More than 50,000 residents
and several major industries in Enid, Oklahoma
rely
on the city's Water Pollution Control Facility
to provide a clean, safe and secure
water supply.
Since opening in 1908, the
plant has undergone numerous additions,
upgrades and expansions to better serve the
growing community and satisfy increasingly
stringent state and federal environmental
regulations. In 1988, the WPC was expanded
with the addition of a "Tertiary Treatment"
facility.
An important feature of the
facility is a remote supervisory control and
data acquisition (SCADA) system which uses
wireless modems. Ultimately, the WPC intends
to expand
the SCADA system to provide complete computer
control of the entire one-quarter
square mile facility. This ongoing project has
called for overcoming a variety of technical
and logistic challenges, most recently in 1996
with the Tertiary Treatment equipment.
Tertiary Treatment is a final step that
degrades secondary effluent in order to reduce
Ammonia Nitrate. It is the final cleansing
process before water is released to a stream.
This stage of treatment is vital not only
because it prevents algae bloom (which can
obstruct the receiving stream), but also
because the process can compensate for any
problems or shortcomings in the
prior stages of solids treatment. According to
Dana Watkins, systems integrator at WPC, "Two
hydraulic traveling bridges, each with four
pumps were running 24 hours per day to keep
the level of solids even throughout the
clarifier.
This posed two significant
concerns." First, without automated control,
any problem that caused a shut down of a
bridge during the night would not be detected
until the next morning – by which time
gasification could occur and solids could leak
into the stream. Secondly, the
bridges were traveling at a continuous pace,
regardless of variations in the flow rate,
which had implications for energy consumption.
"By connecting the SCADA system and networked
PCs to the bridge, we could adjust the
activity during low flow periods, which would
result in tremendous energy savings and
actually improve the treatment process by
allowing the solids
to settle a bit more," explains Watkins. The
challenge to connect the main RS-485
industrial computer to the bridges 50 yards
away was first addressed through a traditional
hard wiring approach. The communication cable
that would carry the SCADA system instructions
to and from the bridges was connected to power
cables running through the bridges. The
configuration proved inadequate for the
sometimes harsh environmental conditions.
"The next big wind storm to
hit Enid twisted the cable and snapped it in
two," recalls Watkins. "That convinced us that
radio control was our only option." The
proprietary industrial control system at WPC
was manufactured byTeletrol, a company that
did not offer wireless equipment for its
protocols. Watkins turned to the Hopper DS, a
915 Mhz wireless modem featuring Direct
Sequence Spread Spectrum technology. The
Hopper DS can transmit and
receive data at speeds up to 19.2 Kbps, across
distances exceeding six miles with a
conventional 1 dB omnidirectional antenna. The
non-standard 9-bit protocols used by the
Teletrol SCADA system did not deter the
implementation. Engineers customized the
modems for compatibility with our SCADA system
– and still saved us $2000 on the
bid price," says Watkins. The wireless
technology at WPC has been in place since
January, 1997. Watkins has observed that the
initial objectives have been met, and in some
cases, even exceeded. Now, should a bridge
fail, an alarm will sound. If the shut-down
occurs during an unattended period, a signal
is routed to a paging system. Watkins can go
on line from his home and restart the bridge
before any problems occur. Watkins has also
written a Windows program in Visual Basic that
displays an animated graphic of the bridge
movement, as well as the pump and spray
activity, for easier monitoring and quick
response. Another new program
will prompt the bridge to restart itself in
the event of a power failure. Plus, the
automated monitoring of flow rates has
resulted in the efficient shut-down of the
pumps on the traveling bridges four to six
times a day. "None of these improvements would
have been possible without the wireless
modems. Shutting down the pumps at 30-minute
intervals up to six times a day will save us
$2500 in energy costs per year, and actually
improves the clarification process" concludes
Watkins.
