Uncovering the Mysteries of the Floc Blanket: An Exploration with Inlet Jets, Flocculators, and Polyaluminum Chloride Precipitates

Abstract

Floc blankets are used in water treatment plants to improve plant performance and aid in sludge removal and consolidation. Maintaining a fully functioning floc blanket in sedimentation tanks requires that the floc blanket remain fluidized and sludge prevented from building up on the bottom of the tank. A laboratory water treatment system was used to evaluate the range of energy dissipation rates (EDR) of the inlet to the sedimentation tank to determine when the settled effluent turbidity of the system would exceed drinking water quality standards.. Increasing the inlet jet EDR up to approximately 300 mW/kg did not increase effluent turbidity of the system. Small inlet jets with high EDR can be used to ensure resuspension of the floc density current without adversely affecting water treatment plant performance.

The design of flocculators is based on mean shear (G) and hydraulic residence time (&thetas;) and the product (G&thetas;). Guidelines for these values are conservative and designs outside the suggested range could lower plant capital costs. The following flocculator parameters were evaluated: 1) increasing G (range of 74-251 s^-1) while decreasing &thetas; from 269 to 80 s (maintaining a constant G&thetas;) and 2) maintaining a constant G (72 s^-1) and varying &thetas; (24 to 1425 s). Flocculator &thetas; below recommended design guidelines performed well indicating that shorter flocculators could be used in the presence of floc blankets.

Three potential hypotheses by which residual particles aggregate with other flocs in the floc blanket were considered: residual particles aggregate with 1) other residual particles, 2) small flocs (transitional), or 3) large flocs (hindered). Three coagulant doses (0.625, 1.25, and 2.5 mg/L) were tested with four hydraulic flocculators with constant G&thetas; but varying G (72, 126, 251, and 340 s^-1) and &thetas; (269, 159, 102, and 59 s) on the combined system. A classification system of flocs in the floc blanket was defined based on floc size and time-scale. Results strongly suggest that hypothesis 2 is valid, however, research is needed on floc sizes at specific locations within the system.