Environmental Engineering Reference
In-Depth Information
at remote locations. Each remote computer can control mechanical processes
(pumps, valves, etc.) and collect data from sensors at its remote location, thus
the phrase
supervisory control and data acquisition
. The central computer is the
master terminal unit
(MTU). The operator interfaces with the MTU using soft-
ware known as the
human-machine interface
(HMI). The remote computer is the
programmable logic controller
(PLC) or
remote terminal unit
(RTU). The RTU acti-
vates a relay (or switch) that turns mechanical equipment on and off. The RTU
also collects data from sensors. In the initial stages, utilities ran wires (hard-
wire or land lines) from the central computer (MTU) to the remote computers
(RTUs). Because remote locations can be located hundreds of miles from the
central location, utilities began to use public phone lines and modems, leased
telephone company lines, and radio and microwave communication. More
recently, they have also begun to use satellite links, the Internet, and newly
developed wireless technologies. Because the sensors of SCADA systems pro-
vide valuable information, many utilities established connections between
their SCADA system and their business system. This allowed utility manage-
ment and other staff access to valuable statistics, such as water usage. When
utilities later connected their systems to the Internet, they were able to provide
stakeholders with water/wastewater statistics on the utility's web pages.
SCADA Applications in Water Systems
As stated above, SCADA systems can be designed to monitor a variety of
equipment operating conditions and parameters, such as volumes, flow rates,
or water quality, as well as to respond to changes in those parameters either
by alerting operators or by modifying system operation through a feedback
loop system without having personnel physically visit each process or piece
of equipment on a daily basis to check it to ensure that it is functioning prop-
erly. SCADA systems can also be used to automate certain functions, so they
can be performed without having to be initiated by an operator (e.g., inject-
ing chlorine in response to periodic low chlorine levels in a distribution sys-
tem or turning on a pump in response to low water levels in a storage tank).
In addition to process equipment, SCADA systems can also integrate specific
security alarms and equipment, such as cameras, motion sensors, lights, data
from card-reading systems, etc., thereby providing a clear picture of what is
happening at areas throughout a facility. Finally, SCADA systems also pro-
vide constant, real-time data on processes, equipment, location access, etc.,
so the necessary response can be made quickly. This can be extremely use-
ful during emergency conditions, such as when distribution mains break or
when potentially disruptive BOD spikes appear in wastewater influent.
Because these systems can monitor multiple processes, equipment, and
infrastructure and then provide quick notification of or response to problems
or disruptions, SCADA systems typically provide the first line of detection
for atypical or abnormal conditions. For example, a real-time, customized
operator interface screen can display critical system monitoring parameters,
