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6.14 CHAPTER SIX

actions that are required for charge neutralization occur within seconds, and the induction
system disperses the coagulant into the raw water stream extremely rapidly.
Induction mixing systems can be in-line in a piping system or submerged in a chan-
nel. The selection of the type of system is primarily governed by induction system design
and the design water flow. By combining the chemical feed, mixing, and control system
into one unit, induction mixing systems can also save power costs. Due to the uniqueness
of these systems and the proprietary nature of the equipment, the evaluation and design
of these systems should be closely coordinated with the equipment supplier.

FLOCCULA TION PROCESS DESIGN

Building optimum size floc requires gentle mixing in the energy gradient range of 20 to
70 s-1 for a total period of approximately 10 to 30 min. Direct filtration requires a small,
dense floc that can be formed at the higher end of the energy range. For settling in con-
ventional basins and in units with settling tubes and settling plates, lower energy levels
are applied to produce a large, dense floc that will resist breakup during contact with weirs
and plates. Often, polymers are used to help form denser floc.
Floc begins to form within 2 s of coagulant addition and mixing. If high turbulence
or shear is subsequently applied to the water, the formed flocs may be fragmented, and
broken floc may not readily settle or re-form.
Optimum floc that is efficiently settled or filtered is usually formed under conditions
of gradually reducing energy. In large plants, it may be difficult to distribute water to floc-
culation basins or filters without quiescent stages and high-energy stages. Conduits han-
dling mixed water should minimize head losses, but may, on the other hand, include wa-
ter jets or air mixing to maintain G at values of 100 to 150 s- 1 before the water is transferred
to the flocculation stage.
The gentle mixing process of flocculation is designed to maximize contact of desta-
bilized particles and build settleable or filterable floc particles. It is desirable to maintain
shear forces as constant as possible within the process. As a result, flocculator mecha-
nisms tend to be slow and to cover the maximum possible cross-sectional area of floc
basins.
It is desirable to compartmentalize the flocculation process by dividing the basin into
two or more defined stages or compartments, as illustrated in Figure 6.7. Compartments
prevent short-circuiting and permit defined zones of reduced energy input or tapered en-
ergy. To prevent short-circuiting, baffles are typically placed between each stage of floc-
culation. For mechanical (nonhydraulic) flocculation basins, baffles are designed to pro-
vide an orifice ratio of approximately 3% to 6% or a velocity of 0.9 ft/s (27 crrds) under
maximum flow conditions.

Incidental Flocculation
As coagulated water is transferred to flocculators in small plants, distances are short
enough that incidental flocculation is negligible. But in large plants transfer may involve
distances of more than 100 ft (30 m) through low-velocity conduits, weirs, or other means
of distributing water equally to each flocculation basin or compartment. This travel in
large plants involves turbulence, and flocculation and incidental flocculation take place.
If velocities or levels between the conduits and the flocculation basin are not limited, floc
may be fragmented and plant efficiency impaired. Higher coagulant feed rates may be re-
quired to overcome fragile floc problems.