Oxygen transfer comparison of jets and coarse bubble aeration in concentrated sludge

The South Truckee Meadows Water Reclamation Facility (STMWRF) in Washoe County, Nevada, commissioned a biosolids facility with jet aerated aerobic digestion. The jet aerators were not performing as designed, so they were tested on‐site in the new tanks in both clean and process water according to ASCE standards. The aerators failed by substantial margins. To make up for the aeration shortfall, Washoe County elected to replace the jet aerators with coarse bubble aerators and to add the additional blower capacity necessary to meet the oxygen requirements in this configuration. After partial replacement, with one basin containing coarse bubble diffusers and the other still containing jets, the efficiency of both systems was tested in process water. The jets and coarse bubble diffusers had similar results for OTE and αSOTE, whereas the coarse bubble diffusers had substantially higher aeration efficiency after accounting for the jet pump power draw. Overall, the project demonstrated the deleterious effects of highly concentrated non‐Newtonian sludge on the coalescence of fine bubbles and ultimately jet aeration efficiency, confirming the incompatibility of fine bubbles and thick sludge. Our results can be extended to other bioreactors operating at MLSS concentration above 1%.


INTRODUCTION
Although the trend in the wastewater treatment industry is moving toward energy recovery with the implementation of anaerobic digestion and biogas use, there are still hundreds of facilities across the United States that rely on aerobic digestion. Aerobic digestion relies on the effective transfer of oxygen to the process fluid, which is typically thickened waste activated sludge (TWAS), to reduce reactor volume until the biomass is sufficiently stabilized. Despite the relative abundance of aerobic digesters in service, there are limited data available on field oxygen transfer rates specific to aerobic digesters. Some refer to efficiency data collected in field tests by Krampe and Krauth (2003) on fine bubble diffusers in membrane bioreactors with high MLSS concentrations to guide aerobic digester aeration system design. However, data specific to aerobic digesters or specific to other types of oxygen transfer systems like coarse bubble (CB) diffusers or jet aeration (JA) in thickened sludge systems are absent from the published literature. As a result, aerobic digesters are commonly designed by "rules of thumb" or with input from aeration equipment manufacturers.
Fine bubbles, regardless of their origin, travel upward in water once they reach their terminal velocity. If they originate from jets or turbines, the fine bubbles initially travel faster than the terminal velocity and reach their terminal velocity once the initial additional energy is dissipated through hydrodynamic resistance. In the case of fine bubbles generated from fine-pore diffusers, the bubbles travel with initial velocity that may be higher or lower than the terminal velocity depending on the pressure in the air line, and then once the terminal velocity is reached, the bubbles travel upward at the velocity determined by the liquid viscosity and the bubble size. In the case of air lines at a pressure substantially higher than the hydrostatic pressure outside the fine-pore diffuser, the pressure differential between the air line and the surrounding water, minus the diffuser pressure drop (dynamic wet pressure [DWP]), is converted into initial bubble velocity through Bernoulli's equivalence because the pressure inside the bubble must equal that of the liquid outside the bubble due to Boyle's law.
The non-Newtonian nature of sludge suspensions appears more and more evident once the sludge concentration is increased. In their analysis of membrane bioreactors, Rosenberger et al. (2002) reported that the MLSS threshold below which MBR sludge apparent viscosity appeared independent of shear stress was 1% of MLSS, and they used the same shear rates representative of aeration tanks ($40 s À1 ) as in Wagner et al. (2002). Once the sludge is sufficiently thick, the bubbles' rising velocity field is subject to the shear-thinning nature of the concentrated sludge, which dictates that lower viscosity is achieved with higher shear. Because the interfacial shear between gas and liquid is proportional to the bubble rising velocity (Dur an et al., 2016), coarser bubbles will rise ever faster by inducing lower interfacial viscosity as an effect of their velocity. This inseparable link explains why fine bubbles in thick sludge must necessarily coalesce into coarser bubbles to experience reduced shear during their faster rise. Thus, any fine bubble, regardless of its origin, will have to coalesce to follow the path of least resistance (Baquero Rodriguez et al., 2017). This supports the hypothesis that at some point, the concentration of sludge would be sufficiently high to promote bubble coalescence and de facto nullify the difference between fine bubbles and coarse bubbles.
