Wastewater aeration basins

Sequencing Batch Reactors (SBRs) are a variation of the activated sludge process that combine treatment steps into a single basin. Based on a fill-and-draw method, major phases of the SBR process include a cycle of fill (with or without aeration), react, settle, decant, and idle. Today, successful flow control strategies have made SBRs state-of-the-art technology. SBRs are commonly used today and are especially beneficial for many industrial applications.

SBR Aeration And Mixing Systems

Like all activated sludge processes, SBRs need adequate aeration to properly treat wastewater, and biological nutrient removal requires mixing without aeration. Both aeration and mixing equipment must be installed at each tank or basin. Typical aeration/mixing systems include fine bubble diffusers combined with mixers or jet aeration systems. For both systems, centrifugal or positive displacement blowers provide low pressure air for oxygen transfer.

Fine bubble diffusers work by pumping the air through a grid of diffusers that cover the bottom of the aeration tank. The diffusers have micron-sized holes, letting air pass through and rise to the surface. Fine bubble diffusers exhibit high clean water aeration efficiency. This is due to the large amount of surface area of the tiny bubbles, leading to a high oxygen transfer rate. Jet aerators transfer oxygen by simultaneously introducing large volumes of liquid (mixed liquor suspended solids or MLSS) and air through a series of jet nozzles. The liquid stream is furnished by recirculating the MLSS using centrifugal pumps. Intense mixing and a high degree of turbulence  rapidly dissolves the air into a fine dispersion  of gas and liquid. The plume mixes with and entrains the surrounding liquid. Each individual jet plume travels horizontally along the basin floor, then rises to the surface. Both technologies work well. However, jet aeration systems have multiple advantages for the operation and maintenance of SBRs.

Less Maintenance And Better Efficiency

Fine bubble diffusers certainly have superior oxygen transfer in clean water. They are super-efficient at plant startup. Yet, they tend to lose efficiency quickly due to biofouling  or inorganic fouling. To maintain their high efficiency, fine bubble systems must be monitored and cleaned periodically. Cleaning requires emptying the basin, which may be problematic for some facilities especially if no redundant basins are available. Also, the diffusers must be replaced as  they wear out, perhaps as  often as  every five years. Diffuser cleaning and replacement becomes a major project for the operators. To reduce the high cost of taking a basin out of service, there is a trend the past few years to supply retrievable fine bubble diffusers.  The diffusers are supplied on multiple racks with each requiring an individual lift-out mechanism. These systems have a much higher capital cost, and typically it is not practical to supply them in a full floor-coverage configuration which reduces aeration efficiency.

In  comparison, one  of  the  biggest advantages of jet aeration is its ability to transfer oxygen in a highly polluted environment typical of most industrial effluents. The hydrodynamic conditions within  the jet results in higher alpha factors than fine bubble diffusers.   The high alpha factor combined with  the clean water oxygen transfer performance results in the most energy-efficient design for industrial applications.

With jet aeration, the openings are large — one to two inches in diameter. Plugging is rarely a problem. To ensure consistent oxygen transfer, jet aerators include an airlift backflush system  to run biweekly for five to ten minutes as a preventive maintenance technique. Backflushing can be automated to make the process even easier. Also, the jet aeration components are made of corrosion resistant vinylester resin fiberglass and  stainless steel. These systems are robust, designed for a 20-year life span. Operators won’t need to regularly empty a basin for maintenance when using jet aeration systems.

Improved Operational Control

More flexibility allows operators to better control the SBR process.   Jet aeration systems are extremely adjustable.

With jet aeration, the liquid flow rate is constant, while the air rate is adjustable. Air flow and aeration gassing rates can be operated at a very wide range.  Operators can change the air-to-liquid ratio. Increasing or decreasing the air flow while keeping the liquid flow constant gives the operator the ability to meet minimum and maximum oxygen requirements. For instance, at the start of the fill phase, more air may be required but, as pollutants are removed during the react phase, less air may be needed. Adjustments to the air can be programmed or manually changed as desired.

If anoxic conditions are needed for nutrient removal, the air can be turned off for a specified time. The liquid jet stream will continue to mix the tank contents. Whereas fine bubble diffuser systems need mixers to provide nutrient removal, none are required with jet aeration.

Additional Process Control  And Efficiency Slot  Injector™  systems are superior jet  aeration systems that  improve energy efficiency and allow additional operational control. These jets have slot- shaped propulsion nozzles and mixing chambers that provide a greater shear surface for mass transfer.  This  results in more efficient gas dissolution while maintaining  all of  the  features of  a conventional jet system.

Slot injectors operate at a much higher pressure on the liquid side. They  can be used with variable frequency drives (VFDs) on the motive pumps for energy savings. As oxygen demand reduces over time, an operator can turn down both the air and the pump flow. Pump flow can be reduced by up to 30 percent at any time during the fill or react cycle without compromising the basic mixing functions of the system. This greatly reduces the operating horsepower for a more energy- efficient operation.

The operator has more control over the process  as well. With slot injectors, they can also simultaneously turn down the pump, reducing the liquid stream, and turn up the air flow. This creates larger bubbles, with the potential to improve bioflocculation in  applications where gravity settling can get challenging at times.

Overall, Slot Injector™ systems are 10 to 15 percent more efficient than standard jet aeration systems. Since 2006, well over 100 industrial facilities have been using this superior jet aeration system.

Choosing  The Best SBR Aeration System

Many factors must be considered when designing or modifying an SBR facility. Aeration and mixing are the key factors in SBR performance. A system that puts operators in control while improving efficiency will improve the environment while saving costs. Jet aeration systems are proven to be flexible,  robust,  and efficient.