Wastewater Plant Automates Facilities With PCs, MMI & PLC Control
Recent plant automation has turned the Mattabassett District Facility into a revenue generator.It is common for smaller municipalities to band together to create a regional public service facility. It is not so common for them to do such a good job that the project turns into a money-making operation by providing services to other communities in neighboring cities and states.
That is exactly what The Mattabassett District has accomplished. This state-of-the-art wastewater treatment facility in central Connecticut was created as a regional entity in 1961. Over the years it has continually been upgraded to meet the wastewater handling needs of 135,000 customers in the cities of New Britain, Cromwell and Berlin and has given the Mattabassett River back to nature. In the process, the facility has also become the fourth largest capacity plant in the state, has joined the ranks of the top 10 percent of treatment plants in the country-and most importantly, has turned into a revenue generator that earns the region approximately $4 a minute, 24 hours a day, 365 days a year.
This unique status was achieved because district management took a long hard look at the plant's manual operations five years ago and decided to reduce the projected large increases in labor overhead by automating plant activities. However, instead of purchasing a turnkey distributed control system this goal was reached more cost-effectively by hiring system integrator NIC Systems Corporation, in nearby Plantsville, CT, to develop and implement our own solution. A network was created of off-the-shelf 486/33 personal computers (PCs), standard industrial programmable logic controllers (PLCs) made by Allen-Bradley, and the InTouch man-machine interface (MMI) software from Wonderware Corporation to provide the supervisory control and data acquisition (SCADA) capabilities needed.
The end result has been a reduction in the labor force from 18 operators and maintenance staff to 13 people. Originally, it was projected that 35 operations staff and 10 to 15 maintenance people were needed to meet 1994 operational requirements. The entire plant is now run with a staff of 13 people. This is a minimum of one operator per shift which frees up staff to do other tasks.
Standard Treatment Procedures
The Mattabassett District plant is designed to treat an average wastewater flow of 20 million gallons per day (mgd) and a peak flow of 40 mgd. The plant uses standard settling techniques as the primary treatment of the wastewater, with aeration and regenerating biological treatment plus disinfection as secondary treatment. Treated water is discharged into the Connecticut River and the biosolids or activated sludge that result from this treatment are dewatered and incinerated.
The plant receives wastewater via a trunk sewer that serves the entire geological watershed of the New Britain, Cromwell and Berlin area, plus shipments of liquid wastewater sludge, industrial washwater and landfill leachates from surrounding communities such as Middletown, New Haven, New London and Simsbury. Dewatered sludge is now also incinerated from communities in nearby states to keep the incinerator running at capacity.
The incoming effluent is first passed through bar screens to remove large debris; then six 200-horsepower pumps move it up 37 feet to the operations level where a pair of detritors remove various grit (sand, coffee grounds, egg shells, etc.) by slowing the flow until heavier particles sink to the bottom of the tanks for removal by collection arms. Pneumatic ejectors carry this grit to a storage tank for subsequent incineration. Pulverizers then shred and remove any remaining large solids or sticks and the wastewater flows through a channel that aerates it, prevents deposition of solids, and splits it for distribution into four primary clarifiers and flocculation and sedimentation basins. Each clarifier has a volume of one million gallons and detains the flow for 2.4 hours to remove about half of the suspended solids and reduce the biological oxygen demand (BOD). Heavier organic solids and most of the remaining inorganic solids are settled out as sludge.
The effluent continues on to four one-million-gallon aeration tanks and the beginning of the secondary treatment process. Four 2,400 volt, 700 horsepower blowers provide compressed air to fine-bubble diffusers at the floor of the aeration tanks. The effluent is mixed with activated sludge from secondary clarifiers to form a biological mass called "mixed liquor," thus beginning a continuously regenerating biological loop. During the five-hour flow through this system, the mixed liquor microorganisms settle to the bottom of the tanks where they are returned to the aeration tanks to close the biological loop. The clear liquid atop the four 120-foot-diameter secondary clarifiers overflows V-notch weirs and flows on to a final mixing chamber, where chlorine is added to disinfect the treated effluent before it flows into the outfall line below the surface of the Connecticut River. The river itself provides enormous dilution and oxidizing power with its 17 billion gallons per day flow.
The sludge from the treatment is gathered in two half-million-gallon storage tanks where it passes through a grinder and is pumped to four belt filter presses. The presses remove about 75 percent of the water. The dewatered sludge is conveyed to the fluidized bed incinerator. There it is burned, along with the grit removed earlier, on a 19-ton charge of "boiling" sand (at 1500 F). The sterile ash and excess bed sand from the incinerator are removed and disposed of at the district's ash disposal site or used for cover material at other landfills.
