Process control of anaerobic reactors is difficult due to the complexity of the methabolic pathways in the microbial consortium and to the difficulty of detecting and monitoring process instability in short time, before the biomass is poisoned by incoming toxicants. Process control based on the Ranter biosensor is based on the following principle : the wastewater that can potentially induce an overload or contains a toxicant is first tested on a small "upstream" digester (the Ranter). This reactor makes possible to detect the potential instability and, if necessary, to divert the concentrated and/or contaminated wastewater to a buffer tank and consequently to protect the active biomass of the full-scale reactor. It is generally accepted that methanogens are the most sensitive microorganisms in anaerobic digestion. Among these bacteria, the acetoclastic methanogens are of primary importance because some 70% of the converted chemical oxygen demand (COD) mass flow passes through acetic acid. Therefore the first objective in the development of the Ranter biosensor has been to monitor the metabolism of acetoclastic methanogens in the presence of toxicants. This article presents the theoretical background required to evaluate the toxicity effects by determining the kinetic constants of the considered microorganisms from experimental data. The results of two series of calibration tests, performed in order to obtain a preliminary evaluation of the biosensor response to overload and toxicity conditions, are reported. In a second article, calibration tests will be described which refer to two prototypes of the biosensor tested in different operating conditions. The crucial point related to the Ranter, i.e., its comparison with a "normal" laboratory-scale digester to simulate a full-scale plant, will be the subject of the third (and last) article, which is planned to describe the development of this instrument.