While online monitoring of water treatment processes has been commonplace for some time, the increasing accessibility and viability of SCADA systems and cloud-based solutions has improved opportunities for water and waste water treatment plants to use online monitoring to reduce operator workload and improve water safety. Online monitoring is a key enabler for automation and frees operators to focus on the delivery of safe water by reducing the reliance on grab sampling and manual operation of plant.
Online monitoring supports the consistent operation of plants and implementation of the schemes' quality management plan.
Depending on the configuration of the instrument, a number of key functions can be performed to assist operation, such as:
monitoring the performance of key treatment barriers
enables direct control of plant to maintain performance
informs operators of changing conditions that may affect performance
provides enforcement of critical limits for key processes.
The use of instrumentation to monitor performance of treatment barriers is well understood and practiced throughout Australia. The monitoring of filtration processes using online turbidity measurement, chlorination processes with free and total chlorine measurement, as well as flow and level instrumentation are commonplace, and provide crucial information to operators to inform treatment decisions and corrective actions.
Barriers to automation
While not fully embraced, many plants utilise direct control of plant equipment to reduce operator burden and operator error. A key enabler for direct control is the stability and reliability of instrumentation. Few plants would consider manual control of pumps - for maintenance of levels within tanks - a value-added use of their time, and this is largely due to the well known and understood parameters that affect the stability of level instruments, as well as the large and mature market for sensors that enables OEMs to provide products that suit many and varied applications.
Not all parameters enjoy such stability however, and this remains an obstacle for increased automation at many facilities. Some important parameters related to the control of water treatment parameters may exhibit unexpected behaviour in some conditions. Many instruments may be sensitive to changes in flow, pressure or other parameters. When instruments are used to inform operator decisions on the operation of treatment processes, these fluctuations can be viewed in context or verified with grab sampling, however the introduction of automation into water treatment plants demand a level of trust that instrumentation is reliable and automated actions reflect typical operator decisions.
An important and often overlooked characteristic of well controlled automated processes is whether the instrumentation installed provides sufficient control to make automated processes reliable and useful to operators. Most water treatment processes are sensitive to incoming conditions and operate in a variable environment, and key barriers often depend on the performance of supporting processes for reliable operation. Placing supporting instruments and automated controls early in the process may significantly reduce the likelihood of automated shutdowns as processes approach their critical limits. Insufficient controls and instrumentation will increase the frequency of automated shutdowns and alarms, in turn leading to a lack of trust of automation, as operators spend more time restarting plants and containing issues than purported work that the automation is saving.
Filtration is a typical example where instrumentation and control in supporting processes can assist in the control of the barrier. Typical measures employed to improve the reliability of the process include:
flow pacing of dosing using flow measurements
control of coagulant dose using streaming current detectors
control of pH correction dose using coagulation pH
adjustment of coagulant dose considering subnatant or settled water turbidity
monitoring of raw water turbidity for early warning of poor conditions
Attempting automation of filtration processes using effluent turbidity alone is likely to lead to significant issues.
During procurement and installation, the factors that affect variability of plants are often overlooked, while a specification that covers typical performance only is often considered. Selection of instrumentation should consider variation in the process and ensure that not only are the instruments are fit-for-purpose, but also that their installation, maintenance and ongoing verification supports reliable operation. Sensors that do not reliably perform increase operator workload and reduce willingness to introduce automation.
Planning for new or replacement instruments should consider the variance in the process, typical operating conditions and requirements for any automated control. The technology selected and configuration of the monitoring system can be guided by these conditions, rather than selecting a technology and reviewing how it can be implemented.
While a detailed review of a system is required to fully understand the variation in operating conditions, typical considerations during the planning phase for new instruments may include:
Parameter Variation: Is the level of expected variation of the parameter to be measured known? Is accurate measurement required for all of the expected process range?
Accuracy: What level of accuracy is required to control the process?
Pressure: Does the instrument rely on pressure generated during operation of the plant? Does this pressure change during operation? Will the variation in process pressure cause variation in the instrument reading? Is a change to the measurement location or a sampling pump required to control pressure variation?
Temperature: Is the measurement of this parameter affected by temperature? Is temperature compensation integral to the instrument?
Detritus: Is this portion of the process prone to a build-up of detritus, floc or sediment? Is a strainer, filter or sediment trap required? Is a non-contact instrument available and more suitable?
Turbulence: Is there sufficient mixing prior to the sampling point to ensure a representative sample? Is a lance required to ensure that poorly mixed water from the boundary layer of the pipe is not affecting results? Is the instrument sensitive to turbulence and requires location in a laminar region of the system?
Gases: Does the pipework configuration protect the sensor from 'running dry'? Does the pipework configuration prevent the accumulation or passage of air or other gases that may affect the reading or reliability of the sensor?
Atypical events: Does the process experience atypical events that may damage the sensor, such as clean-in-place or air scouring? Does the event warrant an automated isolation of the sensor?
Communication Protocols: Is a relay contact or 4-20mA loop connection appropriate, or is remote configuration required (HART, TCP, ModBUS etc)? How is a failure of the instrument detected?
Location: How far is the sensor from the measurement point? Can the sensor be reasonably accessed for calibration, cleaning and replacement?
Instrument Lifecycle: Are spare parts readily available? Are suitably experienced technicians available for support?
Arming operators with information is a key contributor to successfully implementing automation. Operators should be able to transparently identify automated plant actions based on information available, enabling quick response to unanticipated alarms, shutdowns or other automated actions. Local display and well designed HMI systems are important tools to enable the root cause of issues as they occur. Good HMI design practices include:
display of instrumentation readings near relevant control areas/functions
colour coding of the status of equipment and instruments (e.g. running in auto, manual operation, shutdown, warning, fault)
the use of high level overviews and detail control areas
alarm logs noting date, time and a plaintext description of the fault
the use of asset IDs in the HMI to assist in remote trouble shooting
the availability of historical trend data for instruments and running state of equipment on a single chart
Another consideration is the benefits of offsite information. Many HMI solutions exist to enable operators to view critical operational information, manually operate equipment or change process setpoints on phones, tablets or laptops. This enables supervisors and experienced operators to diagnose warnings or alarms offsite and guide onsite staff to control issues. Additionally, operators with offsite duties can monitor performance while away from the plant, improving their ability to respond to changing conditions.
The potential benefits to well implemented automation on water quality are enormous. Online, continuous measurements enable real time understanding of processes by operators, allowing earlier action by operators and automated routines built into SCADA systems to prevent process from losing control in many cases. The increased granularity of data available assists in preventing the release of unsafe water from the plant, and the increased historical information allows improved interrogation of past performance.
For guidance assessing and implementing monitoring and control solutions in your treatment plant, don't hesitate to call Viridis for assistance.