high loads of other particles in the resulting pellets. These other particles can interfere with subsequent purification and microscopic examination of the parasite cysts and oocysts.

Other filters having absolute pore size ratings smaller than the size of the target cysts, oocysts, and spores are alternatives for concentrating parasites from water. These filters are preferred because they are expected to achieve absolute retention of the protozoan cysts, oocysts and spores and because their physical characteristics facilitate easier and more efficient recovery of the retained microorganisms by simpler elution methods than cutting apart and macerating the filter material. Formats for these filters include flat track-etched polycarbonate disks, cellulose acetate membranes (that are dissolved in acetone to recover Cryptosporidium oocysts), pleated capsule filters (1 um pore size polyether-sulphone filters in a polycarbonate housing), and ultrafilters (spinning cartridge and hollow fiber units). Such filters, as well as the smaller water sample volumes, are now recommended by the EPA, and some of them are specified in the recently developed Method 1622 (EPA, 1998). Another type of filter being used to concentrate Cryptosporidium from water is a compressible "sponge" filter. This filter is compressed into a water pipe to achieve a small pore size, and water is allowed to flow through the compressed filter for a period of time. The filter is recovered from the pipe, and the parasite cysts and oocysts are readily washed off of the now decompressed sponge-like filter medium for further processing and analysis.

The most widely used methods for initial concentration and recovery of viruses from water employ microporous filters that retain viruses primarily by adsorption to the filter medium (Sobsey, 1976; Sobsey, 1982; .De Leon and Sobsey, 1991). These filters retain viruses by both electrostatic and hydrophobic interactions between the surfaces of viruses and the filter media. Formats used for virus adsorbent filters include membranes, disks, and pleated cartridges. The media used initially as virus adsorbent filters were negatively charged cellulose esters, fiberglass, and other materials. These filters adsorb viruses efficiently only at lower (acidic) pH levels (pH of 3 to 6) and/or in the presence of multivalent cation salts, such as divalent calcium or magnesium or trivalent aluminum salts. Relatively large volumes of conditioned water are passed through the filter, and viruses adsorb to the filter medium surfaces. Subsequently, filters that are electropositive near neutral pH and adsorb viruses directly without acidifying or adding cations salts to the water were developed for virus concentration (De Leon and Sobsey, 1991).

Electropositive filter media are composed of charge-modified fiberglass sold commercially as disks or pleated cartridges, fiberglass filter disks that are coated with precipitated aluminum or iron salts, or positively charged, natural quartz fiberglass that one packs into a column to make an adsorbent filter. The current EPA-approved ICR method to detect culturable enteric viruses in drinking water supplies specifies use of commercially available, electropositive filter (EPA, 1996). Viruses adsorbed to both electronegative or electropositive filters are subsequently eluted and recovered by passing a relatively small volume of aqueous elution medium through the filter. Viruses in the resulting filter eluates are assayed directly or after further steps of concentration, purification, and extraction.