Computers, SCADA, Software
Operators at the Mid Halton Waste Water Treatment Plant (WWTP) in Oakville, Ontario, Canada, have been using a Supervisory Control and Data Acquisition (SCADA) system to significantly improve efficiency.
For more than three years, they have increased flexibility and reduced expenses at the facility. The operators are connected via computer to Programmable Logic Controllers (PLCs) that monitor and control the plant equipment. Simple reprogramming allows them to change, add or enhance process control.
Another big benefit of SCADA is that operators can extract much more data from programmable controllers. Gone are the days when Mid Halton WWTP's operators had to take individual readings from each flowmeter by walking through the plant. Now the PLC totals all flows in the plant - data easily retrievable by an operator seated comfortably at his desk. And to give operators information on when to change day units, the PLC also calculates motor run times.
In addition, programmable controllers monitor many more alarms. These alarms might be triggered by high level or low pressure switches. They might also be set off by a pump's failure to start or a valve not closing. The PLCs give operators an early warning that something has gone wrong - before the process can be impaired by a failure.
The panel faces of older control systems are replaced by a computer-based operator interface. Indicator lights, chart recorders, push buttons, selector switches and alarm panels are displayed as active graphics on the computer screen.
While SCADA systems are costly - in terms of equipment, software, and operator education - savings in plant operation are significant enough to justify the expense in most cases.
SCADA Saves Time
The Mid Halton plant operates on a single shift and has a half-shift on weekends. The operators start each shift in front of the computer screen, examining each of the PLC's 30 graphic displays. Then the system allows them to call up recent historical trend data recorded during the previous night using customized graphs - a feature that many operators believe is one of their most useful trouble-shooting tools. This historical trend data includes plant flow, waste-activated sludge flow, digester temperature, and dissolved oxygen.
Graphs may be created using any span of time, and any combination of readings can be displayed on the same graph. It is very simple to call up old data, and even years of data can be stored on the computer. Older data can be easily retrieved from archived diskettes, tapes or other storage media.
Operators look at totals from the previous day to learn what the flows were, making records of the data. Then they make process decisions for the day, entering new setpoints or select new equipment duties.
Thirty to 60 minutes has passed since the beginning of the shift. The remainder of the day can be spent doing maintenance work, cleaning the plant, hosing down tanks, pumping down the wet well, walking the plant to check for abnormal conditions, taking samples, performing lab tests, doing administrative work, or discussing process or equipment problems. The programmable controller takes care of most of the hour-to-hour tasks that formerly consumed the operator's daily schedule.
Improving efficiency has been foremost on the minds of the Mid Halton operators. One control strategy change that they proposed, tested and implemented, using their SCADA system is examined here. At each step along the way, the SCADA system was essential to the change process. It helped them detect the problem, test the strategies for improvement, and then effect changes chosen by the operators.
Focus on Dissolved Oxygen
Once they modified the raw sewage pump behavior to produce a continuous flow into the aeration tanks, the SCADA system began to reveal another opportunity to improve the energy efficiency of the operation. The more complex analysis led to a simplification of the DO control process, saving costs in several areas.
The original process strategy in the aeration tank is known as "plug flow" - where a quantity of return activated sludge (RAS) and primary effluent is mixed and moved through the aeration tanks in stages. In plug flow, each stage has a different oxygen demand. The analysis confirmed that "step feed" - where oxygen demand was more uniform throughout the aeration process - would produce several improvements.
The solution that the operators tested was to simplify the control requirement. Plug flow was replaced by step feed. Primary effluent is passed through the aeration tank in parallel, while RAS is brought into the first cell and passes through the next cells in series. The four aeration cells have a more even oxygen demand.
Valves on the air supply lines were manually adjusted so that supply to all four cells was virtually the same - a manually operated, tapered aeration. This left the blower motor speed as the only control factor. With DO oscillations leveled out, four benefits were gained, and the plant:
- Used less energy, saving costs (the DO setpoint was adjusted from 2.0 mg/L to 1.0 mg/L);
- Experienced less strain on the system, since the DO was steadier;
- Saved maintenance costs for motorized valve modulation;
- Benefited from reduced risk of system failure, through the avoidance of air mass flowmeters and valve actuators.