Master of Environmental Studies (MES)
Geography & Environmental Studies
Faculty of Science
Chemically mediated phosphorus removal, done during wastewater treatment, is an effective means of reducing nutrient loads to sensitive environments. Although this method of treatment is widely used, the mechanism of removal is poorly understood. Moreover, phosphorus regulations for wastewater effluents are moving to concentration ranges of 10-100 μg P/L (total phosphorus). This is much lower than current regulations (∼0.1 ∼0.2 mg P/L as TP) required by municipalities (Takács et al., 2006a; Murthy et al., 2005). 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 of 10 cm and 1 m were performed. The ascorbic acid method (Standard Methods 4500-P. E.) was modified for a 10 cm path length by using a colour-forming reagent volume that was 30% of the volume recommended by Standard Methods (4500-P. E.) and a colour development time of 1 hour. The same reagent volume is recommended for a 1 m path length with colour development overnight (24 hours). Additionally, a filtration protocol was found to be 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 gave variable results, likely due to a build-up of iron hydroxide colloids (less than 450 nm) on the filter. Phosphorus is associated with the iron hydroxide and remains on the surface of the filter as the colloids build-up and block the pores of the filter. This resulted in less phosphorus passing through the filter to be measured as dissolved. For jar tests designed to evaluate chemically mediated phosphorus removal, the recommended protocol to achieve the lowest reproducible orthophosphate concentration employs a 47 mm diameter Millipore filter, filtration at a rate of 250 mL/hour and a filtered volume of 10 mL.
To improve the understanding of the chemically mediated phosphorus removal process and integrate modeling of phosphorus removal into computerized plant models such as Bio Win (EnviroSim™ Inc.), laboratory tests employing a factorial design were conducted. A 24 factorial design allowed two extremes of four significant factors in the nutrient removal process to be considered simultaneously. The four factors (with thehigh and low values in brackets) were solution pH (pH 6 and 8), iron dose (5 mg Fe/L and 10 mg Fe/L), mixing intensity (G = 23.5 s-1 and 376 s-1), and water hardness (∼44 mg/L as CaCCh and ∼170 mg/L as CaCOs). The 16 experiments required by the 24 factorial design were each performed over a 24-hour period, monitoring the residual orthophosphate concentration as a measure of effective P removal. The results confirmed factors of the process that were thought to affect phosphorus removal: faster mixing resulted in better initial removal and higher dose (5 compared to 2.5 in terms of molar Fe:P dose) resulted in an order of magnitude difference in residual orthophosphate concentration. Statistical analysis revealed that the most significant factors (p4 system are highly variable and depend on the factors of dose, pH, mixing, and water hardness.
Gilmore, Rebecca L., "Laboratory Studies in Chemically Mediated Phosphorus Removal" (2009). Theses and Dissertations (Comprehensive). 933.