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The Evolution of Plug Flow BNR at Durham AWWTP

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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.
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Document Type: Research Article

Publication date: 01 January 2006

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