Technical Bulletin 174: Analysis of Panel Diffuser Technologies
The emphasis on reducing total cost of ownership of biological wastewater treatment plants has driven the technology for aeration and mixing systems to seek ways of higher efficiency and superior performance. Panel diffuser systems have been developed as a result and offer some interesting capabilities in aeration mixing technology. Panels along with disc diffuser systems and tubular diffuser systems offer the full spectrum of aeration technology availability. Panels have produced the ability to reach some very high O2 transfer efficiencies in clean water. A significant increase in capital cost may be required in order to achieve these high efficiency levels. In addition, panels offer several unique mechanical characteristics that require care in their application and deployment.
It should be noted that panel diffusers have a specific spectrum of performance that is quite attractive for specialty applications. Systems that require the absolutely highest O2 transfer efficiency in clean water will probably select panels because their ability to get high densities deliver maximum SOTE. Systems that have normal aeration mixing requirements may well be better served using discs or tubular diffuser systems. In general, panels must be evaluated as one of three types of diffusers available, then selected for those applications which are the best value. The disc or tubes should then be selected for those systems where they deliver performance and they compete most effectively. In general, panels will be selected for probably 20% of the projects or less based on the process requirements of treatment plants, the geometry of the basins, and the cost of energy and cost of money for investment and maintenance of the systems.
When panel systems are selected there are many features for the design and application that should be reviewed. There are different types of panel configurations, different types of panel mounting systems, different operating characteristics of the panels. In clean water they all deliver a very high O2 transfer efficiency; however, the reality of installing panel systems in activated sludge or other field applications can dictate the selection which brand or which model of diffuser panel is superior. The enclosed analysis presents several factors that are critical in a proper evaluation of the panels and indicates the application and impact of these characteristics on the total cost of ownership or performance of the system.
Items for evaluation and analysis of panels include the following:
1. Are the Units Mounted Directly on the Floor or Raised Off the Floor?
Panel systems are available for mounting flush on the floor or configured to allow mounting off the floor with traditional pipe supports. It becomes very important to evaluate the mounting characteristics considering the operating conditions in wastewater that can have substantial amount of solids and/or grit that may impact the long term performance of the units.
Mounting the units flush on the floor offers gains in O2 transfer efficiency by minimizing pumpage and allowing free rise of gas bubbles from the membrane. This minimization of the liquid pumpage is very positive in clean water O2 transfer testing applications but should be viewed with caution when considering the field transfer characteristics in the field application. Clearly mounting the diffusers as deep as possible gets the highest O2 transfer efficiency but it also increases the operating pressure that must be considered in a total value analysis.
Perhaps the greatest concern in a system mounted on the floor is the impact of inorganic solids that accumulate on the unit when air is turned off or can accumulate on or around the diffusers in some cases simply because the low energy applied for high efficiency requirements and the limited mixing associated with having units directly on the floor.
2. Purge Assembly
All fine bubble diffusers systems require means to purge liquid and solids out of the piping or out of the diffuser system to assure long term life and proper operation. Even the most effective panel systems will accumulate water inside the diffuser assembly on extended shutdown or during times when there is major humidity in the air with the hot air being delivered to the unit and cooled. This condensate and this water that gets inside the system must be removed efficiently in order to sustain maximum O2 transfer efficiency.
Purging of flat panels that are mounted on the floor is extremely difficult. The panels become the lowest item in the piping system and any condensate or liquid or solids will accumulate in those panels. By comparison panels that are mounted above the floor can effectively purge liquid out of the system routinely using the traditional manual or automatic purge assemblies. Pipe mounted panels like EDI curvilinear StreamLine® panels are especially easy to use a purge assembly and avoid the difficulties of accumulating water in the panel systems that are floor mounted. It should be noted that water accumulation in flat floor mounted panels can have a substantial impact on uniformity of air release, efficiency, and of course the operating pressure of the system which translates into energy consumption. All systems should be reviewed on their ability to properly purge liquid and solids from the system.
