Improving Filtration of Cryptosporidium

Abstract

ryptosporidium in water supplies is a widespread problem in the United States and elsewhere. This protozoan causes severe gastroenteritis that is potentially fatal to infants and immunocompromised individuals and that does not respond well to available therapies. Cryptosporidium was detectable in 77% of rivers and 75% of lakes sampled in a US survey. 1 Moreover, Cryptosporidium oocysts were detectable in 83% of pristine water sources (no human activity in the watershed) and 28% of treated drinking water samples. In the United States, > 431,000 confirmed cases of cryptosporidiosis have been reported since the disease was identified in 1976. 1 Most (403,000) of these cases were diagnosed during the 1993 outbreak in Milwaukee, Wis. Some 250-500 million infections occur annually in Asia, Africa, and Latin America. 2 The infective stage of the disease is the oocyst, and the infective dosage (generally reported as 10-100 oocysts) may be as low as a single oocyst. 3 The oocysts resist disinfectants and most environmental conditions, thus making their physical removal necessary. Chemical conditioning and optimal operation of filtration systems can remove > 99 percent of oocysts from water. 4,5 Suboptimal flocculation can substantially decrease performance, however, and drop removal to 96% at a superficial velocity of 293 m/d (5.0 gpm/sq ft). 4 Fogel 6 obtained removals of only 2-43% at hydraulic loads of 24-475 m/d (0.4-8.2 gpm/sq ft) when chemical conditioning was not practiced.Cryptosporidium oocysts carry a negative surface electrical charge in the pH range of natural water sources, 7-9 as do natural filter media such as sand and diatoma-C Surface coating of hydrous iron aluminum oxide on Ottawa sand was investigated as a means of improving the removal of Cryptosporidium oocysts from water by filtration. Coating the sand increased the zeta potential from -40 to +45 mV at pH 7.0, increasing the potential for interaction with the negatively charged (-25 mV at pH 7.0) Cryptosporidium oocysts. Water seeded with Cryptosporidium oocysts was passed through parallel columns of uncoated and coated sand at superficial velocities of 200-800 m/d (3.5-14 gpm/sq ft) and column lengths of 100-400 mm (4-16 in.). In every trial, coated sand removed significantly more oocysts than uncoated sand. Filter coefficients of coated sand were 2.9 times as high as those of uncoated sand, indicating that coating can significantly improve the removal of Cryptosporidium by granular media filters.

Influence of Surface Characteristics on the Stability of Cryptosporidium parvum Oocysts

“…For example, using microelectrophoresis, Brush et al (2) reported finding neutrally charged C. parvum oocysts. In contrast, other workers have reported that the surface of the C. parvum oocyst is exclusively negatively charged at circumneutral pH values (10,20,23,22,29,33). It should be noted that washing procedures were not clearly outlined in some of these studies.…”

“…1b and c may help explain previous conflicting reports concerning surface charge (3,10,20,23,22,29,33,36). For example, using microelectrophoresis, Brush et al (2) reported finding neutrally charged C. parvum oocysts.…”

“…Unfortunately, the results reported previously for electrophoretic mobility have been incongruent. For example, some studies have shown that C. parvum oocysts have an isoelectric point near pH 2.5 (10,20,22,23,29,33), while other reports (2) have demonstrated that the oocyst surface remains neutral at pH values between 2 and 10. Thomas and coworkers (36) recently reported that oocysts suspended in CaCl 2 solutions had near-zero electrophoretic mobility.…”

Physicochemical aspects of Cryptosporidium surrogate removal in carbon block filtration

“…The attachment step in adsorptive filtration involves physical–chemical interactions such as electrostatics and van der Waals forces between the pore walls and the particles. The importance of electrostatic interactions to the removal of particles in granular media filters has also been discussed in the literature (Shaw et al, 2000; Stenkamp & Benjamin, 1994; Tobiason & O'Melia, 1988). In granular media filtration systems for municipal water treatment, the media and the particles to be removed typically both have negatively charged surfaces, resulting in a repulsive interaction (Stenkamp & Benjamin, 1994).…”

Analysis of Cryptosporidium parvum oocyst transport in porous media

“…chlorine) used in the treatment is ineffective to inactivate C. parvum oocysts since the oocyst wall is thick and impervious. Therefore, physical removal such as filtration plays a major role in controlling the oocysts present in surface water (LeChevallier et al, 1991a,b;Shaw et al, 2000).…”