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Three to four years of operating data at the Rock Creek Advanced Wastewater Treatment Facility (AWTF) and Durham AWTF were examined with to determine the reality of achieving very low phosphorus levels in a wastewater plant effluent. Both plants are required to achieve a median total phosphorus (TP) level of 0.07 milligram phosphorus per liter (mg P/L) in their effluent during the period analyzed. The Rock Creek plant phosphorus removal system uses a combination of alum addition in chemically enhanced primary treatment, tertiary clarifiers, and granular media filters. The Durham AWTF phosphorus removal system uses biological phosphorus removal, alum addition in tertiary clarifiers, and granular media filters.

Analysis of the data shows that both plants can and do achieve reliable phosphorus removal to these levels. However, there were distinct differences. The biological phosphorus removal system at Durham appears to add an increased level of variability in the process performance as compared to the purely chemical removal systems in place at Rock Creek. However, the alum usage at Rock Creek is significantly higher (approximately 70 milligrams per liter [mg/L] total versus 45 mg/L total at Durham).

Both plants exhibit a minimum soluble phosphorus level (0.01 to 0.02 mg P/L) that does not appear to be affected by increased alum addition. Both plant “overdose” the alum beyond that needed for simple soluble phosphate removal in order to “dilute” the tertiary clarifier solids with non-phosphorus bearing solids. This practice reduces the particulate phosphorus concentrations, thus allowing the plants the reliably meet their effluent targets. At the Durham AWTF the median value for the effluent TP was 0.047 mg P/L. At the Rock Creek AWTF, the median value for the effluent TP was 0.043 mg P/L.

Document Type: Research Article


Publication date: January 1, 2005

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