Aluminum-Based Water Treatment Residuals Sorb Phosphorus in Urban Wetlands: Preliminary Study.

The City of Boise, Idaho owns the Hyatt Wetlands Reserve, an urban 18 ha site consisting of 9 ha of wetlands.  The City has expressed interest in engineering 0.45 ha of the wetlands to detain and treat urban stormwater runoff for capturing soluble P.  The City also generates Al-based water treatment residuals (Al-WTR) as a waste product from drinking water treatment; Al-WTRs are proven sorbents for P.  Thus, an opportunity exists to blend the use of a typical waste material within close proximity to where it is generated, with the improvement of water quality by potentially reducing urban runoff P.  Our proof-of concept research objective was to identify relatively long-term Al-WTR application rates for sorbing soluble P based on P concentrations similar to those found in Boise’s urban runoff.  A column study was established similar to engineering plans for the wetlands (e.g., gravel overlain by soil overlain by Al-WTR treatments), with treatments consisting of 0, 62, 124, or 248 Mg ha^-1 Al-WTR; red fescue was established in the columns, and columns without plants or Al-WTR were used as controls. Infiltration rates were improved via plants regardless of Al-WTR application.  Plants reduced soluble P leaching; Al-WTR further reduced soluble P leaching.  Fescue yield, P concentration, and P uptake, and Olsen extractable P increased with increasing Al-WTR rate. Amorphous extracts and sequential extractions showed that most P was bound to the Al-WTR. Given the rates, and the calculated P storage capacity, Al-WTR rates of 124 or 248 Mg ha^-1 appear promising for capturing P and reducing transport below the engineered zone.