DETERMINING IRON TO PHOSPHATE RATIOS AS A TOOL FOR PREDICTING THE EFFECTIVENESS OF ARSENIC TREATMENT SYSTEMS

Phosphates can interfere with arsenic removal by competing with arsenic for iron oxide adsorption sites during iron-based treatment of drinking water. Significant levels of phosphates naturally occur in certain locations in a number of countries where groundwater with high concentrations of arsenic is being used for drinking water. The concentrations of phosphates in some areas are sufficiently high to potentially reduce removal efficiencies of some critical iron-based arsenic treatment systems that currently are among the most simple and low-cost treatment approaches. This impact on arsenic removals can vary by country, by district and by individual wells.

This presentation discusses a new data analysis approach for predicting the effectiveness of iron-based arsenic treatment systems for areas or individual tubewells with significant phosphate concentrations. In March-April of 2007, MIT, CAWST and GUS (now LEDARS) conducted pilot field-testing in Bangladesh of the Kanchan Arsenic Filter developed in Nepal by MIT and the Nepali NGO ENPHO. Because of the varying groundwater phosphate levels in different districts in Bangladesh, MIT and CAWST analyzed arsenic studies conducted by a number of organizations. A detailed analysis relative to phosphate issues was carried out of studies that included laboratory analytical results for phosphates, iron and arsenic in wells located in high arsenic areas in five countries: Bangladesh, Vietnam, Cambodia, Nepal and Argentina. The individual groundwater quality data and any arsenic removal treatment data from the studies were entered into a spreadsheet for trend analysis.

The trend analysis revealed that the iron to phosphate ratio (mg/L of influent total iron divided by mg/L of influent phosphates-P) was a good indicator of the arsenic removal performance of iron/arsenic treatment systems. Low iron to phosphate ratios were indicative of lower percent removals of arsenic and high iron to phosphate ratios were indicative of high percent removals of arsenic. It is believed that location-specific iron to phosphate ratios can potentially be an effective new tool to predict the relative arsenic removal efficiency of iron-based systems. Such information could lead to treatment design adjustments, as well as increased follow-up arsenic testing of treated water for locations predicted to be the most challenging.