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Aeration Systems

Aeration Systems - Theory of different type and process of Water aeration and its measurements and comparisons between its technology Part 2

Aeration Systems – Liquid Aeration part 2

Aeration System Types 2
• Surface aerators can be further classified into high speed, low speed, with horizontal or vertical shafts.
• High-speed surface aerators use motors without gearboxes and a propeller that looks like a boat propeller. They are usually smaller than 50kW and operate at 900 to 1200 RPM
• Low-speed aerators always use a gearbox, can be as large as 150kW and operate at 40 to 60 RPM

Aeration System Types 3
• Diffused aeration systems are classified as a coarse bubble of a fine pore.
• Fine pore diffusers produce 1 to 3 mm bubbles by passing gas through a punched membrane or porous stone. They are called fine pore to distinguish them from turbine aerators that create fine bubbles using mechanical shearing action
• Diffused aerators are also classified by geometry, such as “full floor coverage” or “spiral roll.” Full floor coverage systems spread the air across the entire tank bottom while spiral roll systems may have diffusers in narrow bands, often at the tank wall, and induce a rolling action of the fluid.

Surface Aerators
• Surface aerators belong to the first generation of oxygen transfer technologies. They are typically characterized by high OTR and low SAE values (in the range of 0.9-2.1 kgO2 kWh-1). Surface aerators shear the liquid into small droplets that are spread in a turbulent plume at several meters per second. The traveling droplets are in turbulent contact with the atmospheric air and typically oxygenate to at least half-saturation. As soon as they land on the free liquid surface, they mix with the liquid bulk, producing a typical DO pattern as in Fig. 2. There is no way to measure the mass of oxygen absorbed from the air around the aerator. Therefore, the SOTE or OTE cannot be defined. Efficiencies can only be quantified as SAE or AE.

• Surface aerators always “pump in a circle.”
• This means that there is always a DO gradient in the tank
• For entirely aerobic conditions, the fluid returning to the aerator must have positive DO, and it must be sufficiently high to keep the flock centers aerobic
• In nitrifying systems, especially fully loaded or overloaded systems, it is common to observe simultaneous nitrification-denitrification because the circulating fluid becomes anoxic at some point in the circulation pattern.

• High-speed surface aerators find their greatest application in lagoons or oxidation ponds. Often an overloaded lagoon is upgraded by adding surface aerators.
• Lake water depth is restricted when using surface aerators. The impeller must be at least one meter above the bottom and sometimes more depending on the lagoon materials. Surface aerators in too shallow water will “dig a hole” in the bay bottom, destroy liners, kick up rocks and soil causing treatment problems and damage the aerator (see picture)
• At greater depths, high-speed surface aerators require draft tubes, which extend the influence of the aerators mixing to lower depths. Surface aerators are rarely used at higher than 4 to 5 m depth unless they are equipped with lower propellers or draft tubes

• Low-speed aerators are more efficient but require greater support and are most successfully used when mounted on piers on decks.
• More engineering and planning are needed to use surface aerators, such as designing the structural supports, baffles and tank walls/bottom. Long delivery time is common
• Remember than a method to transfer the heavy aerator
(> 10,000 kg) Into and out of the tank or lagoon must be provided – heavy duty piers or crane access.
• Surface aerators with lower propellers can be successfully used in very deep tanks (~10m) and are commonly used with deep tanks in the high purity oxygen activated sludge process (HPO-AS) in the United States.

Empirical Design Considerations
• The surface spray or “umbrella” must never strike the tank walls or the cover if it is a covered reservoir.
• Reduced efficiency occurs, and erosion gradually destroys the tank (even concrete) or lagoon walls
• Manufacturers have empirical information on the diameter and height of the umbrella for their equipment
• Similarly, manufacturers have information on the zone of influence – horizontal and vertical, of the aerator
• Warranties usually include oxygen transfer rates as well as minimum fluid velocities (> 0.3m/sec), uniform TSS profiles, but never uniform DO profiles
• The design engineer’s job is not to determine the empirical design parameters, but to verify them, with independent testing, by witnessing shop testing, or observing operation in existing treatment plants.

Horizontal Shaft Surface Aerators
• Horizontal shaft aerators, called brushes or rotors, find application in oxidation ditches, and sometimes in lagoons
• They provide aeration as well as imparting a circulating velocity in the ditch (> 0.3m/sec at the bottom)
• Power input can be modulated by varying liquid depth or rotor submergence
• There are several manufacturers that provide vertical shaft aerators for ditches but require special geometry.
• In some existing installations, these aerators are being phased out for mixing pumps with fine pore diffusers

Other Considerations
• Surface aerators provide the greatest evaporation and, therefore, provide the greatest cooling. This is especially true in dry climates. Wind velocity is an important parameter. Surface aerators may cause a 4oC temperature reduction compared to fine pore aerators for the same conditions. This can be important to maintain nitrification in winter.
• Occasionally surface aerators are chosen simply because of their cooling ability, such as in petroleum refinery wastewater treatment in warm climates, or to avoid heat impacts of effluents on receiving waters
• Power draw is a function of propeller submergence. High water can overload fixed mounted aerators and burn on the motors
• As we shall see, surface aerators have higher alpha factors. They do not have the greatest clean water efficiency, but the higher alpha factors partially compensate.
• Surface aerators can usually be designed so that maintenance can be performed without dewatering the tank or lagoon.

