Relation between redox potential and oxygen levels in activated-sludge reactors

Alain Heduit (1) , Daniel R. Thevenot (2)

1 UR HBAN - Hydrosystèmes et Bioprocédés
2 LABAM - Laboratoire de Bioélectrochimie et Analyse du Milieu

The importance of dissolved oxygen level in determining the platinum electrode potential in activated sludge has been clearly demonstrated by current-potential curves plotted at different oxygen concentration. Tests have been carried out, in the laboratory and in full scale treatment plants, to define the relationship between the platinum electrode potential at equilibrium (Eh) and the dissolved oxvgen [02) concentration in the activated sIudqe. These two parameters obey a law in the form Eh = a + b log [O2]. The measured values of coefficients a and b differ widely to those found from the oxygen reduction reaction in water (a = 0.8 V at pH 7 and b = 15 mV per decade). Factors a and b mainly depend on the sludge loading, the aeration conditions and the sludge concentration. Using non-polished stationary platinum ring electrodes, the following values of a were obtained (at pH between 7 and 7.6): + 410 mV/NHE for sludge aerated for several hours without feeding, + 265 mV/NHE for over-aerated/low-loaded sludge (Cm = 0.2 kg BOD.kg MLVSS-1.day-1). Factor b would seem to lie between 55 and 65 mV when the sludge is continuously aerated without feeding. At low loads with excess aeration, it lies between 70 and 90 mV. When the medium is slightly septic at low dissolved oxygen concentrations (insufficient daily aeration time, high sludge concentration or aerators shut down for too long periods), factor b increases and can reach 200 mV. In the same way, at high loads, factor b can become 150 mV. These results demonstrate the importance of dissolved oxygen concentration in the mechanisms which determine the metal electrode potentials in activated sludge. They also illustrate the role that the electroactive species play in the process. The type and concentration of these species depend on parameters such as the sludge loading, the overall oxygen supply, the aeration sequence and the sludge concentration.

Mots clés: control; instrumentation; waste water treatment plant; oxygen; activated sludge; redox potential

Domaines: Sciences de l'environnement; Chimie analytique

Alain Heduit, Daniel R. Thevenot. Relation between redox potential and oxygen levels in activated-sludge reactors. Water Science and Technology, 1989, 21, pp.947 - 956. ⟨hal-01084520⟩