Unlike most centralized wastewater treatment plants, decentralized plants typically operate for long periods without onsite staff. Without staff supervision or remote monitoring of critical performance data, operational failures can go undetected for significant periods. Real-time sensors connected to a supervisory control and data acquisition (SCADA) system or equivalent telemetry system can transmit critical performance and operational information to a remote operational center. This setup enables operations or management staff at the remote location to assess the operating status and performance of a facility and to respond to upsets, process or mechanical failures, or other nonroutine situations. Other potential benefits include
consistent effluent quality through early notification of influent and effluent quality and volume changes, enabling treatment process modification;
reduced labor costs due to fewer site visits;
reduced chemical and energy costs by using trending data to optimize processes — for example, using monitoring data to determine when chemical addition is necessary or when filter backwash is required; and
automatically generated management and regulatory reports.
With all of these potential benefits, why do so few decentralized wastewater treatment plants use this approach?
To help answer this question and encourage the use of real-time remote monitoring, the Water Environment Research Foundation (WERF; Alexandria, Va.) funded a study of on-line sensing and data acquisition technologies applicable for use in decentralized wastewater treatment systems to provide real-time information.
The results of the study are to be published early in 2010. The study is slated to include a literature and technical review of the use of on-line sensors and SCADA technologies within decentralized wastewater treatment systems. It also identifies monitoring needs for treatment facilities, the current capabilities of sensors and SCADA, and areas where further research would be beneficial.
Addressing Costly Misconceptions
The main stumbling block regarding the use of real-time remote monitoring of decentralized wastewater treatment plants seems to be the perception that it is not cost-effective. System costs include the capital needed to install sensors and data transfer and storage equipment, maintenance costs for sensors, and monthly costs associated with data transfer and servicing. Another significant issue for real-time monitoring of remote plants is the perception that it is difficult to keep on-line sensors clean and calibrated over long periods, which is essential if costly site visits are to be minimized.
Another factor adversely affecting the use of this technology is that many decentralized wastewater facility owners and operators know little about the benefits of operating their systems with real-time remote monitoring. Education and awareness programs, such as technical papers, workshops, and conferences, can attempt to provide this knowledge. However, even with good awareness of the technology available, many regulators will not accept real-time remote quality monitoring data for reporting requirements in lieu of traditional laboratory analysis.
As part of the WERF study, a small number of facilities using real-time remote monitoring was identified in the United States, Canada, Australia, and Europe. In cases where such equipment is currently used, monitoring typically is limited to simple, traditional parameters monitoring the on–off status of mechanical equipment or liquid levels. A few facilities were found using on-line sensors to monitor such water quality parameters as dissolved oxygen and chlorine concentrations.
The facilities reviewed use a telemetry-based alarm warning system, a Web-based telemetry remote monitoring system, or SCADA for data transfer of monitored parameters. The telemetry systems reviewed either had transfer of data in real time or a system that transferred packets of data once or more per day. For the facilities identified, the systems were considered to be cost-effective, and no significant issues with the sensors were identified, indicating current perceptions of high costs and excessive sensor maintenance requirements may be unfounded.
Real-time remote monitoring enables problems to be detected before they become events and reduces the risk of system or performance failure. While using telemetry systems that send data packets a few times a day to monitor failure situations is useful, sending data from sensors in real time allows for remote monitoring of water quality or process information.
The complete list of parameters for which on-line monitoring would be beneficial depends on the treatment system in use and permit requirements in place at a specific location. However, the parameters that should be monitored at all facilities include equipment status, liquid level, pressure, and flow. These parameters are indicators of system failure. Influent, effluent, and process quality parameters to be monitored will vary among facilities.
The data collected can be used to help optimize chemical addition systems. Effluent quality data are useful if regulators accept real-time remote monitoring data for reporting purposes; if not, the data are useful to announce changes in the process that could lead to failure.
Most permit requirements include limits on effluent total suspended solids and biochemical oxygen demand (BOD). A simple turbidity sensor can indicate total suspended solids levels, and a more complex BOD sensor, a total organic carbon sensor, or a chemical oxygen demand sensor can be used for BOD. Additional monitoring may be required for nutrient concentrations and to control disinfection.
Other examples of monitoring of process performance include using pressure sensors for filter systems to regulate automatic backwash and to identify problems with filter blockage, and using dissolved-oxygen sensors to control recirculation rates automatically in fixed-film reactors to optimize biological denitrification.
The capabilities of sensors that can be used for real-time monitoring vary with sensor type and manufacturer. Consideration should be given to the accuracy, reliability, and installation requirements for sensors, as well as to SCADA interface requirements. Part of the WERF study is to develop a matrix comparing these factors across a range of sensor types and sensor manufacturers.
Additional information on instrumentation capabilities can be found through the Instrumentation Testing Association (Henderson, Nev.) at its Web site, www.instrument.org. Once the sensor type has been decided based on these factors, the cost of ownership should be determined using a life-cycle cost analysis, which should track the costs associated with installing, operating, and maintaining an instrument (inclusive of labor and associated chemical costs) over the useful life of the instrument. This cost analysis can provide information that can be used to develop realistic operating budgets and justify procurement and installation of instrumentation.
There is a range of available telemetry and SCADA communication technologies that can be applied at decentralized wastewater treatment facilities to assist in remote real-time monitoring, alarming, and control. Many factors must be considered in determining the type of SCADA or telemetry system to implement. These include the sensor type, communications method, remote control and alarming methodology, and data collection, storage, and archival methods.
Several companies provide Web-based telemetry systems with proprietary control panels for remote monitoring. Some of these systems operate in real time, and others send packets of data several times a day. A telemetry-based system providing an alarm to service providers has a lower cost than real-time monitoring using SCADA but is unlikely to be suitable for all decentralized facilities, particularly those receiving seasonally variable flows and loads and where on-line effluent quality monitoring is used. In such cases, using SCADA may be a better option. A number of companies specializing in SCADA for wastewater treatment plants can design and install a SCADA system best suited to the needs of an individual facility.
Real-time monitoring using on-line sensors and Web-based telemetry or SCADA for decentralized wastewater treatment plants can be cost-effective and is a good risk-management tool. Real-time remote monitoring likely will become more common in the future as
the complexity of treatment systems increases due to more stringent regulations,
instrumentation and communication devices become more available and reliable, and
labor costs increase.
The cost of remote, real-time monitoring should decrease as the number of decentralized systems serviced and service providers focused on decentralized wastewater facilities increase.
Further research may be needed to promote the acceptance of real-time remote monitoring by the decentralized community. Further field testing of sensors and development of best practices for SCADA and Web-based telemetry systems would help. In addition, a cost–benefit analysis for a range of systems would help decentralized wastewater treatment plant owners and operators evaluate whether remote, real-time monitoring is practical for them.
Lynne Maclennan is a process specialist, and Stephen Nutt is a partner at XCG Consultants Ltd. (Oakville, Ontario).
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