Jet aerators find application in industries and have rarely been used in municipal treatment plants. They have low alpha factors like fine pore diffusers, do not foul and suffer low SAEs due to the need to pump water and compress air.
• The following slide shows efficiencies of various aeration devices in conditions typical of municipal wastewater treatment plants. Three columns are provided: The left most is the clean water aeration efficiency or SAE.
• The next two columns show process water efficiency, at 2 mg/L DO concentration. Two columns are needed for the effectiveness of the fine pore, and fine bubble devices will vary with SRT. Long SRT systems remove surfactants more rapidly, which elevates the alpha factor.
• The data in the table are supported by published tests, but there can be site-specific considerations that alter the results. The table results should not be used as a general guideline and not for design. Aeration efficiency should always be verified by transfer testing.
Air Blowers 1
• Blowers are compressors operating at low pressure, and they are needed for all subsurface aeration systems.
• Blowers often restrict the flexibility of an aeration system, because they are limited “turn up” and “turn down” range.
• There are two kinds of blowers: positive displacement (PD) and centrifugal.
• PD blowers are considered constant flow, variable pressure devices
• Centrifugal blowers are considered constant pressure variable flow devices
• The same PD blower can be operated over a large range of pressures, requiring only a larger motor to operate at higher pressure.
• It is harder to change the pressure of a centrifugal blower. New types have inlet guide vanes and outlet diffusers that provide a larger range.
• When treatment plants are upgraded from coarse to fine pore diffusers, a frequent problem is oversized blowers that operate at too low discharge pressure. This occurs because the air requirements are much less, but the pressure drop (DWP) of the fine pore diffusers is high. A detailed blower analysis must be performed if the existing blowers are to be reused.
Air Blowers 3 – A Comparison of Types
• More economical at small scale
• Noisy – the low frequency “thud” associated with the rotary lobes is harder to dampen. Three lobe blowers partially overcome this objection
• Vibration transmissions to piping and supports sometimes problematic
• Motor overloads with excessive discharge pressure, requiring current protection on motors
• Higher discharge pressures possible
• Economic at all scale but especially for large installations
• Also noisy but the continuous, higher frequency spinning sounds are easier to dampen.
• Operation at excessive flow overloads the motor and operation at excessive pressure causes a surge, which may result in the destruction of the blower. Over current and vibration, detection controls are required for safe operation
• Blower specification is a most important task. Blowers are among the most expensive equipment purchased for a treatment plant and require trained mechanics for maintenance.
• Often consulting firms will have a single individual or a sub-consultant to work with blower specification
Converting Clean Water Transfer Rates to Process Transfer Rates
• Turning clean water to process water transfer rates involves several straightforward equations that use parameters such as DO, temperature, barometric pressure, humidity.
• Two water quality parameters present more difficulty.
• The first is the Beta factor or the reduction in equilibrium oxygen concentration (saturation concentration at the operating hydrostatic pressure) due to contaminants in the wastewater. The modern approach is to use the salinity of the wastewater, which is easily measured and use “handbook” salinity versus saturation tables.
• The alpha factor is more difficult. It ranges from 0 to 1.0 (values greater than 1.0 can be obtained in laboratory situations with small vessels or sea water without surfactants).
• There is a great deal of research, none entirely conclusive on alpha factors. We have performed a large fraction of this work.
• The mechanisms and theory of transfer rate reduction due to surfactants is beyond the scope of this course, but we refer you to some of our recent publications
• The following figure shows the essence of what we have learned and how it affects treatment plants.
• Because the increased alpha at high MCRT is the more rapid and efficient removal of surfactants. It is readily observed in “plug flow” aeration tanks.
• In plug flow aeration tanks, the alpha factor at the influent zone of the aeration tank may be only 0.3 but at the effluent area, it may be as high as 0.8
• This dramatic change requires aeration tapering. Unfortunately, the alpha factor is lowest where the uptake rate is highest.
Role of Selectors
• Almost all modern activated sludge designs include a selector. In the past 15 years, this innovation has become a standard feature.
