Received 20 November 2001; Revisions requested 30 November 2001; Revisions received 13 December 2001; Accepted 16 December 2001 Key words: biofilm, nitrification, PVA, silicone membrane Abstract A combination of cell immobilization and membrane aeration approaches was used in a biological reactor to treat NH + 4 in wastewater. Nitrifying microorganisms, immobilized by polyvinyl-alcohol (PVA) and attached to the surface of a silicone membrane tube, were used to develop a novel reactor for nitrification. The immobilized biofilm had a rubber-like elasticity and resisted shear stress over 5 months of operation. The reactor removed 95% of ammonium, added at 1.97 g N m -2 d -1 , with O -2 enriching the membrane. Introduction NH + 4 removed from wastewater by membrane aera- tion bioreactors has become a common technology. For example, Brindle et al. (1998) designed a reac- tor in which nitrifying bacteria grew on a supporting membrane and where O 2 was supplied from the bot- tom of the biofilm and the O 2 utilization efficiency was therefore much higher than that obtained by tra- ditional aeration methods. Among the materials used to construct the membrane, silicone membranes are non-porous but with high gas permeability (Côté et al. 1988) and can be operated at a high gas pressure thus contributing to high transfer efficiency (Ahmed & Semmens 1992). Moreover, the O 2 transfer rate was higher when the permeable membrane was operated with a biofilm, for example with nitrifying bacteria on the outside (Rothemund et al. 1994). Cell im- mobilization techniques have recently been extended to biological wastewater treatment systems. Chen & Lin (1994) reported many materials such as polyacry- lamide, sodium alginate, agar and polyvinyl alcohol have been extensively applied in cell immobilization. In the beginning of this study, nitrifying bac- teria were directly cultivated on a gas-permeable membrane. However, the biofilm was unexpectedly sloughed off. Polyvinyl alcohol (PVA) was then introduced to entrap nitrifying microorganisms on the membrane and thus prevented such a loss of the biofilm. Moreover, immobilizing microorganisms yields a high cell density medium and can be used easily to separate solids from liquids in a settling tank. Immobilizing cells in PVA with the presence of starch or calcium alginate were found to improve gas perme- ability by modifying the structure, and thus favor the entrapment of microorganisms in matrix of the PVA (Chen et al. 1996). In this study, PVA-immobilized nitrifying biofilm attached to the surface of a silicone membrane tube was used as the basis of a bioreactor for nitrification of synthetic wastewater. A silicone membrane was used as both an O 2 diffuser and a nitrifying bacte- ria carrier, and PVA performed as an immobilizing agent to strengthen the nitrifying biofilm on the sil- icone membrane. The effects of the partial pressure of O 2 and NH + 4 loading on PVA-immobilized nitri- fying biofilm reactor were considered. The results of