Civil Engineering Reference
In-Depth Information
Air Vent
Softened Water
Outlet
Chemical Inlets
REDUCED PLAN
Test Cocks
Raw Water Inlet
Chemical Inlets
Raw Water Inlet
Draw-Off Valve
for Enlarged Catalyst
Fig. 23-24.
Schematic diagram of a reactor for lime sludge pelletization (Courtesy of USFilter)
hardness. Upward velocity is limited to about 3 ft / min (0.015 m / s) at the top of the
cone to prevent carryover of catalyst particles.
12
Lime is injected into the reactor while the raw-water flow is gradually increased
from a low initial rate to design capacity. The lime reacts with calcium bicarbonate
and carbon dioxide to form calcium carbonate, which precipitates on the suspended
particles.
Claims have been made that the size of the calcium carbonate-coated particles can
reach 1.6 mm diameter; however, operating experience has shown that maximum sizes
are in the 0.7 to 1.0 mm range.
12
Theoretically, reactors should be capable of continuous operation. This requires a
fine balance between the blowdown of sludge pellets and the addition of new, granular
catalyst to maintain a constant-volume bed. In practice, the balance is difficult to
accomplish, so the reactors are generally operated in a batch mode.
Treated water turbidity is used as a measure of treatment effectiveness. When high
turbidities are experienced, often after about 40 days of operation, the reactor run is
terminated. At the end of the run, the contents of the reactor, water, and sludge pellets
are discharged into a storage and drainage facility. After drainage, the pellets can be
treated as solids.
The pelletized sludge is approximately 60 percent solids by weight as it leaves the
reactor. The entrained water can be readily drained, with the resulting product being
90 percent solids by weight. If the weight of the entrained catalyst is accounted for,
each 1.0 cu ft (0.028 m
3
) of drained sludge contains approximately 105 pounds (48
kg) of calcium carbonate.
12
A comparison of the volumes of conventional and pelle-
tized sludge shows that the volume of pelletized sludge is 10 to 20 times less than the
volume of undewatered conventional sludge.
The limitations on this approach are: magnesium content should be less than 85
mg / L as CaCO
3
; turbidity should be less than 10; and, in cold climates, the reactors
must be enclosed in heated structures. Excessive magnesium forms magnesium hy-
