Research Report COMPARATIVE ANALYSIS OF PARALLEL IFAS AND ASP REACTORS: OXYGEN TRANSFER AND UPTAKE, NUTRIENT REMOVAL, CARBON AND ENERGY FOOTPRINT Diego Rosso 1,*, Sarah E. Lothman 2, Alan L. Stone 2, Don Howard 3, W. James Gellner 4, Paul Pitt 5 1 Department of Civil and Environmental Engineering, University of California, Irvine, CA 92697-2175 2 Hazen and Sawyer, P.C., 4011 WestChase Blvd, Raleigh, NC 27607 3 Hazen and Sawyer, P.C., 11311 Cornell Park Drive, Cincinnati, OH 45242 4 Department of Water Reclamation, Greensboro Water Resources, 2350 Huffine Mill Road, McLeansville, NC 27301 5 Hazen and Sawyer, P.C., 498 7 th Avenue, New York, NY 10018 *Corresponding author (e-mail: bidui@uci.edu) ABSTRACT A full scale IFAS pilot with AnoxKaldnes media and coarse bubble aeration (IFAS tanks only) was installed at the T.Z. Osborne Water Reclamation Facility located in Greensboro, NC, and a year-long study carried out to quantify nitrification kinetics, aeration requirements, process performance, and identify potential operational issues. As part of the full-scale evaluation, an off-gas test(s) was performed as described by the ASCE Protocol. In order to remove kinetic limitations associated with DO diffusional gradients through the biofilm, the IFAS system was operated at an elevated dissolved oxygen concentration. The IFAS process is characterized by elevated air flux and air use per unit load treated, due to elevated mixing requirements and the high DO required to prevent oxygen diffusion limitations within the biofilm, with associated lower OTE and αsote. In theory, when OTE is the same, the air used per pound of COD removed is expected to be the same. Nevertheless, the mixing requirements specified by the IFAS manufacturers affect air use. Throughout the coarse bubble aeration zones, the IFAS has consistently higher air flow and therefore the relative air use compared to ASP. Also, the IFAS has roughly double the air use for mixing when compared to the ASP. Nitrous oxide in the off-gas was measured in the Winter test, and at this time the data is still insufficient to support any conclusion on the two processes. Keywords: Activated Sludge, Integrated Fixed-Film, Oxygen Transfer, Nutrient Removal, Energy Footprint INTRODUCTION The T.Z. Osborne Water Reclamation Facility (TZO), located in Greensboro, NC is owned and operated by the City of Greensboro. This 40 MGD facility discharges to the Haw River, a tributary of Jordan Lake. Several alternatives are being evaluated as strategies to meet stringent forthcoming regulatory nutrient limits resulting from legislative approval of the Jordan Lake Nutrient Reduction Rules. As part of this
evaluation, one of the alternatives being considered is the implementation of integrated fixed film activated sludge (IFAS). Due to the limited number of full-scale operating facilities in the United States and the City s desire to experience operating the process, a year-long study was conducted to quantify the nitrification kinetics, aeration requirements, process performance, and potential operational issues. The City tested an IFAS pilot in side-by-side configuration with the existing activated sludge process (ASP). In IFAS systems, coarse bubble diffuser systems for aeration are typically installed. The diffuser system consists of stainless steel headers with small holes in the bottom of the header. The design of the system is intended to minimize the need for in tank maintenance. Because of the presence of the IFAS media, the coarse bubble oxygen transfer efficiency may be expected to increase due to bubble hold up in the tank and the splitting of the bubbles when it passes through the areas filled with media, although this phenomenon has yet to be confirmed. For the TZO demonstration, the previously existing fine bubble diffusers in the three IFAS cells were removed and replaced with the manufacturer recommended system. IFAS systems must also be operated with an increased dissolved oxygen (DO) concentration relative to the existing activated sludge basins to facilitate a bulk liquid DO diffusion gradient through the biofilm such that the biofilm is fully aerobic. The target dissolved oxygen concentration recommended by manufacturers are typically 3 to 4 mg/l. Both the coarse bubble system and the increased target dissolved oxygen increase the amount of air required for IFAS systems. Oxygen transfer tests and analyses are key components for improved process design and optimization (Plano et al, 2010). Independent studies on biological aerated filters using off-gas analysis were conducted in small scale (Harris et al., 1996) and full depth reactors (Stenstrom et al, 2008), but not for the IFAS process. Oxygen uptake rate (OUR), a key parameter obtained from off-gas testing, is typically used as indicator of microbial metabolism. OUR in IFAS systems was previously evaluated (Maas et al., 2008). Those studies suggested that COD and nutrient removal rates can be controlled by determining OUR. Maas et al. (2008) found from OUR tests that the biofilm on the media perform the majority of nitrification in an IFAS system. Both studies, however, performed tests at laboratory scale and did not use off-gas testing procedures. Off-gas tests were previously performed by an IFAS manufacturer (Infilco Degremont; Viswanathan in 2008). To our knowledge, this is the first independent off-gas investigation on an IFAS system to date. Energy use is a growing concern at wastewater treatment facilities and aeration has been identified as one of the most energy intensive processes on site (Rosso and Stenstrom, 2005). Furthermore, in the case of TZO, aeration energy was a key factor for the plant upgrade. It is therefore critical to comparatively quantify the ASP and IFAS aeration efficiencies and energy costs. The overall goal of this research is to increase our understanding of oxygen transfer rates in IFAS systems and ultimately use the knowledge gained in the decision making process for full-scale IFAS implementations.
