Phosphorous removal improvements and cost reductions leveraging cationic polymers and anionic polyacrylamides in Chemically Enhanced Treatment Wetland (CETW) systems

Many wetland systems worldwide struggle with high nutrient influx from urban and agricultural inputs that often disturbs ecosystem balance. In the Florida Everglades, concentrations above 15 μg P L−1 have been shown to affect ecosystem balance and have long term effects on downstream periphyton, detritus, soil and macrophyte. Current Stormwater Treatment Areas (STAs) have been shown to effectively lower P levels on average to 41 μg P L−1 (SD of 31 μg P L−1). These levels, released into downstream Water Conservation Areas, exceed a long-term goal of 10 μg L−1 total P targeted for STA outflow concentrations. Here, we look at enhancing P removal through chemical addition to treatment wetland systems through a series of in-situ mesocosm studies informed through laboratory jar tests. In-situ mesocosm results showed Fe and Al metal-based coagulants lowered filtered total phosphorus (FTP) levels but they did not create settleable flocs, thus resulting in no change or even higher unfiltered total phosphorus (UTP) levels when compared to non-dosed control mesocosms. Proprietary cationic polymer blends and anionic polyacrylamide (PAM) added post coagulant dosing were tested to enhance P-removal performance. Cationic polymer addition significantly lowered FTP when combined with Al-based coagulants, suggesting enhanced charge neutralization, and improved UTP removal when combined with Fe-based coagulants, signifying improved bridging or electrostatic patch formation to create larger/denser floc. The addition of PAM after coagulant dosing generally did not improve FTP removal but did improve UTP removal when dosed at levels >0.5 mg L−1. For Fe-based systems, doubling PAM dosage (from 0.5 to 1.0 mg L−1) was slightly more beneficial for UTP removal than doubling coagulant blend dosage. For Al-based systems, doubling coagulant blend dosage was slightly more effective for UTP removal than doubling PAM dosage, but the difference was not significant. The chemical cost savings from utilizing PAMS were large: doubling coagulant blend dose increased chemical costs by $28,000–$100,000 per 1B L of water treated depending upon blend, while doubling PAM dose marginally increased chemical costs by $1700. Wetlands enhanced with chemical addition achieved levels as low as 20–25 μg L−1 UTP and <10 μg L−1 FTP under field conditions. These were lower than average achievable levels by non-dosed mesocosms (84 μg L−1 UTP and 43 μg L−1 FTP). While the goal of 10 μg L−1 total P was not met by these chemically enhanced treatment wetland (CETW) systems, strategic placement to treat water prior to entrance into STAs can potentially help STAs meet this criterion by extending the STAs' effectiveness and lifespan. More importantly, these systems can help mitigate downstream ecosystem imbalance and combat the formation and migration of a moving P front.

Duke Scholars

Author Curtis J. Richardson Environmental Sciences and Policy