The impact of alum based advanced nutrient removal processes on phosphorus bioavailability

Because eutrophication is a widespread consequence of wastewater discharges, there is a strong impetus to develop new approaches to remove phosphorus (P) from wastewater treatment plant (WWTP) effluents. We examined the effluents from a pilot plant that is testing various alum based processes for achieving > 99% P removal, however, it is not known how these advanced P removal technologies affect the bioavailability of P (BAP). We tested how the percent BAP (%BAP) varied with different P removal levels using an algal growth bioassay methodology. This facility reduced total P concentrations from ≈ 500 μg L−1 in the pilot plant influent to 19 ± 4 (±SD) μg L−1 in the final effluent, and our results showed that as the level of P removal increased, the %BAP of the product declined sharply, r2 = 0.98. Prior to alum treatment, the influent had an average %BAP of 79 ± 13%, and after three steps of alum-based removal the %BAP averaged 7 ± 4%. Thus, this alum based P removal process was very effective at sequestering the P forms that most readily stimulate algal growth. Further, our results show the final BAP of the effluent was only ≈ 50% of the “reactive” P concentration. These results have important implications for nutrient management and trading schemes.

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► Advanced alum-based phosphorus removal greatly modified phosphorus speciation.
► Bioavailable P declined from 80% in secondary effluents to 10% in final effluents.
► The bioavailable P concentration was less than reactive P in the final effluents.
► Our results will help to better predict wastewater impacts on surface waters.