METHODOLOGY Nomenclature The following terms are used in this paper, consistently with ASCE (2007): OTE = Oxygen transfer efficiency in percent OTR = Oxygen transfer rate in mass of oxygen per unit time α factor = The α factor is the ratio of the value of oxygen transfer coefficient (KLa) measured in process water to the KLa measured in clean water. The smaller the value of the α factor, the greater the reduction in transfer rate due to contaminants in the process water. When the term αf is used, it refers to used diffusers, which usually have additional reduction in transfer efficiency due to biofouling or changes in diffuser material properties, such as membrane hardening or softening αsote = Oxygen transfer efficiency (OTE) corrected for all process conditions such as DO, salinity, barometric pressure, etc., except for the α factor. This is the single most useful parameter to describe the performance of an aeration system under process conditions. The αsote divided by the SOTE is equal to the α factor Off-Gas Tests In-Process Oxygen Transfer Testing or off-gas testing was performed in the same fashion as described by the US EPA (1989). It is based upon the original off-gas method developed by Redmon, et al., (1983). Off-gas testing is accomplished by capturing a quantity of gas being released from the surface of the aerated mixed-liquor. This gas, called off-gas, is passed through an analyzer that measures oxygen partial pressure (equivalent to mole fraction). The carbon dioxide and water vapor in the off-gas can either be measured or removed from the gas by chemiadsorption. In the tests performed at TZO, the carbon dioxide and water vapor were removed from the off-gas prior to analysis by chemiadsorption, i.e. using a desiccating column filled with silica gel granules and sodium hydroxide pellets. Figure 1 illustrates the testing layout. Two tests were performed, in the Winter (January 2010) and Summer (June 2010) seasons.
Figure 1. Off-gas testing layout. This non-invasive testing method allows the concurrent measurement of the actual oxygen transferred to the water (oxygen transfer efficiency [OTE, %], oxygen transfer rate [OTR, kgo 2 /h], and oxygen uptake rate [OUR, mgo 2 /L/h]) The oxygen transfer efficiency (OTE) can be calculated from the off-gas oxygen mole fraction and the known ambient air mole fraction (20.95%). The gas flow rate is not needed to perform this calculation, although it is desirable for performing flow-weighted averages over an aeration tank or across several aeration tanks. At each measurement the apparatus self-calibrates by zeroing with an initial sampling of atmospheric air. The off-gas was collected in two floating hoods providing a capture area of 32 ft 2 each. Two testing locations were chosen in each reactor cell in order to gather a representative off-gas sample from each cell. Only the last cell of each process was tested in the middle. For each testing location, two readings were taken. Figure 2 shows the hood locations. It is generally recommended that a minimum of 2% of the tank area be sampled, the TZO tests exceeded 3% coverage.
