Fundamentals

     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 grade level  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."

ACTIVE BIOMASS vs  MLSS vs MLVSS

     The volatile solids content of the mixed liquor in an activated sludge process typically is considered to represent the mass of microorganisms that is available for waste treatment.  However, not all volatile solids are active microorganisms.  Inert cell matter and volatile but nonbiodegradable influent solids also contribute to the mixed liquor volatile solids.  Oxygen uptake occurs when active microorganisms consume biodegradable constituents of the wastewater.  Respirometer-aided methods can provide oxygen uptake measurements which can be used to estimate actual active biomass in mixed liquors.  Case studies show that mixed liquor volatile solids typically range from 5 to 25% active microorganisms.

     We can't go much higher in MLVSS than about 2,000 mg/L and still get good oxygen transfer in the aeration basins. 

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. 

     Minimum depth manufacturer's specs for 1 and 2 Hp aerators can be say 1.2 meters (4ft. depth).

     Minimum depth say for larger 10 and 15 hp units can be say 3m (10ft. depth).

     In order to be able to use them in [even] shallower basin anti-erosion assemblies must be incorporated.

     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 approximayely 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 exisiting wastewater treatment plant at an edible oil plant having a detention time of ... [only] ten (10) minutes.

The following approach can be used to estimate oxygen requirements

Ro = (1 - b * Yg ) * Rs  + b * d * X ,     mg O2/L-hr          
           
 where: 

Rs = rate of COD conversion, mg COD/L-hr (usually COD load * removal efficiency), 
Ro = rate of oxygen uptake, mg/L-hr,
X = microorganism concentration, mgVSS/L
Yg = biomass yield coefficient, mass VSS/mg COD removed (usually 0.3 kg VSS/kg COD removed)
b         = 1.42
d = endogenous decay rate (usually 0.1/day)

     So for 42 mg COD/L-hr maximum COD loading rate ( = 1 g COD/L-d) and an assumed 2,000 mg/L VSS in the mixed liquor and an endogenous decay rate of 0.00417/hr (= 0.1/day), the oxygen uptake rate would be:

Ro = (1 - 1.42*0.3) * (42 * 90%) + 1.42 * 0.00417 * 2000 mg/L = 34 mg O2/L/hr 

     The aeration equipment would have to equal or exceed this rate to insure positive DO in the aeration basin. Of course, you will need to make sure you calculate the loading rates correctly in the zone of interest. For example, for plug flow type processes, the COD loading rate would be that in the first section of the basin. 

     Most manufacturers and equipment vendors have readily available software for preliminary designs and will generally assist you with proper equipment/unit selection including recommended/minimum/maximum depth, oxygen dispersion diameter, complete mix diameter and so on.

ACTUAL IN-WASTE/FIELD OXYGEN TRANSFER RATES

Rightly or wrongly, AOR/SOR ratios of about 0.7 are frequently used for quoting or budgeting direct-drive, surface mechanical aerators. However, until alpha, beta, theta, elevation and required residual D.O. are known the 0.7 factor is nothing but a glorified guess. It should be obvious you don't want this type of guessing especially in the light of present day availability of highly qualified treatability assessments. 

It's illustrative to see how each aerator manufacturer decides to showcase their units. The following argumentation was proffered by a manufacturer of both surface aerators and submersible aerator blower combinations:

"Based on a flow rate of 1200 m3/day and a BOD of 1043 mg/l and TKN of 11 mg/l we calculate that the AOR will be 4385 lbO2/day. Using a correction factor to SOR = 8570 lbO2/day = 357 lbO2/hour. 

With our surface floating mechanical aerators at a transfer efficiency of 2.5 lb/Hp/hour you will need 142.8 BHp. Due to the shallow basin [swd=3.5m] we recommend three (3) 50 Hp (37.3 kW) aerators of stainless steel float design. 

If you can change your basin size to 16.5m square by 6.1m liquid depth then we would recommend one (1) 30 Hp submersible aerator mixer supplied air from one (1) 75 Hp blower with accessories. Total energy draw will be 30HP for the submersible aerator mixer and 64HP for the blower for a total of about 94 Hp (70 kW).

The submersible aerator mixer can either be hydraulic driven or electric driven at the same cost. The submersible aerator mixer and blower combination should save over 40 Kw of energy costs and this should more than pay for the capital cost difference in less than two years." 

MIXING AND MECHANICAL AERATION SYSTEMS

Adequate contact must be provided between organic wastewater constituents and the microorganisms.  Mechanical aerator manufacturers often provide sizing charts or layout guidelines including recommended water depth, oxygen dispersion diameter and complete mix diameter estimates, the following being sample formulas for low-speed, floating surface mechanical aerators:
Mixing diameter (feet)                  =  2 x ((  646 x H.P.)/DEPTH)^0.5
Oxygen dispersion diameter (feet) = 2 x ((6490 x H.P.)/DEPTH)^0.5
 

One surface aerator manufacturer's rule of thumb suggests that the HP/mg power density required for mixing with up-draft, direct-drive aerators is up to 1 HP/1,000 ft3 or about 133 HP/mg.

MIXING AND DIFFUSED AERATION SYSTEMS

     A common rule of thumb indicates the air volume required to mix a tank is 0.12 cfm of air per sq.ft. of flat floor area in the tank  This is a way of showing or representing the actual energy requirements per square foot of tank floor.  The reason the 0.12 cfm per square foot of tank is used is to eliminate the variable of depth from the calculation.  Regardless of the depth of the tank one ends up with a proportional energy per unit volume.  Using a value of 20-30 cfm of air per 1,000 cu.ft. as some engineers do, may be a disproportionate amount of energy requirement for deep tanks which may not be realistic.  One must understand the 0.12 value as pertaining to biological solids only.  If it were a tannery wastewater with a lot of heavy material or difficult solids, one could suggest the energy level for mixing be increased to 0.15 or possibly even approach 0.2 cfm per floor square foot.  This is a determination based on the characteristics of the solids to be handled.  For instance, for aerobic digesters one could/would specify 0.2 to 0.3 cfm per square foot because of the high concentration of solids and more difficult mixing conditions.

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