Why Use A Brush Aerator?

Horizontal rotor aerators, commonly referenced as “brush” aerators due to the rotor’s resemblance of a brush, are ideally suited for a wide range of applications in an equally wide variety of reactor types, styles, and configurations.  Originating in the “oxidation ditch” type reactors in Europe, the initial fixed- rotor concept was adapted to a floating platform in the southeastern United States for use in the shallow-pond aquaculture industry.  From there, the floating platform horizontal rotor aerator transitioned to installation municipal wastewater lagoons, particularly facultative and partially-mixed lagoons, where a highly oxygenated surface layer is desired, yet the depths of the pond are allowed to become anoxic or anaerobic.  However, applicability of the floating horizontal rotor is not limited to just facultative or partially-mixed lagoons.  Indeed, well-mixed hydraulic conditions may be established and maintained within a lagoon, or other reactor, using horizontal rotor aerators.  This expands the flexibility and suitability of floating platform horizontal rotor aerators into wastewater treatment processes including influent equalization, aerobic treatment, digestion, and sludge storage.

The flexibility of the floating platform horizontal rotor aerator is due to its operational performance, inclusive of its oxygen transfer capability and discharge, or mixing, capability.  While these capabilities are often discussed exclusively, it is important to understand that they are inter-related.

With regard to oxygen transfer performance, a key factor in achieving superior oxygen transfer performance is to maximize the dissolved oxygen differential across the aerator unit.  Stated differently, the greatest oxygen transfer efficiency is achieved by initiating the transfer process with a bulk liquid containing minimal, preferably no, dissolved oxygen. This condition maximizes the rate of oxygen flux through the surface of the water droplet.  When coupled with a rotor and blade design that maximizes droplet formation and minimizes droplet size, the result is a superior oxygen transfer capability.

To achieve a minimal dissolved oxygen concentration entering the aerator rotor, the aerator must be capable of rapidly moving the highly oxygenated liquid away from the rotor’s immediate area of influence to allow minimally oxygenated liquid to be drawn into the aerator. Horizontal rotor aerators, by design, have a significantly greater liquid displacement capability due to the surface area of the rotor actively involving the bulk liquid.  Because of this, the horizontal rotor aerator, both fixed-platform and floating platform configurations, are capable of displacing between 3 and 10 times the liquid of other types of conventional mechanical surface aeration.

The most significant advantage of the horizontal rotor over other types of mechanical surface aeration equipment is the horizontal rotor’s discharge profile. Unlike the splasher-type aeration equipment, with its primarily vertical mixing profile, or the aspirator-type aeration equipment, with its significant vertical mixing component, the horizontal rotor aeration equipment discharges primarily parallel with the pond surface, with only minor vertical mixing in the immediate vicinity of the aerator unit.  When coupled with the greater discharge capacity, the floating platform horizontal rotor aerator is able to establish and maintain a highly oxygenated surface layer within a basin, while allowing the depths of the basin to remain a relatively quiescent and oxygen limited environment. The depth of active mixing is managed by the horizontal rotor aeration horsepower applied to a specific basin volume.

Conventional rules-of-thumb suggest an applied mixing energy of 8 HP of conventional mechanical surface aeration equipment per million gallons of basin volume is needed to achieve a hydraulic regime considered to be partially mixed. However, the lack of a significant vertical mixing component and a displacement rate of 3 to 10 times that of other equipment allows horizontal rotor aeration equipment to achieve partially mixed conditions with the application of 4 HP of aeration equipment per million gallons of basin volume.

The horizontal rotor aerator’s discharge profile does not limit its applicability to surface aeration and mixing processes typical of partially mixed aerated lagoons. Indeed, horizontal rotor aerators are capable of establishing and maintaining well-mixed conditions typical of activated sludge treatment processes, aerobic digestion processes, influent equalization basins, etc., as demonstrated by their widespread use in oxidation ditch-type extended aeration activated sludge processes. Well-mixed conditions are achieved through intentional constriction of the aerator’s discharge profile, either through physical constriction such as a reactor wall or a “hydraulic” constriction such as the proximity of adjacent aeration equipment.  The aerator’s rotor design dictates a relatively constant discharge rate for a given blade submergence.  Instead of allowing the discharge energy to extend over a large area at a limited depth, as desired in surface aeration and mixing applications, the constrictions drive the mixing energy to a greater depth within the aerator’s area of influence.   Thus, for a constrained horizontal rotor aerator, feed water for the rotor is drawn from below the rotor rather than from the surface perpendicular to the direction of discharge.

Using conventional mechanical surface aeration equipment, applied mixing energies between 75 and 200 HP per million gallons of basin volume are necessary to achieve “well-mixed” hydraulic conditions within a reactor.  In contrast, applied mixing energies between 30 and 100 HP per million gallons of basin volume are capable of achieving “well-mixed” conditions when using horizontal rotor aeration equipment.

The level of applied mixing energy is generally expressed in the arbitrary terms of horsepower per unit volume. However, this approach negates the mixing performance of the selected equipment. A more reasonable approach, which considers equipment mixing performance in determining the appropriate size of the aeration system to be used, is the concept of turnover time. Turnover time is the theoretical length of time necessary to circulate a unit volume from the aeration equipment, through the reactor volume, and returning to the aeration equipment. In general, a basin is considered well mixed when the total volume passes through the aeration equipment at least six times each hour (i.e. a turnover time of 10 minutes or less).  Because of the significantly greater discharge rate of the horizontal rotor aerator, at least 3 times that of high-speed “splasher-type” aerators and more than 10 times that of aspirator-type aerators, the desired degree of mixing within a reactor may be achieved with significantly less energy input.