Figure 2. Testing Hood Positions
Plant Operation The plant operating conditions should be noted when comparing performance to other test results or other plants. At the time of both tests, there were no unusual plant conditions. Table 3 shows the key operating parameters for the plant. At the time of testing, the two tanks were operating using the same wastewater influent, and despite the roughly double load on Tank 12, comparable effluent concentrations for COD and ammonia. Oxygen transfer efficiency can be dramatically affected by plant conditions. For activated sludge plants, the transfer efficiency is reduced at high F/M or low MCRT operation. Soluble substrates (i.e. organics) and other soluble materials (e.g. surfactants) in the aerobic zones can suppress the α factor, and therefore depress αsote. It is well known that high MCRT systems using fine bubble diffusers have higher α factors and α factors at the effluent end of a plug flow aeration tank are higher than the influent zone. This trend is consistent with observations on α factors. Also, coarse bubble diffusers are typically associated with elevated α factors but low SOTE values. Table 3. Plant Operating Conditions During Testing January 2010 June 2010 Parameter Tank 11 ASP Tank 12 IFAS Tank 11 ASP Tank 12 IFAS Flow Rate, MGD 2.9 6.0 3.0 6.0 COD, mg/l Primary effluent 182 182 337 337 Final effluent 37.8 36.8 31.7 35.4 MLSS (mg/l TSS) 3,110 1 1,215 1 2,780 1,655 MLVSS (mg/l VSS) 2,375 1 928 1 N/A 2 N/A 2 Temp, MLSS ( o C) 14.2 14.5 26.8 26.8 NH 4 -N Primary 12.9 12.9 23.8 23.8 Final 5.7 5.6 7.6 5.0 1 Average of values collected Tuesday-Friday of testing week since testing day was a holiday 2 MLVSS not measured for June 16, 2010. June 17, 2010 MLSS values were 2,840 and 1,815 for Tanks 11 and 12, respectively. MLVSS values were 2,220 and 1,390 for Tanks 11 and 12, respectively.
RESULTS AND DISCUSSION Off-Gas Test Results The testing results are summarized in Table 1. The oxygen transfer efficiency (OTE, %) shows the actual transfer efficiency at the existing temperature, and dissolved oxygen (DO) concentration. The αsote (%) shows the transfer efficiency, adjusted for temperature, barometric pressure, DO, salinity, and process contaminants. The results are shown below as the flow-weighted average values of OTE, αsote, and oxygen uptake rate (OUR, mg/l-hr). In the case of oxygen transfer efficiencies, the average is flowweighted based on airflow. Due to a sudden change in weather conditions with a violent thunderstorm on day 1 of the summer testing (June 16), the test was interrupted after the bulk of the data was collected (i.e., positions 1-6 for both tanks). These positions measured during day 1 correspond to the process sections where the IFAS reactors were operated. On day 2 of testing, data for the remainder of the process equipped with the fine-pore diffuses in both tanks were measured (i.e., positions 7, 8, 9 for both tanks). Since the off-gas velocity for one of the data points of the IFAS could not be measured due to inclement weather, the missing point was excluded from the flow-weighed averages. Table 2 shows a comparison of results for the January and June testing. Note that although the air use and the OUR are significantly different during the two tests, the IFAS has an OUR and air use roughly double than those of ASP. The results from the second test confirmed that the IFAS is characterized by elevated air flux due to mixing requirements specified by the process manufacturer, with associated lower OTE and αsote. It is critical to note that, for Tank 12, only the IFAS reactor cells (Cells D, E, F as shown in Figure 2) contain coarse bubble diffusers. The remaining aerobic cells in Tank 12 (cells G, H, I) contain fine bubble diffusers. In order to provide an accurate comparison, the results are also normalized as air used per unit load removed (SCF/lb LOAD ). Figures 3-7 show the OTE, αsote, OUR, Air Flux, and Air Use for both tanks tested. The differences between the IFAS tank and the ASP are apparent. The IFAS tank has higher transfer rates and OUR, higher air flux and air use per unit load removed, and lower OTE. Note that in all figures, IFAS cells are installed only at testing positions 1-6, and positions 7-9 are ASP for both tanks. From Figures 3 and 4 is evident that OTE is clearly different for the ASP and IFAS processes in both tests, while SOTE shows no definite distinction between the processes. This is due mainly to the elevated DO required for the IFAS process. In order to reach such elevated DO, the air flow rate required is high, therefore lowering the OTE for IFAS. Moreover, high DO operations are disadvantaged for OTE, since the DO is closer to oxygen saturation. Due to the lower driving force for oxygen transfer, while OTE is disadvantaged for the IFAS process, SOTE is compensated with a zero DO correction and therefore is in the same range for both processes. The DO correction accounts scales the more difficult oxygen transfer when operating closer to the saturation value. Nevertheless, although the overall process efficiency ( SOTE) may appear similar for both processes, the energy footprint of each process is quantified by the air flux and air use per unit load removed (Figures 6 and 7).
