Practical identification of the dissolved oxygen dynamic in activated sludge plants

Abstract

This paper presents the application to a full-scale waste water treatment plant (WWTP) of a practical methodology to identify the oxygen supply and consumption terms, which regulate the dynamic behaviour of the dissolved oxygen concentration (SO) in activated sludge reactors. This methodology is based on a periodic adjustment of the selected model for oxygen mass-transfer coefficient (KLa) and a continuous estimation of the oxygen uptake rate (rO), maintaining constant the adjusted KLa model, in order to uncouple the effects that the consumption and supply terms have on the SO concentration. The adjustment phase of the KLa model is based on typified excitations of the system through the aeration system of the plant, which allow the estimation of KLa for different values of the aeration intensity parameter, for instance, air flow rate in air diffusion systems, revolution speed in surface aerators, etc. Once the KLa model is adjusted, it is possible to evaluate the KLa value in process conditions at any time. The continuous estimation of rO is carried out starting from the oxygen mass balance in the activatedsludge reactor. This practical methodology has been successfully verified in the Badiolegi WWTP in Azpeitia (Spain), which biological process consists of a double stage AB process, with two different aeration systems: fine pore air diffusers in the A-stage and surface aerators in the B-stage.