• For high SRT plants, anoxic selectors denitrify, participate in phosphorous removal and prevent the growth of filamentous organisms
• For small SRT plants, they participate in phosphorous removal and prevent the growth of filamentous organisms
• In both cases, our data show they improve alpha factors, presumably by the uptake of soluble contaminants into the biomass
Normalized Standard Oxygen Transfer Efficiency for selected plants operating with different layouts. Labels refer to the diffuser Status: NEW (within one month from installation), USED (between 1 and 24 months of operation), OLD (over 24 months in operation), and CLEANED (within one month from a cleaning event). The effect of diffuser aging outweighs the increase in performance due to process upgrade (from conventional to N-only and NDN).
• Aeration systems need to be periodically monitored to maintain transfer efficiency.
• For fine pore systems, the impacts of fouling must be monitored and diffusers cleaned or replaced as needed. The integrity of plastic piping systems must be assured.
• For coarse bubble systems, less monitoring is required, but system integrity must be evaluated – corrosion monitoring, structural monitoring.
• Blowers require routine maintenance and large blowers should be included in asset conservation programs.
• For surface aerators, motors, gear boxes, and impellers must be periodically evaluated to track wear and avoid outright failures through preventative maintenance.
Fine Pore Systems
• Off-gas testing to determine transfer efficiency and OUR is one of the key ways to monitor system performance
• Sample diffusers, collected from aeration tanks, should be routinely analyzed for pressure drop, fouling and changes in material properties.
• System pressure should be tracked to predict when cleaning will be necessary
Sustainable Aeration Practices
• Lagoons typically find their best application in remote areas, where land is inexpensive and population density if low
• Such locations usually have fewer individuals trained to operate and maintain treatment plants.
• How does one perform mechanically simple aeration for lagoons and other treatment systems?
• It is now possible to use diffused aeration in large lagoons and install the equipment without dewatering the lagoon.
• Using a diffused aeration system has several potential advantages, including less heat loss, reduced misting and less impact from freezing
• Several manufacturers make competing equipment, so the selection is possible.
• A design algorithm has been developed to allow iterative solution of the design equations (section 9.5) in the text
• The key variables are MCRT, and the “trial” air flux rate and the number of diffusers being used.
• The first step is to select a trial air flow rate based on Eqn 7. AFR can be approximated by a “guessed” efficiency (αSOTE). The number of diffusers and the active or “bubbling” area of a single diffuser is used in Eqn 7, along with the depth and number of diffusers.
• From Fig., locate the intersection of the MCRT and he normalized air flux, to obtain the αSOTE on the left axis.
• Check to see if the guess αSOTE matches the αSOTE you read from the left axis. It will take 2 or 3 iterations for the solution to converge. Vary either the SRT or the number of diffusers to iterate to convergence.
• Aeration systems are the key to the success of any biological process.
• They consume the most energy of any part of an aerobic process and the potential energy savings warrants close attention to design and maintenance details.
• The recent work at standardization of methodologies (ASCE standards) has taken a lot of the guesswork out of the design process
• A key aspect of any design is its practicality and workability. One does not want to force the plant operators to operate in a certain way or region because of an inflexible aeration system
• Key concepts
– Fine pore diffusers are the most efficient way of aerating municipal wastewater. They strip the fewest volatile organic compounds and cool the water less than other methods. They require routine maintenance
– Coarse bubble diffusers are much less efficient – expect to use twice the energy of fine pore diffusers, but in spite of this disadvantage, they are sometimes the best solution. Examples were coarse bubble diffusers may be the best choice are for aeration of viscous fluids such as found in aerobic digesters and MBRs operating at high MLSS (> 8,000 mg/L). In plants where it is not possible to dewater the aeration tanks, coarse bubble diffusers may be preferred.
– Surface aerators are less efficient, but there are some applications where they are the best choice.
– Select aeration equipment not just by energy efficiency, but also by maintainability, odor production, and heat loss.
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