Effects of Inclusion of Modified Mixing Devices on Effluent Quality in Aerated Lagoons: Case Study at Wingate, IN WWTP

Transcription

1 Effects of Inclusion of Modified Mixing Devices on Effluent Quality in Aerated Lagoons: Case Study at Wingate, IN WWTP Ernest R. Blatchley III, Ph.D., P.E, BCEE Professor, School of Civil Engineering and Division of Environmental & Ecological Engineering Purdue University West Lafayette, IN 4797 INTRODUCTION Lagoons are commonly used for treatment of municipal wastewater in small, rural communities. The motivations for their use in these settings include low operation and maintenance costs, as well as availability of inexpensive land. Aerated lagoons are used as an alternative to other lagoon systems (e.g., facultative lagoons). Aerated lagoons typically include mechanical devices to promote mixing and O 2 transfer, thereby facilitating biochemical oxidation of reduced substrates. Aerated lagoons are relatively simple to operate and accomplish effective removal of suspended solids (TSS) and carbonaceous biochemical oxygen demand (CBOD); however, control of reduced nitrogen, including ammonia-n, can be problematic in lagoon systems, especially during periods of prolonged cold weather. This is believed to be attributable to the relatively slow growth rates that are typical of nitrifying bacteria, as well as their relative intolerance of cold conditions. On the other hand, some success in accomplishing nitrification in aerated lagoon systems has been reported in cold regions among systems where attached growth is promoted. For example, Richard and Hutchins (1995) reported results of a study in which a ringlace medium was included in an aerated lagoon in Winter Park, CO, resulting in significant increases in nitrification rate (as indicated by an increase in the concentration of nitrate-n in the effluent), even under conditions where the water temperature was just above freezing. They attributed this behavior to an increase in total system biomass, which was presumed to include the community of nitrifying bacteria. Promotion of attached growth in their system also yielded reductions in effluent TSS and BOD. In an aerated lagoon system, several possible fates of substrates (including N) can be identified, including: 1. Uptake by the microbial community for incorporation into new cells 2. Incorporation into settled solids 3. Liquid gas transfer 4. Biochemical oxidation (or reduction in the sludge bed) 5. Effluent discharge. To varying degrees, all of these fates can be influenced through process design and control. For example, consider the basic dynamics of liquid gas transfer, as described by the two-film theory. Under this model, the rate of transport between the two phases is described as follows: ( ) (1) where:

2 F = net flux of compound between phases = net mass transport rate of compound between phases, per unit interfacial area K l = overall mass transfer coefficient, based on liquid-phase concentration C eq = liquid-phase concentration that is in equilibrium with (bulk) gas phase C = actual liquid-phase concentration. When C = C eq, the system is at equilibrium and no net transport will be observed. When C < C eq, net transport will be from gas liquid phase. When C > C eq, the opposite will be true (i.e., net transport will be from liquid gas phase). In general, the difference between the equilibrium and actual conditions is used to represent the driving force for transport between the two phases in contact. Any change to the system that affects one or more terms in this equation can be expected to also affect the net rate of transport between the gas and liquid phases. For example, the inclusion of mechanical mixing (normally applied to the liquid phase) is known to decrease resistance to transport on the liquid side of the gas:liquid interface. For volatile compounds, this can lead to a substantial increase in the overall mass transfer coefficient. In addition, some mixing devices can increase the gas:liquid interfacial area, thereby promoting mass transfer. Independent of mechanical mixing, it is also possible to influence the rate of mass transport by changing system chemistry, so as to alter the equilibrium condition. For example, ammonia-n is known to participate in a simple acid-base reaction: Like all acid-base reactions, equilibrium conditions for this reaction are established essentially instantaneously, and are governed by ph. The equilibrium condition for this reaction determines the fraction of ammonia-n that is present as NH 3, as well as the fraction that is present as NH 4 +. The equilibrium for this acid-base reaction is defined as follows: (2) [ ][ ] [ ] (3) At T = 2 C, the acid-dissociation constant for this reaction has a value of (Stumm and Morgan, 1996). Therefore, because NH 3 is volatile and NH 4 + is not, knowledge of equilibrium for this reaction provides information about the distribution of ammonia-n, defined as: [ ] [ ] (4) that is present in the volatile form (NH 3 ) and the non-volatile form (NH 4 + ). Figure 1 illustrates this equilibrium distribution. From this illustration, it is evident that as ph increases to approach the pk a of equation (3), we should expect the efficiency of removal of ammonia-n from water to increase, simply because a larger fraction of the ammonia-n will be present in the volatile form, thereby increasing the driving force for liquid gas transfer.

