The wastewater plant in Slippery Rock, Pennsylvania, is a primary/secondary plant that uses a combination of two-stage trickling filters and Orbal aeration with ammonia denitrification. Originally built in the 1920s, the plant has undergone three major upgrades. The latest occurred in 1982 at a cost of $2.6 million to complete. Most of the existing structures in the current system are more than 30 years old. Over the past few years, increasingly stringent licensing requirements have made it necessary to upgrade again. The difference, this time, is that the upgrade is being accomplished without any major construction, outside consultants or huge outlays of money.
Slippery Rock's plant, with a maximum capacity of 1.2 mgd, is fairly typical of older domestic wastewater facilities. Influent passes through a bar screen, then through an aerated grit chamber, a pre-aeration channel and a Parshall flume. It enters horizontal primary settling tanks, moves on to and through the first stage trickling filter, flows to a gravity sump, and is then returned to the second-stage trickling filter where most of the BOD reduction is achieved. From the filter, the stream enters an Orbal basin for ammonia nitrogen removal and is then split between a circular clarifier and a rectangular clarifier. Finally after passing through a chlorine contact chamber, the effluent leaves the plant.
Sludge from the primary settling tanks goes directly to two anaerobic digesters. Secondary sludge from the secondary clarifiers is either returned to the Orbal basin or sent to the aerobic digester. Both the anaerobic and aerobic digesters' sludge is conveyed to a belt press installation for polymer addition and dewatering. All of the 175 tons of sludge the plant produces annually are trucked to a landfill.
Two main considerations influence every wastewater treatment decision at the Slippery Rock plant. First, the effluent is a tributary to Slippery Rock Creek, which is used extensively for recreational purposes-fishing, boating and swimming. This fact alone is the basis for the ongoing tightening of the parameters for renewal of the facility's Part 1 NPDES permit.
Second, while the municipality has a population of just 3,000 permanent residents, it is also home to Slippery Rock University, which has a student body enrollment of approximately 7,800. Therefore, for eight months of the year the municipality's population more than triples. Average flow during the school year is 526,000 gpd, but during the remaining four months the average flow is just about 300,000 gpd.
When I took over, the operation was facing more-stringent licensing requirements, and the efficiency and effectiveness of the plant had to be improved. Money was not available to invest in a completely new treatment system, nor did I think a new system was necessary. From reading in trade journals and related materials, I believed we could upgrade the existing plant and manage the resources available in a way that would produce the needed results without spending millions of dollars.
Problems to be Addressed
Several considerations had to be addressed immediately. First there was no way of accurately measuring flow coming into the plant with the existing flowmeter. In addition, it was not possible to control the flow during wide fluctuations due to population shifts, and periods of heavy rain could drive the flow up to a totally unmanageable rate of 3.5 to 4 mgd for three or four hours at a time. We also had an overflow problem that was so bad the overflow actually spilled into the chlorine contact plant. At the same time, the new licensing requirements called for much better control over ammonia nitrate-cyanide levels, but we could not establish how cyanide was being produced in the plant. Stopgap fixes were not an alternative. The only real solutions were those that would work in the long run and be cost-effective.
The first action I took as superintendent was to buy a PC for gathering data and handling reporting tasks. Eliminating a lot of paperwork, the PC made things easier. The computer now has 15MB of RAM and 220 MB of hard drive, and is continually upgraded. All plant information is kept on this computer, from overtime for employees to EPA reports. Even all maps and drawings, which used to lie scattered over the office, are on the computer. In the past, if we had a problem and had to locate a bad line, it was virtually impossible. Now, we simply pull the map up on the computer for the details.
Also at this time, we replaced the distributors on the trickling filters with an all-aluminum type. These new units provide better, more consistent distribution of the wastewater in treatment over the tank and all but eliminated messy, time-consuming maintenance.
Community Cooperation a Big Help
We enlisted the help of the local community in conducting inflow and infiltration (I/I) tests to address the problem of unmanageable flow to the plant during heavy rain. We set off smoke bombs in the lines to see where there might be broken pipes, connected drain spouts, basement traps and driveway run off. Where we found problems, we let the residents know they would be charged for the water generated that would flow to the plant. Community cooperation has been tremendous, and as a result, the flow into the plant during heavy rain has been reduced to a manageable level.
Next, we installed a Milltronics OCMII ultrasonic influent flow meter. This instrument indicates current average flow, and the information can be downloaded to the PC and graphed to show the status of the processes in the plant. The use of this flowmeter proved the benefits of accurate instrumentation, and further study suggested it was possible to control DO in the aeration process, chlorination, total suspended solids and blanket level every day, around the clock, as well. I became committed to working toward an automated plant by combining new technology with old plant systems to attain maximum efficiency.