The oxygen transfer efficiency (OTE, %) is the ratio of the oxygen transferred to water (OTR, kg O2 h À1 ) and the oxygen fed in the air line (W O2 , kg O2 h À1 ): The volumetric oxygen transfer rate is proportional to the oxygen transfer coefficient k L a (h À1 ): where DO(t) = dissolved oxygen concentration at time t (g m À3 ) DO sat = dissolved oxygen concentration at saturation (g m À3 ) V = water liquid volume (m 3 ) The volumetric oxygen transfer coefficient is the product of the oxygen transfer coefficient k L (m h À1 ) and the specific area a (m 2 m À3 ). The mass transfer coefficient k L is proportional to the interfacial renewal or turbulence (Danckwerts, 1951;Higbie, 1935), whereas the mass transfer area a is the ratio of the bubble surface and the liquid volume, accounting for the gas holdup ϕ (dimensionless): In Equation (3), d B is the bubble diameter (m). After bubbles coalesce, they accelerate, and the resulting increase in terminal velocity is associated with an increase in the gas transfer coefficient as quantified by the similitude in Frössling's equation (1938): where the Sh is the Sherwood number (k L a = gas transfer coefficient; d B = bubble diameter; D = diffusivity), Pe is the Péclèt number (u B = bubble terminal velocity), and b 1 ,b 2 are fitting coefficients. The gas transfer coefficient is in the numerator of Sh, and the bubble velocity is in the numerator of Pe. The range value of b 2 is 1/3 to 1/2 for contaminated and clean water, respectively (Rosso & Stenstrom, 2006). Consequently, the increase in velocity of coalesced bubbles corresponds to an increase in their gas transfer coefficient k L , albeit less than linearly. Whether the fine bubbles originate from jets, turbines, or fine pores, the specific area decreases substantially once they coalesce in thick sludge because the mass transfer area drops more in proportion with the increase in bubble diameters. This paper presents full-scale clean water and process water oxygen transfer efficiency testing for both coarse bubble diffusers and jet aerators in an aerobic digester.
The tests were performed in the same tank under the same sludge conditions and thus provide a valid oxygen transfer efficiency comparison between the two technologies. With these results, we demonstrate the incompatibility between fine bubbles and thick sludge due to the non-Newtonian shear-thinning nature of the sludge.

Facility and process description
The South Truckee Meadows Water Reclamation Facility (STMWRF) is located in Reno, NV, and is owned and operated by Washoe County. The liquid treatment at the facility consists of headworks, two oxidation ditches, secondary clarifiers, tertiary sand filters, and chlorine disinfection. In the past, the solids generated at the facility were pumped into the collection system of the nearby Truckee Meadows Water Reclamation Facility (TMWRF), and no solids were processed in situ.
On-site solids processing began with the February 2016 start-up of a jet aerated aerobic digester. The JA system was designed to destroy 2960 kg VS d À1 (6510 lb VS d À1 ) at an assumed oxygen demand of 2 g O2 /g VS with 75% of the time assumed to be aerobic. The aeration system was designed with anoxic periods to reduce the mass of total nitrogen returned to the influent, but these unaerated periods also increased the required OTR of the system by 33%. The design point for volatile solids destruction (VSD) was established at 270 deg-day, originally a 15-day SRT at 18 C, and corresponded to an expected volatile solids destruction (VSD) of 35%. The jet manufacturer recommended the alpha factor (design value = 0.80) and claimed a standard oxygen transfer efficiency in clean water (SOTE) of 25.4% in 5.9 m (19.5 ft) deep basins (corresponding to a specific efficiency of 4.30% m À1 or 1.41% ft À1 of submergence). This resulted in a jet system in each digester cell sized for 1360 Nm 3 h À1 or 800 SCFM. Each jet manifold discharged going through eight nozzles. The blowers were conservatively oversized/designed to produce 1530 Nm 3 h À1 or 900 SCFM. A summary of the process design is provided in Table 1.

