Chemically Mediated Phosphorus Removal: Optimization of Analytical Methods

Phosphorus regulations for wastewater effluents are moving to concentration ranges of 10-100 μg P/L (total phosphorus). The analytical methods for low level phosphate analysis need to be optimized to perform reliably and accurately at these low levels. To accomplish this, absorbance measurements with long path lengths, 10 cm and 1 m cells, are performed. The ascorbic acid method (Standard Methods 4500-P. E.) was modified for a 10 cm path length to use a colorforming reagent volume 30% of the volume recommended by the Standard Method and a color development time of 1 hour. The same reagent volume is recommended for a 1 m path length with color-development overnight (24 hours). Additionally, a filtration protocol is necessary to better define the “dissolved” fraction in synthetic iron and phosphate solutions used to model chemically mediated phosphorus removal. It was found that a 0.45 μm syringe filter gives variable results, likely due to a build-up of tiny (less than 450 nm) iron colloids on the filter. For jar tests designed to evaluate chemically mediated phosphorus removal, the protocol to achieve the lowest reproducible orthophosphate concentration employs a 47mm diameter Millipore filter, filtration at a rate of 250 mL/hour and a filtered volume of 10 mL.