There are various ways to deliver air to a secondary wastewater treatment process. Some involve blowers feeding diffusers on the bottom of a basin. Others are mechanical, introducing air through a stirring action.

Now Aqua-Aerobic Systems offers OxyStar aspirating aerators for municipal and industrial wastewater applications. Rated at 3 to 30 hp, they can be used to deliver oxygen and mixing to oxidation ditches and also to lagoons, aerobic digesters and equalization basins.

The units provide efficient fine-bubble aeration by circulating and mixing basin or tank contents. They are well suited for retrofit in process upgrades or to supplement or replace aeration systems that no longer perform adequately. Alan Rice, product manager with Aqua-Aerobic Systems, talked about the technology in an interview with Treatment Plant Operator.

What was the rationale for bringing this technology to the market?

Rice: Aqua-Aerobic Systems has a long history in manufacturing surface aerators, and there is a market need for directional aerators. A number of processes, including oxidation ditches, require directional aeration and mixing, for which our typical aerators would not be a good fit.

How do you define a directional aerator?

Rice: Our traditional aerators spray water in a radial pattern. OxyStar units pump water downward and forward at a 45-degree angle. Oxidation ditches have long, narrow channels that need this directional mixing to maintain a certain velocity throughout the channel and to hold the solids in suspension.

In basic terms, how do these devices work?

Rice: They mix and oxygenate the water by rotating a submerged propeller. As the propeller spins, it creates a low-pressure zone below the surface. That pressure gradient aspirates air through a hollow shaft and discharges it into the water. There the turbulence created by the propeller shears it into fine-bubble aeration.

Would this technology also be suitable for traditional secondary aeration basins?

Rice: That is a different treatment process used mostly in larger communities. Oxidation ditches are designed for smaller communities, and that is the No. 1 application for these aspirating aerators.

Where does this product fit in with the other applications mentioned?

Rice: It fits well in cold-weather areas where other surface aerator technologies could cause ice buildup. The aspirator is completely subsurface, which means we’re not splashing water or creating aerosols or mists. In a northern state during winter, that moisture can potentially freeze and coat plant equipment with ice. In addition, for plants in residential areas, those aerosols and mists can be picked up in the wind and bother local residents.

Do these devices produce air bubbles as fine as those from traditional bottom-mounted membrane-type aerators?

Rice: It creates fine bubbles by a different mechanism. Rather than create bubbles at the bottom of the basin, we draw air into the basin and shear it with a propeller. That creates fine bubbles according to the U.S. EPA definition.

Why is this technology more suitable in certain applications than bottom-mounted membrane aerators?

Rice: These units are especially designed for shallow operation. The efficiency of membranes on the floor is highly dependent on how deep the basin is. They are very efficient at 20 feet deep, but they’re not efficient at all at 5 feet deep. The efficiency of our units is independent of depth, and so it makes a lot of sense in shallow operations.

What other major advantages does this technology have?

Rice: It is very mechanically simple. There are only three moving parts — a motor shaft, a coupler and a propeller — and they are all above the surface. In addition, the motor bearings are sealed for life, so there is no regular maintenance to perform.

What is involved in installation of these units?

Rice: They are very simple to install. The units float on the surface, so all that’s needed is a crane, a hammer drill and a set of bolts. You drill the bolts into the side of the basin, set the units in, and the work is essentially complete. They can be installed within hours. The primary use for these aerators is in retrofits.

Can they be installed without taking an oxidation ditch out of service?

Rice: Yes. Installations can be completed while the basin is full and operating.

How would you characterize the energy efficiency of these aerators?

Rice: They have energy efficiency similar to brush rotor aerators.

What features make the aerators durable?

Rice: They are long-lasting because there is so little maintenance to do. There are no submerged seals or bearings that can be damaged by grit or solids in the wastewater. The pontoons are molded polyethylene and built for buoyancy, and they are connected with a stainless steel framework.

Is there a sweet spot in the size of facilities for these units?

Rice: They make the most sense for facilities smaller than 3 to 4 mgd. On the lower end, we can fit almost any size facility, but the sweet spot would be 0.1 to 1 mgd.

How many of these units does it take to serve a 1 mgd facility?

Rice: For mixing we would typically use two 30 hp units. For oxygen transfer, we would require four units.

So far, how have customers responded to this technology?

Rice: They have spoken highly of it. Most have used it to replace old equipment. A small town in Alabama replaced an old brush aerator with the OxyStar and reported a doubling of mixing capacity. They have basically no maintenance issues. Customers have been happy with the performance and the low maintenance of our unit.