2003 Progress Report: Microbial Pathogen Removal During Bank FiltrationEPA Grant Number: R829010
Title: Microbial Pathogen Removal During Bank Filtration
Investigators: Ryan, Joseph N. , Harvey, Ronald W. , Elimelech, Menachem
Institution: University of Colorado at Boulder , United States Geological Survey , Yale University
EPA Project Officer: Page, Angela
Project Period: September 1, 2001 through August 31, 2004 (Extended to August 30, 2005)
Project Period Covered by this Report: September 1, 2002 through August 31, 2003
Project Amount: $506,006
RFA: Drinking Water (2000) RFA Text | Recipients Lists
Research Category: Drinking Water , Water
The overall goal of this research project is to develop a model of oocyst transport in porous media that can accommodate the physical and geochemical heterogeneity present in alluvial valley aquifer used for riverbank filtration. The specific objectives of this research project are to determine the effect of: (1) microbe size on transport; (2) physical heterogeneity of the porous media on transport; (3) geochemical heterogeneity of the porous media on transport; (4) microbe release from unfavorable attachment sites; and (5) high pumping rates on microbe release. Our incomplete understanding of processes and properties affecting pathogenic microbe transport during riverbank filtration currently is limiting our ability to predict the effectiveness of this water treatment option. We propose a series of fundamental experiments designed to better understand the effects of microbe size, physical and geochemical heterogeneity of the porous media, and high pumping rates on the transport of Cryptosporidium parvum oocysts in alluvial valley aquifers used for riverbank filtration.
Laboratory flow-through column experiments were conducted to explore the effects of physical and geochemical heterogeneity on the transport of formalin-treated C. parvum oocysts in pure quartz sand of varying grain size and ferric oxyhydroxide-coated quartz sand. The results show that: (1) the interactions between oocysts and quartz grains of different size are independent of grain size as long as the grains are carefully purified; and (2) the amount of ferric oxyhydroxide coating controls the efficiency of attachment for oocysts as it does for other colloids and microbes.
Experiments testing the transport of oocysts and polystyrene latex microspheres of three sizes (1.0, 2.8, and 4.6 µm) through a random physically and geochemically heterogeneous aquifer tank 5 m in length were conducted. The results showed that oocyst and microsphere transport in the tank favored pathways of larger grain size (higher hydraulic conductivity) and lower ferric oxyhydroxide content (lower collision efficiency). Physical heterogeneity was more important than geochemical heterogeneity in controlling oocyst and microsphere transport. Microspheres broke through at the same time as the oocysts, but the microspheres were removed much more rapidly than the oocysts. The tracer transport was modeled using a two-dimensional transport model.
Further laboratory flow-through column experiments on oocyst transport in pure quartz sand showed that straining is contributing to the removal of oocysts in fine-grained sands. Straining was demonstrated by comparing the transport of polystyrene latex microspheres (0.32 to 4.1 µm) to oocyst transport. Removal was consistent for microspheres from 0.32 to 1.9 µm, and increased significantly for 4.1 µm microspheres.
Stagnation point-flow experiments were conducted to explore the effect of ionic strength on oocyst deposition. The dynamics of oocyst deposition in these experiments—the oocysts come into contact with the glass deposition surface, but do not remain in the position of first contact—clearly indicate the oocysts are depositing in the secondary minimum of the Derjan-Landau-Verwey-Overbeek (DLVO) potential energy profile. Under these conditions, oocyst attachment is readily reversible.
During the next year, our experimental work will focus on laboratory column and stagnation point flow experiments. The laboratory column experiments will explore the effect of: (1) microbe size; (2) physical heterogeneity on microbe transport; and (3) flow rate on oocyst release. A goal of these experiments will be to determine the importance of straining. The stagnation point flow experiments will continue to explore the dynamics of oocyst transport. The results of the aquifer tank experiment will be modeled using an adapted two-dimensional model of microbe transport that accounts for physical and geochemical heterogeneity.