The Durham Advanced Waste Treatment Facility AWTF treats wastewater to very low phosphorus levels (median effluent of less than 0.1 mg P/L) using a combination of biological phosphorus removal followed by tertiary treatment with alum. Historically, the biological phosphorus removal process has shown a relatively high level of instability. Testing was done during summer 2005 to determine the reasons for this instability. The testing showed the biological phosphorus removal process was very sensitive to mechanical upsets and aeration control problems. It is hypothesized that this sensitivity may be the result of the competition between polyphosphate accumulating organisms (PAOs) and glycogen accumulating organisms (GAOs) within the biological phosphorus removal process. In addition, a method of crossseeding between aeration basins was developed to speed the recovery of biological phosphorus removal in an upset basin.
The Durham AWTF in Tigard, Oregon, is run by Clean Water Services. The Durham plant is rated at 27 million gallons per day (MGD) and uses a combination of BPR and tertiary treatment with alum to achieve a median effluent limit of 0.1 mg total phosphorus/L. In addition, they have a 0.1 milligrams per liter (mg/L) ammonia limit, but no total nitrogen limit during the dry season. In the wet season, the plant does not have any nutrient limitations. During summer 2005 a full-scale BPR optimization program was developed and operated, with the goal of determining whether process changes could be made to optimize the operation of the existing BPR system.
The Durham AWTF secondary treatment system consists of four fully independent biological phosphorus removal systems operated in parallel. All aeration basins (ABs) have essentially the same volumes with seven separate cells. Figure 1 illustrates the differences between the ABs. All the basins were operated in A2O mode, except AB #1, which was operated in the Johannesburg (JHB) configuration.
Historical Biological Phosphorus Removal Performance
The optimization program’s goal was to increase the reliability and performance of the Durham AWTF. Historically, the BPR system was subject to frequent upsets that resulted in relatively high ortho-phosphate (OP) levels in the secondary effluent (Johnson et al., 2005). Figure 2 illustrates the historical performance of the Durham BPR system. It should be noted that during the years prior to 2005, secondary alum addition was practiced to reduce secondary effluent OP levels during BPR upsets. So it is likely that the actual frequency of BPR upset conditions is higher than indicated in this figure. The percentage of the time that the secondary effluent OP was less than 0.1 mg P/L has historically averaged approximately 60 percent of the days.