In parallel with the wastewater processing, the district's chemistry laboratory monitors plant effectiveness in meeting government agency standards and provides continuous operational information for use in plant process control. The lab staff analyzes upstream pipeline inputs and river cleanliness and maintains a close relationship with the state Department of Environmental Protection.
Although the original impetus for upgrading the treatment plant was to keep labor costs under control, plant management and the consulting engineers determined early on that there was "no such thing as being a little bit pregnant." The District did as much as they could afford within the budget limits. Working with Project Engineer Bob Kaine and his staff at NIC Systems, the whole system was designed and installed within six months. It has been running at 100 percent uptime for over a year. However, the District is still not taking advantage of all its capabilities, so they plan to continually upgrade the system far into the future.
The current automation system uses two Allen-Bradley PLC5 controllers plus one Allen-Bradley PLCJr with three remote drops as the control systems for pumps, motors and fan blowers throughout the plant. Data from nearly 2,000 I/O points is fed over a Wonderware A-B DDE Server, via the A-B Data Highway, to a LANtastic 6.0 network of seven IBM compatible 486/33 PCs that run the MMI software.
This network provides supervisory control for the six 200-horsepower raw sewage pumps that bring influent from the trunk sewer to the plant operations level; the filtration and detrition equipment with associated pneumatic ejectors; aeration systems; mixing equipment; triplex sludge pumps; grinders; belt filter presses; and piston pumps that transport the sludge. In addition, all plant facilities are monitored and real-time data is acquired so that operators can tell at a glance the plant's status. Operators can also call up real-time and historical trend screens to spot any out-of-bounds conditions. A workstation is also provided in the laboratory so that Chemist Liz Walters, the lab manager, can set waste rates and directly monitor water quality. This assures that lab data and plant data are coordinated and all staff are working from common information.
One unusual addition to the system is a set of video cameras that monitor the output of the sludge belt filter presses so operators can tell at a glance if there is a flow rate problem. This process provides a visual check for operators. However, a future enhancement will be to bring the video images into the MMI via a PC video card so that the MMI can monitor the density and contrast of the pixels in the video image and pop up an alarm if it detects that a belt is plugged or if the sludge is too watery.
One of the biggest benefits of the system is that people no longer have to walk several miles a day, just to turn equipment on and off. In the old system, operators had to leave the console and walk as much as a half mile just to turn on belt presses, blowers, pumps and so forth. These tasks can now be launched from the console itself. In cases where a sequenced start is required, the software now manages everything in its proper order so operators just have to click once to set the process in motion.
This new system is comparable to flying an airplane using an auto-pilot. One primary operator on each shift can now manage the entire facility from the control room and still have plenty of time to perform other tasks. Overtime costs have been virtually eliminated and enhancements like providing a live video link will only make the supervisory job even simpler.
Another popular feature is the modem link provided on the SCADA network. This allows management to dial into the network during off-duty hours if there is a problem. If I get a call from an operator at night, I can dial in from home and usually manage the situation using the MMI screens on my home computer.
The historical data files and trending capabilities of the software also allow Maintenance Supervisor Gary Simpson greater latitude in scheduling regular preventive maintenance work. This system also provides a greater level of accountability that they never had before. Emergency responses are better because the alarms sounded are now related to a specific problem which saves time troubleshooting. In addition, with the new cumulative runtime records for major equipment, maintenance work can be scheduled during non-peak times, when there is less disruption to operations.
Another feature is that a proper audit trail is created so that if anything goes wrong, it is documented what happened, when and why. Any operator actions that are taken are archived electronically and operators on other shifts automatically know what staff on another shift has done. This gives the District greater consistency in facilities management.
Only on the 10th Floor
As successful as the control system has been, the Mattabassett District has only begun to tap its potential. I tell people we bought the World Trade Center and we are only occupying the first 10 floors. All of the status reporting has been automated, but the same is not yet true for the control systems. Complete one-touch startup capabilities for very complex systems like the incinerator, the 1400 kW emergency power generator, air compressors, boilers, and even plant-wide systems like security and fire alarms would like to be added.
However, thus far this facility is the only wastewater plant along the northeastern seaboard that uses a SCADA system to this extent. The facility has become so efficient that they are now taking on work from other districts in Vermont, New Hampshire, Rhode Island and Massachusetts. About 70 percent of the current incineration volume comes from other districts. This helps the District maintain efficient incineration because the furnace runs almost continuously, and generates revenue that operating costs.
The ultimate proof of the system's success is contained in a pair of charts developed by the State of Connecticut. One chart shows staff size relative to volume of water processed, and The Mattabassett District ranks first at about 1.75 people per million gallons processed per day. The other chart shows plant operating costs per 1,000 gallons treated. On this chart, The Mattabassett District ranks second at a cost of about 20 cents per 1,000 gallons of water treated.