Flocculation experiments were carried out by using a Couette type of v
iscometer for applying uniform fluid shears to freshwater suspensions
of estuarine fine-grained particles. The suspensions were buffered by
a maleic acid/ammonium hydroxide system. The objectives of the study a
re to investigate the relationship between the steady-state sizes of f
locculated particles and applied shear at various pH values. The resul
ts showed that the rate of particle flocculation increased with the ad
dition of the buffer. With a particle concentration of 100 mg/l and pH
of 6.0, the floc size decreased with the increasing shear. While with
a pH of 7.5, the floc size increased with the shear until it reached
a maximum value at the shear of 400 s(-1), and then the size decreased
as the shear increased further to 500 s(-1). The maximum size shear w
as found at 200 s(-1) for pH 7.9. By increasing the pH value to 9.0, t
here was no particle growth with any applied shear. The electrophoreti
c mobility of the particles was measured. It was found that for any co
nstant shear the floc size is inversely proportional to the absolute v
alue of the mobility. That is, the more repulsive the force between pa
rticles, the smaller the floc size. The effect of pH on floc size was
not uniform within the pH range tested. The results demonstrated that
the higher the sheat the more significant the changes of floc size due
to the pH effect.