The 1994 Cryptosporidium Criteria Document described a filtration method using polypropylene wound yarn filter with a 1-:m porosity. This collection method can be used for large volume samples with varying turbidity. Some tested 10 cartridge filters varying in composition (polypropylene, nylon, rayon, and cotton) and porosity (0.5 and 1.0 :m) for removal of Cryptosporidium- and Giardia-sized particles.
Although retention of 3- and 7-:m particles was greater using filters with a 0.5 :m porosity, they tended to clog, limiting the amount of water that could be filtered. The use of cotton, nylon, and rayon filters led to the most efficient removal of Cryptosporidium and Giardia-sized particles. The authors tested the filters using 1 gallon (3.78 L) volumes of water. Wound fiber filters may not necessarily be superior to wound filters for samples greater than 1 gallon in volume. To further minimize losses during filtration, the filter housing was matched with the filter, and a screw press was used to wring the filters. Concentration of the eluate was best performed at centrifuge speeds of 6,700 to 10,000 x g.
Also described in the 1994 Cryptosporidium Criteria Document was the use of cellulose acetate membrane.
(CAM) filters. Others compared recovery rates of a method using CAM filters to the ASTM
ICR method using wound yarn filters. Prior to filtration by either method, Cryptosporidium and Giardia were spiked into environmental water samples varying in quality and turbidity. Cyst and oocyst recoveries decreased with increasing water turbidity, regardless of the filter type. Overall, the cellulose acetate method gave higher recoveries; however, because the parasites were stained on polycarbonate filters, microscopic confirmation was not possible. Therefore, the authors recommended the use of the ASTM ICR method for environmental sample
Drinking Water Criteria Document Addendum: Cryptosporidium March 2001
75 analysis and the cellulose acetate method for spiking studies. Adlome and Chaglam modified the CAM method by including an acid dissolution step following filtration. This modification resulted in a 70.5% recovery of oocysts spiked into 3 liters of treated municipal water. Graczyke used cellulose acetate filters, followed by filter dissolution and ASTM ICR method processing to test recovery of Cryptosporidium
from spiked drinking water. The overall mean recovery rate was reported as 77.7%. Further studies
indicate that the acetone dissolution step does not compromise viability or infectivity.
EPA’s Method 1622 requires a capsule filter (USEPA, 1999b) these filters contain a pleated polysulfone membrane with a 1-:m porosity. Filter is a 6-cm-diameter by 21-cmlong capsule with a surface area of 1,300 cm2. Clancie compared throughput and recovery rates of this capsule filter with those of polycarbonate membrane filters, vortex flow filtration, and cellulose acetate membrane filters which were dissolved post-filtration. All four filters were challenged with 10 liters of municipal raw and finished waters. The cellulose acetate membranes and polycarbonate membranes were blocked at 8 and 2.5 liters, respectively, at a raw nephlometric turbidity unit (NTU) of 5. The polymer vortex flow and capsule filters were able to process the entire 10 liters of raw water and gave recovery rates of 11-57% and 8-78%, respectively. In finished waters from five utilities, the vortex flow recovered 18-69% of the seeded oocysts, while the capsule filter recovered 45-117%. The researchers concluded that the capsule filter performed best with the various water matrix conditions tested. Other membrane filters composed of glass fiber have been evaluated.