Water Environment Federation (WEF)

The Use of Oxidation-Reduction Potential as a Means of Controlling Effluent Ammonia Concentration in an Extended Aeration Activated Sludge System

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This study developed a strategy to control effluent ammonia from an activated sludge system, using oxidation-reduction potential. By controlling effluent ammonia concentrations, disinfection of treated wastewater is more cost effectively achieved by chloramination rather than break-point chlorination. The system under consideration was an extended aeration oxidation ditch. The study takes into account the concepts of nitrification and denitrification, as well as intrinsic characteristics of oxidation-reduction potential, while also keeping ease of operation in mind. Data were gathered by varying the rate of aeration of the basin from two extremes, while collecting samples along the timeline. Oxidation-reduction potential, dissolved oxygen concentration and ammonia concentration data were collected for evaluation. Using these data a preliminary control strategy was developed. The control strategy focused on operating the system in a continuous flow and varying aeration scenario. Two attempts to control the oxidation ditch were made. Using oxidation-reduction potential as an indicator, adjustments were made to the aerator controls in order to accommodate changes in organic loading, and maintain a constant effluent ammonia concentration. The first attempt was met with success and used to fine tune the strategy for the second attempt. The second attempt experienced more success than the first in controlling effluent ammonia concentrations, thus confirming the original hypothesis of the study.

Electrode Potential or Oxidation Reduction Potential (ORP), as it is commonly referred to, has proven to be an effective method of monitoring wastewater processes and in some instances has been used as a control parameter. This thesis shows that ORP can be effectively used as an ammonia control indicator for use in an extended aeration oxidation ditch process. This use of ORP as an indicator to control ammonia, led to the development of a control strategy for the extended aeration oxidation ditch under consideration.

In the context of this study, ORP is a measurement of the ability of the system being observed to either accept electrons (reduce) or donate electrons (oxidize). ORP sensors measure this ability in milivolts. When positive, the measurement indicates the degree to which the system is oxidative, and when negative, indicates the degree to which it is reductive. When an activated sludge system experiences high organic loading, oxygen is consumed and a reducing environment occurs (Rabinowitz, 1985).

Some oxidation ditch systems experience significant diurnal variation in dissolved oxygen concentration due to constant aeration rates for varying waste loads. During periods of low organic loading, DO levels may be sufficient to support complete nitrification resulting in no ammonia present in the effluent of the oxidation ditch. Conversely, anoxic or near anoxic conditions during periods of high organic loading preclude nitrification and effluent ammonia concentrations increase. Fluctuations in ammonia concentrations make disinfection difficult to achieve with a uniform chlorine dose. Ammonia is utilized in the disinfection process; the combination of chlorine and ammonia allows for the formation of chloramines, which are effective disinfectants. During periods of no effluent ammonia, it may be necessary to provide break point chlorination. One of the benefits of effective control of the ditch DO as proposed would be the ability to insure sufficient ammonia to allow for effective disinfection without the expense of breakpoint chlorination. Optimizing power used in aeration is another benefit of effective control.

The objective of this study was to develop a control strategy for the operation of an oxidation ditch process using the measurement of the Electrode Potential (ORP) and dissolved oxygen of the mixed liquor. A strategy is needed to operate the process to maximize COD removal and nitrification and denitrification while allowing ammonia to remain in the effluent to aid in the disinfection process. These objectives were accomplished by first, conducting an intensive sampling regime, second, reviewing sampling data and planning a control strategy, and finally, attempting to control effluent ammonia concentrations from the oxidation ditch. All of these objectives were met an are documented herein.

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