3 1. [NH 3 ] [NH 4 + ].8.6 [i]/c T,N Figure 1. Equilibrium distribution of ammonia-n (C T,N ) as a function of ph at T = 2 C. For ph values below 9.3, the majority of ammonia-n will be present as NH 4 +. ph Temperature can influence the rate of virtually any physico/chemical or biochemical process. Specifically, the rate constants and equilibrium constants of reaction and transport processes typically demonstrate temperature dependence. Therefore, seasonal changes in temperature can be expected to influence many aspects of the behavior of wastewater treatment systems, which typically depend on a combination of physico/chemical and biochemical processes. In a general sense, biochemical nitrification will proceed when conditions are favorable for growth of nitrifying bacteria. Because these organisms are relatively slow-growing, they typically require long (cell) detention times in the system (Metcalf and Eddy, 23). In addition, because nitrification can result in expression of substantial oxygen demand, it is necessary to provide sufficient oxygen to support this process. This usually requires an increase in oxygen transfer rate, relative to a system where biochemical nitrification does not take place. WASTEWATER TREATMENT IN WINGATE, IN The town of Wingate, IN constructed their wastewater treatment system in 1984 using funds from a construction grant. The facility, which is located roughly 1.2 miles northeast of the town of Wingate, includes a three-cell aerated lagoon that discharges treated water to Charles

4 Ludlow Ditch. The facility receives septic effluent from residential and commercial activities in Wingate. The Wingate wastewater treatment system was originally configured with two 5-HP arrow mixers in the first lagoon, with one 3-HP mixer in each of the second and third lagoons (16 HP total). In this configuration, the system accomplished acceptable treatment with respect to BOD and TSS. However, the performance of the system has been inconsistent or poor with respect to removal of ammonia-n, particularly during periods of extended cold weather. Discharge limitations on ammonia-n were included in the Wingate NPDES permit beginning in the winter of 211. Therefore, modifications to the system and/or the method of operation will be required to comply with these pending permit limits. A conventional approach to this problem involves construction of a mechanical wastewater treatment facility to replace the lagoons. Such a system can accomplish reliable treatment, such that consistent permit compliance can be accomplished. However, these systems are more complicated and expensive to operate than lagoons, and the capital costs of such a system are likely to represent an unacceptable financial burden for the community. Another option is to alter the lagoon system to improve its performance, particularly as related to removal of ammonia-n. The specific alteration that is being examined at Wingate is the inclusion of alternative mixing devices, and inclusion of media to allow for development of an attached-growth community in the lagoons. This approach is conceptually similar to the approach reported by Richard and Hutchins (1995). As described previously, such a system should allow for a substantial increase in the total biomass within the system, possibly including an increase in the population of nitrifiers. Relative to a conventional mechanical (or package ) system, this modification to the existing lagoon system has substantially lower capital costs. In addition, the basic operation of the lagoon system remains largely unchanged. To examine the effectiveness of this approach, a long-term experiment was initiated at the Wingate WWTP as a collaborative effort involving the Town of Wingate; Bradley Environmental (BE); Commonwealth Biolabs (CB); the Indiana Department of Environmental Management (IDEM); and Purdue University (School of Civil Engineering). Participation on the part of Purdue University originally involved Professor M.K. Banks. However, Professor Banks has left Purdue University and is unable to continue her participation in this project. PROJECT HISTORY The project was initiated in July 21 with installation of a single BE 1-HP pump (see Figure 2) in the third lagoon at the Wingate facility. Data collection was initiated in December 21, with analyses being performed by CB. In February 211, six additional BE 1-HP pumps were installed in the first lagoon. Soon thereafter (February 211), 1-HP enclosed biochemical reactors ( BOBBER, see Figure 3) were installed in each of lagoons 2 and 3 (one each). In October 211, the BOBBER in lagoon 3 was moved to lagoon 2, and four additional BOBBERs were installed. The 1-HP BE pumps draw water from the lagoon through an 8 port and is discharged back into the lagoon through an array of radially-oriented PVC pipes (see Figure 2). In lagoon 1, the six 1-HP BE pumps are distributed roughly uniformly across the surface of the lagoon. Lagoon 2 is now configured with six BOBBERs, which are also distributed roughly uniformly across the surface of the lagoon. For these systems, water is again drawn toward the device through a series of radially-oriented PVC pipes. However, in the BOBBER system the water is discharged into a 6 -diameter black plastic sphere that contains a medium with a high specific

5 surface area which provides extensive surface area for development of attached growth within the system. Figure 2. Schematic illustration of 1-HP BE mixing devices installed at Wingate WWTP (left); digital image of 1-HP BE surface mixing device (images provided by Bradley Environmental). Figure 3. Digital images of BE BOBBER devices (photos provided by Bradley Environmental). METHODS In addition to routine collection and analysis of samples for monthly reporting of system operation and performance, sample collection was initiated in December 21. Effluent samples from all three lagoons were collected roughly every other week from the Wingate facility and transported to the CB labs for analysis. Analyses conducted by CB labs included the following: Ammonia-N: performed by basification of samples to ph > 11 (to convert all ammonia-n to NH 3 ), followed by analysis with an ammonia-selective electrode. The voltage signal

