ASP-Final-E-version
- 1. Application Note XAUK WW201-01 0116 USING AMMONIA AND NITRATE MEASUREMENT DATA Activated Sludge Process Optimisation
- 2. Starting a discussion on saving energy in activated sludge through dissolved oxygen control is unlikely to win prizes for originality or innovation. However, successfully achieving this through effective instrumentation and control algorithms can reap rapid rewards. The reason that proper control of aeration is so important is illustrated through the figure presented below (figure 1). WTW Online Monitoring for Wastewater Application Note XAUK WW201-01 0116 ASP OPTIMISATION Ammonia and Nitrate measurement Easy removal of ISE from sludge liquor for inspection “When compared to the lifecycle cost of the ISE system, payback time for this technology is under one year.” Figure 1. Breakdown of operational energy requirements as a percentage of entire plant demand. Data taken from: Energy Awareness in Wastewater Systems, DERF,June 4 2008 ASP 55.6% Activated Sludge Aeration (ASP) Primary Clarifier and Sludge Pump Heating Dewatering Influent Pump Station Secondary Clarifier and RAS Process Water Postaeration/Chlorine mixing Lighting Thickener and Sludge Pump Effluent Filters Headworks % Energy Requirement
- 3. This breakdown is nothing new, so targeting an activated sludge plant as a key optimization location is, as admitted above, hardly innovative. Before discussing the advances in instrumentation, let’s go briefly back to basics. The requirement for aeration in activated sludge is to enable the conversion of ammonia to nitrate and finally to nitrogen. The first of these stages requires oxygen, the second carbon. This is better illustrated in a diagrammatical form as below. For sites incorporating swing zones, these Ammonia and Nitrate measurements are even more important as they can be used to trigger zone activation, change of zone process and monitor the concentrations of each parameter leaving the swing zones and entering the final aeration stages to ensure the plant is operating within the required margin of safety to maintain discharge consents. For those sites which do not have Ammonia consents but focus on carbon, the argument is still a valid one. The plot in figure 4 shows comparative degradation speeds of carbonaceous material and ammonia against dissolved oxygen concentrations in the liquor. Measurement of Ammonia at the inlet to the ASP and in the aeration basin would enable control of carbon through an ammonia degradation surrogate. XAUK WW201-01 0116 Application Note | ASP Optimisation Figure 2. Microbial Pathways Schematic for Conventional Activated Sludge for Nitrogen removal All plants benefit from increased levels of data, from the most basic with a single aeration lane right up to flagship sites incorporating multiple lanes with swing zones. Being confident in the validity and reliability of the data enables application options to be considered. In its most basic form and without any control logic, the Ammonia and Nitrate loading data could simply be used to monitor inlet characteristics and the performance of the nitrification-denitrification stages, alerting an operator when an action is required. While an enhancement over a static set-point, this is not going to win medals for efficiency or energy savings, and relies on operators being available when the action is required. Obviously, the next stage is then to automate this function. How complicated this is made depends on how the control scheme is developed. The level of Ammonia can simply be used to adjust the set-point of the blowers enabling targeted DO levels. Incorporating the MLSS measured at the inlet to the lane enables further refinement of these targeted levels as two variables are then used to calculate load. Adding Nitrate measurement allows for tracking of the performance of the nitrification-denitrification stages, further expanding the knowledge of the process and potentially identifying problematic regions. Figure 3. Complete sensor set-up for ASP control comprising: MLSS on inlet, AmmoniaorcombinedAmmonia/NitrateandDOineachlane,Nitrateonoutlet