Project chronology
During process selection, a two-cell aerobic digester was chosen for in situ solids processing. The facility design included a rotary drum thickener to thicken the digester contents either by WAS pre-thickening or recuperative thickening and dewatering screw presses for digestate. The solids facility began operating in February of 2016 ( Figure 1). Operation of the JA system began as the solids were fed to the facility. Once the digester cells reached their design depth and a concentration of 1.6% solids, it was apparent that the design dissolved oxygen concentration of 2.0 mg L À1 was unattainable. Dissolved oxygen T A B L E 1 Process design summary. readouts during this time period were generally between 0.1 and 0.3 mg L À1 , which is below the range the optical dissolved oxygen probes provide reliable measurements. Subsequent tests to determine the facility's volatile solids reduction (VSR) indicated that the VSR was near zero. The facility management recognized that the lack of VSR was almost certainly due to the aeration system not achieving the design oxygen transfer rate. Digester commissioning continued with TS concentration not exceeding 1.2%. Commission testing for the digesters was periodically disrupted to intentionally operate at 2% TS so that the rotary drum thickener and dewatering screw presses could be tested at their operating targets. The utility approached the aeration system manufacturer about the underperformance in summer 2016. The manufacturer initially imputed the underperformance to an unidentified factor, such as the presence of surfactants inhibiting oxygen transfer in the liquid stream. The utility requested that the manufacturer perform a full-scale clean water test in one of the new digester tanks, because the customary shop test in clean water had not been performed prior to equipment installation. The utility retained an ex parte expert to witness the clean water test and perform the off-gas test of the JA system in the adjacent digester tank. The manufacturer retained their ex parte witness.

Oxygen transfer testing
Oxygen transfer in clean water was measured for several airflow rates according to standard protocol procedures (at the time of testing ASCE, 2006;now ASCE, 2022). The same tank was tested in process water (i.e., sludge) using the off-gas technique according to the ASCE testing protocol (at the time of testing ASCE, 1997;now ASCE, 2018). Results are reported as SOTE (%) for clean water and αSOTE (%) for process water. Analogously, the oxygen transfer rates are SOTR (kg O2 /h) and αSOTR (kg O2 /h). The alpha factor can be calculated as the ratio of either oxygen transfer efficiencies or rates in process and clean water, respectively: To perform the clean water test, one of the tanks was cleaned, and an apparatus of pulleys and steel cables was constructed in situ to spatially distribute the six DO sensors (YSI model 58 connected to electrochemical sensors with gas-permeable membranes) throughout the water volume (two sensors for each of these depths: 1.5 m; 3.0 m; 4.5 m). The sensors were field calibrated at the beginning of each of the to 2 days of testing. Figure 2 illustrates the DO sensors layout. The cobalt chloride (CAS # 7791-13-1) and sodium sulfite (CAS # 7757-83-7) necessary for scavenging the oxygen in the clean water tests were mixed in a stock of water above the top deck in a plastic container approximately 100 L in capacity. The concentrated liquid stock was then injected into the jet manifold intake with a pump connected to a hose (connection intentionally made for this test). The jet pump and manifold were used to evenly spread the chemicals throughout the water volume. Samples were collected to verify the cleanliness of the tank and so rule out contamination of the clean water. Total organic carbon (TOC) was measured ex situ in triplicates, and the TOC value was 0.73 +/À 0.16 mg L À1 (AVG +/À STDEV). A total of five tests were performed: three at the medium airflow (1530 Nm 3 h À1 or 900 SCFM); one at the minimum airflow (1360 Nm 3 h À1 or 800 SCFM); one at the maximum airflow (1820 Nm 3 h À1 or 1070 SCFM). The three tests at the medium airflow were within 2.3% error, which was considered satisfactory. Because the results were considered repeatable, only one replication was carried out at the minimum and maximum airflows.
All tests were valid given that all six DO probes functioned correctly until the end of the test.