6 from analysis of a basified sample was compared with the voltage signals that were developed from a series of standards to define the ammonia-n concentration in the sample. Nitrification rate: 1 mg (as N) NH 4 Cl was added to a 1 ml sample. The sample was then aerated for 24 hours, after which the ammonia-n concentration was measured, as described above. Media nitrification rate: Twenty randomly-selected beads of media were transferred from a BOBBER to a 1 ml solution of hard synthetic water. The assay described above was then performed to determine the rate at which ammonia-n was removed. Heterotrophic bacteria: These were quantified using a conventional plate method. Algae: Algal cells were counted under a microscope using a Sedgewick-Rafter counting cell. NO - 2 : Nitrite was quantified through formation of an azo dye produced at low ph by coupling diazotized sulfanilamide with N-(1-naphthyl)-ethylenediamine dihydrochloride (NED dihydrochloride). The concentration of the azo dye was measured spectrophotometrically by comparison with measurements from a set of standards. - NO 3 + NO - 2 : Nitrate in a sample was reduced to NO - 2 using metallic cadmium, followed by the complexation and colorimetric analysis described above. NO - 3 concentration was then estimated by subtraction of the NO - 2 signal described above. Other parameters (ph, T, BOD, TSS, DO) were measured using conventional methods. RESULTS AND DISCUSSION Microbial Quality Figure 4 provides a time-course summary of measurements of microbial quality in the Wingate WWTP. The inclusion of the mixing devices appears to have resulted in an increase in the heterotrophic bacterial community, especially in lagoons 1 and 2. This observation is consistent with the findings of Richard and Hutchins (1995). In contrast, the concentration of algal cells appears to have been reduced by inclusion of these mixing devices. The changes in algal content were reflected in measurements of algal cell counts and chlorophyll a, and were most evident in lagoons 2 and 3. Among the factors that could reduce algal content in a lagoon is mechanical mixing. Efficient mixing of a lagoon will result in destratification. Under these conditions, algal cells will be forced by the mechanical action of the mixing devices to move between the upper and lower layers of a lagoon. Penetration of visible light from the sun, which is required for photosynthetic activity by algae, is likely to be limited to the upper reaches of a lagoon. Therefore, algae will experience an environment in which photosynthesis becomes more difficult than in a stratified lagoon. In a stratified lagoon, it is possible for algae to proliferate in the upper portions of the lagoon; however, algal growth in the lower layers of a lagoon is likely to be limited by lack of sunlight. It is possible that other factors may have contributed to the changes in algal cell counts and chlorophyll a that were observed in the Wingate lagoons. However, it appears likely that mechanical destratification may have contributed to these observations. A more detailed discussion of mixing behavior in the lagoons will be presented later in this report.

7 Heterotrophic Bacteria (cfu/ml) /1/21 3/1/211 Lagoon 1 Effluent Lagoon 2 Effluent Lagoon 3 Effluent 6/1/211 9/1/211 12/1/211 3/1/212 Algal Cells (cells/ml) /1/21 3/1/211 6/1/211 9/1/211 12/1/211 3/1/ Chlorophyll a /1/21 3/1/211 6/1/211 9/1/211 12/1/211 3/1/212 Figure 4. Time-course measurements of microbial quality in effluent samples from the three lagoons at the Wingate WWTP. For each panel, the vertical dashed lines indicate the last three modifications to the system. Top panel represents measurements of heterotrophic bacteria; center panel represents algal cell counts; bottom panel illustrates measurements of chlorophyll a.

