Tuesday, November 14, 2006 - 8:15 AM

Phosphorus Sequestration: Using Alum in Municipal Wastewater Treatment Wetland Mesocosms.

Lynette Brown1, John R. White2, and K. R. Reddy1. (1) Univ of Florida, Soil and Water Science Dept, 106 Newell Hall, PO Box 110510, Gainesville, FL 32611, (2) Wetland Biogeochemistry Inst, Dept of Oceanography & Coastal Sciences, Louisiana State Univ, Baton Rouge, LA 70803

The Orlando Easterly Wetland is one of the oldest and largest constructed treatment wetlands in the US, located in Christmas, FL.  Recently, phosphorus (P) concentrations have increased during the winter months resulting in concern over the P binding capacity of the soil.  One method to increase the P binding capacity of wetland soils is through the addition of alum. A three month mesocosm study was performed to determine the effectiveness of alum in immobilizing P in organic soils.  Eighteen mesocosms were established, triplicate experimental and control units for Typha sp., Scirpus californicus, and SAV (Najas guadalupensis dominated).  Water column pH, soluble reactive P (SRP), total P, ammonium, and DOC were measured.  Soil characteristics, including the size and activity of the microbial pool, and plant nutrient and Al content were determined at the initiation and completion of the experiment. Mean water column SRP in the alum treatment (0.032 ± 0.02 mg L-1) was lower than the controls (0.103 ±0.06 mg L-1) with corresponding removal efficiencies of 83% and 44%, respectively.  Significantly higher Al concentrations (1407 ± 699 mg kg-1) in the surface soil of the alum treatment resulted in a lower soil pH (5.00 ± 1.12). The surface soil of the alum treatment had a lower microbial biomass P (1.17 ± 0.94 mg kg-1) than the control (3.52 ± 1.99), resulting in lower sediment oxygen demand (6.26 ± 1.66 mg kg-1 h-1) and potentially mineralizable P rates in the alum treatments. This research suggests that application of a slow drip alum system in treatment wetlands may prevent release of P from the soil back into the water column while continuing to bind influent P as the alum settles through the water column, however treatment may have a negative impact on the microbial community.