For the process water test, the tank headspace acted as off-gas hood (Figure 3). The vent where the gas escaped the tank top was connected to a flexible duct, and the off-F I G U R E 3 Illustration (section view) of the gas collection setup in the west digester tank (Digester No. 2). The tank top, acting as off-gas hood, is highlighted in green. DO1, DO2, electrochemical DO sensors and corresponding analyzers (YSI mod. 58); LDO, luminescent DO sensor (process sensor and analyzer).
F I G U R E 2 Illustration (section view) of the dissolved oxygen sensors' layout in the east digester tank. All sensors were placed in the tank volume using a custom-made system of pulleys and steel cables. All cables had one pulley at the entry hatch where the sensors were first calibrated. The dashed lines represent the steel cables. When viewed from the top, the cables are aligned along the reactor's diagonal. gas samples were collected at the end of this duct. The entire surface of the tank contributed to the sampling. To calculate alpha factors, the measurements were conducted at the same or similar airflows as in the clean water tests. The alpha factors were calculated as the ratio of the process water oxygen transfer rates (αSOTR,) and the clean water SOTR. Process water measurements were also performed at 1020 Nm 3 h À1 (600 SCFM) and 670 Nm 3 h À1 (390 SCFM). The alpha factors for these last two cases were calculated extrapolating the linear fit of the clean water SOTR values (r 2 = 0.992).

Evaluation of coarse bubble diffusers
Two scenarios were developed to correct the JA underperformance problem. The evaluation found that it would be more cost-effective to replace the underperforming JA system with a system consisting of higher capacity CB diffusers plus additional blower capacity than it was to correct the issue with a new JA system.
The jet pumps and manifold were left in the digester to facilitate digester mixing during planned off-aeration cycles, whereas the airflow from the blowers was separated and sent to an independent CB diffuser grid (Figure 4). The diffusers were first installed in a single digester to confirm performance with additional off-gas testing prior to installing diffusers in the second digester. With the use of a redundant positive displacement blower, the airflow to the CB equipped digester was operated up to 3350 Nm 3 h À1 (1970 SCFM).
The "chicken feeder" type CB diffusers installed were 24 in. long and manufactured by Stamford Scientific International, Inc. (SSI). The final design included 66 diffusers in the east digester. The SOTE of the CB diffuser system was measured in an independent clean water shop test at 16.64% and 16.03% (test performed ex situ by ATC according to the ASCE, 2006 standard) by testing the same diffusers in a shop tank at specific airflows of 50 Nm 3 h À1 diff À1 (24.8 SCFM diff À1 ) and 60 Nm 3 h À1 diff À1 (30.2 SCFM diff À1 ), corresponding to full-scale airflows of 2790 Nm 3 h À1 (1638 SCFM) and 3390 Nm 3 h À1 F I G U R E 4 Coarse bubble diffuser layout around the existing jet header.
(1995 SCFM), respectively. The shop tests were performed with diffusers installed at the same submergence as the digesters.

RESULTS AND DISCUSSION
Initial testing results on the jet aeration system The in situ clean water test results showed that the JA failed to reach the design SOTR of 95.3 kg O2 h À1 (210 lb O2 h À1 ) at the manufacturer's design airflow rate of 1360 Nm 3 h À1 (800 SCFM), even at the maximum blower speed. The results shown in Table 2 indicate that the measured SOTE was 18.26% instead of the design submittal of 25.4%.
After the clean water tests, off-gas tests were conducted on the JA system in the adjacent digester. Because the digesters are covered, the off-gas from the vent was measured, thereby producing samples representative of the entire tank. At the time of testing, the sludge in the digester was approximately operating at a TS concentration of 1.2%. Using the results from the process water and clean water tests, alpha factors for the JA system were calculated over the span of airflow rates, as plotted in Figure 5. The average alpha across all flow rates never exceeded 0.40 and exhibited a behavior independent of airflow, which is consistent with the behavior of coarse bubbles and droplets (Rosso & Stenstrom, 2006) or of fine bubbles from fine pore diffusers in processes with very low food to mass ratio (Gillot & Héduit, 2008).