8 Alkalinity and ph These two parameters are intimately linked to each other, and to the fundamental biochemistry of the lagoons. In a broad sense, many processes will influence (carbonate) alkalinity and ph in an aerated lagoon system. However, three important processes will include oxidation of carbonaceous BOD, oxidation of nitrogenous BOD, and photosynthesis. Biochemically-mediated oxidation of carbonaceous substrates will involve a wide array of compounds. Using a simple carbohydrate as an example of a carbonaceous substrate, the role of inorganic carbon in this process can be illustrated: In this reaction, aerobic microorganisms combine a carbohydrate and oxygen to yield CO 2 and H 2 O as a means of gaining access to chemical energy. The expression of NBOD involves a community of microbes that participate in a symbiotic process to oxidize ammonia-n to nitrate, with nitrite as an intermediate: (5) (6) (7) The Nitroso bacteria may include species such as Nitrosomonas or Nitrosococcus, while the Nitro bacteria that participate in this process may include Nitrobacter or Nitrospira (Metcalf and Eddy, 23). In addition to oxidation of reduced substrates, both of these processes also result in consumption of alkalinity, either through production of CO 2 (which functions as an acid) or through the direct production of H +. In many respects, photosynthesis opposes these oxidation processes, or works to complete the elemental cycles of carbon, oxygen, and nitrogen. The following expression is representative of the stoichiometry of photosynthesis: In this process, energy in the form of visible radiation (usually from the sun) will drive the photosynthetic process to yield carbohydrates and molecular oxygen as products. In addition, inorganic carbon in the form of CO 2 is consumed in this process, thereby reducing the acidity of the solution. Given the complexity of the microbial community and the soluble substrates in a system such as an aerated lagoon, it is likely that other processes will influence alkalinity and ph. However, the processes listed above (and their analogs) are likely to be important contributors to the overall behavior of alkalinity and ph. Therefore, changes in the lagoon environment that alter the microbial population, particularly as related to the organisms that are responsible for BOD expression and photosynthesis, can be expected to influence lagoon alkalinity and ph. Figure 5 illustrates the time-course behavior of alkalinity in the Wingate lagoons. In the 12-month period preceding the completion of the modifications to the lagoons, a cycle of alkalinity was evident, whereby alkalinity was generally lowest in mid-summer, and highest in fall and winter. Inclusion of the entire mixing system at Wingate appears to have resulted in a (8) (9)

9 decrease in the seasonal fluctuation of alkalinity across the lagoons, relative to the preceding year Alkalinity (mg/l as CaCO 3 ) Lagoon 1 Effluent Lagoon 2 Effluent Lagoon 3 Effluent 12/1/21 3/1/211 6/1/211 9/1/211 12/1/211 3/1/212 Figure 5. Time-course measurements of alkalinity in effluent samples from the Wingate WWTP lagoons. Figure 6 provides an illustration of influent and effluent ph as a function of time (top panel), as well as illustrations of the difference between influent and effluent ph ( ph) across the system. Effluent ph was higher than influent ph for the entire monitoring period. If this interpreted in terms of the processes of biochemical oxidation and photosynthesis, these results imply that photosynthetic activity has a greater effect on ph than expression of BOD. As described above, the inclusion of the modified mixing systems has led to a reduction in the concentration of algal cells, while the concentration of heterotrophic bacteria appears to have increased. The increase in biomass also has been accompanied by a decrease in effluent BOD and ammonia-n concentration (to be discussed later). These changes would be expected to yield a decrease in CO 2 uptake by photosynthesis, along with an increase in CO 2 and H + production resulting from CBOD and NBOD expression. Collectively, these changes would be expected to yield a decrease in effluent ph along with a smaller value of ph. Both of these changes are

10 evident in the pattern of data illustrated in Figure 4, particularly for the period since October 211. However, it is important to recognize that the full configuration of the lagoons with all mixers operating has only been in place for roughly 6 months, and as such it is not possible to define the behavior of this system in terms of an annual cycle ph Influent Effluent 7. 1/1/21 4/1/21 7/1/21 1/1/21 1/1/211 4/1/211 7/1/211 1/1/211 1/1/212 4/1/212 ph /1/21 4/1/21 7/1/21 1/1/21 1/1/211 4/1/211 7/1/211 1/1/211 1/1/212 4/1/212 Figure 6. Time-course measurements of influent and effluent ph at the Wingate WWTP (top panel). Bottom panel illustrates difference between influent and effluent ph ( ph) as a function of time.

11 The behavior of the bacteria that are responsible for nitrification is known to be related to ph. Specifically, ph is known to influence nitrifier activity via changes in the form and availability of inorganic carbon, activation or deactivation of nitrifying bacteria, and inhibition by formation of NH 3 or HNO 2. Villaverde et al. (1997) examined nitrifier activity in an attached-growth system and found that the optimum ph for ammonia-oxidizing bacteria was near ph = 8.2 (see Figure 7, left). This observation was consistent with earlier findings of Alleman (1984). Villaverde et al. (1997) also observed that free ammonia (NH 3 ) inhibits the activity of nitrite-oxidizing bacteria (see Figure 7, right). Figure 7. Observations of the effect of ph on activity of nitrifying bacteria (from Villaverde et al., 1997). Left panel illustrates activity of Nitrosomonas spp. as a function of ph. Right panel illustrates accumulation of NH 3 -N as a function of ph (left vertical axis) as well as accumulation of NO 2 - -N as a function of ph (right vertical axis). Nitrogen A primary objective of this project was to examine the ability of the process modifications to improve removal of ammonia-n. Figure 8 illustrates influent and effluent ammonia-n as a function of time. The inclusion of the complete set of mixing devices, which was completed in October of 211, appears to have resulted in improved removal of ammonia-n form the lagoons in winter.