- 4. KEY DEVELOPMENTS IN ISE TECHNOLOGY While measurement of incoming load or load at a key point in the aeration basin is an obvious improvement over the basic DO set-point control, the suggestion that the load measurement may be performed through the use of Ion Selective Electrodes to provide live Ammonium and Nitrate measurements has been contentious to date. Good control is impossible without accurate and reliable data, something which ISEs have typically only managed briefly before beginning to drift. Alongside the issue of drift, the electrodes themselves have not been robust enough for wastewater applications resulting in a change-out frequency of around 3 months and a real risk of damage to the electrode surfaces during cleaning, rendering them useless. All of these issues have combined to give the humble ISE a, perhaps well-deserved, bad reputation amongst process scientists and operators. However, the ISE should not be written off as an option. A number of recent advances in electrode technology, both in terms of function and robust construction, mean that the ISE sensor deserves a second consideration. The first issue that has been addressed is the robustness of the electrodes themselves through incorporation of an integrated metal grid into the active surface, bringing an added benefit of straightforward electrode cleaning with nothing more advanced than a nail brush. This has been coupled with improvements to the internal functionality of the electrodes enabling a dramatic increase in the potential in-process operational lifetime. Electrodes are now being supplied with a 12 month warranty and an expected accurate functional life of 2 years with a recommended replacement cycle of 18 months: a massive improvement over the previous 3 month replacement cycle. Increased stability has been achieved through modifications to the reference electrode. This reduction in drift is impressive and greatly reduces the need for manual checking and maintenance. Figure 6. Plot of Relative Slope against Weeks Operational illustrating very little drift in sensor readings over extended period of useXAUK WW201-01 0116| Application Note | ASP Optimisation Figure 4. Degradation rates of Carbon and Ammonia against DO concentrations Figure 5b. ISE sensor installed in ASP. Chain mounting allows oscillation of sensor head for self-cleaning.Air cleaning can be incorporated to enhance fouling removal where needed This increase in stability would be of little benefit without a comparative development in accuracy, repeatability and response time. Trials of the ISE technology against wet chemistry analysers showed that when applied in activated sludge processes the performance of the two technologies are comparative, with the ISEs providing the benefit of continuous live measurements. Figure 5a Cleaning the sensor head with a nail brush. Weeks Operational RelativeSlope%
- 5. REAL WORLD APPLICATION Following extensive trials, ISE technology has been applied to 30 sites across a major utility, where measurements of Ammonia and Nitrate are being used to enable a dynamic DO control scheme: a method which has generated energy savings in the aeration lanes on each site of between 15 and 20%. “When compared to the lifecycle cost of the ISE system payback times for this technology are under one year. With all of these improvements brought to fruition, perhaps it is finally time for the ISE to take its’ place amongst the instruments which efficient sites really can’t be without?” For further information, please contact: Xylem Analytics UK Limited 2 Focal Point, Lacerta Court Letchworth SG6 1FJ United Kingdom Website: www.xylemanalytics.co.uk Email: salesuk@xyleminc.com Telephone: (+44) 1462 673581 Fax: (+44) 1462 673582 Xylem Analytics UK manufactures environmental monitoring instruments and systems. Formerly known as YSI, the group is a market leader with a reputation for high levels of accuracy and reliability. The company’s water quality measuring instruments are designed for both laboratory and field use. In addition to bench top and hand-held instruments, Xylem Analytics also builds monitoring systems and wireless networks that are able to operate in remote locations and challenging environments. The company’s water quality monitoring sondes can be fitted with a wide variety of sensors including dissolved oxygen, PH, temperature, turbidity , conductivity, salinity, ORP, chlorophyll, open- channel flow, vented level, ammonia, chloride, rhodamine, depth photo synthetically active radiation. Xylem brands represented include; Aanderaa, MJK, SonTek, WTW and YSI. Tel +44 (0) 1462 673 3581 | salesuk@xyleminc.com | www.xylemanalytics.co.uk Xylem Analytics UK Ltd. | Letchworth | SG6 1FJ | United Kingdom © 2015 Xylem, Inc. All rights reserved. XAUK WW201-01 0116 All trademarks registered to Xylem Inc. or one of its subsidiaries. Figure 7. Comparison of Ammonium and Nitrate measurements from ISE sensors and Analysers. Please note that the analysers are on 30 minute measurement cycle.