Using the Oxidation Index (OXI) as an Activated Sludge Process Control Tool

Wastewater treatment plant operators have increased mean cell residence time (MCRT) during the colder months and decreased it during warmer months to address bioactivity changes. However, MCRT and related MLSS only address the biomass contained within the system and not the time in which microorganisms have to function while passing through the aeration tank. The Oxidation Index (OXI addresses both factors and estimates the total treatment power being applied. However, the modified OXI (MOXI) uses MLVSS instead of MLSS for biomass concentration and a temperature factor based upon the Arrhenius equation. A third version of the OXI concept relates the treatment power (MOXI) to the organic loading through the power-Ioadratio (PLR). Data from four plants representing four very different loading conditions were analyzed to determine if OXI, MOXI, or PLR could be related to sludge settleability expressed as sludge volume index. Two of the four plants showed promise for OXI/MOXI/PLR as a process control tool.