12 6 5 Influent NH 3 -N Effluent NH 3 -N Air Temp Water Temp 3 2 NH 3 -N (mg/l) Temperature ( o C) 1 1/1/21 4/1/21 7/1/21 1/1/21 1/1/211 4/1/211 7/1/211 1/1/211 1/1/212 4/1/212-1 NH 3 -N (mg/l) /1/21 4/1/21 7/1/21 1/1/21 1/1/211 4/1/211 7/1/211 1/1/211 1/1/212 4/1/212 Figure 8. Influent and effluent ammonia-n (left vertical axis) at the Wingate WWTP as a function of time (top panel). Superimposed on the top panel are records of air and water temperature at the plant (right vertical axis). Bottom panel illustrates the difference between influent and effluent ammonia-n ( NH 3 -N) as a function of time.

13 The results of these measurements are in qualitative agreement with the report of Richards and Hutchins (1995), in that promotion of attached-growth and an overall increase in biomass within the system appears to have yielded improvement in removal of ammonia-n from the system. Also included in Figure 8 (top panel) are measurements of air and water temperature at the Wingate facility. These measurements are included in this graph because the behavior of nitrifying bacteria is known to be adversely affected by cold temperature. The bottom panel of Figure 8 illustrates the change in ammonia-n concentration ( NH 3 -N) as a function of time. There is considerable variability in this signal, but a clear seasonal pattern is evident, whereby removal of ammonia-n diminished during the winter months. This pattern generally holds across the entire data set, but the reduction in ammonia-n removal was less pronounced in winter than in previous years. It is important to recognize that the winter of was unusually mild in central Indiana, in terms of air temperature. On the other hand, water temperature at the Wingate facility during the winter of was similar to water temperature in the preceding winter season, yet removal ammonia was improved in winter relative to previous years. One other issue to consider regarding the temperature signals is heat transfer. The physics of heat transfer are similar to those of mass or momentum transfer. Systems that increase mass transfer (e.g., by improved mixing) are likely to increase heat (and momentum) transfer. In a general sense, the dynamics of heat transfer between air and (liquid) water can be described mathematically by a relationship of the following form: ( ) (1) where, F H = flux of heat between air and water = rate of heat transfer from air to water per unit air:water interfacial area K H = overall heat transfer coefficient T air = air temperature T water = water temperature. In general, the rate of heat transfer between phases will be determined by the product of the interfacial contact area, the heat transfer coefficient, and the difference between air and water temperatures. The mixing systems included at the Wingate facility almost certainly increased the interfacial contact area between air and water, as well as the heat transfer coefficient (because of improved mixing). Interestingly, water temperature during winter was similar to the water temperature during winter , despite the fact that air temperatures during winter were substantially lower. In other words, the driving force for heat transfer ( T) was smaller in winter This suggests that heat transfer was improved by the new mixing devices. If this is true, then it is possible that water temperature could be substantially reduced by the system during a period of prolonged cold weather, as is common in central Indiana winters. It is not clear how this may affect performance of the system with respect to nitrification (or other aspects of treatment), but this is an issue that should be monitored in the future. Figure 9 illustrates the time-course behavior of ammonia-n (top), nitrite (center), and nitrate in effluent samples from the three lagoons at Wingate. Ammonia-N was removed in all

14 three lagoons. As described above, inclusion of the full set of mixing devices resulted in improved ammonia-n removal, particularly in the winter months. Similarly, these changes appear to have improved removal of nitrite, including during the winter months. NH 3 -N (mg/l) Lagoon 1 Effluent Lagoon 2 Effluent Lagoon 3 Effluent 5 12/1/21 3/1/211 6/1/211 9/1/211 12/1/211 3/1/ NO 2 - -N (mg/l) /1/21 3/1/211 6/1/211 9/1/211 12/1/211 3/1/ NO 3 - -N (mg/l) /1/21 3/1/211 6/1/211 9/1/211 12/1/211 3/1/212 Figure 9. Time-course measurements of effluent ammonia-n (top), NO 2 - -N (center), and NO N (bottom) at the Wingate WWTP.

15 The nitrate-n signal indicates that NO 3 - concentrations in all three lagoons are higher than they were prior to introduction of the mixing devices. This is consistent with promotion of biochemical nitrification within the lagoons. The pattern of the NO 3 - signal is such that the concentration consistently decreases as water moves through the facility. This may be an indication of denitrification activity within the lagoons. This pattern of behavior appears to be somewhat more regular after October 211 than before this date Influent Effluent 12 CBOD (mg/l) /1/21 4/1/21 7/1/21 1/1/21 1/1/211 4/1/211 7/1/211 1/1/211 1/1/212 4/1/212 CBOD (mg/l) /1/21 4/1/21 7/1/21 1/1/21 1/1/211 4/1/211 7/1/211 1/1/211 1/1/212 4/1/212 Figure 1. Time-course record of influent and effluent CBOD (top) and change in CBOD ( CBOD, bottom) at the Wingate WWTP.