Table 2. Comparative summary of results for the off-gas tests performed in January and June 2010 T.Z. Osborne WRF Summary of Results JANUARY 2010 Tank OTE 1 SOTE 1 Air Flux 1 DO OUR 1 Air Use 1 No. (%) (%) (scfm/ft 2 ) (mg/l) (mg/l-hr) (ft 3 /lb LOAD ) 11 (ASP) 11.5 13.5 0.4 1.0 39.0 40.9 12 (IFAS) 6.1 10.1 1.4 3.6 85.7 80.4 JUNE 2010 Tank OTE 1 SOTE 1 Air Flux 1 DO OUR 1 Air Use 1 11 (ASP) 11.3 15.6 0.46 2.2 53.4 24.1 12 (IFAS) 7.9 13.4 1.41 3.8 106.8 37.7 1 airflow-weighted average values Figure 3. Comparative OTE profiles for ASP and IFAS during the January and June tests. IFAS cells are installed only at testing positions 1-6, and positions 7-9 are ASP for both tanks.
Figure 4. Comparative SOTE profiles for ASP and IFAS during the January and June tests. IFAS cells are installed only at testing positions 1-6, and positions 7-9 are ASP for both tanks. In Figure 5, the higher OUR for the IFAS is due to the higher load. Figure 6 presents the air flux for both tanks tested. The air flux in the IFAS basin can exceed three times that of the activated sludge control basin. This directly affects power consumption and the oxygen transfer efficiency. At locations 7, 8, and 9, where both basins have fine bubble diffused aeration, air flow rates for both basins are similar. Figure 7 presents the air use (expressed as volume of air used per unit load removed) and relative ratio for both tanks. In theory, when OTE is the same, the air required per pound of COD removed and/or NH 3 -N nitrified should be the same regardless of the process. Nevertheless, the mixing requirements and elevated DO requirements specified by the IFAS manufacturers significantly impact air use. Throughout the coarse bubble aeration zones (positions 1 to 6), the IFAS has elevated air flow rate and therefore the relative air use is in the range of 1.4 to 1.7. This trend is consistent between the January and the June tests.
Figure 5. Comparative OUR profiles for ASP and IFAS during the January and June tests. IFAS cells are installed only at testing positions 1-6, and positions 7-9 are ASP for both tanks. Figure 6. Comparative Air Flux profiles for ASP and IFAS during the January and June tests. IFAS cells are installed only at testing positions 1-6, and positions 7-9 are ASP for both tanks.
Figure 7. Comparative Air Use profiles for ASP and IFAS during the January and June tests. IFAS cells are installed only at testing positions 1-6, and positions 7-9 are ASP for both tanks. Energy footprint considerations can be concluded directly from figures 6 and 7. The elevated air use associated with IFAS directly relates to energy footprint increase, as there is no mechanical mixer in the IFAS reactors. During design and operation, therefore, the IFAS is expected to have an energy footprint between 1.6 and 2.0 times higher than the conventional activated sludge system Figures 8-11 show the comparative nutrient profiles for both tests.
Ammonia Profile 01/18/2010 (mg/l) 15 10 5 0 Primary Cell A Cell C Cell D Cell E Cell F Cell I Clarfier Basin 12 Basin 11 Ammonia Profile 06/16/2010 ( m g /L ) 10 20 0 Primary Cell A Cell C Cell D Cell E Cell F Cell I Clarfier Basin 12 Basin 11 Figure 8. Comparative ammonia profiles for both tests
Nitrate Profile 01/18/2010 (mg/l) 6 5 4 3 2 1 0 Primary Cell A Cell C Cell D Cell E Cell F Cell I Clarfier Basin 12 Basin 11 Nitrate Profile 06/16/2010 (mg/l) 10 8 6 4 2 0 Primary Cell A Cell C Cell D Cell E Cell F Cell I Clarfier Basin 12 Basin 11 Figure 9. Comparative nitrate profiles for both tests
Nitrite Profile 01/18/2010 (mg/l) 0.3 0.25 0.2 0.15 0.1 0.05 0 Primary Cell A Cell C Cell D Cell E Cell F Cell I Clarfier Basin 12 Basin 11 Nitrite Profile 06/16/2010 (mg/l) 0.5 0.4 0.3 0.2 0.1 0 Primary Cell A Cell C Cell D Cell E Cell F Cell I Clarfier Basin 12 Basin 11 Figure 10. Comparative nitrite profiles for both tests
Ortho-P Profile 01/18/2010 (mg/l) 20 15 10 5 0 Cell A Cell C Cell D Cell E Cell F Cell I Clarfier Basin 12 Basin 11 Ortho-P Profile 06/16/2010 (mg/l) 50.00 40.00 30.00 20.00 10.00 0.00 Cell A Cell C Cell D Cell E Cell F Cell I Clarfier Basin 12 Basin 11 Figure 11. Comparative ammonia profiles for both tests
Figure 12 shows the comparative results of N 2 O partial pressure readings for ASP and IFAS in the January 2010 off-gas test. Samples were grabbed from the off-gas stream and stored in previously evacuated sealed 10ml vials, and measured in a controlled laboratory with a gas chromatograph equipped with an electron capture detector (Shimadzu, Carlsbad, CA). These samples were collected only in select points in the Winter test, due to laboratory availability. These results show that for repeatability of the measurements was guaranteed, although no significant difference between the ASP and IFAS was found. IFAS reactor 2 had much higher N 2 O partial pressure, hence overall emission, in the off-gas. IFAS reactor 3, however had comparable emission to the same position in the ASP. The activated sludge grid following the IFAS reactors in Tank 12, when compared to the correspondent position in Tank 11, had slightly higher off-gas partial pressure. The relative ratios of the measurements is shown in Figure 12 as well. These results show the need for longer-term measurements for N 2 O in this process. Figure 12. Comparative results of N 2 O partial pressure readings for ASP and IFAS in the January 2010 off-gas test. Samples were grabbed from the off-gas stream and measured in a controlled laboratory with a ECD gas chromatograph (Shimadzu, Carlsbad, CA).
