Before considering how to best implement automation redundancy in a wastewater treatment plant, the first step is to ask whether it is needed at all. There are several factors to examine before making the decision.
What is the storage capacity or retention time of the facility? If the plant has a large retention time or storage capacity, then a more basic level of redundancy may be the best solution. However, if the plant is on-line full-time and storage capacity is minimal, then full automated redundancy may be in order.
What economic, political and ecological effects might occur in the event of a full-scale automation system shutdown? If a plant is small and can operate reasonably well in manual, then the impact may be minimal. This might mean a basic level of redundancy is most appropriate. However, a larger facility, or a plant in a high-visibility or critical location (such as a tourist area) may want to have full redundancy for safety or political reasons.
What is the technical expertise of the staff? Many plant staffs have little or no ability to rapidly diagnose or correct automation anomalies — they rely on systems integrators as their primary means of support. For some facilities, the integrator might be a considerable distance away. These facilities may want automation redundancy as a way to remain functional while waiting for the integrator to respond.
Other plants have some level of expertise on staff and are less reliant on integrators. These plants may have people who understand how to diagnose and maintain an automation platform, and may have spare parts in stock ready for service. Here, the decision to use redundancy may relate more to process issues than to staff expertise.
Assume that a wastewater treatment plant is using UV disinfection. If the control system becomes inoperative, the UV system could be shut down, allowing pathogens to leave the facility untreated. With a redundant control system in place, the backup would immediately take over for the failed system and eliminate the possibility of a shutdown. Thus a redundant control system would help minimize the possibility of a system failure.
How easy is it to get technical support to the facility? Could a snowstorm, hurricane, avalanche or other realistic disaster block the route that support personnel would take to the plant? If so, then a redundant system can increase reliability during these times.
What is being protected against? Sometimes reliable process quality is the deciding factor, while at other times the deciding factor may be data integrity. The answer to this question will dictate which type of redundant system is best for the facility.
For example, if operational data are kept on a single computer, then loss of that computer could lead to a loss of historical data. This may mean required reports would not be completed properly. If this possibility is of concern, it is important to use computers in a redundant configuration.
What is the budget? It always costs more to make any given control system redundant. Redundancy adds a chassis, power supplies, communication cards and a processor. Furthermore, these additional devices require panel real estate, which may further increase system costs. The selection of a redundancy system must include a critical examination of the project budget.
Municipalities and wastewater agencies have different viewpoints on automation redundancy. Some believe the low level of risk does not justify an investment in redundancy. Others want to use every means necessary to protect their treatment capability. In most cases, the differing perspectives stem from the answers to these important questions.
Grant Van Hemert, P.E., is an application engineer with the Water and Wastewater Competency Center of Schneider Electric, a manufacturer of power and automation control systems with United States headquarters in Palatine, Ill. He can be reached at grant.vanhemert@us.schneider-electric.com.
