Settling velocity of Cryptosporidium parvum and Giardia lamblia

Understanding transport behavior of Cryptosporidium parvum oocysts and Giardia lamblia cysts (together referred to as (oo)cysts) in overland flow is important for beneficial uses of receiving water bodies. Like sediment, (oo)cysts are subjected to deposition once they are present in overland flow or low flow environments like reservoirs, wetlands and sedimentation basins. The objectives of this paper are to present the theory and experiment to determine the free settling velocity (vs) of (oo)cysts and to compare experimental settling velocities to estimates using Stokes’ law. A settling experiment was designed to quantify the vs of (oo)cysts in an aqueous column. C. parvum oocysts used were spherical with average diameter (±1SD) of 6.6±1.1 μm. G. lamblia cysts were oval shaped (average eccentricity =1.48±0.19) with average size of 11.8 ±1.3 μm. Average densities were 1009 kg m−3 for C. parvum oocysts and 1013 kg m−3 for G. lamblia cysts. Observed experimental settling velocities are 0.27 μm s−1 and 0.67 μm s−1 for C. parvum and G. lamblia, respectively. Estimated average settling velocities using Stokes’ law were 0.36 μm s−1 for C. parvum and 0.84 μm s−1 for G. lamblia. R-squared values of the observations from the settling experiments with the Stokes’ law estimation are 0.87 and 0.88 for G. lamblia and C. parvum, respectively. Our results suggest that Stokes’ law can be used to estimate settling velocities of (oo)cysts. Qualitatively, the low settling velocities indicate that (oo)cysts will very slowly settle out of suspension.