Page 199 - Water and wastewater engineering

5-2 WATER AND WASTEWATER ENGINEERING

5-1 INTRODUCTION

Principal components of the design of water and wastewater treatment plants are the selection of
appropriate chemicals, calculation of their dosage, selection of their physical state (gas, liquid,
or solid), how they are to be stored, how much is to be stored, and the type of feed equipment
to be used. The selection of appropriate chemicals and calculation of dosage will be discussed
in Chapters 6, 7, 13, 22, 23, 25, and 26. This chapter summarizes some of the alternatives and
design criteria for storage and handling as well as consideration of safety and security issues.
Of the over 50 chemicals used in treating water and wastewater, four chemicals have been
selected for illustration purposes: aluminum sulfate (alum), ferric chloride, lime (CaO), and chlo-
rine. Detailed discussion of the other chemicals may be found in Anderson (2005) and Metcalf
& Eddy (2003).

5-2 REDUNDANCY AND CAPACITY PROVISIONS
Redundancy
The requirement for redundancy is dependent on whether or not the application of the chemical is
noncritical and, therefore, interruptible or, critical, and, therefore, noninterruptible. Where chemi-
cal feed is necessary for the protection of the water supply or, in the case of wastewater, where
the receiving body could be permanently or unacceptably degraded, the chemical is considered to
be noninterruptible. For example, coagulants and chlorine in water supplies are noninterruptible
(GLUMRB, 2003). Chemicals used for corrosion control, taste-and-odor control, and fluoridation
are interruptible.
For small plants where one feeder may be adequate for the range of anticipated flows a mini-
mum of two feeders shall be provided for noninterruptible chemicals (GLUMRB, 2003 and U.S.
EPA, 1974). In larger plants, where it is necessary to have three or more feeders, there should
be one feeder for each application point plus one or more standby units in reserve of sufficient
capacity to replace the largest unit when a unit is out of service (GLUMRB, 2003).

Capacity
The required capacity of feeding equipment is based on two requirements: ability to meet the maxi-
mum dosage required and capability to feed that dosage at the maximum flow rate while still main-
taining reserve units. Multiple units of different capacity may be required because the minimum
feed rate may be less than that provided by the turn-down ratio, that is, the ratio of the maximum
feed rate to the minimum feed rate, of standard manufacturer’s equipment. This may be especially
true during low flows at the beginning of the design life of the plant. In this case, a larger number
of units may be required to cover the range of the feed equipment. It may be more economical to
plan chemical feed equipment for a shorter design life than the entire facility with an incremental
increase in the number of units or replacement of smaller units as the flow rate increases.

5-3 DELIVERY, HANDLING, AND STORAGE
Delivery and Handling

Dry Chemicals. For small plants, dry chemicals are purchased in bags or barrels and delivery
is by truck to a loading dock. For large plants, dry chemicals are delivered by truck or railcar.