Page 217 - Water and wastewater engineering

5-20 WATER AND WASTEWATER ENGINEERING

TABLE 5-7
EPCRA threshold planning quantities
Threshold planning
Chemical quantity, kg

Chlorine 45
Chlorine dioxide Not listed
Anhydrous ammonia 225
Aqua ammonia Not listed
Hydrogen peroxide (52%) 450
Sulfuric acid 450
Ozone 45

Preventive maintenance includes regularly scheduled times for equipment to be taken out
of service for replacement of worn parts, calibration, and so on. Frequently, this type of work
is scheduled in the winter to take advantage of low flows. In addition, feeders, feed lines, and
instrumentation are to be checked routinely during each shift.
Good housekeeping includes prompt cleaning of spills and removal of chemical dust.
Although regulatory agencies will dictate that certain records be kept, operation of the plant
often requires more information than is reported. For example, the status of the chemical inven-
tory and the operating performance of each piece of the chemical handling and feeding equip-
ment should be logged and conveyed to the next shift operator (Kawamura, 2000).

Hints from the Field. Operation and maintenance personnel who have to live with the results
of the engineer’s design have offered the following suggestions:

• Schedule 80 PVC and CPCV are the most commonly used materials for sodium hypochlo-
rite piping. Early installation of these systems failed because of leaks at the solvent welded
joints. Special glues designed for use with NaOCl must be used to guard against this type of
failure.
• Quicklime storage silos should always be cylindrical. Because of its hygroscopic nature,
lime will invariably cake in the silo. In one case, the working volume of a 200 Mg square
silo was effectively reduced to 35 Mg. Vibration and other attempts to loosen the caked
material were ineffective. The additional expense of a cylindrical silo will be repaid many
times by the reduced O&M costs of trying to loosen the caked lime.

• As shown in Figure 5-6 , place the slaker directly beneath the lime storage silo to minimize
dust in transporting the lime to the slaker.

• Grit in the lime can be removed after slaking by the simple expedient of placing a milk
crate lined with hardware cloth in the exit stream ( Figure 5-6 ).
• Transport the slaked lime to the mixing device with an eductor and flexible hose ( Figure 5-6 ).
Pumps will cake with lime, and rigid pipes will clog. The eductor eliminates moving parts,
and the flexible pipe makes it easy to spot blockages and either break them in place or quickly
replace a section for out-of-service cleaning.