The design SOTR of 210 lb O2 h À1 and the design α of 0.8 imply a design αSOTR of 168 lb O2 h À1 . The tested SOTR of approximately 165 lb O2 h À1 and tested α of approximately 0.38 imply a tested αSOTR for the jet system of 62.7 lb O2 h À1 , only 38% of the implied design αSOTR. This underperformance resulted in an aeration system incapable of supplying the oxygen required at this facility for effective solids stabilization. Furthermore, no dissolved oxygen could be left as residual because the oxygen supply was insufficient. The suspicion of surfactants or polymers present in the tank (which would have affected the test results) was ruled out by ex situ TOC testing that yielded concentrations of 0.73 +/À 0.16 mg L À1 .
Initial off-gas test of the coarse bubble diffuser system The redesigned CB diffuser system was placed in service in the east digester and was initially filled with WAS from the oxidation ditches and decanted to thicken the contents. After the first week of operation, decanting was replaced by recuperative mechanical thickening with a rotary drum thickener. The digester total solids were gradually increased, with concentration in the range of 0.8%-1.1% and between 1.2% and 1.6% before an off-gas test was performed at concentration of 1.37%.
The results of the initial testing were surprising ( Figure 6). Alpha factors were 0.26 and 0.27 at the airflows where SOTE was directly tested (2790 Nm 3 h À1 or 1638 SCFM and 3390 Nm 3 h À1 or 1995 SCFM, respectively) and was projected to be even lower at lower airflows. Alpha values this low for CB diffusers in relatively thin sludge were not expected.
The abnormally low values observed during the initial test were imputed to oxygen transfer inhibition due to the additives used in mechanical thickening. This hypothesis appeared reasonable because the polymers added to the influent to the thickener selectively accumulate onto the solids, which are later returned to the digester, thereby increasing the concentrations of said polymers inside the reactor. Because the polymer addition is expected to affect the apparent viscosity exhibited by the sludge suspension at its concentration and at its polymer concentration, there is a reasonable expectation that oxygen transfer will be affected, which has been documented in a fundamental study on the rheological effects and gas transfer effects of solids suspensions (Dur an et al., 2016).
In the immediate period after the east digester cell was placed into service, the dissolved oxygen levels in the tank were high. This changed immediately after mechanical thickening replaced decanting as the primary means of thickening sludge. WAS load and blower flow rate were held constant across this period, and the concentration did not vary substantially. However, the DO dropped from an average of approximately 4 mg L À1 from to an average below 1 mg L À1 (Figure 7).

Comparative off-gas testing
Upon installation, the coarse bubble diffusers were tested in process water (east digester), whereas the existing JA system was operating in the west digester. At the time of testing, the two digesters had been operated as similarly to each other as was possible for more than 1 month and were essentially aerating the same sludge. The digested sludge was approximately 0.9% thick in both digesters. The comparative alpha values from the two tests are shown in Table 3.
The average alpha value for the CB diffuser system was slightly higher than the average alpha value for the jet aerators, and the average OTE from the two systems was essentially the same (7.25% and 7.26%). These numbers support the hypothesis that the fine bubbles released from jets in clean water coalesce in sludge and therefore exhibit much lower k L a due to the decrease in the specific area associated with coalescence. Furthermore, we recognize the fact that the tests were conducted here with operating solids concentrations in mind, and therefore, a systematic assessment of bubble coalescence versus increasing TS concentration was outside the scope of the project. In the future, fundamental investigations are necessary to identify the critical threshold where the fluid changes its nature to shear thinning.
Results for OTE versus airflow and αSOTE versus airflow are plotted in Figures 8 and 9. Note also that the range of applicable airflows in the digester with CB diffusers spanned from 1350 to 3350 Nm 3 h À1 (790-1970 SCFM), whereas the jets were limited to the range 1020-1860 Nm 3 h À1 (595-1093 SCFM). F I G U R E 5 Alpha versus flow rate for the off-gas test on the aerobic digester equipped with JA. The values in brackets represent the averages of three repeats.
Had the facility begun operations with minimal loading, the underperformance of the JA system may have gone unnoticed. As this was a replacement facility, the high loading rates at initial commissioning immediately exposed the issue.
