By Oliver Grievson
After a century of operation, the activated sludge process is still being fine-tuned to meet today’s challenging treatment and efficiency targets.
This year, as many know, sees 100 years since the activated sludge process was first developed by Arden & Lockett in Manchester, United Kingdom. Since then, technological developments in the way the process is designed and operated have been governed by a number of different pressures including population growth and the need to intensify the process, the need to make the process more financially efficient, and, of course, the need to tighten consents in order to protect the environment. The use of instrumentation to monitor the process and feedback on how it is performing has been a necessary step.
Instrumentation and process automation in activated sludge have developed hand in hand. Initially, they were developed with variable speed drives for aeration blowers and PID (proportional-integral-derivative) loops to control the amount of dissolved oxygen in the process, and more recently with ammonia control systems and advanced process control.
This has evolved into the use of instrumentation and process models to control the activated sludge process. Mathematical models have been developed by the International Water Association (IWA) for the control of the process, and this has been fundamental in the evolution of modern advanced-process control systems.
A Short History Of Instrumentation In Activated Sludge
The development of dissolved oxygen probes was the first innovation within the activated sludge process initially with the advance of the Clark polarographic sensor in 1956 by Dr. Leland Clark (YSI, 2009), and then the first optical sensor in the late 1990s, developed by Environmental Instruments in the U.S. This was originally called the Fluorprobe and has evolved over the years into instruments such as the RDO by Partech.
The respirometer, a complement to the dissolved oxygen probe, was first developed in 1996 by Dr. John Watts, who was with Minworth Systems Limited at the time. The technology has been commercially available for many years with the advantage that it can be used for both online control of the dissolved oxygen requirement of the process and also for toxicity measurement allowing for interventions on the process to take place should a shock load be detected in the influent of the plant. The most notable systems that are available in the world today are those of Strathkelvin Instruments in the U.K. and Challenge Technologies in the U.S. The ASPCon (activated sludge plant controller) is the most recent award-winning development by Strathkelvin Instruments, bringing respirometry to mainstream control.
The developments of suspended solids monitors in 1973 as a way of measuring the quantity of the biomass in the process and the ammonia monitor in the late 1990s have meant that the process has been able to run more and more efficiently over the years.
The key to instrumentation and its use within the activated sludge process is the development of activated sludge models. The first model was developed by the International Association of Water Quality (one of the groups that formed the IWA) in 1983. The first model, ASM1, covered chemical oxygen demand (COD) removal, oxygen demand, bacterial growth, and biomass degradation. This was extended further in 1995 to include biological and chemical phosphorus removal in ASM2 and was further developed in the late 1990s to ASM2d to include the aerobic uptake of phosphorus. The IWA models formed the basis of commercially available models such as BioWin, GPSX, and Stoat. These in turn form the basis of many of today’s advanced-process control models.
The Modern Activated Sludge Plant And The Role Of Advanced Process Control (APC)
The state of instrumentation and process control in the modern activated sludge plant is very much dependent upon the size and the complexity of the plant itself. The vast majority of the smaller conventional activated sludge plants have only basic instrumentation including dissolved oxygen and flow, mainly for the control of the dissolved oxygen concentration to ensure that the plant is compliant and the energy consumption is kept in check.
The intelligence of instrumentation and process automation and control has been put in place at larger treatment plants. Water companies have done a variety of things, including:
Dissolved oxygen control was one of the first initiatives to make the process more efficient with run and dwell systems on surface aeration systems and with PID loop control with fine bubble systems.
Organic load control of the carbonaceous load within the activated sludge plant and ammonia control was the next initiative that was brought to the activated sludge plant with the development of accurate instruments for ammonia analysis. This had led to mixed results with some water companies successfully implementing it and others less so.
Controlling the amount of biomass within the process became a possibility with the development of suspended solids monitors. This is key not only for energy reduction, but also for keeping the treatment plant compliant.
Within the past five to eight years, the principles of individual process control on the activated sludge plant have been gathered into integrated advanced-process control systems in the U.K. by two commercial organizations and one or two of the water and sewer companies. The water companies themselves have tried to do this with PID loop controls and in some cases with cascade loop control systems, some of which have had some success in Europe.
