EXAMINING THE BIOAVAILABILITY OF CHEMICALLY REMOVED PHOSPHORUS AT THE BLUE PLAINS ADVANCED WASTEWATER TREATMENT PLANT

The goal of this study was to determine if simultaneous precipitation in the biological process lead to phosphorus deficiency within the biological process. Tests were conducted to understand the bioavailability of chemically removed phosphorus during simultaneous precipitation, using controlled laboratory experiments. The bioavailability was examined during aerobic and anoxic conditions, and for heterotrophic and autotrophic growth. To evaluate the impact of chemical precipitation, aerobic/ autotrophic growth was monitored using the WERF High F/M Method for estimating the maximum specific growth rate of nitrifiers. To monitor anoxic growth, a new High F/M anoxic batch test method was developed at DCWASA. The method is analogous to the WERF High F/M Method for estimating the maximum specific growth rate of nitrifiers. Methanol was added for denitrification in the anoxic test. These laboratory tests suggest that the chemically bound phosphorus is bioavailable for aerobic and anoxic processes and for heterotrophic and autotrophic metabolism. Experimental evidence and equilibrium modeling of the chemical P removal process indicate that the lowest achievable OP concentration in excess of metal (ferric and alum) cations is in the 0.01 mgP/L or lower range. This study and plant data provides direct evidence that microorganisms can utilize this very low OP concentration for their cell synthesis needs. In a mathematical modeling environment, this is described by a nutrient switch that should have a lower value than the solubility of the chemically bound phosphorus. In absence of direct measurements, it is recommended that the phosphorus nutrient limitation half saturation concentration be set to a significantly lower value than 0.01 mgP/L. Although chemical removal can achieve very low residual concentrations of orthophosphate, the kinetics of dissolution/ dissociation of this precipitate is rapid. The rapid dissociation and equilibration of the chemically removed phosphorus apparently always supplies sufficient orthophosphate to the relatively slower growing anoxic/heterotrophic and aerobic/autotrophic biomass, and that the biomass can sequester the very low (