Factors Influencing Selector Success And Failure

This paper explores factors leading to failure of an anaerobic selector compared to success at an adjacent wastewater treatment plant in the Seattle, Washington region. An anaerobic selector had been tested at bench and pilot scale at the West Point Treatment Plant in 1988. Settleability during selector operation proved worse than during periods of operation without the selector. In contrast, pilot-scale operation of an anaerobic selector at the South Treatment Plant in Renton was successful and full-scale operation of a selector has been able to control chronically poor settleability at this plant. Modeling of the two different plant configurations was conducted using the commercial BioWin™ model and a multi-species model developed by the senior author. The BioWin™ modeling confirmed the signs of enhanced biological phosphorus removal at the South Treatment Plant: disappearance of soluble phosphorus from the plant effluent, phosphate release in the anaerobic zone, and increase in the ratio of phosphorus accumulating organisms (PAO) relative to other heterotrophic organisms. These signs failed to appear in the modeling of the West Point plant. This suggests that biological phosphorus removal is key to improving settleability with an anaerobic selector at this plant. The multi-species model provided a qualitative explanation for the poor settleability experienced at West Point in non-selector operation. It was also used to check potential feasibility of an aerobic selector for West Point. It failed to predict, however, the experimentally determined result that settleability was worse during anaerobic selector operation than it had been before. It likewise failed to adequately simulate phosphorus uptake.