CONCLUSIONS Off-gas testing was conducted at the T.Z. Osborne Water Reclamation Facility (located in Greensboro, NC) on January and June 16, 2010, using the ASCE Standard Protocol. The test was performed to determine the comparative oxygen transfer efficiency of the activated sludge (ASP) and the integrated fixed-film activated sludge (IFAS) processes, and to confirm the winter measurement results during the warm season. The plant has 12 aeration tanks, all of which are equipped with fine-pore diffusers except for the IFAS reactor cells which are equipped with coarse bubble diffusers. The IFAS process is operated in Tank 12, side-by-side to the ASP in Tank 11. Off-gas testing of the IFAS and ASP basins was performed in order to better understand the operational cost significance of operating each technology in order to aid the City of Greensboro in selecting a process to help them meet stringent nutrient limits. The results show that in both tests the ASP had lower DO, air flux and air use than the IFAS, which in turn was characterized by lower OTE. Due to the elevated DO requirements for the IFAS, the normalized efficiency ( SOTE) was comparable between the two processes, after data normalization to standard conditions (i.e., zero DO). Nevertheless, the air used (as mass of oxygen per mass of load removed) by the IFAS is between 1.6 and 2.0 times higher than the ASP, corresponding to the same ratio for energy footprint. Nitrous oxide in the off-gas was measured in the Winter test, and at this time the data is still insufficient to support any conclusion on the two processes. REFERENCES 1. ASCE (1984, 1991, 2007) ASCE Standard: Measurement of Oxygen Transfer in Clean Water, ISBN 0-87262-430-7, New York, NY. 2. ASCE (1997) ASCE Standard: Standard Guidelines for In-Process Oxygen Transfer Testing, ISBN 0-78440-114-4, New York, NY,. 1. Harris, S.L., T. Stephenson and P. Pearce (1996) Aeration Investigation of Biological Aerated Filters Using Off-Gas Analysis, Water Science and Technology, 34, No. 3-4, pp. 307-314. 2. Maas, C.L.A., W.J. Parker, R.L. Legge (2008). Oxygen Uptake Rate Tests to Evaluate Integrated Fixed Film Activated Sludge Processes, Water Environment Research, 80, No. 12, pp. 2276-2283. 3. Plano, S., Rosso, D., Benedetti, L., Weijers, S., De Jonge, J., Nopens, I. (2010) Towards dynamic activated sludge modelling without need for calibration, Proc. IWA World Water Congress, Montreal, QC, Canada. 4. Redmon, D.T., Boyle, W.C., and L. Ewing. L.(1983). Oxygen Transfer Efficiency Measurements in Mixed Liquor Using Off-gas Techniques, JWPCF, 55, pp.1338. 5. Rosso, D., R. Iranpour and M.K. Stenstrom (2005). Fifteen Years of Off-Gas Transfer Efficiency Measurements on Fine Pore Aerators: Key Role of Sludge Age and Normalized Air Flux, Water Environment Research, 77, pp 266-273, 2005. 6. Rosso, D., and Stenstrom, M.K. (2005) Comparative Economic Analysis of the Impacts of Mean Cell Retention Time and Denitrification on Aeration Systems, Wat. Res. 39, 3773-3780.
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