Aluminum-based water treatment residual use in a constructed wetland for capturing urban runoff phosphorus: Column study

Aluminum-based water treatment residuals (Al-WTR) have a strong affinity to sorb phosphorus. In a proof-of-concept greenhouse column study, Al-WTR was surface-applied at 0, 62, 124, and 248 Mg/ha to 15 cm of soil on top of 46 cm of sand; Al-WTR rates were estimated to capture 0, 10, 20, and 40 years of phosphorus from an urban watershed entering an engineered wetland in Boise, Idaho, USA. Creeping red fescue (Festuca rubra) was established in all columns; one set of columns received no Al-WTR or plants. After plant establishment, once per week over a 12-week period, ~1.0 pore volumes of ~0.20 mg phosphorus/L was added to each column. Infiltration rates were measured, leachate was collected and analyzed for soluble phosphorus, and fescue yield, phosphorus concentration and uptake were determined. After plant harvest the sand, soil, and the Al-WTR layer were collected and analyzed for Olsen phosphorus, amorphous aluminum, iron, and phosphorus storage capacity (PSC), and soluble+aluminum+iron-bound, occluded, and calcium-bound phosphorus phases. Infiltration rate increased only due to the presence of plants. Leached phosphorus decreased (50%) with plants present; Al-WTR further reduced soluble phosphorus leaching losses (60%). Fescue yield, phosphorus concentration and uptake increased with increasing Al-WTR rate, due to Al-WTR sorbing and potentially making phosphorus more plant available; Olsen-extractable phosphorus increased with increasing Al-WTR rate, supporting this contention. The PSC was reduced with the 62 Mg/ha Al-WTR rate but maintained with greater Al-WTR rates. The 124 and 248 Mg/ha Al-WTR rates also contained greater phosphorus associated with the soluble+aluminum+iron and occluded phases which should be stable over the long-term (e.g., decadal). It was recommended to apply Al-WTR near the 124 and 248 Mg/ha rates in the future to capture urban runoff soluble phosphorus in the Boise, Idaho engineered wetland