RAPID MIXING IN WATER TREATMENT

ployed for the express purpose of achieving complete homogenization of a coagulant chemical with the stream to be treated. It is accomplished by applying turbulence to the combined streams and has been done in a variety of ways (e.g., mechanical agitators or baffled basins). It is hoped that this article will show that the general design criteria are not compatible with the nature and rates of the chemical and physical reactions that occur when Al(III) or Fe(III) are used. Inefficient rapid mixing may have two harmful effects: (1) wastage of chemical and (2) slower particle aggregation rates for a given chemical dosage. To understand this problem, the following must be taken into consideration : ( 1 ) the type and rates of chemical reactions which Al(III) or Fe(III) undergo, (2) the chemical and physical factors which affect these rates, (3) how different mixing regimes affect these physical and chemical factors, and (4) how the various reaction products of Al(III) and Fe(III) affect the collision efficiency of particles. These considerations are necessary because the physical-chemical reactions not only appear to be extremely rapid, they are essentially irreversible. Also, the rates and reaction products depend upon local concentrations, while the collision efficiency is highly dependent upon the type of reaction products adsorbed to the colloid surface and the concentration and distribution of them upon the colloid surface.