Nitrification-denitrification processes and technologies in new contexts

Recently, a number of new processes and configurations concerning nitrogen removal have appeared in the domain of water treatment. Processes such as SHARON, ANAMMOX, de-ammonification, OLAND and the nitrification-denitrification by methanotrophs, are emerging. A series of new tools to study and optimally use nitrifiers is under development. Examples are 16S rRNA-probes, nitrifier pheromones and online biosensors for ammonium, and nitrate. Furthermore, there are strong indications that nitrifiers can be used to bring about, by means of their radical generating potential, indirect bio-catalysis, and by this mechanism degrade certain chemicals. A series of so-called bio-supplements capable of stimulating or protecting nitrifiers is now documented and available in practice. Moreover, several challenging questions should urgently be addressed. Indeed, nitric oxide is perhaps of major significance as an environmental disinfection agent. There is also a possibility that the appearance of nitrite in microbial communities is a signal of overall ecosystem malfunctioning and uncoupling. In addition, the recently proposed hypothesis of closely coupled nitrification-nitrogen assimilation in forest soils needs to be explored for various other natural ecosystems. Finally, it is of considerable interest to examine to what extent nitrifier cells score as live food sources in the aquatic and soil food chains.