The hardest aspects of comparing efficiencies among submerged aeration technologies are the many variables in capital expense (CAPEX), operating expense (OPEX), and complexity of wastewater makeup. Instead, consider comparing the physics of different aeration technologies and the relative costs associated with them. Here are five major categories for evaluating industrial wastewater aeration efficiency.
Oxygen Transfer Rates
The key to efficient aeration is maximizing the oxygen transfer in the mixed liquor. There are three major factors to oxygen transfer rates:
- bubble size, which dictates the surface-area-to-volume ratio of entrapped air (Figure 1),
- the behavior of the gas/liquid interface where oxygen transfer occurs, and
- the effect of flow turbulence on the surface renewal rate of the liquid-film interface.
Figure 1. Twenty-seven 3-mm diameter bubbles containing the same volume of air as a single 9-mm diameter bubble offer 3x the surface area for potential oxygen transfer.
This guide provides a good tutorial on the physics of oxygen transfer rate with relation to liquid-film mass transfer coefficient (KL), the overall mass transfer coefficient (KLa), and the driving force of jet aeration.
Jet aeration’s turbulence enhances the liquid surface renewal of aeration bubbles to provide better mass transfer of oxygen than the molecular diffusion relied upon with the laminar flow rising vertically from a fine-bubble diffuser. If heavy concentrations of organics in the water impede molecular diffusion, the oxygen transfer rate will drop. Jet aeration turbulence minimizes the effects of depressed oxygen transfer due to impeded molecular diffusion (Figure 2).
Figure 2. Unlike the laminar flow of bubble-diffusion techniques, jet aeration turbulence at the air/liquid interface of a bubble improves the surface renewal rate of liquid around the bubble. This negates the limiting effects of surfactants and other organic substances in industrial wastewater that can hamper oxygen transfer rate.
Slot injectors™ are a type of jet aerator that use a slot-shaped nozzle and mixing chamber provide a larger shear surface for mass transfer than a circular jet of identical cross-section. This unique shape results in a high percentage of gas dissolution and superior oxygen transfer rates, with a 10-20 percent reduction in energy usage as compared to conventional jet aerators (Figure 3).
Figure 3. By distributing the same amount of air in smaller volume of fluid than conventional jet aeration, and using VFD controlled pumps, Slot Injector™ technology can improve upon jet aeration energy efficiency by up to 20 percent.
Challenging Wastewater Environments
There is no debate that fine-bubble air diffusers are an effective and cost-efficient approach in municipal wastewater treatment plants (WWTPs) where biochemical oxygen demand (BOD) runs between 100 and 200 mg or oxygen consumed per liter. In industrial wastewater environments, however, where typical BOD can often run between 1,000 and 2,000, fine-bubble performance can struggle due to a low “alpha” factor and biological or chemical fouling issues with the diffusers.
The advantages of jet aeration in those challenging industrial applications arise from the integration of fine/medium-bubble aeration entrained in a jet of liquid. When that aerated liquid is discharged horizontally into the aeration basin, it travels a longer path that extends contact time while providing turbulence to improve the surface renewal rate for enhanced oxygen transfer efficiency.
While dirty-water oxygen transfer efficiency is difficult to measure as a quantitative value, jet systems have gained a reputation for exceeding predictive performance in situ at challenging industrial environments. These include applications where bubble diffusers have struggled — from chemical plants, refineries, leachate treatment systems, textiles, and pharmaceuticals, to food and beverage applications. Many of these applications are upgrades/retrofits to replace fine-bubble aeration systems that lasted only a few years due to fouling or low oxygen transfer rates from chemical precipitates or from the more concentrated mixed liquor.
Disruptive Chemistries
An important question to answer before choosing an industrial wastewater aeration technology is, “What’s in the wastewater?” Beyond with BOD and nitrogen removal, certain elements tend to reduce oxygen transfer efficiency. Look for aeration technologies that can combat the following problems.
- Oil and grease can cause problems in food and beverage applications, and it is imperative that upfront operations remove all emulsified and floating fats, oil and grease. If emulsified oil and grease get into the aeration tank, they will wreak havoc with any type of submerged aeration device.
- Heavy organic loads that typically require a more concentrated mixed liquor for proper biological treatment can quickly cause biofouling, even in new diffuser systems.
- Surfactants, particularly anti-foam additives used in industrial processes, can seriously compromise the efficiency of oxygen transfer of any submerged aeration device. Other manmade or naturally occurring surfactants typically depress the oxygen transfer of fine bubble diffuser systems but rarely have a negative effect on jet aeration systems.
- Dissolved solids precipitation can spawn specific problems in certain industrial applications. While anaerobic treatment processes are often used to remove up to 80 percent of high BOD levels, dissolved solids like calcium often are impacted by changes in solubility as a result of the process. With CO2 being a natural byproduct of aeration, certain applications will tend to form calcium carbonate, which can precipitate out of the water and coat aeration diffusers, piping systems, tank walls, and equipment in a matter of a year or two. In extreme applications where >1000 mg/l of calcium are present, jet aerators have been prone to fouling but only after 8-10 years of service, which is an acceptable length of time for most industries.
Equipment Logistics
While differences in bubble-size between fine-bubble diffusers and jet aeration techniques are minimal in mixed liquor, pumping liquid along with air through the jet diffuser nozzles provides distinct advantages as there are orders of magnitude fewer diffusers required. From an in-basin equipment perspective it typically requires far fewer aerators, pipes, and supports, making it a much simpler installation. A fine bubble diffuser system requires “full floor coverage” with diffusers packed tightly together along the entire surface of the basin floor. When you add in all the associated pipe and supports, the installation time is also much greater.
Compared to diffusion, a jet aeration retrofit can reduce both CAPEX and OPEX while increasing wastewater handling capacity in an existing aeration basin. For example, in a coarse-bubble diffuser application using a 200 hp of blower capacity, jet aeration can double treatment capacity with the addition of a 100 hp pump to the existing 200 hp blowers in operation, instead of requiring another 200 hp blower of capacity with associated piping system, potential civil works and motor control centers. The total 300 hp energy requirement for the new capacity would also be lower than the 400 hp energy costs of the blowers required for the coarse-bubble diffuser upgrade. In applications where CAPEX is the major concern relative to OPEX, industrial users may opt to install a coarse-bubble diffuser and accept the relatively higher energy costs.
Turndown capacity is another aspect of equipment logistics that can boost overall efficiency without compromising performance. Slot injector aeration systems with VFD controlled air blowers and liquid pumps have turndown capability that can save up to an additional 20 percent of energy during periods of lower oxygen demand compared to jet systems that can only turn down the air blowers.
Maintenance Impacts On Aeration and Cost Efficiency
Higher exit velocities, the scouring of the liquid/air stream, and larger nozzle openings (measured in inches) all make jet aeration nozzles less prone to fouling than coarse-bubble or fine-bubble diffusers (where pore openings are measured in millimeters). Self-cleaning jet aeration nozzles with an airlift backflush system can also drastically reduce the frequency and cost of routine cleaning maintenance (Figure 4). The self-cleaning jet-aeration approach has a proven 20-year design life in the field, well beyond that of submerged diffuser technology.
Figure 4. Self-cleaning jet aeration technology enhances the length of service life by reversing the flow of air and water to drive accumulated materials out of the system without the need to take the aeration basin out of service.
Finally, regardless of the aeration technology chosen — bubble diffusers or jet aeration —removable aeration headers are becoming the trend to simplify maintenance and eliminate the high cost of taking basins out of service.
