Improved process controls and new equipment revitalize 10-year-old WWTP
Every municipality would appreciate having a brand-new wastewater treatment plant (WWTP) complete with the latest technology and energy-efficient equipment, but budgets are tight and the feasibility of a new plant is not always realistic. The quality of water, however, can benefit from improved process control. Energy cost reduction is the mantra preached daily across the country. If only it was possible to attain this benefit without funding and constructing a completely new WWTP.
BVAA Upgrade Package
This was the situation facing Brokenstraw Valley Area Authority (BVAA), located in the hills of northwestern Pennsylvania. The solutions implemented were an upgraded control system and equipment providing state-of-the-art process control, energy efficiency and creature comforts for the plant operators. The modest investment in the new equipment achieved the water quality needed without the financial commitment of a new plant.
The upgrade package included:
• Three high-efficiency Airzen blowers;
• Three ABB ACS800 variable frequency drives (VFDs);
• A local rack extension of the existing Automation Direct 405 programmable logic controller (PLC);
• A rewritten PLC program;
• Two new Maple System HMI5070TH 7-in. color touchscreen graphic operation terminals;
• A wireless Ethernet network; and
• A cellular-based SCADA system.
The BVAA WWTP was built in 2000 using the sequencing batch reactor (SBR) treatment process, considered by many to be the best of some 60 processes in use worldwide. The original system used full-voltage motor starters, a text-based operator interface and a PLC program that often prevented better control options due to its inflexibility.
The engineers at Lake Eng. of Edinboro, Pa., and Wm. T. Spaeder and Fralo Control Systems, both of Erie, Pa., collaborated with BVAA Plant Technician Joel Turk to develop an improved SBR. The new process control PLC program provided better control of the dissolved oxygen (DO) level in the effluent. The blowers and the VFDs worked together to maintain a stable and accurate DO content. The new system also greatly reduced an undesirable, chain-like algae called “filamentous.”
“My ability to fine tune the SBRs individually has made a huge difference in settleability and filamentous control,” Turk said.
With the DO under tight control, the next improvement was better sequencing of the reactor phases. Fralo Control Systems’ new PLC program emulated a gear-shifting motif that allowed the speed of the process to be controlled by the influent rate. Tank levels are sampled on a regular basis, then the PLC determines which of six speeds to use in the sequencing of the SBR phases. All parameters are available for modification through the touchscreen and all operations can be performed manually in the event of equipment failure.
The original controller was an Automation Direct 405 PLC with two local racks. To avoid the cost of a new controller and to save the extensive input/output (IO) infrastructure, Fralo retained the 405 processor and all the wired IO. A third local rack with new analog cards to support the new VFDs and an H4-ECOM100 Ethernet network card to connect to the new wireless Ethernet network were added.
Energy efficiency pays dividends every day. In this case, the addition of the new VFDs and the PLC program provided energy savings by reducing the number of motor starts per hour and avoiding demand charges from the electrical utility.
“We have noticed a significant energy savings with the new operating program,” the plant operator said.
The BVAA plant layout required the operator to perform his duties outside in the weather—not too grievous on a sunny summer afternoon, but a challenge in the middle of a snowstorm in the dark. After discussions with Turk, Fralo proposed installing two identical color touchscreen terminals linked together by Banner Eng. DX80 wireless Ethernet radios. One touchscreen replaced the original text screen outside; the second screen was installed inside the treatment plant lab, allowing the operator to monitor and control the operation from the comfort of the office.
The final improvement was the addition of a cellular-based SCADA system from Mission Communications. The SBR plant is fed by five lift stations, each of which was fitted with a M-800 RTU. This allows the plant operators to monitor each of the lift stations and the SBR. Flow rates, tank levels, equipment statuses and alarms are reported in real time and are available for viewing on mobile devices.
BVAA is an excellent example of how technology, human ingenuity and collaboration can salvage an aging treatment plant to deliver the benefits of a new plant without the financial commitment of a new facility.
http://www.wwdmag.com/sites/default/files/imagecache/article_slider_big/BrokenstrawPlantwWorker.jpgBVaa’s new process control PLC program provided better control of the dissolved oxygen level in the effluent.
http://www.wwdmag.com/sites/default/files/imagecache/article_slider_big/BVAA%20Screen%201.jpgA cellular-based SCADA system allows the plant operators to monitor each of the lift stations and the sBr.
http://www.wwdmag.com/sites/default/files/imagecache/article_slider_big/BVVAPonds.jpgThe modest investment at BVaa achieved the water quality needed by the municipality without the commitment.