Overflow problems were solved by installing Milltronics continuous ultrasonic level monitors, that automatically control pump speeds in the wet wells. At the same time variable frequency drives for the pumps were installed.
An auxiliary aerator was added to the Orbal ditch to cope with high loads during winter days, but we needed to know when to bring it on line with pinpoint accuracy. In 1992, after considerable investigation, Royce Instrument Corporation was given the opportunity to install a demonstration unit of the firm's dissolved oxygen (DO) analyzer, Model 9010 with a self-cleaning sensor, for a 30-day trial. If the DO dipped below three ppm, the DO analyzer signaled the auxiliary aerator to come on. When DO returned to the three ppm mark, this aerator was shut down. The system immediately worked well and the demonstrator was purchased and left in place, where it has continued to perform as required. Its maintenance needs have been light, with a membrane change on the sensor and a recalibration of the analyzer every six months.
The next instrument added was the Model 7010/78 suspended solids monitor. We had no idea what the percent solids in the denitrification unit was at any time during a 24-hour period, unless an involved lab test was carried out. Now, the information is available in a few seconds. The self-cleaning system, which needs no calibration, aids in controlling the solids between the wasting option and the return to the Orbal ditch. These values are much more consistent, and there is better control over the ammonia nitrate-cyanide balance. Since the new limit on ammonia nitrogen is 1.5 and cyanide is 0.005, a carefully balanced solids return is very important.
In 1994 an interface level analyzer (ILA) was installed. This sludge blanket device is used to operate six Limitorque actuators that control the valves in the clarifier to maintain a blanket of between two and three feet. When the blanket is maintained at this level, the clarifier works at peak performance and produces the least amount of pin floc and sludge bulking. The ILA is ultrasonic and virtually maintenance-free. Before it was installed, time-consuming manual tests with telescopic valves had to be relied upon. The sludge blanket now can be held to within a tenth of a foot, and a sufficient inventory of sludge can be maintained without upsetting the process.
That same year we installed a Milltronics ultrasonic level controller to control the gate to the Orbal system. This is used primarily for the denitrification stage of the process. There are three rings in the system: the outer ring removes 75 percent of the ammonia nitrogen from the process stream, the middle ring removes 20 percent, and the innermost ring removes the final 5 percent. As the level in the channel between the two sections of the plant rises, the level controller opens the gate automatically, and provides better control.
The plant also maintains two Sigma composite samplers, and field work is carried out with portable samplers from the same manufacturer. In addition, we have both an automatic gas chlorination system and a back-up, pressure-driven chlorination system in the event the gas system goes down.
Advantages are Clear
The benefits of adding instrumentation have been tremendous. The ammonia nitrogen level is now less than 0.5, while cyanide concentration has dropped dramatically to below the mandated 0.005 level. Total percent solids are within limits, and controlling the solids has reduced the number of different kinds of pathogenic bacteria, which reduces chlorine use. In fact, changes to the plant as a whole have reduced chlorine addition to the effluent by 75 percent, and dechlorination is not required. The plant now is capable of maintaining the process through loading swings caused by population shifts in town, and sludge management is under control. Reduced sludge handling costs are anticipated since the process produces a better grade of sludge that compacts better.
Instrumentation has definitely upgraded the Slippery Rock plant. In addition, the instruments can also be tied to a PC for data collection. In fact, the next upgrade will be to acquire a new computer software program. The program we have looked is designed to collect data and present it on a standard spread sheet. The program archives data automatically, which will provide tracking of the plant's performance over time. The system is intended for smaller and medium-sized plants without full-blown SCADA systems, and uses an RS485 link to the instruments on line. It will provide a window to the entire operation on the display screen.
Some operators tend to shy away from automating their plants, fearing they may be putting themselves out of a job. But the instruments are useful tools that help get the job done successfully. I personally do not believe in total computerization. What we do at Slippery Rock is fine-tuning. Which is only possible with accurate information from carefully chosen instrumentation. The plant is automated, but the PC is just for gathering data and archiving information; it does not control the process. If one instrument goes down, all else keeps working.
Our plant requires three of us working eight hours a day, five days a week, and three or four hours a day on weekends to keep at optimum efficiency. We take good care of our equipment, upgrading and overhauling it ourselves, rather than hiring outside contractors.
Five years ago our mandate was to upgrade without huge outlays of capital. This was accomplished with automation through instrumentation. We could not have met the new regulatory limits without this instrumentation. Much has already been achieved, but the work continues, for upgrading an old plant like this is an ongoing process. My advice is to take one step at a time, and build on each step, to achieve optimum efficiency in the most cost-effective way possible.
About the Author:
Gary L. Davis is the sewer department superintendent at the wastewater treatment plant in Slippery Rock, Pennsylvania.