Water Treatment: A place for process control
August 10, 2015
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Process and instrumentation control is becoming commonplace. As a result, there are many means by which water and wastewater operators can select equipment, not to mention many complexity and price levels.
The numerous choices can result in incorrect decisions and incompatible equipment — both of which can result in additional costs and/or malfunctioning equipment.
The decision to add process control is the basis of system controls. It can be as simple as float control or timers on a pump. Manual control is provided by means of adjusting the swing on a float or the timer interval. Confirmation of pump operation and cycles can be provided with hour meters and cycle counters. With a system of limited complexity, this is entirely feasible and acceptable. However, as systems require additional control, or the data is needed to support a regulatory filing or forensic review of operations, the choices can become unwieldy.
If a controller is seeking to measure the flow over a 24-hour period, he or she can complete two trips to the site on consecutive days or store that information for retrieval at a later date on a memory card. Similarly, if a controller is trying to provide for a consistent water level, he or she can spend that time in person or have controls such as a pressure transducer to support pump operation, or speed control with greater system complexity.
One of the first steps to consider when integrating process or instrumentation control is how important the data is, and to what use is required. Operators collecting information and have no basis to interpret the data, or no budget to spend the time required, might consider looking at simpler systems. Interpretation of the data is critical to obtaining the most value and to aid in providing process controls. In the decision-making process, operators will also want to determine if process controls are required to be adjusted automatically, and if data storage is the minimum requirement.
Take, as an example, a small water-treatment system consisting of pumps, filters, disinfection and storage. A simple operation would have a float controlling the pump on and off, visual gauges to know when the filters require replacing and a chlorine system that turns on when the water pumps turn on. This is the majority of water systems, and for the most part, works effectively. However, that is an assumption based on visual inspections periodically to measure the tank level (does it have water in it?) and the chlorine residual.
Outside of that site visit, operators only hear of problems and not consistency in operations. Operators are unable to determine what happens outside of the site visits unless there is a complaint from a homeowner. With such a system, regulatory authorities are asking for monitoring and control points, and potentially some improved control. One of the first considerations is the extent you want to collect additional information.
You could provide a well data logger to measure the ground water level and determine the recharge — and, over a longer term of several years, the long-term trend on the ground water level, if there is over pumping. The data loggers can be retrieved from the well and the data downloaded periodically. Or you could have a surface data logger transmitting the information to a central controller. Adding pressure transmitters and a flow meter provide additional data, but require either a data logger or controller to obtain additional usable data.
As the degree of complexity increases, operators will want to consider whether the information available is suitable to collect only as historical data. This can be achieved with a multi-parameter universal controller that can display current information and store collected data points for historical interpretation. The end result is little change in operations control, which can be comforting for smaller systems, yet a large database of information for longer-term review and assessment. Further improvements will result in changes to the operation. A chlorine monitoring probe, for example, can provide continuous feedback on chlorine residual. As the residual varies from the desired set point, a controller can adjust the chemical feed pump to speed up or slow down accordingly to maintain a consistent residual and optimize chemical use.
As the complexity increases yet again, the need for a programmable logic controller (PLC) arises to establish control points for the various instruments. Depending on the equipment selected, operators will be providing voltage or amperage control, or discrete on and off settings. Reliability and robustness are strong considerations in choosing an appropriate controller, as well as the ability to prepare the ladder logic programming.
Several brands of industry standard PLCs can be combined with touch screens to provide full automation control and visual trending of current and historical information. However, based on cost and complexity, they may not be suitable or required for smaller systems. There is no easy answer when it comes to controllers and automation of water and wastewater systems.
Good planning up front and understanding the extent of the needs is a critical first step. When upgrading existing systems, the same applies — although those needs might be more limited. Decide if the needs are data storage for historical review and trending, or if there is a need to control equipment by instrumentation. This will aid in the selection of data collection, or use of a PLC. If this is a retrofit, consider the options for a simple low-cost automation system and panel, or if industry standard equipment is required. This will help determine the level of involvement of the engineering design team and scope of the project.
Good luck and plan carefully.
Mike Seymour has more than 25 years’ experience in water and wastewater treatment for small decentralized systems throughout B.C. He is currently involved with the BC Water and Waste Association. Seymour is also the principal with MSR Solutions Inc. in Victoria. He can be reached at 250.479.5164.