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 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
• 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.