A new bioreactor (shell-and-tube co-immobilized cell bioreactor) for integrated nitrogen removal from wastewater was developed. The configuration of this bioreactor was similar to a shell-and-tube heat exchanger, and consisted of a bundle of parallel tubes made up of polyvinyl alcohol (PVA) gel, containing nitrifying and denitrifying bacteria, fixed on to the tube sheets. The tube bundle was contained in a cylindrical shell which was provided with two channels, one at each end. Ammonia nitrogen wastewater was introduced into the shell-side space surrounding the tubes. At the same time, air was pumped through an air-stone into wastewater for nitrification. Ethanol solution was pumped into one channel. It flowed through the tubes into the other channel and was withdrawn into the ethanol solution tank for recycling. An integrated nitrogen removal process was studied using this bioreactor. The nitrogen removal process could be much simplified when it was carried out in this bioreactor, because only a small amount of ethanol diffused into the wastewater from the recycling ethanol solution, and the BOD5 value in the effluent was lower than 30 mg l(-1). In addition, no accumulation of ammonia, nitrite and nitrate in the recycling ethanol solution was observed. The total inorganic nitrogen (TIN) concentration was below 3 mg l(-1) after the ethanol solution was recycled in the bioreactor for a week. The ammonia oxidation rate when nitrification and denitrification occurred simultaneously in a bioreactor was about twice as fast as when only nitrification occurred (denitrification was restrained). The operational system remained stable for more then 3 months and the mechanical strength of the tubes was excellent even after 3 months of continuous operation. The existence of a short-cut of nitrogen removal was indicated, in which ammonia nitrogen was first converted into nitrite and subsequent denitrification proceeded directly from nitrite using the co-immobilized mixed culture. (C) 2003 Published by Elsevier Ltd.