Second-Generation ATAD Provides Solution to Upgrade at Constrained Site

The Douglas County Sewer District No. 1 (District), in central Washington State, has been dealing with a significant odor problem ever since they installed an Autothermal Thermophilic Aerobic Digestion (ATAD) process as part of a multifaceted facility upgrade completed in 1998. Based on the District's experience, and experience elsewhere, it appears that a set of reduced sulfur compounds generated in these early, first-generation ATAD systems could not be removed by odor control technology available at the time. Most of the first-generation ATAD facilities of the late 1990s have significant odor problems.Setting the odor problem aside, there are significant advantages of the ATAD process, including a relatively small process volume and the capability of retrofitting existing tanks with the ATAD technology. Because of the intrinsic value of the ATAD, specialty vendors have developed second-generation ATAD processes that retain the underlying advantages of the ATAD process but avoid even short periods of anoxic conditions in the reactor. These newer designs are much more robust in aeration capability, with oxidation reduction potential (ORP) control of the aeration process. As a result, no penalty is paid for the required larger blowers, as they are turned down automatically via ORP control when not needed.After experiencing firsthand the frustration of a non-performing first-generation ATAD odor control system, the District opted to upgrade it with an improved, second-generation system. However, prior to doing so, they wanted assurance that the new ATAD system would operate without odors. If first-generation ATAD type odors were detected following a new project replacing the existing first-generation ATAD system, then the District wanted to have strong, quantitative and mutually agreed-upon standards to settle disputes if any were to arise.This paper describes the methods that were used to assure the District that reasonable odor standards would be enforceable in the event of non-performance on the part of the secondgeneration ATAD vendor. Non-performance is defined in quantitative terms to allow the District to stand on a strong foundation in the unlikely event of a construction dispute.