Supervisory Control and Data Acquisition Systems

In an age where the population continues to grow and the planet’s resources are limited, the importance of reducing mankind’s environmental footprint becomes even more important. In the world of wastewater treatment, environmental protection efforts can be seen in the tightening down of limits, including nutrients, at the outfalls of our water reclamation facilities. The limits on ammonia and total-nitrogen, phosphorus, oxygen demanding pollutants, solids, and other outputs continue to be pushed lower to mitigate our environmental impact in our valuable waterways.

To the cities, towns, and small communities that operate water reclamation facilities, the operating cost and staff required to meet these limits continues to increase. The advanced controls required to biologically treat raw sewage may require multiple operators on hand or 24/7 operations staff to meet the treatment plant’s needs. The control of pumps, blowers, motors, presses, conveyors, screens, and other treatment equipment can become overwhelming to communities of all sizes.

A treatment plant’s influent never stops but it also is in a constant state of flux; this requires careful operation and coordination with operations staff to not disrupt the plant’s overall health and process. For this reason, a plant-wide SCADA system can provide around-the-clock support and relief for operations staff.

SCADA systems (Supervisory Control and Data Acquisition systems) provide varying levels of automated control and also provide operations staff with valuable information to make important decisions regarding the health of the plant. The system can determine when it is time to turn up a blower, turn down a cycle rate, open a gate, and more everyday tasks. A well-designed SCADA system can manage the everyday control operations within the plant. The operators aid the system by providing control parameters, valuable maintenance, and additional operations support in the lab and in the collections system.

Much like a human operator, a SCADA system requires raw data, but SCADA can continuously collect and analyze data from process analyzers, flow meters, and other instruments in order to make informed decisions. Unlike a human operator, SCADA can provide a continuous monitoring chain that allows SCADA to make corrections to the process as quickly as it receives the information.

This article will cover some of the items critical to a successful SCADA system and others that allow plants to see valuable energy savings and a reduction in overall operating costs.

Flow monitoring is a critical component for operation’s staff to utilize to make decisions in the plant. Magnetic flow meters (mag meters), doppler radars, flumes, and other devices monitor the flow throughout the plant and provide an idea of how the plant is operating. To a SCADA system, flow monitoring equipment can provide trending data in order to meet the monthly data reporting requirements by the State’s regulatory agencies.

Flow data can also help SCADA know when to turn on a standby pump in the raw sewage pump station or even when to modify a pump’s speed. Should operations staff know the concentration of solids in the waste activated sludge line (WAS line), SCADA can estimate the volume and mass of solids being removed from the process. Flow metering provides an accurate picture of what is coming into the plant, what is inside, and what is leaving. SCADA utilizes this data to make process changes, troubleshoot problems, and help communities understand their current plant capacity and when it is time to expand.

Dissolved oxygen probes (DO probes) are one of the most critical analytical components of the aeration system. Verifying the process has enough DO is required to ensure biochemical oxygen demand (BOD) and ammonia are being adequately removed. DO probes monitor the “free oxygen” remaining in the aeration basin after the microbial “workers” in the tank have done their job in reducing BOD into CO2 and ammonia into nitrate. A carefully controlled DO concentration along with adequate mixing is the most important indicator that the process is healthy and sufficient air is being added to the basin.

SCADA systems can utilize the DO reading to control air supply to the process. SCADA can speed up or slow down blowers to control the flow of air, open and close valves to push air to parts of the process that need it, and throttle back where demand is low. Aeration is typically 40 – 70% of the plant’s electric bill and results in high operations costs during times of low flow. SCADA can utilize the DO readings to throttle the aeration system, saving energy and in turn, operating costs. A full time operator would have to work around the clock monitoring the DO readings and making modifications to the aeration system to meet the same efficiency SCADA can perform.

Oxidation-reduction potential probes (ORP probes) provide operators and SCADA an overall health status of the process throughout the plant. Headworks buildings, anaerobic basins, anoxic basins, aeration basins, sludge holding tanks, and digesters are some of the many places you can find ORP probes in the system. Like the DO probe, SCADA can utilize the ORP reading to gauge the overall health of the process.

ORP probes provide a critical parameter to help operators know if their anaerobic tank is truly anaerobic and a good environment to promote phosphorus accumulating organisms for enhanced biological phosphorus removal. ORP probes can help verify appropriate recycle rates to the anoxic tank to optimize nitrogen removal. ORP is an indicator of the health of the process and can help operators and SCADA make modifications to the process.

Nitrate and ammonia probes are the new kids on the block. These probes provide feedback on the health of the process by directly monitoring the chemical concentrations we are trying to reduce. Monitoring of nitrate and ammonia in the aeration basins can allow the DO set point to be dialed down even lower than with a DO probe alone as the probes are providing a direct measurement of the efficiency of the process. These online analyzers have the ability to monitor and trend the ammonia and nitrate within the process. These parameters can help ensure nutrient compliance and that the plant is operating as it should. To a community where low discharge limits are critical and fines for not meeting these limits are high, these probes can provide an extra safety net of maintaining compliance while being energy efficient.

Biogas flow meters are a surefire way to understand the health of your anaerobic digester. The production of methane is the last critical component to the anaerobic digestion process and verifying the digester is producing consistent methane helps operations staff understand the nuances every anaerobic digester has. A reduction in biogas can trigger an alarm that something is going on with the digester and control needs to change. A drop in methane may mean the digester is going sour, the temperature is too low for the environment, the mixer on top has failed, the heat exchanger is plugged, the recirculation pump has failed, or the tanker that brought in raw solids yesterday that pumped directly into the digester upset the anaerobic process.

There are many parameters SCADA can analyze to control the treatment process, but flow monitoring, dissolved oxygen probes, oxidation-reduction potential probes, nitrate and ammonia probes, and biogas monitoring flow meters are some of the most common. A SCADA system acts as an extra set of eyes and hands on the process, enabling communities to stay compliant without 24/7 staffing. SCADA is not a replacement for a well-trained operations staff, but can be a very beneficial tool allowing a good operator more consistent monitoring than humanly possible. A proper SCADA system may require more general maintenance of probes and equipment while autonomously controlling the process around the clock, but is an invaluable tool for communities to meet regulations.

Careful design and coordination with communities and operators provides the foundation for a successful SCADA system. A tailored design is required, as no two water reclamation facilities are the same. Additionally, many communities have different preferences on the level of sophistication they would like their SCADA system to have. Our engineers provide guidance to communities in defining what SCADA system would best serve their community. The key to a good SCADA design is to work smarter not harder.

More questions about SCADA systems? Check out our water services page or email us at info@msconsultants.com