Page 207 - Water and wastewater engineering

5-10 WATER AND WASTEWATER ENGINEERING

and mixing it with water into one system. They include a quicklime feeder, water control valve,
grit removal device, and a reaction vessel.
The slaking reaction is highly exothermic. The reaction vessel is designed for the high
heat release rate. It is completely contained to protect the operator from “boil up” of the
slurry.

Example 5-3. Select a feeder for the lime described in Example 5-1 . The quicklime is lumpy.

Solution:
a. From Example 5-1 , the average purity of lime is 87% and the daily lime consumption is
3,575 kg/d.

b. The feeder must be capable of handling a lumpy material. The first choice of feeder from
Table 5-2 is “loss-in-weight.”
c. Check the capacity:

⎛ 1 ⎞ ⎛ 1 ⎞
3
(3575 kg/d ) ⎜ ⎝ 24 h/d⎠ ⎝ 850 kg/m ⎠ ⎟ 0..18 m /h
⎟ ⎜
,
3
3
where 850 kg/m is the average bulk density of lime from Appendix A. This is within the
4
3
operating range of 6 10 to 2 m /h for the loss-in-weight feeder.
Liquid Feed Systems
A typical liquid feed system consists of a storage tank and/or a day tank, pump, meter, and distri-
bution system as shown in Figure 5-4 .
The characteristics of liquid chemical feeders are summarized in Table 5-3 on page 5-12.

Piston and Diaphragm Pumps. The capacity of these pumps depends on the stroking speed
and the length of the stroke. In contrast to the piston pump where the piston is in direct contact
with the chemical, diaphragm pumps, as the name implies, use the movement of a diaphragm to
move the fluid.

Progressive Cavity Pumps. These pumps use a combination of an eccentric rotation of a shaft
combined with stator elements to move the fluid. They are particularly suited to moving viscous,
shear sensitive fluids, pastes, and gritty slurries.

Eductors. A stream of water passing through a venturi in the eductor creates a vacuum that
draws the liquid chemical into the eductor. Because the eductor is incapable of flow rate control,
the chemical must be metered in some fashion. This system has found success in moving lime
slurry from a slaker to a mixing system.