Use of ORP for Monitoring and Control of Aerobic Sludge Digestion

The oxidation‐reduction potential (ORP) was monitored within aerobic sludge digesters undergoing alternating aerated and nonaerated conditions. Observations were made on both bench‐scale and pilot‐scale reactors. Batch tests were also conducted on both reactors, involving a time series of chemical analyses, in conjunction with automated monitoring of reactor ORP and dissolved oxygen. The results indicated that a characteristic and reproducible ORP profile was associated with the cycled operation of the digesters. These real‐time ORP profiles had a number of distinctive features, directly related to changes in system chemistry and biological activity. The ranges of aerobic and anoxic respiration, as well as fermentation, were clearly defined by slope changes on the ORP profile. They also corresponded to the presence of the appropriate electron acceptors. The transition between the different ORP ranges was observed to be quite different during reaeration and deaeration. ORP was related to low levels of dissolved oxygen and nitrate. The data indicated that ORP could be used to measure very low oxygen concentrations. The reproducibility of the ORP profiles and sensitivity of the measured potential, to changes in biological or chemical activity, appears to make it an ideal parameter for automated monitoring and process control.