Hach Lange, in conjunction with MWH Global, developed the Water Treatment Optimization system (WTOS), which is very much an advanced-process control system based upon instrumentation. The WTOS system utilized Hach Lange instrumentation and a process model based upon the ASM1 activated sludge model (Thornton et al., 2010).
One of the first full-scale installations controlled with the WTOS system was a 250,000 population equivalent four-stage Bardenpho plant with methanol addition in the second anoxic zone. The system and controller that was developed for this treatment plant looked to monitor and automate the whole process including the nitrification and methanol dosing. This first installation conducted a trial over a 10-week period and managed to achieve a 20 percent reduction in the amount of aeration, control of the amount of ammonia that was discharged, and a 50 percent reduction in the amount of methanol that was consumed.
Since its first implementation in 2008, this technology has developed even further with other control modules including a nitrification module (which includes sludge age control) specifically designed for the activated sludge plant, as well as modules that are designed for other plant processes.
The second approach to advanced process control has again been based on model-based controllers but is less reliant on instrumentation and more reliant on the intelligence of the system as a whole; any failings in the implementation of APC have been due to poor data quality from the instruments. This approach put more intelligence into the control system to identify when an instrument becomes unreliable, and for the system as a whole to replace the unreliable data with an inferred value based upon the readings being received from other instruments within the system.
For example, the control model “knows” what each DO sensor should measure at any given time, given the influent flow, blower load, valve positions, manifold pressures, and treated water quality. If any probes report values that are significantly different from those that are expected, an alarm is raised, and the inferred value is used to exercise control of the process. Optimized control can be maintained even when real-time measurements become unreliable.
The multivariate process approach has advantages of being a system based upon the control element and is much more widespread within the plant, taking into account the whole treatment facility rather than just the activated sludge plant on its own. Case studies of this approach in three U.K. water and sewage plants realized savings between 20 and 35 percent of the aeration costs while also reducing the risk of compliance failure as the treatment plant operates more efficiently under automated control.
It is clear that the advent of instrumentation within the wastewater industry has propelled the development of control systems that have given the industry significant savings in the way it operates (a) treatment processes and (b) activated sludge plants in particular.
The Future Of Activated Sludge
So what is the next step in the next 100 years of the activated sludge plant?
As the demands on the wastewater industry get tighter and tighter, activated sludge plants will be expected to do more. We are seeing this with more and more treatment plants moving toward activated sludge and its different variants including the various forms of biological nutrient removal (BNR), enhanced biological phosphorus removal (EBNR), membrane bioreactors (MBRs), and integrated fixed-film activated sludge (IFAS) to realize these tighter demands.
For some of these variants, control systems have already evolved; for others, control systems will need to be developed.
There are also a number of different instruments that various suppliers within the industry have either developed or are in the process of doing so, including nitrous oxide sensors and a new generation of respirometers and biological monitoring systems to check on the health of the biological part of the process. Where these sensors will fit into the process remains to be seen.
What is clear, however, is that the activated sludge plant cannot be considered on its own, but rather as a much wider part of a larger system. The industry as a whole is starting to develop a philosophy of the treatment plant as a production facility and the production facility as part of the whole wastewater network, from the treatment system to the customer’s discharge point, to the point of return to the environment. This could see the demands upon the treatment plant change again. The activated sludge control system will become part of a treatment plant control system ensuring a consistent product feed and knowledge of what a facility is expected to receive.
What challenges will the activated sludge plant of the future hold? To match the needs of the stakeholders — whether it’s the customer or the environment as a whole — instrumentation, process automation, and control will need to play a major role.
The author wishes to acknowledge the contributions of Angus Fosten of Partech, Simon Mazier of Perceptive Engineering, and Robert Lagrange, formerly of Endress+Hauser, for their assistance in preparing this paper.
YSI Incorporated, (2009), The Dissolved Oxygen Handbook, YSI Thornton A, Sunner N, Haeck M (2010), “Real Time Control for Reduced Aeration & Chemical Consumption – A Full Scale Study, Water Science & Technology,” Vol. 69, pp. 2169-75.
Oliver Grievson is currently the Flow Compliance & Regulatory Efficiency Manager for the U.K. water company Anglian Water. He also runs the “Water Industry Process Automation & Control Group” (WIPAC) on LinkedIn and is a member of the Wastewater Committee of the Foundation for Water Research. His career has stretched from working in a water laboratory to managing water and wastewater facilities around the world.