MONITORING TOXICITY IN ANAEROBIC DIGESTERS BY THE RANTOX BIOSENSOR - THEORETICAL BACKGROUND

Process control of anaerobic reactors is difficult due to the complexi ty of the methabolic pathways in the microbial consortium and to the d ifficulty of detecting and monitoring process instability in short tim e, before the biomass is poisoned by incoming toxicants. Process contr ol based on the Ranter biosensor is based on the following principle: the wastewater that can potentially induce an overload or contains a t oxicant 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 wastewate r to a buffer tank and consequently to protect the active biomass of t he full-scale reactor. It is generally accepted that methanogens are t he most sensitive microorganisms in anaerobic digestion. Among these b acteria, the acetoclastic methanogens are of primary importance becaus e some 70% of the converted chemical oxygen demand (COD) mass flow pas ses through acetic acid. Therefore the first objective in the developm ent of the Ranter biosensor has been to monitor the metabolism of acet oclastic methanogens in the presence of toxicants. This article presen ts the theoretical background required to evaluate the toxicity effect s by determining the kinetic constants of the considered microorganism s from experimental data. The results of two series of calibration tes ts, performed in order to obtain a preliminary evaluation of the biose nsor 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, w ill be the subject of the third (and last) article, which is planned t o describe the development of this instrument. (C) 1997 John Wiley & S ons, Inc.