How conditions and coagulant type can effect Cryptosporidium removal by filtration

Dec. 3, 2002
This article, based on a paper presented at the 10th Gothenburg Symposium, describes the effect of different coagulant types on the removal of Cryptosporidium by filtration.

Dec. 3, 2002 -- This article, based on a paper presented at the 10th Gothenburg Symposium, describes the effect of different coagulant types on the removal of Cryptosporidium by filtration.

Multiple water treatment strategies are critical in the effort to effectively remove pathogens including Cryptosporidium parvum (C. parvum) from drinking water. While traditional disinfection technologies incur considerable costs and have practical limitations, filtration by granular media can provide an excellent barrier when operated properly. But for filtration performance to be effective, appropriate chemical pre-treatment is vital, and efficacy suffers if that pre-treatment is sub-optimal.

It is recognised that the interaction of C. parvum oocysts with chemical coagulants contributes to the effectiveness of the pathogen's removal by filtration, but only limited information is available regarding the charging mechanisms. Recent studies have suggested oocyst surfaces contain glycoproteins and trace amounts of fatty acids that can have ionisable groups such as carboxylates or phosphates. This means that the interactions between the surfaces of oocysts and the chemical properties of coagulants used in drinking water treatment are likely to be mechanically different and may affect subsequent treatment processes differently.

Recent work has suggested that enmeshment in precipitate may be the primary mechanism of oocyst removal when ferric chloride is used. In contrast, the same work suggested that the chemisorption of hydrolyzed aluminium species was an important mechanism when alum was used as a coagulant. These investigations suggest that the specific interactions between alum and the oocyst surfaces might provide benefits in oocyst removal. While some studies have demonstrated that improvements in C. parvum removal by filtration could be associated with specific coagulants, others have underscored the general optimisation of chemical pretreatment rather than specific coagulant selection.

Previous studies have demonstrated comparable levels of C. parvum removal by filters treating alum-coagulated water at both warm (~20-26C) and cold (~2-3C) water temperatures. Similar studies conducted at the same pilot plant demonstrated that oocyst-sized polystyrene microspheres were reasonable surrogates for C. parvum removal by filtration.

In the study described here, pilot-scale filtration studies were performed to investigate the relative impact of coagulant type on the removal of Cryptosporidium and oocyst-sized polystyrene microsphere surrogates by granular media filtration. The impacts of in-line alum, ferric chloride, and chitosan coagulation on subsequent filtration were investigated.

MATERIALS AND METHODS

Pilot-scale treatment plant
Two glass filter columns 50mm in diameter were operated at a loading rate of ~10.4 m/h (~4.3 US gpm/ft2) in a constant rate, rising head mode during the filter evaluations. Each of the filters contained 508 mm of anthracite over 203 mm of sand. The filters treated dechlorinated tap water with 3.5 NTU of kaolinite-induced turbidity. The raw water was coagulated in lime and then filtered. One filter treated alum-coagulated water at a dose of 5 mg/L alum at pH 6.9. A second filter treated ferric chloride-coagulated water at a dose of 3 mg/L at pH 6.9. Investigations were also conducted with Chitosan-coagulated water at a dose of 1 mg/L.

During the experiments, formalin-inactivated oocysts and oocyst-sized polystyrene microspheres were added to the filter influent to yield concentrations of ~105 oocysts/L and microspheres/L respectively. The oocysts were added to the raw water and were subsequently coagulated. The addition of oocysts to the raw water did not substantially increase particle loading to the treatment system.

Operational conditions
The effects of alum, ferric chloride, and chitosan coagulation on Cryptosporidium and oocyst-size microsphere removal by filtration were investigated during three experimental conditions: stable operation, sub-optimal coagulation, and no coagulation. Stable operating conditions were periods of optimised treatment during which filter effluent turbidities did not exceed 0.1 NTU. Sub-optimal coagulation conditions represented a coagulant misfeed resulting in a 50 per cent reduction in applied coagulant dose. The no coagulation experiments represented a complete coagulation failure.

All of the experiments were conducted after two to four hours of stable operation after filter ripening. Cryptosporidium oocysts and oocyst-sized polystyrene microspheres were seeded into the raw water for one hour during each of the experiments. Filter effluent and effluent samples were collected at four time points, each approximately ten minutes apart, throughout the seeding period. Removal calculations were based on these influent and effluent concentration pairs.

Analytical methods
(a) Cryptosporidium parvum
Stock suspensions of formalin-inactivated C. parvum were vortexed for 30 seconds, and then a small portion of the suspension was removed to enumerate the oocyst concentration. The stock concentration was determined by averaging triplicate counts using a hemocytometer and light microscopy. The entire grid (1mm2) was used for oocyst enumeration at 400x magnification.