Aeration Graph Sample

Aeration Graph Sample

 

Diffused Aeration
• Diffused aeration, sometimes called subsurface aeration, is divided into two categories:
– Coarse bubble, with orifices of 5 mm to 12 mm, producing large, non-spherical, rapidly rising bubbles that can be as large as 50 mm in diameter
– Fine pore, producing mostly spherical bubbles 1 to 3 mm in diameter, through porous plates, discs or domes (ceramic or plastic), or punched plastic or rubber membranes
• In former times, coarse bubble diffusers dominated the municipal field, but now fine pore diffusers are dominant, and they save at least half the power of coarse bubble system providing the same SOTR.

• Coarse bubble diffusers need little maintenance, and a system might be installed and operated five or more years without maintenance. Problems requiring maintenance are corrosion of piping or diffusers, line breakage, but rarely diffuser plugging since the orifices are so large.
• Fine pore diffusers always require cleaning. The frequency varies and is site-specific, and may vary from 6 months to 2 years. It is almost always necessary to dewater tanks to clean diffusers and downtime is rarely less than a week, which means plants must have redundant tanks or ways to reduce plant load during cleaning
• The choice between fine and coarse often depends on the plant’s ability to clean diffusers. If cleaning is not possible, fine poor diffusers are a very poor choice.

Coarse Bubble
• Coarse bubble diffusers were, in former times, installed in simple rows, which created a kind of spiral roll across the tank. Additional rows of diffusers created cross roll or “ridge and furrow” flow patterns.
• Modern coarse bubble installations place diffusers as uniformly as possible across the floor of the tank. These systems called “full floor coverage” are significantly more efficient that a cross roll configuration.
• Full floor coverage can provide as much as 3%/m SOTE while a spiral roll system may be as low as 1%/m.
• Both systems create large circulating liquid velocities in the tank, as much as 2m/sec at the surface

Fine Pore Diffusers
• Fine pore diffusers are called fine pore, as opposed to fine bubble, to indicate that the fine bubbles are created by a porous media, such as a ceramic stone
• Other devices, such as turbines and jets, can produce fine bubbles by hydraulic shear
• Fine bubble diffusers dominate the marketplace in Europe and North America. They save more than half the power required of coarse bubble diffusers, and are more efficient than surface aerators as well, although surface aerators still have preferred applications (e.g., HPO Activated Sludge).
• Fine pore diffusers in new installations are always mounted in full floor configurations. For retrofits, fine pore tube diffusers have been mounted on swing arms and air headers

Geometries
• Ceramic domes (~ 170 mm dia)
• Ceramic discs (~ 220 or 170 mm diameter)
• Membrane discs (~220, 170 mm dia and sometimes larger)
• Ceramic tubes (50 mm dia by 360 mm long)
• Membrane tubes (50 to 100 mm dia by 360 to 720 mm long)
• Membrane panels (usually polyurethane, 1 m wide by 4 m long)
• Membrane strips (usually polyurethane, 20 mm wide by 4 m long)
Membrane tubes and strips can usually be obtained in custom lengths and diameters

Pressure drop
• Both coarse and fine bubble diffusers present a pressure drop. The operating pressure of a diffused air system must include pressure drop in pipelines, the hydrostatic pressure of the water at the diffuser submergence, and the pressure drop of the diffuser.
• A diffuser pressure drop is called the “dynamic wet pressure” and includes both the pressure loss through the diffuser but also the surface tension of the fluid being aerated.
• Coarse bubble diffusers have very low DWP, generally only 5 to 10 mbar. Fine pore diffusers always have more, with ceramic devices having DWPs from 15 to 30 mbar. Membrane devices have higher DWPs, as much as 45 mbar. Consultant manufacturers data and verify DWP when clean water testing
• Fouled diffusers have much higher DWP. The DWP of a fouled diffuser can be more than twice its new DWP.
• When diffusers are highly fouled, several undesirable results are likely:
– The diffuser, especially if it is a membrane diffuser, may rip or tear away from it’s binding
–Centrifugal blowers may go into surge as the pressure increases beyond the safe range
–The motors on positive displacement motors may overload and burn out.
–In all cases, the transfer rate of the diffusers decreases and the treatment plant may be unable to maintain its rated capacity

Combined Types: Diffusion and Mechanical Shearing
• There are several types of aerators that use a combination of bubbles and mechanical energy to create fine bubbles without using small orifices
• Turbines are one major example that uses an impeller to shear coarse bubbles into fine bubbles
• Jet aerators are another type, which injects air into the throat of a Venturi to create fine bubbles
• There are other less common types, such as impeller blades that aspirate air into a discharge plume, rotating blades with porous surfaces, etc.

Turbines 2
• Turbines are commonly used in new treatment plant designs. The exception can be industrial designs, where a very high OUR must be satisfied.
• Turbines can have high SOTE, but are lower than surface aerators in SAE. The reason is the penalty associated with two prime movers – blower motor and mixer motor
• Finally, turbines make fine bubbles and have low alpha factors, like fine bubble diffusers. They do not have fouling problems.

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