Discover how distributed control
systems can help municipal
authorities manage operations
more efficiently and cost effectively
BY DOUG JOHNSON
At the highest level, municipal water and waste- water system operators are tasked with ensuring public health and safety. But they also face daunting challenges as they go about the daily operation and management of sophisticated physical, chemical and biological
processes — all in a highly-regulated environment that must
operate 24/7. More and more, municipalities are turning toward distributed control systems rather than PLCs to help
address operational challenges.
While PLCs have been applied successfully to municipal
water and wastewater treatment systems for quite some
time, as operations continue to increase in complexity and
regulations become more stringent, municipalities are realizing the full benefits of utilizing DCS technology as the foundation of a longer-term automation master plan.
Although both technologies have evolved over the years,
it’s interesting to note that PLC systems were initially introduced for discrete manufacturing processes, such as automotive manufacturing — operations which could be shut down,
problems fixed as needed, and manufacturing then resumed.
On the other hand, DCS systems were developed to control
continuous processes that could not easily be interrupted,
such as power generation or water/wastewater treatment.
Given the public bidding practices of many utilities, capital
expense is often the primary basis of selecting an automation
system. However, this can be misleading. It is important for
municipalities to take a broader view that goes beyond the
capital expense alone and considers a number of other relevant factors, including total cost of ownership over the expected useful life, overall system capabilities, including security as
well as other advanced features and functions, maintenance
and upgrades, and long-term service and support.
TOTAL COST OF OWNERSHIP
The initial cost of installing either a DCS- or PLC-based sys-
tem is generally considered to be equivalent, as long as the
systems are being installed in a new facility. However, if older
control technologies are being replaced, some DCS suppliers
can provide migration tools that can make the upgrade simpler
and more cost effective than a wholesale “rip and replace.”
Likewise, the cost and time associated with commissioning
each type of system can be quite different. Since a DCS is
fully integrated with hardware and software that is devel-
oped, tested and maintained by a single supplier, commis-
sioning is relatively straightforward. PLC-based systems are
often assembled by a systems integrator with hardware from
one supplier and software from another, making it difficult to
achieve the level of integration and overarching system-level
support that is possible with a DCS.
Redundancy tends to be the norm for continuous process
operations, making it a core strength of DCS technology.
While many PLC-based systems offer a redundancy option,
the additional cost — including equipment, configuration
and maintenance — can be significant.
Software licensing is another important cost consideration.
PLC vendors typically license a limited number of tags, and ad-
ditional costs are therefore often incurred as the system grows
over time with process changes or plant expansions. Reputable
DCS suppliers generally do not charge “per tag” licensing fees.
It’s important to remember that annual software license
costs can vary greatly among suppliers, and annual fees need
to be well understood when considering how much a system
will cost over time. It’s particularly important to understand
HMI software licensing costs with PLC-based systems.