The aim of this work is to linearize and control a non linear model of a biological nitrogen removal process. The paper illustrates the use of a linearized model in the design of an observer (software sensor) and a linear control technique based on a phenomenalogical model of the process. This model describes the complete dynamics of autotrophic and heterotrophic biomasses, biodegradable organic and nitrogenous matters. The control approach structure is combined with the estimation algorithm, for the on-line reconstruction of unmeasured biological states of the bioprocess. The efficiency of both the control and estimation are demonstrated via computer simulations.