Aeration Systems – Liquid Aeration part 2
Aeration System Types 2
• The surface aerator can be further classified into high-speed, low-speed, with horizontal or vertical shafts.
• High-speed surface aerator use motors without gearboxes and a propeller that looks like a boat propeller. They are usually smaller than 50kW and run at 900 to 1200 RPM
• Low-speed aerator always use a gearbox, can be as large as 150kW and work at 40 to 60 RPM
Aeration System Types 3
• Diffused aeration systems are classified as a coarse bubble of a fine pore.
• Fine pore diffuser produces 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 aerator that creates fine bubbles using mechanical shearing action
• Diffused aerator is 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 the diffuser in narrow bands, often at the tank wall, and induce a rolling action of the fluid.
• Surface aerator belongs 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 aerator shears 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 aerator 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 see simultaneous nitrification-denitrification because the circulating fluid becomes anoxic at some point in the circulation pattern.
• High-speed surface aerator finds their greatest application in lagoons or oxidation ponds. Often an overloaded lagoon is upgraded by adding surface aerator.
• Lake water depth is restricted when using the surface aerator. The impeller must be at least one meter above the bottom and sometimes more depending on the lagoon materials. Surface aerator 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 aerator requires draft tubes, which extend the influence of the aerator mixing to lower depths. Surface aerator are rarely used at higher than 4 to 5 m depth unless they are equipped with lower propellers or draft tubes
• The low-speed aerator is more efficient but need greater support and are most successfully used when mounted on piers on decks.
• More engineering and planning are needed to use the surface aerator, 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 aerator 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 decide 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 aerator
• Horizontal shaft aerator, called brushes or rotors, fine application in oxidation ditches, and sometimes in lagoons
• They offer aeration as well as imparting a circulating speed in the ditch (> 0.3m/sec at the bottom)
• Power advice can be modulated by varying liquid depth or rotor submergence
• There are several manufacturers that offer vertical shaft aerator for ditches but need special geometry.
• In some existing installations, these aerators are being phased out for mixing pumps with fine pore diffuser
• Surface aerator gives the greatest evaporation and, therefore, give the greatest cooling. This is especially true in dry climates. Wind speed is an important characteristic. Surface aerator may cause a 4oC temperature reduction compared to fine pore aerator for the same conditions. This can be important to support nitrification in winter.
• Occasionally surface aerator 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 aerator and burn on the motors
• As we shall see, surface aerator have higher alpha factors. They do not have the greatest clean water efficiency, but the higher alpha factors partially compensate.
• The surface aerator can usually be designed so that maintenance can be performed without dewatering the tank or lagoon.