3. Cost of the Membrane
All panel systems require a high membrane area in the tank for maximum performance. Some diffuser assemblies use quite large membranes and the damage of one membrane can be a major expense. Typically, systems that have modest sized panels are preferred for ease of handling, and the economics of membrane cost. Modest sized membranes are much more likely to be changed and managed in case of a damaged unit vs. very large panels that can be major cost per panel membrane unit or for the membrane only. Recent bids show cost of flat panel system membranes only cost MORE than a complete upgrade to a completely new EDI economical curvilinear total system.
4. Membrane Changing in the Field
Maintenance of panel diffusers is required just as any other diffuser assembly. The evaluation of any panel assemblies should consider how easy it is to install, remove, or replace membrane units. Systems that have flat panels that use sheet membranes are typically extremely difficult to change and in many cases are physically impossible to change in the field. Typically, any system that has a flat panel configuration membrane change requires entire units to be removed from the tank, with several hours of installation at a protected environment such as the plant maintenance sheds. For a review of the panel membrane at least two of the system manufacturers recommend that the panels be returned to the factory for membrane change out and warranty of the membranes. Most of the panel manufacturers indicate that any replacement of the membrane in the field by other than the factory personnel or any change of membranes that were not returned to the factory for factory personnel to install the warranty will be void!
It should be noted that the EDI system that uses the curvilinear panels can be changed routinely in a matter of minutes in the field, by normal operating personnel and are specifically designed to allow this ease of membrane change. In addition to the ease of actually changing the membrane the curvilinear sealing mechanism is such that it is easily accomplished as part of that membrane change. Typical membrane change-out can be achieved with the curvilinear panels in 2 minutes for pipe mounted units and typically 5 minutes or less on the linear type units with full membrane warranty when changed by operating personnel. Compare this maintenance feature with returning to factory or even need to replace the entire diffuser assembly.
5. Membrane Materials
Most panel membrane materials are designed to use polyurethane as the only material Polyurethane is a wonderful membrane for many applications in wastewater treatment plants, but it is not a universal membrane. Applications must be reviewed in each case as there are places where other membrane materials would be more effective.
Flat panels that employ sheet membranes are almost exclusively limited to polyurethane as it has the necessary tensile characteristics while use of other materials would balloon or cause damage because of the high internal stress. The larger the panel the greater the balloon force to resist.
EDI curvilinear panels are low hoop stress by comparison. Curvilinear membranes are available of EPDM, silicone, polyurethane, high temperature polyurethane, or the PTFE Matrix or Matrix Plus™ materials specifically designed for superior performance in severe applications.
6. What are the Mounting Options Available with the Panels?
There are 3 basic ways to mount panel systems including a linear piping arrangement in the StreamLine® or the Gold Series™ systems. Pipe mounted units are generally mounted perpendicular to the air supply piping and may limit the amount of air supply piping. The third type panel is individual units that are fed with small Ø hoses off a main air supply with the units sitting directly on the floor. Each panel type has unique characteristics. Generally, the pipe mounted units or the linear units that are mounted up off the floor have superior operating characteristics for purging, avoiding of sand and grit as well as ease of installation, operation, and maintenance.
7. Are the Systems Retrievable?
The use of EDI curvilinear panels can be effectively employed with ModuleAir™ assemblies and easily installed wet, retrieved and reinstalled to allow routine maintenance. Systems that are flat panels typically are not retrievable and are generally mounted to the floor requiring multiple basins or accepting limited access to the system.
EDI curvilinear panels also can be installed with floating lateral systems and suspended diffuser assemblies for a wide spectrum of applications including lagoons, variable liquid level systems and those cases where retrievable features are desired for large basins.