16 CBOD - Figure 1 illustrates the behavior of CBOD at the Wingate WWTP. In general, effluent CBOD has consistently been below 2 mg/l, and the performance of the Wingate facility with respect to CBOD removal or control was not substantially affected by inclusion of the process modifications. Figure 11 illustrates the total BOD signal at the Wingate facility. As compared with the CBOD signal described above, there is an obvious improvement in TBOD as a result of inclusion of the full set of modifications. This is consistent with the improvements in nitrification described above. Substantial variations in the TBOD signal are evident in lagoon 1. In absolute terms, these variations are dampened as water moves through the system Lagoon 1 Effluent Lagoon 2 Effluent Lagoon 3 Effluent TBOD (mg/l) /1/21 3/1/211 6/1/211 9/1/211 12/1/211 3/1/212 Figure 11. Time-course record of TBOD in all three lagoons at the Wingate WWTP. Particles - Figure 12 illustrates behavior of total suspended solids (TSS) in the influent and effluent of the Wingate facility (top), as well as changes in TSS across the facility ( TSS) during the monitoring period. The inclusion of the full set of modifications appears to have yielded an improvement in effluent TSS, in that there appears to be a slight downward trend in effluent TSS since October 211. However, the TSS signal does not appear to have changed markedly since October 211. It is not entirely clear why this is so. The influent TSS signal was

17 quite variable in samples collected after October 211, but within this variable signal there appears to be a slight downward trend in influent TSS. With the relatively long residence time that characterizes the Wingate lagoons, it is reasonable to expect some dampening of the TSS signal by simple equalization. It is difficult to conclude from this data set that any significant improvement in TSS removal can be ascribed to the process modifications. 1 8 Influent Effluent TSS (mg/l) /1/21 4/1/21 7/1/21 1/1/21 1/1/211 4/1/211 7/1/211 1/1/211 1/1/212 4/1/212 1 TSS (mg/l) /1/21 4/1/21 7/1/21 1/1/21 1/1/211 4/1/211 7/1/211 1/1/211 1/1/212 4/1/212 Figure 12. Time-course record of influent and effluent TSS at the Wingate WWTP (top) and changes in TSS ( TSS) across the Wingate facility (bottom). Figure 13 provides a more comprehensive summary of the behavior of suspended particles at the Wingate facility. The data presented in Figure 13, in which suspended particles are characterized by measures of TSS (top panel), settleable solids (center panel), and turbidity (bottom panel), indicate improved particle removal as a result of inclusion of the process modifications. These observations are consistent with those reported by Richard and Hutchins (1995).

18 TSS (mg/l) Lagoon 1 Effluent Lagoon 2 Effluent Lagoon 3 Effluent 1 12/1/21 3/1/211 6/1/211 9/1/211 12/1/211 3/1/212 6 Settleable Solids (mg/l) /1/21 3/1/211 6/1/211 9/1/211 12/1/211 3/1/ Turbidity (NTU) /1/21 3/1/211 6/1/211 9/1/211 12/1/211 3/1/212 Figure 13. Time-course record of effluent particle concentrations from the three lagoons at the Wingate WWTP, as indicated by TSS (top panel), settleable solids (center), and turbidity (bottom).

19 Sludge Blanket Depth - Collectively, the improvements in NBOD removal and suspended solids removal imply that sludge production within the Wingate facility should increase as a result of inclusion of the process modifications. Figure 14 provides a summary of sludge depth measurements that have been performed periodically at the Wingate WWTP roughly once per month, beginning in May 211. No obvious trend of increasing sludge depth is evident from these measurements. Therefore, if changes in sludge accumulation in the Wingate facility do result from the process changes, it appears that these will be evident on a longer timescale than is illustrated in Figure 14.

20 Sludge Depth (inches) Location A Location B Location C Lagoon 1 5/1/211 7/1/211 9/1/211 11/1/211 1/1/212 3/1/212 Sludge Depth (inches) Location A Location B Location C Lagoon 2 5/1/211 7/1/211 9/1/211 11/1/211 1/1/212 3/1/212 Sludge Depth (inches) Location A Location B Location C Lagoon 3 5/1/211 7/1/211 9/1/211 11/1/211 1/1/212 3/1/212 Figure 14. Time-course measurements of sludge depth in the lagoons at the Wingate WWTP: lagoon 1 (top), lagoon 2 (center), lagoon 3 (bottom).

