CULTIVATION OF NITRIFYING BACTERIA IN THE RETENTOSTAT, A SIMPLE FERMENTER WITH INTERNAL BIOMASS RETENTION

The retentostat was developed for long-term continuous, axenic cultiva tion of microorganisms at those low growth rates which prevail in most natural habitats and which cannot be established properly in chemosta ts. How a microbial population approaches 'zero-growth' was studied in axenic cultures of Nitrosomonas europaea with complete biomass retent ion at 25 degrees C and constant input of a nutrient solution containi ng ammonium (0.57 mM) as energy source. Since only cell-free filtrate left the reactor, biomass accumulated until a stable maximum of 2.7 X 10(9) cells ml(-1) (398 mg l(-1) dry matter) was reached after about 5 weeks. In this state, growth rate approached zero, and the ammonium i nput just met the substrate demand required for maintenance energy (1. 43 mu mol NH3 - N mg dm(-1) h(-1)). The potential of the retentostat f or studying interactions between different microorganisms was demonstr ated with a cascade of cultures of Nitrosomonas, Nitrobacter, and a de nitrifying Pseudomonas. Thereby the ammonia was completely eliminated from artificial wastewater.