Page 132 -

MIXING, COAGULATION, AND FLOCCULATION 6.23

The process may be designed to include intentional backmixing in the flocculation
stage, and the clarification stage may be designed for "sludge blanket" conditions, where
flow from the flocculation zone moves upward through a layer, or blanket, of sludge in
the settling zone. Designs may include a conical section in the mixing and flocculation
stage and increased area in the upflow zone in the clarifier stage.
A single-unit solids contact type of flocculator-clarifier may have advantages for some
applications. These units perform best in waters that can develop a dense, fast-settling
floc, but the process may be difficult to control in some waters. In general, they work
very well on waters that are stable in water quality and with treatment systems that re-
quire steady water demands. "Flashy" river waters or water systems that require wide
treatment rates can be problematic. Due to these issues and the potential variations be-
tween raw waters and system demands, at a minimum, bench-scale tests should be per-
formed at all facilities considering these types of unit, and full-scale pilot tests are rec-
ommended for large plants.

Sludge Recirculation and Solids Contact. The intentional introduction of preformed
floc or sludge into the mixing and flocculation stage is a feature of most proprietary wa-
ter treatment equipment. High concentrations of suspended solids in the flocculation pro-
cess (and in the sludge blanket of the clarifier) can provide improved efficiency in
reducing particulates, colloids, organics, and certain ionized chemicals.
Recirculating sludge and reintroducing filter wash water into the mixing and floccu-
lation stages of nonproprietary designs may improve efficiency and reduce chemical re-
quirements. Sludge and wash water quality is an important consideration when one is de-
termining the reuse of process residuals. Sludge recirculation may be difficult to optimize
at the pilot plant stage of investigation. Providing flexibility for sludge recirculation and
solids contact should be considered at the design stage of new plants.

Contact Flocculation. The ability of a coarse media bed to act as a flocculation system
is well demonstrated. The time required to build optimum size and density of floc may
be reduced where there is close contact with preformed flocs. The time factor is impor-
tant for plants treating very cold water and for portable plants and pressure plants. Con-
tact flocculation may be applicable for difficult high-suspended-solids water or water with
low total dissolved solids that may not respond readily to metal-ion coagulants.
A typical application of the process is to pass coagulated water through a coarse me-
dia or gravel bed, either by gravity or under pressure. The gross detention time may be 3
to 5 rain at 5 ° C. Flow may be either upflow or downflow, as illustrated in Figures 6.13
and 6.14. The system should provide for removing excess accumulated floc, usually by
means of air scour, similar to filter air scour.
Pilot plant investigations should precede design, or the criteria for contact flocculation
should be based on proven applications on similar water sources. Several manufacturers
offer equipment or processes that use the principles of contact flocculation.

PROCESS MONITORING AND CONTROL

Design dosages for chemicals should be based on experience with similar types of waters
or, preferably, with jar tests and pilot tests. Design dosages of coagulants can be deter-
mined from jar tests. The effectiveness of coagulation and flocculation can also be mon-
itored by several parameters, including turbidity, zeta potential, streaming current, parti-
cle counts, and bench and pilot tests (discussed in other chapters of this volume).