During the filtration investigations, C. parvum oocysts were measured in filter influent and effluent samples. Filter influents were analysed in 2.5 mL volumes. Filter effluents were analysed in volumes ranging from 5 mL to 1 L, depending on the operating condition studied. Sample volumes were chosen to yield between ten and 2,000 oocysts per membrane.

All of the samples were directly filtered through 25 mm, o.40 µm polycarbonate membranes utilising a previously described method and standard immunofluorescence assay. Presumptive microscopic analysis for C. parvum was performed at 400x magnification. Recovery data from the water matrix indicated approximately 75 per cent recovery of oocysts, comparable to results reported elsewhere.

(b) Polystyrene microspheres
FluoresbriteTM carboxylated YG fluorescent-dyed, oocyst-sized polystyrene microspheres were used as non-biological surrogate indicators for C. parvum removal. The YG dye matches the fluorescence filter settings of fluorescein, similar to FITC for C. parvum. The microspheres were concentrated and enumerated concurrently with C. parvum, by the method generally described above. Recovery data from the water matrix indicated approximately 75 per cent recovery of microspheres, comparable to results previously reported elsewhere.

(c) On-line parameters: turbidity and particle counts
Turbidity was monitored at the filter influent and effluent locations using on-line turbidimeters calibrated using dilute formazin solutions as specified by the manufacturer. An IBR particle counter measured total particles from 1-150µm at the filter effluent location.

RESULTS AND DISCUSSION

C. parvum removals by filtration preceded by in-line alum, ferric chloride, and chitosan coagulation during stable operation, sub-optimal coagulation, and coagulation failure were recorded and analysed. During stable (optimised) operating conditions, similar levels of C. parvum removal were observed in the pilot-scale filters, regardless of coagulant type (alum, ferric chloride, or chitosan). The importance of maintaining proper coagulation was clearly demonstrated. Compared to alum and chitosan, ferric chloride may result in slightly lower C. parvum removals by filtration during sub-optimal coagulation conditions, and further analysis is necessary to determine whether the differences are statistically significant. A possible explanation for such differences may be the different mechanisms of interaction between the coagulants and the oocysts. In agreement with other reports, this study demonstrated almost no oocyst removal by filtration during complete coagulation failure, regardless of coagulant type.

Oocyst-sized polystyrene microsphere removals by filtration preceded by in-line alum, ferric chloride, and chitosan coagulation during stable operation, sub-optimal coagulation, and coagulation failure were also recorded and analysed, providing similar data to the oocyst data above, however there was considerably more variability. The overall trends regarding the impact of coagulant type and coagulant conditions were similar between both the oocyst and microsphere sets of data. Microsphere removals by filtration preceded by alum, ferric chloride, and chitosan were similar. Again it appeared that, compared to alum and chitosan coagulation, ferric chloride coagulation may result in slightly lower removals of microspheres by filtration during sub-optimal coagulation conditions, and when no coagulant was present, there was little removal of microspheres by filtration.

Oocyst and microsphere surface charge and compressibility may contribute to the observed differences in oocyst and microsphere removals by filtration. In the present study, the overall relationship between C. parvum oocyst and polystyrene microsphere removals during the range of operational conditions investigated was fairly linear. Microsphere removals were similar to oocyst removals, though often slightly lower, regardless of the coagulant type utilised.

CONCLUSIONS

The pilot-scale results from this work indicated that:

• Alum, ferric chloride, and chitosan coagulation generally resulted in similar removals of Cryptosporidium oocysts and oocyst-sized microspheres during optimised operating conditions when filter effluent turbidities were consistently below 0.1 NTU.

• Sub-optimal coagulation conditions with alum, ferric chloride, and chitosan coagulation resulted in deteriorated Cryptosporidium and microsphere removal by filtration, relative to stable operation. Cryptosporidium removal by filtration during sub-optimal coagulation with either alum or chitosan appeared marginally better (relative to stable operation) than that observed during ferric chloride coagulation.

• The observed differences in Cryptosporidium and microsphere removal during sub-optimal coagulation conditions (and possibly during stable operation) may be associated with the different mechanisms of alum, ferric chloride, and chitosan interaction with oocysts during filtration. Further analysis is necessary to determine if these differences are statistically significant.

• Oocyst-sized polystyrene microspheres appeared to be reasonable indicators of Cryptosporidium removal by filtration, regardless of coagulant type.

The authors of the original paper are M. Emelko and T. Brown, of the University of Waterloo, Canada. Monica Emelko PhD. is assistant professor of Environmental and Civil Engineering at the University of Waterloo, Ontario, Canada, and can be contacted at [email protected].

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