Surface water treatment plants are required to reduce filtered water turbidities down to 0.1 NTU. Turbidity measurement is not sensitive to particles in the size range of Giardia cysts and Cryptosporidium oocysts; furthermore, low treated water turbidity does not necessarily indicate their absence in the water. Particle removal measurement in the Giardia and Cryptosporidium size range was used for treatment process evaluation and optimization. Pilot plant experiments demonstrated that preoxidant type and dosing rate of alum and a cationic polymer affected removal efficiency of particles during direct filtration process more than any other process parameter.

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