SHOULD WE KEEP DO FOR NITRIFICATION CONTROL? THE PROOF IS IN THE AMMONIUM ELECTRODES Robert Lagrange 1 *, Sue Baert 2, Amanda Poole 3, Dave Green 3 and Nick Camin 4 1 Lagrange Consulting, 4310 Royal Mustang Way, Snellville, Georgia, USA, 30039 (*correspondence: rplagrange@gmail.com) 2 Wheaton Sanitary District, 1S640 Shaffner Road, Wheaton, Illinois, USA, 60189 3 Baxter & Woodman Inc., 39 South LaSalle Street, Suite 816, Chicago, Illinois, USA, 60603 4 Endress+Hauser Inc., 2350 Endress Place, Greenwood, Indiana, USA, 46143 KEYWORDS Process Control, Optimization, Nitrification, Ammonium, Dissolved Oxygen, Energy Consumption, Compliance, Instrumentation ABSTRACT Aeration often is the process that consumes the most energy in a wastewater plant. At the same time it is an essential process to maintain compliance for BOD and Ammonia limits. It thus deserves a lot of attention. The use of dissolved oxygen measurement to control aeration is widely accepted. The literature has many examples of excellent return on investment in wastewater plants that have implemented this control. There are even theories that a concentration of 2 mg/l is the perfect value. But a DO sensor only tells how much oxygen has not been used in the process and at times this measurement may not bring the expected results. The effluent may end up out of compliance or too much energy can be spent. Examples are available. As one of the main objectives of the aeration tank is nitrification the measurement of the concentration of ammonium or ammonia provides much better information than dissolved oxygen. Ion Selective Electrodes (ISE) installed directly in the tank eliminate the need for sampling line and ensure an easier integration. Their selectivity, accuracy and lower maintenance make them an attractive solution. As demonstrated at Wheaton and Colorado Springs, additional energy savings and compliance are possible when using either ammonia in a cascade loop with DO control or in direct control of the air flow. Direct control either in feed-forward or feed-back provide faster and better response when spikes are present. The return on investment is very good even on smaller plants and compliance is achieved. Dissolved oxygen concentration remains an important factor as it affects the quality of the sludge. The control strategy has to maintain it within limits.
Lagrange, Baert, Poole, Green & Camin 2 INTRODUCTION Many wastewater plants around the world are working hard to reduce their energy consumption and the amount of nutrients discharged in their effluents. As both carbon removal and nitrification consume oxygen it is important to insure that oxygen is present in the right amount in the aeration tank. Dissolved Oxygen Instruments (DO) are typically used for this application. But they are only an indication of the amount of free oxygen available at the measuring point. The measurement of the amount of air sent to the tank provides the additional data to determine the oxygen uptake rate. In this paper the data from Wheaton Sanitary District (Illinois) provides the base for the discussion and data from other sources will be added to confirm the findings: the installation of instruments for automatic control is justified for small to large plants. THE GOOD AND THE BAD OF DISSOLVED OXYGEN MEASUREMENT The good The literature abounds in examples of plants using DO to control the airflow to the aeration tank. The return on investment is good even for small plants. Eagle Lake Sewer Utility District in Wisconsin treats only 0.26 MGD and was able to justify the installation of an optical DO instrument with the additional benefit of reduced maintenance cost (Kohlmann, 2010). As water and wastewater consume about 4% of the energy produced in the USA, power companies have shown a big interest in promoting DO control associated with blowers with Variable Frequency Drive (VFD). The Tennessee Valley Authority (TVA) sponsored demonstrations in real life at existing wastewater plants. A result is shown in Figure 1.
Lagrange, Baert, Poole, Green & Camin 3 Figure 1 TVA demonstration The Installation of DO sensors and VFD s provided a 13% reduction in consumption and most importantly a 39% reduction in peaks with the corresponding rate change.. More recent data shows that reduction in consumption of up to 50 to 60 % can be achieved and 80% when new diffusers are used (Poole, 2011) The bad As we said earlier DO only shows the residual amount of oxygen available in the tank. It certainly does not define the elimination of carbon nor does it tell precisely what is happening with nitrification or denitrification. The Hanover Area Regional WWTP (Pennsylvania) installed Ion Selective Electrodes instruments in their oxidation ditch. For the initial period the data, Ammonium and Nitrate concentrations, were recorded but not available to the operators. They had the information from a flowmeter, a DO and a NADH (Fluorescence of nicotinamide adenine dinucleotide) to operate the plant. The surface aerators had only 2 speeds. (Lagrange, 2009) Figure 2 shows the location of those instruments at about 1/3 rd of the tank.
Lagrange, Baert, Poole, Green & Camin 4 Figure 2: Location of the instruments at Hanover Figure 3 Hanover Data Figure 3 shows the data for almost a week of operation. In the initial part the DO value does not change while both the flow and the ammonium concentration (scaled 0 to 15 mg/l) change. As a result the effluent was out of compliance. Then the aerator speed is finally adjusted between high and low to follow the diurnal flow variations. The ammonium concentration is drastically reduced. Too much in fact as at 1/3 rd of the ditch it is already below 2 mg/l. The plant was expecting to see some denitrification taking place at low aerator speed (nitrate concentration scaled 5 to 25 mg/l). Again it is not happening. The plant was using far too much energy.