The quantification of the aeration efficiency (AE, kg O2 kWh À1 ) of the two systems was beyond the scope of the field project. However, the JA system utilized a pump with break horsepower (BHP) of 15.7 kW (21 HP) at the duty point. This mechanical power was utilized for all F I G U R E 7 Dissolved oxygen and concentration in the east digester after start-up.
F I G U R E 6 Alpha factors from the initial process test on coarse bubble diffusers. These values were calculated as a ratio of process water αSOTE and clean water SOTE. Note that the clean water SOTE was available only for the two highest airflows, which were nonetheless in the range of 16.0%-16.6%. The other values of alpha were calculated using the average of the available SOTE values. airflow conditions, whereas the power draw from the blower was similar for the two systems at a given airflow (because the pressure drops of both aeration systems were comparable). At the peak mechanical power from the blower (56 kW or 75 HP), the jet system would demand 28% more power to supply the same airflow to the digester. The results indicate that this air is not supplied in process water more efficiently than the coarse bubble diffusers, resulting in the jet aerator using more than 28% additional power per unit mass of oxygen transferred into the process water (i.e., AE jets > 1.28 AE coarsebubbles ). The difference between the power uses of the two systems increases as the blower is turned down due to the larger proportion of the jet pump power draw over the total jet power demand.
At the time of testing, the presence of an oxygen inhibiting surfactant could not be confirmed; nonetheless, the standard operating protocols were subsequently modified to include periodic manual digester decanting to remove floating oils from the digester, and polymer usage for thickening the WAS influent to the digester was closely monitored, controlled, and tested. The standard operating protocols include testing and monitoring for pH, airflow, total and volatile solids mass loading, temperature, residual dissolved oxygen, oxidation-reduction potential, specific oxygen uptake rates, VSR, sludge retention time, and target design criteria. In this case, the design criteria of F I G U R E 8 Oxygen transfer efficiency versus airflow rate for jets (west) and CB diffusers (east).
T A B L E 3 Comparative performance of the coarse bubble diffusers and jet aerators, reported for the tests where both clean and process water data were available. These results are from the tests with the process working under normal conditions (i.e., unlike the results reported in Figure 6). 270 degree-days was generally achieved at a 9-day SRT and 30 C with higher than expected temperatures from the exothermic aerobic digestion process compensating for a decrease in solids concentration and SRT.
Potential oxygen transfer inhibition from mechanical thickening Digester start-up data from October 2017 show a correlation between the start of mechanical thickening with the rotary drum thickener at the facility and a dramatic drop in the dissolved oxygen concentration inside the digester. During this period, WAS load to the digester and blower airflow were held constant, and the solids content inside the reactor did not substantially change. As a result, oxygen transfer inhibition from the polymer being returned to the reactor was deemed the most likely hypothesis to explain the sudden inability to maintain a dissolved oxygen residual during this time-period. This hypothesis does not appear in the scientific literature or in industry guidance for the design of aerobic digestion systems. The authors recommend additional studies to evaluate whether thickening polymers inhibit oxygen transfer by performing off-gas testing in a reactor before and after a given amount of commercial thickening polymer is added to the system to directly measure any changes in oxygen transfer efficiency.
Design engineers should be aware of potential oxygen transfer efficiency inhibition from polymers that are beyond the impacts expected from concentration alone when designing aerobic digesters and other thickened sludge aerated systems.

Roles in design and specification
Aeration system design involves several parties: the manufacturer of the aeration technology, the design engineer (consultant), the independent third-party expert who tests or verifies the claims, and the client. The main role of the manufacturer is to supply the aeration system and to verify its performance using a factory or field test (under the witnessing eyes of an independent third-party expert) following standard procedures. The ASCE Standards establish a minimum and consistent set of instructions and procedures to follow so that the results are replicable and no doubt is cast on the claims. This is why standards for testing aeration systems in clean and process water are crucial. F I G U R E 9 Comparative αSOTE versus airflow for jets (west digester) and CB diffusers (east digester).
