The Evolution of Plug Flow BNR at Durham AWWTP

This paper describes experiences with plug flow nitrification and plug flow biological nutrient removal (BNR) at Durham Advanced WWTP. The kinetic advantages of the plug flow configuration for nitrification have been recognized for some time; however, limited plug flow BNR process design information was available at the time (1997).

The case study began with pilot testing in 1998, during which a side-by-side comparison was conducted in one-cubic-yard containers. Based on the results, a new secondary treatment train was designed with an aerobic cell that had baffles to provide plug flow characteristics.

Despite all testing and careful design, the plug flow train appeared to perform at less then 100 percent of its potential after startup in 2000. In particular, biological phosphorus removal appeared to perform less consistently and efficiently in the plug flow train compared to the rest of the plant.

In 2003 a nitrification capacity study was conducted. This included a combination of full scale stress testing and activated sludge modeling. For the study, the new plug flow train was compared with one of the three complete mixed trains. The goal was to quantify the additional nitrification capacity provided by the plug flow design.

This study showed the plug flow train provided 16 percent more nitrification capacity. The full scale stress testing also identified at least one major cause of the observed performance problems: insufficient air distribution control. The diurnal flow and load changes resulted in alternating oxygen demand distribution throughout the plug flow basin. Because of control limitations, DO concentrations would vary between 0 mg/L and 6 mg/L throughout the day. This limitation was corrected the next year by installing additional DO meters and modulating air control valves. Following, one of the existing complete mixed aeration trains was also converted to plug flow.

In August of 2005, the plug flow train was unintentionally given the opportunity to show off its full potential. This occurred when a dewatering centrate storage tank was accidentally drained during the diurnal peak loading time of day. Unlike the complete mixed trains, the plug flow trains only showed a minimal increase in effluent ammonia despite influent ammonia concentration nearing 80 mg/L for several hours.

The Durham Advanced WWTP is a Clean Water Services facility located in the Portland, Oregon metropolitan area. The facility discharges into the nutrient-sensitive Tualatin River. Dating back to the early 90s, the plant has been required to comply with very low effluent phosphorus (<0.07 mg/L) and ammonia (<0.5 mg/L) limits. Clean Water Services has invested substantial effort and capital to improve the treatment process at Durham AWWTP, as well as its economic performance. One of the key components of this strategy was and is maximizing capacity of existing facilities through improved process design, process optimization, and automation.

The Durham AWWTP was first placed into service in 1976. In 1978, the plant was upgraded to chemical phosphorus removal with primary and tertiary chemical addition. In 1990, the plant was upgraded to BNR in an effort to reduce chemical cost. During the planning stages of the last secondary treatment expansion, optimization of the existing and future BNR trains was a main point of focus.

The improvements to the BNR process included the introduction of:

  • Primary sludge fermentation to improved biological phosphorus removal
  • Dewatering centrate storage to better manage the recycle ammonia load
  • The design of a new secondary treatment train (Train 4) in a plug flow configuration.

The new plug flow train was taken into service in 2001 while other improvements of this expansion were completed the following two years. In the years following startup, additional efforts were made to further improve the performance of the new train, as well the rest of the facility. This included full scale nitrification stress testing and air distribution control upgrades. Finally, Train 3 was also upgraded to plug flow mirroring the optimized design of Train 4. Figure 1 shows the process schematic and Figure 2 shows an aeration photo of the treatment plant in 2005.

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