Another pre-treatment strategy that is particularly well suited to situations where AFO wastes and other contaminants are possibly present in surface waters is to replace traditional surface water intakes with wells and other subsurface collection systems located adjacent to the surface water source. These systems are commonly referred to as riverbank filtration (RBF), although these systems can be located at the riverbank, lake bank, river bottom, or lake bottom. They essentially use the subsurface media between the source and groundwater well to remove a variety of contaminants.
Under appropriate circumstances (several dozen feet of unconsolidated, unfractured porous media), RBF systems can reliably remove microbial pathogens, organic contaminants (including taste and odor causing substances and disinfection byproduct (DBP) precursors), hormones, endocrine disruptors, turbidity, and other substances. RBF dampens or eliminates contaminant spikes (e.g., spills or other large accidental releases). RBF can also decrease impacts of diffuse pollution over space, by spreading over time and decreasing contaminant concentration.
RBF is particularly well suited for Cryptosporidium removal (Gollnitz et al., 2005; EE&T, 2004). USEPA allows direct Cryptosporidium removal credit in the LT2ESWTR for new RBF systems. In this instance, Cryptosporidium bin assignment samples are collected from the surface water source and utilities that install RBF are allowed 0.5 or 1.0 automatic credits if they meet certain requirements, or higher credits through demonstration. Existing RBF systems are required to collect Cryptosporidium bin assignment samples from the well, consequently resulting in an indirect credit because the RBF well will likely result in a lower bin assignment than would samples collected prior to RBF. RBF systems typically are followed by conventional surface water treatment processes, including granular rapid-rate filtration. However, USEPA can allow some RBF systems that achieve suitable performance to convey water direct to consumers without subsequent treatment, other than disinfection.
Successful case studies of utilities using RBF in agricultural watersheds include Lincoln, Nebraska, Cedar Rapids, Iowa, and Mankato, Minnesota. As another example, studies reported by Wang et al. (2002), indicate that organic compounds, turbidity, total coliforms, HPC (heterotrophic plate count - also called standard plate count), aerobic spores, algae, diatoms, Giardia, Cryptosporidium, insects, and other contaminants were found in Ohio River water but were either not found or were found at drastically reduced levels in adjacent RBF wells near Louisville, Kentucky.