21 Electrical Power Consumption - Figure 15 provides a graphical illustration of (daily) electrical power consumption at the Wingate facility. Since the inclusion of the process modifications, starting in February/March 211, the overall pattern of electrical power usage has trended downward, but within this data set a pattern of seasonal variation in power consumption is evident. Specifically, daily electrical power usage from April-October appears to be consistently lower than during the period from October- April. To date, the data regarding electrical power consumption suggest that the new system has lower electrical power requirements than the original configuration. This is consistent with the fact that the nominal overall power rating of the new configuration is lower than the original configuration. To be sure of this trend, it would be beneficial to continue to monitor electrical power usage at the Wingate facility. 5 Electrical Power Usage (kw-hr/day) Daily Power Usage 12/1/29 4/1/21 8/1/21 12/1/21 4/1/211 8/1/211 12/1/211 Figure 15. Daily electrical power consumption at the Wingate WWTP. Mixing Behavior - Profiles of dissolved oxygen and water temperature were measured intermittently (roughly once per month), beginning in September 21. For most of these sampling dates, measurements were taken at three locations in each lagoon, which were roughly equally spaced across a cross-section of the lagoon (see Figure 16). At each location, measurements were collected at the surface, mid-depth, and just above the sludge layer.

22 Therefore, on most sampling dates, nine measurements of DO and temperature were collected in each lagoon. Lagoon#1 C B NORTH A Lagoon # 2 Lagoon # 3 C C B A A B Figure 16. Schematic illustration of sampling locations for lagoon profiling measurements. At each location, samples were collected from the surface, roughly 5 feet below the surface, and at the top of the sludge layer.

23 16 Dissolved Oxygen Concentration (mg/l) /1/21 12/1/21 3/1/211 6/1/211 9/1/211 12/1/211 3/1/212 Lagoon 1 Lagoon 2 Lagoon 3 Water Temperature ( o C) 3 Lagoon 1 Lagoon 2 Lagoon /1/21 12/1/21 3/1/211 6/1/211 9/1/211 12/1/211 3/1/212 Figure 17. Results from lagoon profiling measurements. Top panel illustrates dissolved oxygen concentration measurements, while bottom panel illustrates water temperature measurements. Symbols represent the mean of all measurements (n=9 for most dates), while error bars represent one standard deviation for this same set of measurements.

24 Figure 17 illustrates the results of the DO and temperature profiling measurements. For each entry in these graphs, the point represents the mean of the population of measurements for a given lagoon or a given sampling date, while the error bar represents that standard deviation of that same population of measurements. For most sampling dates, DO and temperature measurements were recorded at three locations across each lagoon, and at three depths at each location. Therefore, for most sampling dates, nine measurements of DO and temperature were collected in each lagoon. The DO and temperature data can be used to examine mixing behavior within the lagoons. In a general sense, a well-mixed system is one in which no substantial gradients in composition are evident within the system. This will yield a system where system composition, as defined by constituent concentrations, temperature, etc, show no spatial gradients. In other words, at any point in time, the composition within each well-mixed cell should be the same everywhere. These conditions will be met when the processes that are responsible for mixing within a system are able to move constituents around the system at a rate that is substantially faster than the rate(s) of processes that affect local concentration. The DO measurements (Figure 17, top panel) indicate some spatial variability within the lagoons prior to October 211, when the current configuration was completed. This variation is evident in the magnitude of the standard deviation of measurements on a given date. In theory, the standard deviation of these measurements should be zero in a well-mixed system. Since that time, the magnitude of variations in the DO have decreased markedly, thereby suggesting that mixing behavior has improved with respect to DO. The temperature measurements suggest that little or no thermal stratification is evident within the lagoons. This condition existed before inclusion of the BE mixing devices; therefore, there was no real opportunity for change in this behavior. Well-mixed conditions tend to reduce the likelihood of short-circuiting, although the locations of inlet and outlets relatively to each other can also influence this behavior. Wellmixed systems also are effective for dampening the effects of changes in flow rate or influent composition; in other words, a well-mixed system will equalize flow characteristics, thereby yielding effluent quality that tends to be relatively consistent. Lastly, well-mixed conditions can simplify the analysis of the behavior of a system. It is important to recognize that the measurements of DO and temperature that are illustrated in Figure 17 are only for DO and temperature within the cross-sections of the lagoon that are illustrated in Figure 16. It is possible, though unlikely, that quantifiable gradients in DO or temperature may be evident at other locations in the system. It is also possible that quantifiable gradients in these (or other) parameters may be evident in other parts of the system, such as within the BOBBERs. Mass-transfer behavior and characteristics within the BOBBER systems appear to be largely undefined at this time. The changes in mixing behavior that are evident in the DO profiles appear to be related to the inclusion of the BE mixing devices. These changes took place despite the fact that overall power applied for mixing was reduced. In lagoon 1, overall power was reduced from nominally 1 HP (in the form of two 5-HP arrow mixers) to nominally 6 HP (six 1-HP BE mixers). In lagoon 2, power was increased from 3 HP to 6 HP (six BOBBERS). In lagoon 3, power was decreased from 3 HP to 1 HP (a single BE mixer). With respect to lagoons 1 and 2, it is important to consider not only the nominal power rating of the mixing devices, but also the distribution of this power.