Lagrange, Baert, Poole, Green & Camin 5 EXPERIENCE AT WHEATON SANITARY DISTRICT The Wheaton Sanitary District is also looking at ways to maintain or improve the quality of its effluent while reducing the consumption of energy. A new turbo blower was installed in 2011 to replace an existing multistage centrifugal blower. Then a joint project was started with Baxter & Woodman to find the best control strategy for that specific plant. Endress+Hauser Inc and WTW (now Xylem) provided instrumentation assistance for the project. The plant is designed to treat a dry weather flow of 8.9 million gallons per day (MGD), a design maximum flow of 19.1 MGD and a peak wet weather flow of 45 MGD. The effluent is discharged in Spring Brook with very low flow that dictates the willingness of the District to discharge effluents with a quality well above their discharge permit. The treatment consists of screens, sand, grease and fat removal, primary clarification, trickling filter, aeration, secondary clarification, filtration and disinfection. The District has 5 aeration tanks in parallel. Each equipped with one Air Flow measurement and one DO measurement. For the test one Ion Selective Electrode (ISE) based measurement for ammonium was installed toward the start of tank 2 and another at the end of tank 3 Figure 4: Wheaton aeration showing the location of the instruments
Lagrange, Baert, Poole, Green & Camin 6 Step 1 Air Flow Control After the installation of the new blower the first step was to maintain a constant air flow to each tank. With this in place the DO concentration keeps changing reverse tracking the ammonium concentration measured close to the inlet in tank 2 as expected. During that time the ammonium concentration in tank 3, measured close to the end of the tank, remains constant around 0.4 mg/l very close to the limit of detection of the instrument. This confirms that nitrification is complete. In the final effluent, after some post aeration, the ammonia concentration is at a non detectable level in laboratory samples (<0.05 mg/l) Figure 5: Tank2 in airflow control mode A 28% energy consumption saving was achieved, 10% of which can be attributed to the control itself Step 2 DO Control In that phase the control objective is to maintain a constant DO. With the correct DO set point there should be enough air to keep the solids in suspension and prevent settling in the aeration tank.
Lagrange, Baert, Poole, Green & Camin 7 In that period, as can be seen in figure 6 there were some ammonium spikes the largest and with highest rate of increase was on January 14. The control reacts well with that spike. As the DO measurement is located toward the end of the tank, some 20 feet, the control had difficulty to keep up with the fast change resulting in a very small blip on the effluent ammonium measurement. The change is less than 0.2 mg/l increase. There was a 9% increase in energy consumption compared with the period with airflow control. Figure 6 DO Control Step 3 Ammonium Control Figure 7 compares tank 2 in ammonium control with tank 3 in airflow control. The variations in DO concentration are drastically reduced and in tank 3 the process remains under control with the big spike
Lagrange, Baert, Poole, Green & Camin 8 of ammonium on January 28/29. At the same time with the same wastewater in tank 3 the oxygen is completely depleted and the nitrification is not complete. The use of ammonium in feed-forward provides a better control, more stable process and insures that the concentration of ammonia in the effluent remains at the low level desired by Wheaton. Figure 7 Feed forward NH4 control ISE A BETTER TOOL When normalizing for changes in flow and BOD load the results favor ammonium control Step 2 consumes 5% more energy Step 3 consumes 6% less energy. From Wheaton s point of view achieving a stable process while maintaining their very low ammonia discharge justified the investment to install one ISE instrument in the splitter box ahead of the 5 aerations tanks and modify the control strategy to feed-forward control. The decommissioning of the
Lagrange, Baert, Poole, Green & Camin 9 trickling filters will bring a higher nutrient load to the aeration tank allowing for a more efficient control as the low limit on air flow from the blower will be prevented. As we can see in figure 7 there is margin to reduce the energy consumption. Colorado Springs Utilities Colorado Springs Utilities (Lagrange, 2011; Camin, 2011) with a different set of problems took a different approach. That plant faces very high daily ammonia peak while trying to run only their small compressor. Designed for 20 MGD the actual flow was only 8 MGD at the time of the study, While on DO control (those DO units were installed at construction time) the plant faced some high ammonia concentration in the effluent or at other times low ph as nitrification consume alkalinity. The decision was then made to install ISE measurement for both ammonium and nitrate as some denitrification was implemented to increase alkalinity. The locations are described in figure 9. Figure 8 Concentration of influent ammonia in green and power consumption at CSU
Lagrange, Baert, Poole, Green & Camin 10 Figure 9 Installation point for the instruments at CSU To increase the speed of response of the control loop the air flow is directly controlled based on the ammonium concentration. The control takes into account the DO concentration to maintain it within high and low limits. With that control strategy CSU was able to reduce the cost of energy while the flow increased as shown in figure 10 for the months of November 2009 and November 2010.
