Particle size distribution in the clarification process

A study of particle size distribution was performed in order to predict the clarifying performance in the water clarification process.

A series of jar-tests were run using raw water from the Mark B. Whitaker Water Treatment Plant, Knoxville, Tennessee, and an alum coagulant. Dosages from 10 to 35 mg/1 were tested.

Observations of the floes were made using a 35 mm camera with a macro lens. The floes were measured by projecting the negatives on a large screen. A HIAC/ROYCO Model 4100 was used to measure floe particles smaller than 75 µm.

A computer program was developed to calculate the dimensionless concentration C(t)/C(0) at time t. The program requires as inputs the ranges of particle size and the number of particles in each range. Assuming spherical particles, uniform initial distribution, same floe density, and settling in a quiescent system, the program calculates the settling velocity of each set of particles and the particle concentration at different settling times.

The optimum dosage was determined by a comparison of turbiditytime and concentration-time curves. The first curve was obtained by direct measurement of the parameters during the jar-tests, and the second curve was obtained using the computer program fed with the data obtained photographically and with the particle counter.

The evidence from this research clearly indicates that the initial floe particle size distribution is one of the principal determinants of coagulant dosage control of the clarification process. So the knowledge of this parameter gives us a quick and better way of determining coagulant dosage.