A new procedure for sizing the biological reactors and the final settling of single sludge systems aimed at nitrogen degradation is proposed. Both combined nitrification and pre-denitrification systems are considered. The procedure is innovative as it is able to give the proper size
of the treatment phases as a function of the required performances. A steady-state model taking into account both suspended and dissolved substrates is developed for the biological phases, while the limiting solid flux theory is assumed for the design of the settling phase. A relationship
correlating the two phases is obtained expressing opportunely the sludge recycle and waste flow rates, without recourse to any empirical parameter. The combined nitrification model includes three substrates (soluble and particulate biodegradable organic matter and ammonia nitrogen) and gives
an explicit expression of the biological reactor volume if both the influent flow composition and the kinetic parameters are known and the effluent nitrogen concentrations are fixed. The pre-denitrification model considers four substrates (the above cited and the nitrate nitrogen) and makes
it possible to define the applicability of the system and to express in explicit terms the biological reactor volumes, as in the previous case. Applications of the biological and settling models show: i) a relevant effect of the influent flow rate, the organic and nitrogen load and the desired
effluent characteristics on the size of the treatment system; ii) an important influence of the maximum ammonia nitrogen removal rate and the ratio between the decay rates of heterotrophic and autotrophic biomass on the nitrification reactor volume and iii) an underestimation of the denitrification
reactor volume when zero order in stead of Michaelis-Menten type denitrification kinetics are used.
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