Investigating the Fundimental Basis for Selectors to Improve Activated Sludge Settling

Aerobic, anoxic, and anaerobic selectors have become popular for controlling filamentous bulking in activated sludge systems; however, selectors are not always successful. Regression analyses of data collected from 44 full-scale wastewater treatment plants, with operating selectors, provided criteria for determining when selectors will and will not be successful. Anoxic selectors do not appear to control filamentous bulking in long-mean cell residence time (MCRT) plants. Further, the elimination of all anoxic zones may help to control bulking in these plants. Other design/operating parameters, however, were shown to influence activated sludge settleability in long-MCRT plants. Aerobic selectors in short-MCRT plants do control filamentous bulking if they are small enough to produce a biochemical oxygen demand (BOD) concentration gradient in the aeration basins. Anoxic and anaerobic selectors do control filamentous bulking in short-MCRT plants if the selector volume is large enough and/or the selector mixed liquor suspended solids concentration is high enough. These selector systems do not appear to benefit from a BOD concentration gradient as the aerobic selectors in short-MCRT plants do. Although anaerobic/anoxic selector compartmentalization in these plants appears to improve settleability, this is presumably because of reduced selector short-circuiting.

Chudoba et al., (1973) introduced the aerobic “selector” over thirty years ago. They defined a selector as that portion of the aeration basin, upstream of the main aeration basin that provides a biochemical oxygen demand (BOD) concentration gradient, with either small aerated tanks in series or aerated compartments in series, and suppresses filamentous organism growth in activated sludge. It was later shown that “selectors” could also be anoxic or anaerobic. The reported success of selectors and their relative low cost to construct and operate have resulted in their wide-spread use around the world. Unfortunately, selectors have often proved ineffective at controlling filamentous organisms in activated sludge (Osborn et al.,1986; Wakefield and Slim, 1987; Gabb, 1988; Gabb et al., 1991; Daigger and Nicholson, 1990; Marten and Daigger, 1997, Marshall and Richard, 2000; Davoli et al., 2002; Labek and Rosenwinkel, 2002, etc.). Operating and design guidelines have been published in the literature, but these guidelines may not always be applicable, or even valid.

Materials and Methods
Surveys were sent to 125 activated sludge plants across the United States and the United Kingdom. Out of the plants surveyed, 85 had operating selectors, but only 46 of these plants were reported to have improved settleability following selector installation. More detailed information was requested for those plants with selectors. Because of the detail required by the second data request, only 44 out of the 85 agencies with selector plants were able to fully respond. One year’s worth of data from each of these plants were used in a regression analysis to test the selector operating and design criteria offered by the literature. A full-scale pilot study was also conducted at the East Bay Municipal Utility District (EBMUD) in Oakland, CA, and Orange County Sanitation District (OCSD) in Fountain Valley, CA. In both cases anaerobic selectors were installed in these large, short mean cell residence time (MCRT) activated sludge systems. Operating and design criteria for both plants were compared to literature values to help explain why a selector was successful at one plant but not at the other.

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