Fixed bed reactors are often used in aerobic wastewater treatment. They are advantageously used in order to increase the biomass concentration within the reactor and are preferred when the maximum specific growth rate is close to the dilution rate to be applied. The use of carriers allows increasing significantly the microbial residence time and reduces the washing-out risk at short hydraulic retention times. One of the best given example of aerobic processes based on slow growing microbes is the so-called "nitrification". Nitrification is the most commonly used nitrogen removal process and consists of aerobic ammonia oxidation to nitrites (nitritation) and then to nitrates (nitratation). Nitrosomonas and Nitrobacter genus are the major autotrophic groups involved in ammonia and nitrites oxidation, respectively. Both genus are characterised by a maximum specific growth rate ranging from 0.014 to 0.064 h-1, which are approximately 10 times lower than commonly accepted values for heterotrophic microbes. These specific growth rates implies that in suspended biomass reactor (CSTR, for instance) the hydraulic retention time must be kept higher than 15 to 70 hours, to avoid biomass wash-out.
By comparison, fixed-bed nitrification reactors can work at residence time lower than five hours. Fixed -bed reactors are therefore an important tool for wastewater treatment and nitrification is probably the best example of a biological process that can be advantageously integrated in fixed-bed reactors. Together with biomass measurements, respirometry allows the process characterisation. In fixed-bed reactors these measurements are significantly more complex than in suspended sludge reactors. The aim of this project is to develop new in-situ respirometric tools and associate t hem to stoichiometric measurements for on-line control of fixed-bed processes. Nitrification will be taken as first experimental model and the expected results will be applied to other fixed-bed process.
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