8. What is the Headloss in the System.
Panel systems traditionally have been applied at very low airflow or flux rates and high operating pressures. Panels with maximum O2 transfer efficiency claims are generally mounted on the floor and operate with high headloss membranes in order to get proper air distribution and fine bubble delivery with large panels most challenged for uniformity. This becomes an interesting total cost of operation comparison. Clean water SOTE values on systems can be deceptive as the additional pressure associated with some panel diffusers can cause the actual kilowatt use to be greater than systems with lower SOTE percent! Systems that are slightly higher off the floor and pipe mounted to avoid the mechanical problems and the operational problems of floor mounted units will typically have slightly less O2 transfer efficiency (SOTE percent) but they also have a similar reduction in the pressure of operation so kilowatts can be very favorable for the curvilinear designs and other systems that are mounted off the floor.
It is also true that the curvilinear design diffuser assemblies operate at typical pressures for disc or tube fine bubble diffusers and about ½ the operating pressure of most flat panel type diffusers. This ability to operate at low operating pressure for the curvilinear designs is a major benefit in total cost of ownership and again must be evaluated on a total energy cost, not an SOTE to get a proper value comparison.
9. What is the Alpha Factor for the Panel?
Systems that are routinely installed to give full floor coverage on the floor, minimize pumpage, have very high headloss from needle punch openings widely spaced on the membrane are designed to deliver greatest O2 transfer efficiency in clean water! This is an area that can be deceptive as the clean water O2 transfer efficiency is typically assuming that all fine bubble diffusers operate in the wastewater at a similar reduction in the performance based on the AOR/SOR conversion. Unfortunately it appears that assumption needs to be challenged as systems operating with the very tiniest of bubbles and very least of pumpage or turbulence for highest SOTE in clean water are actually requiring greater energy when applied to field conditions vs. systems that have slightly less clean water O2 transfer efficiency but enhanced mixing. EDI curvilinear panels designed to use slightly larger perforations for long term performance with AOR/SOR values are typically very favorable compared to the low mixing pinhole perforations of the flat floor mounted panels. The alpha factors of the improved mixing by curvilinear panels seem to demonstrate significant improvements in the AOR/SOR because of the improved alpha factors and results in lower energy consumption. It seems this use of the same alpha factors for flat panels vs. curvilinear panels or other fine bubble diffuser platforms may be a serious flaw. Analysis of alpha impact suggests lower values with flat panels and can result in field performance for the flat panel of 20% to 30% less than predicted vs. curvilinear units with improved mixing.
10. Mixing Velocity
Mixing velocity in aeration mixing systems is a function of pumpage and circulation. Flat panel systems are designed to minimize water velocity with free rise rate of the gas bubbles. Any systems that require significant mixing are greatly enhanced by the use of curvilinear units which deliver significantly greater pumpage for solids mixing in digesters or other high solid systems. This greater pumpage and turbulence at the curvilinear diffuser assembly is also quite beneficial in enhancing the alpha factor as suggested in the previous paragraph.
11. A Total Cost of Ownership Must be Calculated!
It is clear that for an economic comparison of various panel diffuser types it is necessary to look at multiple items during a total cost of ownership analysis. Economics is more than clean water O2 transfer efficiency! A true total cost of ownership analysis is going to include an analysis of the first capital cost, installation cost, maintenance costs associated with the mechanical characteristics of the design, and the energy cost of course. That energy cost has to be evaluated much more than just the clean water O2 transfer efficiency. It must be evaluated on the basis of the AOR/SOR and field O2 transfer efficiency in Kw that incorporates operating pressure characteristics and the alpha factor effect on efficiency in field service.
12. Can Systems be Supplied with Completely Noncorrosive Materials?
Many applications in high salinity waters or high TDS waters can be corrosive in the diffuser environment. Can the system be supplied with completely nonmetallic components? Most panel systems that are floor mounted are metal panels or have metal components in order to hold the system together. By comparison the EDI curvilinear panels are all designed capable of being installed with no metal components! Nonmetallic features can be very beneficial.
