Full Scale High Rate Wet Weather Biological Contact Performance

The Durham AWTF in Tigard, OR experiences peak wet weather flows in excess of five times the average dry weather flow. Historically these high peak flows have been handled with a combination of peak flow equalization and step feeding the bioreactor system. In 2005 CH2M HILL suggested the plant try a new operating mode where primary effluent is added to the last zone of the aeration basins, a version of contact stabilization, during high wet weather events. Contact times during the peak flow events were at about 60 minutes. Operating experience with this mode showed excellent BOD and TSS removal. This operating mode increases plant secondary treatment capacity from approximately 70 MGD to at least 100 MGD with no degradation in effluent quality. These results have shown the viability of high rate biological contact in both conventional secondary treatment systems and in dedicated wet weather treatment facilities.

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