With the right instrumentation and controls, a sewer plant can be operated effectively from a remote location, as the Board of Water and Sewer Commissioners of the city of Mobile, Ala., has demonstrated at one of its facilities. The Wright Smith wastewater treatment facility, a trickling filter sewer plant designed to process 12.8 mgd, is similar to hundreds of other plants in the United States. The process requires minimal automation to monitor operations and ensure the quality of treatment Additional instrumentation was installed to improve the frequency of monitoring, and enable detection of alarm conditions beyond what operators on the site could conduct. Video cameras were installed for cases requiring visual inspection.
Determining how to achieve remote operation requires an analysis of operator duties and involving the operators in such an analysis is the key to success. For the analysis of the Wright Smith plant, the consulting engineer assembled a design team that included plant supervision and operators. The team interviewed operating personnel to determine:
Following the analysis, the team developed a new method of control that would allow remote operation. The solution included adding motorized actuators to manual valves, ratio controllers for flow-dependent processes, software timers, variable speed drives for pumps, video cameras for visual inspections, and redundant controls where needed. The designers closely coordinated with the operators to ensure such automation and remote operation were feasible.
To monitor this process, technicians installed computers in the operators' control room and the operation supervisor's office. A server provided views of individual process equipment and the overall plant. Installers also placed computers and video screens at the C.C. Williams wastewater treatment facility, 5 miles away. Communication between the computers at these two locations occurs via the board's intranet. All alarms, visual displays, trending, and other data and functions are duplicated at both sites. Therefore, operators can monitor and control the Wright Smith plant effectively from either location. The software is set up to communicate to pagers when alarms occur. Crucial personnel receive an alphanumeric message identical to that shown on the computer screen alerting them to the problem at hand.
Locally mounted programmable logic controllers (PLCs)—independent from the central computer—control critical operations within the plant. This practice ensures continual operation should problems arise with the computers or with the communication links between the two plants. The setpoints and timer adjustments within the local PLCs all can be adjusted from the computer located at either plant. Automated valves can be operated from the computer, including control of screens at the headworks, pumps at the influent and intermediate pump stations, and the ratio of chlorine and sulfur dioxide to the final process discharge. At the headworks, the levels up- and downstream of the screens are monitored, and high-differential alarms are provided. Alarms also occur if there is high level downstream and no screen motors are running.
At the influent and intermediate pumps, the local PLC monitors temperature and seal leaks for each motor. For protection of the pump, the liquid must reach a safe level in the pit before the local PLC will start the pumps. Operators at the remote locations receive alarms for high temperature or moisture in motors, variable-frequency drive (VFD) faults and high amps, and they can view the status of switch operation. In case of a fault, the operators can remotely switch a contactor that will bypass the VFD and place the motor back in service at full speed.
The primary, intermediate, and final clarifiers all have rotating rakes that are key to their continued operation—alarms occur if any of these motors stop running. Fault conditions such as high torque on the rake also trigger alarms. Sludge from the primary clarifier is automatically pumped periodically to the pre-thickener, based on selectable times. Sludge also travels from the intermediate and final clarifier back to the headworks by opening motorized valves. Operators can select the time, amount of flow, and duration of flow to obtain the desired results. Process material also is recycled from the intermediate clarifier back to the filter by automatically ratioing the speed of the recycle pump with the effluent flow. Operators can set the ratio or revert to manual setting of the speed from the remote locations.
All computers and PLCs are connected to an uninterruptible power supply (UPS). The system monitoring plant power notifies operators when line power has been switched to generate power from an auxiliary generator, or the UPS.
Video cameras are essential to the system because they:
Via cameras with pan and zoom capabilities on 30-foot-high poles, operators can selectively zoom in on any part of the grounds or any piece of equipment. An operator could zoom in on a belt drive to see if the belt is moving or broken. The video camera lets the user make sure if one of the rakes on a clarifier is turning.
When someone requests entry to the plant, operators can zoom in on the remotely operated front gate to question the visitor. Fixed cameras inside the buildings help to protect expensive tools and equipment from vandalism and theft. These cameras include motion-activated contacts that activate a video recorder whenever motion is present.
The automatic monitoring and control system at the Wright Smith plant, completed in 2004, has operated successfully on the new instrumentation and computers. Its success is largely attributable to the intimate involvement of the operating staff throughout the conception, design, implementation, and startup. Continual involvement gave the operators a strong feeling of ownership in the new system.
— May is a consulting engineer with Mobile, Ala. based McCrory & Williams Inc.