The Evaluation of Permeable Cementitious Media for Nutrient Removal

Abstract

Excessive amounts of nutrients in surface and ground waters can interfere with the quality of aquatic and human life. Permeable pavement systems have been studied for contaminant removal, but there are limitations to the current state of knowledge. Laboratory studies were utilized to fundamentally characterize permeable cementitious media for the removal of nitrate-nitrogen and orthophosphate-phosphorus. Batch reactor tests were used to determine nitrate and orthophosphate removal and retention for five permeable reactive concrete mixtures. Additionally, fixed-bed breakthrough experiments were used to determine the nitrate removal for five permeable lightweight cellular concrete mixtures. Finally, one breakthrough experiment was used to determine the nitrate removal of a lightweight cementitious media. It was determined that nitrate and orthophosphate removal isotherms of permeable reactive concrete were linear within the range of concentrations tested. The greatest nitrate removal in batch reactor and column tests were with ordinary portland cement (OPC), 50% replacement with ground-granulated blast-furnace slag, and 3% replacement with titanium dioxide (TiO2). OPC cellular concrete mixtures demonstrated beneficial removal at a lower w/c ratio. The nitrate removal process was characterized as slow for the cellular concrete and lightweight media mixtures during column tests. Multiple rates of nitrate removal observed during column testing may indicate multiple mechanisms of removal. Leach testing indicated that nitrate removal may not be permanent, although over 85% was retained by the TiO2 and OPC mixtures. Orthophosphate removal and retention was effective for all PRC specimen with greater removal for the mixtures containing 3% replacement with TiO2 and 25% replacement with fly ash. In addition to nutrient removal, correlations between nitrate removed and sulfate leached were investigated. It was determined that soluble sulfate slowly leaches from the hydrated concrete and foaming agent in all mixtures. Specimen that were flushed with deionized water experienced lower removal rates than their counterparts. No direct correlation between mass of sulfate leached and mass of nitrate removed was observed in batch reactor testing or cellular concrete tests, although supplementary materials and the foaming agent could affect the relationship.