Improving phosphorus removal in a surface flow wetland and land application system by geochemical augmentation with alum

Abstract

Constructed surface flow (SF) wetlands are commonly used for phosphorus (P) removal. Geochemistry of wetlands provides explicit mechanisms for permanent P-sequestration in sediments. This study had two goals: (1) Find P removal performance and rate at the highest alum doses that do not produce floc in an SF wetland; and (2) Determine potential improvements to P removal performance with low alum doses in a 140-ha land application system downstream from the wetland. The study started with a small fraction of a conventional, flocculation/sedimentation alum dose, then progressively increased the dose to observe initiation of floc formation and removal of P. For flows near 10 megaliters per day in an 0.8 ha SF wetland, doses started 189 L d−1 for two weeks, then increasing by 189 L d−1 every two weeks until the final two weeks at 946 L d−1. At an alum dosing rate of 189 L d−1 (alum concentration of 9.5 mg L−1), there was an order of magnitude improvement in P removal rates over literature values. Floc formation in the wetland was observed at 567 L d−1, but no significant improvement in P removal rates were observed until a conventional alum dose of 946 L d−1 was applied.Alum addition improved P removal performance in the land application system. In 2014, during which there was no alum dosing, the median effluent total P (TP) during the July–September dry season (groundwater dominated outflow) was 0.43 mg L−1. In 2015, (alum dosing August–October) median dry weather TP of 0.18 mg L−1 was significantly lower (p < 0.0001). Alum dosing in 2016 at 189 L d−1 produced a dry weather median of 0.28 mg L−1, which was significantly lower (p = 0.015) than in the 2014 median. Mean daily dry weather TP loads to the land application system were 44 kg d−1 in 2014, 45 kg d−1 in 2015, and 41 kg d−1 in 2016.