Lagrange, Baert, Poole, Green & Camin 11 Figure 10 CSU flow and cost The consumption of energy per MGD per pound of ammonia nitrified was reduced by 12% compared with standard DO control. In the future the Ammonium sensor after the primary will be relocated to the swing zone to combine feed-forward and feedback. Others The city of Peoria, Arizona, operated their Beardsley Road Water Reclamation Facility, a 4 MGD plant, with a cascade control where the output of the Ammonium controller adjusts the set point of the DO control loop. They were able to achieve a 3 months return on investment and improve the sludge quality. (Dabkowski, 2011) Rieger (2012) also demonstrated in three wastewater plants with average flows of 3.5, 10.5 and 54 MGD in Switzerland that the use of ammonium control provides benefits on other processes such as a reduction in the consumption of chemicals for phosphorus removal as more volume can be used in anaerobic phase.
Lagrange, Baert, Poole, Green & Camin 12 SUMMARY Wheaton Utilities District decided to use a control strategy based on feed-forward based on ammonium concentration as it provides them with the best results for their objectives. Colorado Springs used a partial feedback and achieved excellent results. Peoria included the ISE in a cascade control for a high return on investment. Further reading is available. (Grievson, 2012) We would like to leave the final comments to Leiv Rieger Ammonia controlled aeration in an activated sludge system leads to a significant reduction of the total energy consumption in conjunction with decreased effluent concentrations of total nitrogen..the best solution will be the simplest concept that still yields a significant benefit in comparison with the annual costs. LIST OF ACRONYMS: ISE... Ion Selective Electrode DO... Dissolved Oxygen NADH... nicotinamide adenine dinucleotide (fluorescence of) CSU... Colorado Springs Utilities MGD... Million Gallons per Day
Lagrange, Baert, Poole, Green & Camin 13 REFERENCES Camin & al. Ammonia Based Aeration Control Workshop 105 WEFTEC 2011 Dabkowski & al. Ammonia Based Aeration Control Workshop 105 WEFTEC 2011 Kohlmann. Saving costs by continuous monitoring of D.O. in plant Application Note Endress+Hauser Inc. 2010 Grievson Aeration Control in wastewater activated sludge plants using mixed liquor, organic loading and ammonia analysis Water Industry Process Automation & Control Group, March 2012 https://www.onlinefilefolder.com/2s7lvnxaeb7zzw Lagrange & al. The revival of the Ion Selective Electrode Applications in Wastewater Plants ISA Expo 2009 Lagrange & al. A change in control strategy reduces power consumption at Colorado Springs Utilities Energy & Water 2011 Poole & al. Aeration System Automation: Control Strategies to Maximize Energy Savings at Low Capital Cost WaterCon 2012 Rieger & al. Improving Nutrient Removal While Reducing Energy Use at Three Swiss WWTPs Using Advanced Control Water Environment Research, Volume 84, 2012
Lagrange, Baert, Poole, Green & Camin 14 About the Authors: Robert Lagrange, PhD is a Doctor in Physics from the University of Grenoble, France. After 11 years with Endress+Hauser as Business Manager Water and Wastewater Robert is now working part time as a consultant. Robert presented multiple times at the ISA WWACS and is a member of WEF and AWWA instrumentation and control committees Sue Baert, B.Sc. is the Plant Superintendent at the Wheaton Sanitary District where she started as a chemist became the Lab Manager and wrote the Quality Assurance Project Plan for the stream water quality project, collected, analyzed and submitted the data. She has a degree in Biology/Chemistry from the UW-Lacrosse. Sue is involved with the DuPage Salt Creek watershed group as Vice- President. She is an active member of WEF through the local MA Central States Water Environment Association (CSWEA). She was President of the Illinois section of Central States for a one year term, and then Illinois Trustee to the Executive staff for two years. In all, her career in the water/wastewater section has spanned over 22 years. Amanda Poole, M.Eng. is an environmental engineer at Baxter & Woodman, where she focuses on energy reduction and generation measures at wastewater treatment plants. Amanda has been involved in numerous wastewater treatment plant energy audits and aeration energy reduction projects throughout the Chicago area. Amanda received her B.S. and M.S. in Environmental Engineering from the University of Illinois in Urbana Champaign. She has been working in the water/wastewater sector for the past 3 years and awaits her P.E. licensure in December 2012. She is an active member of the Central States Water Environment Association. David Green, AAS EET joined Baxter & Woodman in 2008 as a Senior Systems Integrator. Dave s passion for automation and innovation led him to become the Automation Technical Director. He brings over 16 years of experience designing and providing unique solutions to Automated Systems Integration projects. He has quickly become a technical leader in the organization, and has a hunger for learning new technology and finding the best solutions for automation problems. Nick Camin, BSEE, MBA has worked in the instrumentation field for over thirteen years specializing in environmental industry applications for over ten of them. Nick is a graduate of Purdue University with a bachelor of sciences degree in electrical engineering and he also holds a master of business administration degree from Indiana Wesleyan University. With Endress+Hauser he has held the positions of application engineer, project manager, municipal business manager, regional sales manager and currently he is marketing manager for the environmental industry. Nick is a member of WEF and AWWA instrumentation and control committees.