Page 285 -

10.30 CHAPTER TEN

Key to the design of these systems is how much chemical storage to maintain on-site
and what equipment to back up. In general, the chemical storage is as salt with only small
amounts of stored hypochlorite. Because the generated sodium hypochlorite is very di-
lute, the volume of storage required for sodium hypochlorite is very large as compared to
the salt storage. Usually 30 days of salt storage is supplied, and 3 days of sodium hypochlo-
rite storage is supplied. This keeps the system reasonably economical.
The electrolytic cells are the most expensive part of the system. Sometimes installed
spares are supplied, but more frequently they are not. Rather, the sodium hypochlorite
storage tanks are designed to handle concentrated sodium hypochlorite (12.5%), so that
in the event of cell failure, sodium hypochlorite can be delivered to the site. The disad-
vantage of this is that multiple metering pumps must be supplied because of the large size
variation.

Choosing between Chlorine, Delivered Sodium Hypochlorite, and
On-site Generation

When a facility is researching what chemical to use for disinfection, the most common
choices will be chlorine, delivered sodium hypochlorite, and on-site generation of sodium
hypochlorite.
If a net present-value analysis is performed, chlorine will almost always be the best
choice. Chlorine will have a relatively low installed cost and the lowest chemical cost.
Delivered sodium hypochlorite will typically have the lowest installed cost, but the high-
est chemical cost. On-site generation will have the highest installed cost, but a relatively
low chemical and power cost. Of course, in all these instances local variations can affect
the most economic choice, but this is usually how they fall out.
If a facility is considering delivered sodium hypochlorite and on-site generation, it is
likely that the facility is concerned with safety or otherwise concerned with having chlo-
rine on-site. As such, often new and existing chlorine systems are being eschewed in fa-
vor of delivered sodium hypochlorite and on-site generation systems.
The specific project requirements, including sparing philosophy, local delivered chem-
ical cost, and power cost, will greatly influence the choice between the two options.

Chlorination Control
Proper design of the control system for chlorination facilities is as important as any other
aspect of their design. Methods currently used are manual setting, based on flow and pe-
riodic or continuous residual measurements; continuous feedforward control, based on
continuous flow measurements; feedback control, based on continuous residual measure-
ments; and compound, closed-loop control, based on continuous measurements of both
flow and chlorine residual. Each of these methods is illustrated in Figure 10.12. Details
of control signal manipulation are not shown.
Control Methods. The manual control method is limited by the diligence of the opera-
tor. If either the flow or the chlorine residual changes, the operator must make adjust-
ments; consequently, this method gives a wide range of performance. The feedforward
(flow-pacing) method is a significant improvement because the flow has been eliminated
as a variable, provided the operator is diligent in maintaining the control system. For wa-
ter supplies where chlorine demand is very stable, feedforward control is preferred.
Feedback control is theoretically superior to feedforward control because the chlorine
residual is being directly used to control chlorine addition. Unfortunately, this is not al-