Gravity settling characteristics of Cryptosporidium parvum oocysts in aqueous suspension using in situ static light scattering

Gravity settling velocity measurements are reported for Cryptosporidium parvum oocysts in aqueous suspensions. A non-intrusive technique using laser light scattering was developed to measure in situ settling velocity of particles. Settling experiments were performed on the C. parvum suspended in particle free de-ionized (PFDI) water at a low volume fraction of 3.4×10−4. The oocysts were found to have a log-normal distribution of settling velocities with a mean of 0.029 μm s−1 (geometric standard deviation, σg=2.81). The mean velocities were considerably smaller compared to previously reported values by Medema et al. [Appl. Environ. Microbiol. 64 (11) (1998) 4460–4466] and could possibly be due to variation in oocysts density. Experiments on suspensions with varying ionic strength of the medium showed no marked effect on the settling velocity. These results have implications in transport modeling of C. parvum oocysts in filtration beds and other environmental systems.