25 In lagoon 1, the original configuration involved two arrow mixers, both located near the center of the lagoon, pointing in opposing directions. The new configuration involves six surface aerators, which are distributed roughly uniformly across the lagoon (see Figure 18, left). Similarly, the six BOBBERs in lagoon 2 are roughly uniformly distributed (see Figure 18, right). This more uniform distribution of mixing energy, as opposed to the original configuration, probably results in improved mixing in the lagoons. Figure 18. Digital images of BE mixing devices in Lagoon 1 (left) and Lagoon 2 (right) at the Wingate WWTP. SUMMARY AND CONCLUSIONS The results of sampling and analysis at the Wingate WWTP to date indicate that the inclusion of the BE mixing devices has yielded improvements in effluent quality with respect to suspended particles and ammonia-n. These improvements appear to have resulted from the inclusion of an alternative mixing regime within the system, as well as the inclusion of the BOBBER systems, which promote attached growth. Overall, these systems appear to be using less electrical power than the previous system. The new configuration appears to provide for more efficient and more complete mixing than was accomplished with the previous configuration. This should promote oxygen transfer, and may also lead to improved stripping of volatile gases, such as NH 3. On the other hand, conditions that lead to efficient mass transfer also tend to promote efficient transfer of heat. The data from the winter of indicate the heat transfer from air to water may have been improved by the new mixing devices. It is possible that this could lead to substantial reductions in water temperatures during periods of extended cold weather. The winter of was the warmest on record in west-central Indiana. It is not clear how these systems will perform during a normal or cold winter season, given this apparent improvement in heat transfer. RECOMMENDATIONS The inclusion of the BE mixing devices appears to have yielded improvements in overall process performance, both in terms of effluent quality and electrical power usage. These improvements indicate the potential for these systems to be used in other, similar applications.

26 And given the number of lagoon-based systems that are in use in the U.S. and elsewhere, there certainly appears to be a market for this type of system. At present, there is no well-defined approach to be used in the design of systems based on this technology. On the other hand, design approaches for other, related systems are in place. Therefore, it is likely that the principles of design that have been applied in these related systems could be adapted to the BE mixers. Moreover, the extension of this technology to other facilities, and possibly other settings, represents a logical opportunity for continued collaboration among the participants in this project. In terms of applied research, several specific topics appear to merit attention. These include: Quantification of NH 3 and CBOD uptake rates by the attached-growth community in the BOBBER systems this behavior of the system is likely to be influenced by masstransfer behavior and the composition of the microbial community within the BOBBERs. By defining this behavior and the process parameters that influence this behavior, it may be possible to develop a design procedure for the BOBBER system that can be verified against field measurements. Fluid mechanics in aerated lagoons the BOBBERs and the BE mixers appear to have resulted in improved mixing behavior in the lagoons at Wingate. However, the evidence to define this behavior is incomplete. Numerical simulations (perhaps involving computational fluid dynamics) and physical tests (e.g., tracer tests to allow measurement of the residence time distribution of lagoon cells) may be beneficial as methods of validating the effects of the mixing devices. In addition, these tests may indicate opportunities for improvement of mixing behavior in lagoons. Mass and heat transfer the improved mixing in the Wingate lagoons should yield increased mass and heat transfer. With respect to mass transfer, one area of particular relevance is the ability of these systems to transfer oxygen. However, a closely-related issue is the potential to strip volatile gases, such as NH 3. Heat transfer is likely to be most important in the winter months. Detailed information about heat transfer characteristics of these systems may provide insights into the behavior of these systems in cold-weather months, as well as opportunities to improve this behavior. REFERENCES Alleman, J.E. (1984) Elevated Nitrite Occurrence in Biological Wastewater Treatment Systems, Water Science & Technology, 17, Metcalf & Eddy (23) Wastewater Engineering: Treatment and Reuse, Fourth Edition (G.T. Tchobanoglous, F.L. Burton, and H.D. Stensel), McGraw-Hill, New York. Richard, M. and Hutchins, B. (1995) Enhanced Cold Temperature Nitrification in a Municipal Aerated Lagoon Using Ringlace Fixed Film Media, Presented at the Rocky Mountain American Waterworks Association / Water Environment Association Annual Conference, Sheridan Wyoming September 11th, Stumm, W. and Morgan, J.J. (1996) Aquatic Chemistry: Chemical Equilibria and Rates in Natural Waters, Third Edition, John Wiley & Sons, New York. Villaverde, S.; Garcia-Encina, P.A., Fdz-Polanco, F. (1997) Influence of ph Over Nitrifying Biofilm Activity in Submerged Biofilters, Water Research, 31,