Proceedings of the 4 th Slovenian-Croatian Symposium on Zeolites 73 BACTERIA IMMOBILIZED ON NATURAL ZEOLITIZED TUFF ARE ABLE TO SURVIVE IN THE CONDITIONS OF EXTREME pH Tomislav Ivanković, Jasna Hrenović Department of Microbiology, Faculty of Science, Zagreb, Croatia E-mail: tivanko@zg.biol.pmf.hr ABSTRACT The bacteria in the form of biofilm are more adapted to survive environmental stresses. We prepared biofilm by immobilizing phosphate-accumulating bacterium Acinetobacter junii, a heterotrophic bacteria normally present in activated sludge, on the particles of natural zeolitized tuff. The obtained bioparticles had thick biofilm with number of immobilized bacteria 2.2±0.12×10 9 CFU g -1 . To test the survival of such bacteria in harsh conditions of wastewater treatment plants, such as unfavourable pH, the bioparticles were introduced in reactors containing simulated wastewater of extreme pH values, namely, 2, 3, 4 and 5 during 24h. The bacteria immobilized on bioparticles showed great resistance to unfavourable pH. The bacteria were viable in the biofilm, but also detaching from the biofilm and continued its growth in the media as planktonic cells. At pH 4 the survival of bacteria was 82% of starting number as opposed to the control reactors where survival was 0%. At pH 3, there were 38% of bacterial cells able to survive, while at pH 2 no viable cells were present in the reactors after the incubation. The results proved that bacteria immobilized on natural zeolitized tuff are able to survive the extreme conditions of wastewater treatment plants such as 24h disturbances of pH. This way the survival of bacterial population is enabled while the system upsets are fixed and bioparticles could serve as seeds for regeneration of bacterial population in the treatment plant. Keywords: bacteria, immobilization, pH, wastewater, zeolite. INTRODUCTION Over the last 30 years the preferred process for phosphate (P) removal from wastewater has become the enhanced biological phosphorus removal (EBPR), a process based on the metabolism of P-accumulating bacteria present in the activated sludge. Currently much attention is on the technology of bacterial immobilization. The bacteria that are immobilized on suitable carriers are better adapted to survive periods of environmental stresses [1] and show enhanced metabolic activity in the bioreactors. We tested the ability of P-accumulating bacterium A. junii immobilized on the natural zeolitized tuff (NZ) to survive in the conditions of extreme pH values. The immobilized bacteria form biofilm, a specific community of microorganisms which offer protection from environmental insults and assaults [2]. EXPERIMENTAL Bacterium The culture of P-accumulating bacterium A. junii (DSM 1532) was obtained from the Deutsche Sammlung von Microorganismen und Zellkulturen GmbH. Growth medium The bacteria were incubated in a chemically defined water solution that simulated the real wastewater. The composition was (in mg L -1 of distilled water): Na-propionate 300; peptone 100; MgSO 4 10; CaCl 2 6; KCL 30; yeast extract 20; KH 2 PO 4 88. Before autoclaving (121°C/15 min) the pH of wastewater was adjusted to desired values with 1 M NaOH and HCl.