Evaluating and Selecting Chemical Equipment
DESIGN AND APPLICATION LIQUI D-SOLI DS
SEPA R A TI0N T h e last step, actual selection
of equipment, is not necessarib the most important
T. L . M O O R E EUGENE D. DRISCOLL
THE first coagulation basin was probably an earthen crock into which an ancient Chinese or Egyptian tossed a handful of alum before retiring for the night. In the morning, clear water was ready for his use. Since then, coagulation basins have increased considerably in size and complexity. The units used today have transformed this batch process into a continuous operation, increased the rate and degree of completion for the chemical reactions, and increased the rate at which precipitates are removed. Solids-Conbct hiins-Flow
Control and Chemical F d a n
The first large-scale basins were the so-called conventional flocculation and sedimentation basins. Still in wide use today, this type of equipment is based on providing detention periods of 4 to 6 hours or more, to provide time for floc and sediment to separate from a slow horizontal flow of water. However, because of the relatively large area requirements, use of such units is restricted largely to municipal water supplies. For industrial water supplies, equipment supplied for coagulation and flocculation is almost invariably a type of solids-contact basin. There are a number of sound reasons for this: Such basins operate at relatively high rates and detention times are generally 60 to 90 minutes. Thus, area requirements are 25% or less than those required for conventional-type basins; short detention times also provide $eater flexibility in opera50
I N D U S T R I A L AND E N G I N E E R I N G CHEMISTRY
tion. The basins lend themselves more readily to automatic operation and control, limiting labor requirements and the possibility ofhuman error. Operating results are as good or better than can be obtained in conventional-type units and, although solids-contact basins use mechanical equipment and control elements to a considerable degree, personnel capable of operating and maintaining such equipment is generally available in the labor force of most industrial plants. Design. Solids-contact basins are generally single units in which .a series of operations occur in proper sequence and under adequate control. Raw water and chemicals are mixed with previously formed sludge. This makes a high rate of operation possible and, al‘ I though the manner in which this is accomplished differs with various units, it is the common denominator of all such basins. The problem of incorporating a solids-contact basin into a complete water treatment system varies widely, depending on raw water quality and required purity of the effluent, physical restrictions, such as topography and space available, treated water storage facilities, and extent of fluctuations in demand. Nevertheless, a (Continued on page 52) +y