13. Ease of Installation
Systems of panel configuration need to be reviewed for ease of installation and ease of handling for specific plant geometries. Floor mounted panels tend to be more difficult to install on systems that have uneven floors, have sloping floors, or systems that require the unit to be mounted up off the floor. Curvilinear panels by EDI are much quicker to install as they use the SuperStrut™ method of mounting and/or the saddle mounted assemblies on the piping that can typically be installed and leveled at any elevation in a minimum of installation time. The ability to install on sloping floors and to allow application into basins that have need for drainage and dewatering or cleaning can show that the curvilinear panels by EDI offer major benefit over the panels that are floor mounted.
14. Perforation Choices in the Panel Membranes
Application of panels into various systems should be reviewed with the ability to select membrane perforations for the service intended. Systems that typically use floor mounted panels have only 1 perforation pattern available and 1 material of the membrane itself. This can limit the airflow range of each panel and/or can limit the air handling capacity of the panels. EDI curvilinear panels are unique as they have the ability to have the NanoPore™ perforation with small slits as low 0.2 mm in length up to 2 mm or more in length in order to deliver the capacity needed for the particular process. Processes that require higher air volume because of O2 uptake or other unique applications are well served by having a choice of membrane perforations of NanoPore, MicroPore™, or the high capacity membrane by EDI for the curvilinear units. Perforation selections are also necessary to effectively use panels in digesters or other high solid applications. Curvilinear panels and engineered perforations are superior to flat panels in heavy solids applications.
15. How Much Experience?
EDI curvilinear panel systems have been in continuous operation since 1993 with many thousands of units successfully applied over that period of time. These curvilinear panel systems have been used in floating systems, retrievable systems, fixed to floor system, low density systems, high density systems, and can be engineered and configured to meet each specific project requirements. Demonstrated operating history in excess of 25 years is proof of long-term performance of curvilinear designs.
16. Membrane Stress
What is the stress on the membranes that are to be employed with the panels? Systems that use flat panels and sheet panel configuration are extremely high stress with significant stress concentration at each of the corners and each of the penetrations used to hold the membrane in place. The curvilinear design by EDI is a hoop stress design component which minimizes the stress in the membrane. Curvilinear panels enhance long term performance in operation without damage from the cycling of on and off or the operation at elevated airflows when necessary for the process. Low stress, hoop stress panels by EDI offer extended performance and reliability with that curvilinear design, particularly in on/off applications such as SBR systems.
17. Are Multiple Length or Size Membranes Available?
Most panel systems come in 1 or 2 lengths or sizes. This can limit the application efficiency in round tanks or other small or unusual geometry tanks. By comparison the EDI curvilinear panel is available in lengths from about 0.5 m up to 2.5 m per panel and multiple units in series to fit almost any geometry or any configuration and any density that might be required.
18. Curvilinear Panel Units
Curvilinear panel units mounted above the floor are especially effective in shedding grit or heavy solids that may settle in the tank. In addition, the curvilinear geometry sheds all the biological solids when the air is turned off for a decant operation in an SBR or aerobic digesters as an example. Systems that have flat panels are typically vulnerable to damage or to blinding as a result of material settling out on the flat surface. High MLSS or digester applications are challenging for flat panels.
A particularly important feature of the curvilinear panel is to allow grit to settle past it to the floor without accumulating in any the crevasses along the panel frame that holds it together. On/off operation of panel systems that have frames holding the membrane in place typically create erosion and stress at that location with the inflation and vibration of the membrane during routine operation. This erosion and accumulation of grit in the channel or frames that hold the membrane in place can be particularly damaging to the long-term life of the membrane itself. The curvilinear smooth surface eliminates these operational issues.
This is a partial analysis of the various components that should be considered in an evaluation of the panel diffuser assemblies. Constructing a checklist comparison of panel diffuser features allow evaluation of different panel designs and determine the best mechanical features, the best operating features, and the best overall cost of operation economics. It is clear that in any evaluation of panels the highest O2 transfer efficiency in clean water may not be the primary item for analysis as there are many features that will determine the optimum design and the optimum materials and the optimum panel configuration for each application.