UC Berkeley

Drinking water in low-income countries and stormwater in the U.S. is often contaminated with pathogens, posing health risks to local communities. In response, distributed filters are being used to provide the needed protection from exposure and infectious disease. Two examples of these distributed filters include point-of-use (POU) devices at the household scale for drinking water treatment and bioretention basins at the community scale for stormwater treatment. Sand is a common media used in distributed filters, but pathogen removal is often low due to unfavorable interactions between the sand surface and microorganisms. Removal may be increased, however, if certain amendments are added that create favorable interactions between microorganisms and the filter media, either through electrostatic attraction or hydrophobic interactions. This dissertation focused on the use of three media amendments, including a quaternary ammonium silane (QAS), zero valent iron (ZVI), and biochar, in distributed filters to improve removal of indicator bacteria and viruses from drinking water and stormwater.

In the first part of this dissertation, a QAS-coated sand was evaluated for drinking water treatment in POU filters. Varying influent water matrices were introduced to QAS-coated sand columns at representative flow rates, and the effluent was evaluated for indicator bacteria, indicator virus, and human virus removal. In basic water matrices with 1 mM NaCl and Escherichia coli (E. coli) or MS2 coliphage (MS2), removal was observed to increase with depth and decrease with increasing ionic strength, flow velocity, and experiment length. E. coli attached to QAS-coated sand were observed to have permeable membranes, providing evidence of inactivation. Major limitations, however, were observed when the influent contained humic acid or other viruses. Rapid fouling in the presence of humic acid was likely caused by competition for positively charged binding sites, and low PRD1 and Adenovirus 2 removal may have stemmed from steric interference between viral capsid features and the media surface.

In the second part of the dissertation, ZVI and biochar amendments were evaluated for stormwater treatment in bioretention basins in two long-term experiments. In the first experiment, stormwater columns were operated intermittently to examine E. coli and MS2 removal in conventional bioretention media (CBM) and ZVI-amended media. A 1-year storm (3.2 cm) for Berkeley, CA was introduced to the columns once per week for 5.15 h. Afterwards, the columns were drained and remained in this state until the next week’s storm. This pattern was repeated for 46 weeks. In CBM, E. coli removal increased as the columns aged and correlated with a decrease in hydraulic conductivity. Some evidence that biological activity contributed to E. coli removal was observed in a sodium azide experiment. In a ZVI-amended silica sand, E. coli and MS2 removal was above 1 log through week 11 but decreased quickly afterwards to levels observed in silica sand. The presence of other adsorbates, including natural organic matter (NOM), phosphate (PO43-), and other organisms, likely contributed to the observed decrease in removal. In ZVI-amended CBM, E. coli and MS2 removal remained low throughout the experiment, likely due to the NOM, PO43-, and organisms leached by the compost in CBM. Iron leaching was also observed in ZVI-amended CBM. In both ZVI-amended media types, large hydraulic conductivity reductions and cementation issues raised concerns over long-term permeability in the field.

In the second experiment, columns were operated as a batch system where stormwater was injected into a column and allowed to rest in the media for 24 h. After this rest period, fresh influent was injected into the column again, and the expelled effluent was evaluated for E. coli and MS2 removal. This pattern was repeated daily for 75 d. In biochar-amended silica sand, E. coli removal above 4 log was observed throughout the experiment. In iron-amended silica sand, MS2 removal above 2 log and PO43- removal around 90% was observed after 75 d, and hydraulic conductivity did not decrease as observed in unsaturated columns.

Results from this dissertation provided insights into the limits of media amendments for pathogen removal, identified possible removal mechanisms of indicator organisms in mature bioretention basins, and demonstrated the promise of a batch configuration with increased rest periods and media amendments for long-term removal of indicator organisms.