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Aeration Fundamentals

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AERATION FUNDAMENTALS: AERATOR TYPES, TRANSFER RATES & DEPTHS

     Broadly speaking, aeration systems are popularly classified as either surface aeration systems or submerged type aeration systems.

     Typical examples of surface aeration systems include most frequently floating or pier-anchored mechanical type units, such as direct-drive, high speed units or gear driven, low speed units.  Flow can be either upflow or downflow and either axial or radial/centrifugal. 

     A typical direct-drive, high speed unit consists of a motor, a fiberglass or stainless steel float and an intake/suction cone.  The most common designs can be marine type impellers assisted with fixed/non-rotating diffusion heads or screw centrifugal, Archimedes type impellers.  A good quality, high speed unit can and should deliver say about 2.4 lbs O2/hp/hr, +/-10%, in clean water. 

     A typical gear-driven, low speed unit consists of an electric motor, gearbox, relatively large diameter rotors (say up to 10' or 3.2m), spool and mounting plate for pier-mounted units  Floating type low speed units include knocked-down, float platforms that can be easily assembled onsite.  A good quality, low speed unit can and should deliver say about 3.5 lbs O2/hp/hr  in clean water.    .  

       Surface aerators are typically employed in the relatively shallower ponds, basins or tanks.  Evaporative cooling does take place which may be undesirable or unacceptable  in some contexts.  Volatile organic compound stripping can be significant and/or again unacceptable  

 

     The most popular submerged type aeration systems include diffused aeration systems and submerged, turbine-type aeration and mixing aerator configurations.

     Diffused aeration systems are frequently classified into two major categories according to the diffuser's pore/bubble size, i.e. fine-pore diffusers and medium/coarse diffusers.  

     Medium/coarse diffused aeration systems are used in foul-prone applications.

     Both fine and coarse bubble diffusers can be used in retrievable racks/arrangements, diffuser banks or assemblies either sitting on basin bottoms or evenly suspended to overcome irregular, lagoon-type floors. 

     Submerged, turbine-type aeration systems include slow rotating bottom impellers coupled with grade level blowers.  The submerged impeller draws liquid from the bottom for reactor mixing and effects oxygen transfer/bubble shearing   Blower units provide air to the submerged turbine assemblies, (e.g. 35-40 SCFM per turbine share motor HP, ballpark 50/50 total HP split between blower and submerged turbine) via flexible hoses as needed to satisfy specific operating modes/targets, e.g. just mixing (off) , anoxic stage, SBR phases, filamentous bacteria control

     Submerged  jet aerators, i.e. basically a pump and submerged venturi-type diffuser, call for 8 to 10 m deep water levels.  In this type of system, mixing and air supply can be operated independently of each other, i.e. pump only or  pump and controlled introduction of pressurized air.

     Aspiration type units provide good oxygen transfer but also cause a circular pattern of flow through the reactor. This circulation pattern is OK if the basin type requires circulation, such as oxidation ditches and facultative lagoons, but BNR reactors do not need this circulation. Aspiration type aeration devices also provide a high velocity jet that can cause erosion of the bottom or sides of the basin if the basin has a shallow depth or the unit is too close to the side of the berm.

     Most ATAD processes use aspiration type aerators with oxygen transfer rates well above 2.5 lb/HP-hr. In fact, ATAD processes are made possible by the high oxygen transfer rate capabilities of these aerators..   

     Most types and brands are suitable for AS applications, but each has its own best applications. For example, brush aerators are best for oxidation ditches while fixed diffusers and surface aerators are best for conventional AS systems. The key is to size the unit properly for each application. Once OTR characteristics are established, the sizing is fairly straightforward. Other factors include alpha factor, impact of floc size and settleability, impact on effluent TSS, etc. The key phrase is "if properly sized/selected."

LOADING RATES

     For conventional activated sludge of average rate, i.e. medium rate, it is generally recommended approximately 50 lbBOD/1,000 cu.ft. as maximum.  For process stability and better assurances of performance, [fine pore/fine bubble] diffused aeration systems favor the use of low f/m and that is generally restricted to about 10-15 lbBOD/1,000 cu.ft.  This significantly lower rate sizing tip takes into account process recommendations (extended aeration) as well as diffuser technology old hands recommendations.

BASIN DESIGN AND TANK DEPTH

     Design of the aeration tanks is also important for optimum efficiency.  Proper tank design can make it or break it for any given aeration system selection. It is often the case that the same identical piece of equipment or given amount of hardware will deliver far more return for the investment if only careful/generous process considerations are taken into account, adding buffer treatment capacity.

     For best performance, mechanical surface aerator vendors frequently suggest recommended/minimum/maximum liquid depths for their standard units. If basin is too deep, the aerator may not be able to effectively pump up beyond a given depth thus resulting in idle pockets or even whole layers, at least as regards intended aerobic activity. 

     Fine bubble diffusers can work at 2.5m water depth but deeper basins will give greater efficiency and superior results on capital costs.  Diffusers are directly dependent on liquid depth for their aggregate efficiency.  As a result, if the basin depth is doubled, it will approximately use the same horsepower but it will take only roughly one-half the number of required diffusers, i.e. capital cost of the diffusers is about 50%.  Using deeper basins, e.g. 5m, and larger volumes offer much better assurances of performance.  It must be said that one of the authors once witnessed a probably still existing wastewater treatment plant at an edible oil plant having a detention time of ... [only] ten (10) minutes.

 

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