The consultant designing the aeration system should request third-party verification whenever in-house expertise is insufficient to critically evaluate the equipment guarantee. Given that manufacturers of blowers and aeration systems know their technologies best, they are ideally placed to perform tests at conditions deemed controlled and replicable. This inevitably excludes the tests in process water, for the composition of the wastewater or sludge is not known and cannot be repeated. Although one may attempt to replicate the concentrations in the process water, it is its chemical nature that dictates the difference in oxygen transfer (e.g., the same concentration of soap and sugar would yield fundamentally different alpha factors).
The understanding of wastewater characteristics and operating conditions for a wide variety of aeration technologies is the purview of consulting engineers who are engaged in design due to this expertise that transcends a particular technology. Thus, it is the consultant who must select the alpha factors and in doing so assume the responsibility of delivery. The range of alpha factors, depending on the process conditions, can only be verified a posteriori, that is, after the facility is constructed and the process is operating at its design targets. The assumptions for the range of alpha values can be verified by the third-party expert, but they are ultimately the responsibility of the engineer delivering the design. In no case should the determination of alpha be within the purview of manufacturers, there would be an inherent conflict of interest.
A note on the quantification of the power should be reported here. There are different ways to express and measure power drawn by the aeration system, or power used in the definition of AE or its standard correction in clean and process water (SAE or αSAE, respectively). Wire power, i.e. the electrical power consumed by the motors used for aerators or blowers, is the most frequent choice and aggregates all electromechanical losses. The brake horsepower (BHP), which is the adiabatic mechanical output of a motor or gearbox, is sometimes used and excludes gearbox losses. To avoid confusion and calculation errors, a consistent use of wire power is often preferred. Whenever motors and blowers are specified separately, BHP becomes useful.

SUMMARY AND CONCLUSIONS
The comparative analysis of a JA system and coarse bubble diffusers to aerated concentrated sludge in aerobic digesters yielded the following conclusions.
Both the CB diffuser system and the JA system had relatively low alpha values (0.46-0.52) in sludge with solids concentration of 0.9%-1.2%. The fine bubbles released by the JA system coalesce in sludge to decrease the hydrodynamic resistance to rise by increasing their rising velocity, and therefore, they exhibit similar characteristics to coarse bubbles rather than fine bubbles. Future investigations should focus on the critical threshold where solids concentration alters the Newtonian nature of the fluid and causes fine bubble coalescence to produce coarse bubbles.
The alpha factor varies considerably as a function of solids concentration regardless of the aeration system. Claims of constant alphas regardless of concentration are unrealistic. Furthermore, the claim that fine bubbles released from jets experience similar elevated alpha values cannot be verified.
The two systems had similar oxygen transfer efficiencies at similar test conditions. CB diffusers had a slightly higher alpha and a slightly lower SOTE than jets, which yielded a similar overall efficiency. However, the oxygen transfer rate for the CB diffusers spanned a broader airflow range and could reach values much higher than the JA system. The CB diffuser system was substantially more efficient on the basis of mass of oxygen transferred per unit energy used. Moreover, the CB diffuser system did not require an additional mechanical unit (the jet pump) with its associated cost, energy consumption, and maintenance requirements.
The polymer added during mechanical thickening operations could have a potentially significant impact on the oxygen transfer efficiency for aerobic digesters due to the selective accumulation of the polymer onto solids. More research is needed to both determine the cause of this observed impact and to quantify the effect.
The separation of roles among the manufacturer, design consultant, client, and independent expert is critical for the success of equipment specification. Aeration performance in clean water must be verified independently before the design is completed, and off-gas testing should be used to verify design assumptions.

AUTHOR CONTRIBUTIONS
Paul Steele was the consultant process engineer for the start-up and commissioning of the biosolids facility and the design of the revised coarse bubble diffuser aeration system. He performed QA/QC review of all data and cowrote the paper. Rick Warner was the utility project manager, oversaw the project, and co-wrote the paper. Diego Rosso was the ex parte expert who witnessed the clean water tests, performed all off-gas tests, calculated and plotted the results, and co-wrote the paper.