The control of aeration by measuring the oxidation reduction potential (ORP) of activated sludge, conducted at the YFFINIAC (West of France) sewage plant in 1983, showed this type of regulation to be attractive in that it can optimize energy costs and perfect the removal of carbonaceous and nitrogenous pollutants (CHARPENTIER et al, 1987). From a practical point of view, this new, inexpensive means of regulation has moreover proved easier to implement than the method based on measuring Dissolved Oxygen (D.O.).

Before implementingthis process, we undertoook a pilot study that enabled us to make more accurate the ORP values for activated sludge that must be observed in order to achieve the satisfactory removal of carbonaceous and nitrogen pollutants. Furthermore, the ORP readings revealed points of inflection that can be used for regulation purposes. These have been related to the disappearance either of the ammonia or the nitrates with release of phosphates.

The experience gained with 7 ORP regulating systems, most of them in operation now for several years, and installed in works with vastly different characteristics, shows that the range of ORP values used corresponds to the pilot-scale experiments. It also shows the attractive features of the ORP signal, compared with that of D.O., when used for the regulation and interpretation of the phenomena involved in the biological treatment. This type of regulation leads to improvements in the annual electricity balance, expressed in terms of kWh v. kg of treated BOD5, and has proved to be compatible with a programmed shutdown of the aerators during the winter peak hours when the tariff per kWh is particularly expensive. In conclusion, the diversity of experiments on site has largely contributed towards the definition of those practical methods upon which depends the success of the system.

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