In order to complete and support the industrial application of ORP regulation for Activated Sludge, initiated by the first installation at Yffiniac in 1983, the thermodynamic approach to oxidation-reduction equilibria has been examined using the Pourbaix atlas of electro-chemical equilibria. A "biological window" has been defined in the pH-EH diagram for pure water, including the EH = f (pH) lines for redox pairs relating to the main elements involved in biological treatment (C, N, P, S). The theoretical values have been compared with those observed in various plants chosen for the variety of situations which they represent The recordings and graphs show that values "in the field" are consistent with those given by the thermodynamic approach. Moreover, they illustrate how compounds of carbon, nitrogen and iron influence the redox potential measurement and, as a result, how ORP regulation is useful in controlling them. The way in which transformations of compounds of phosphorus and potassium are affected by the oxidation-reduction conditions of the medium is also demonstrated. Fifteen years of experience with ORP regulation has allowed conclusions to be drawn regarding both the measurement itself and its use. Application of ORP regulation, initially normally reserved for plants with capacities above 10,000 PE, now extends to smaller plants. Many of the new logic controllers have this type of regulation, including SUPEROXYDOSE, specially designed for reliability of operation of small and medium sized plants. Another development, of relevance to larger plants, is the REGUL'N(R) process. (C) 1998 Published by Elsevier Science Ltd. All rights reserved.
