Too Much Air? Understanding the Critical Role of Aeration Systems

Excessive aeration in activated sludge and aerobic digestion processes can waste energy and impede treatment performance.
Too Much Air? Understanding the Critical Role of Aeration Systems
Hallstead Great Bend (Pennsylvania) Joint Sewer Authority.

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Wastewater operators today are saving energy, using fewer chemicals and improving control of their activated sludge process by optimizing their aeration systems, including aerobic digestion.

Aeration zones are critical in the activated sludge process, which includes secondary treatment and nitrogen and phosphorus removal. The question is: Can you over-aerate that process? Many operators would say, “No, the more the better!” But the real answer is: “Yes.”

Too much is not good

Over-aeration wastes energy and can negatively affect process performance. Most wastewater treatment plants operate their aeration zones and aerobic digesters at 1-3 mg/L dissolved oxygen (DO). Anything more may waste DO and energy. Even aerating at 1 or 2 mg/L over the DO setpoint can be extremely wasteful.

Over-aeration can also cause operational problems. Operators love a mixed liquor that settles well. This happens when the microorganisms in the aeration tank excrete a sticky film around their cells as their food gets depleted. Aeration keeps the bugs in suspension, allowing them to collide and ultimately stick together, forming a floc. The floc exhibits a snowball effect: By the time it reaches the secondary clarifier, it is denser than water and settles. Over-aeration can break this floc apart, causing pin floc or small, dispersed floc that does not settle well.

Anoxic zones used for denitrification can also turn partially aerobic, or oxic, due to the high DO in the internal (nitrate) recycle flow. In a biological nutrient removal facility, internal recycle flow can be 400 percent or more of the influent flow. This recycle flow pulls liquid that is high in nitrates from the end of the aerobic zone, then discharges it back to the head of the anoxic zone, where there should be very little or no DO.

An internal recycle saturated with DO could stop the denitrification process because the bugs use the free oxygen (DO) rather than the chemically bound oxygen attached to the nitrate (NO3) ion to respire. The presence of DO inhibits some desirable biological processes, particularly denitrification.

Operators rely on denitrification not only for nitrogen removal, but also for the alkalinity that is released in the denitrification process. If denitrification is inhibited, the operator may have to add chemicals to keep the alkalinity and pH in check. Alkalinity is so important that many operators add a denitrification step even if it’s not needed for nitrogen removal, just to bring back some free alkalinity and oxygen to the system and save money on chemicals.

How much is needed?

It’s a myth that the bugs need air 24/7. Here are two quick points that refute the myth:

  • DO may not deplete immediately after the air is shut off. This depends on the time of day, organic loading, temperature, and the type of diffusers. It may take half an hour or more for the DO to be reduced to zero.
  • Oxygen exists in three main forms in a treatment plant: DO (O2), nitrite/nitrate ions ( NO2, NO3), and sulfate ions (SO4). After the air is shut off and DO approaches zero, there still may be plenty of chemically bound oxygen (NO3) available for BOD removal (denitrification).

Another myth is that the mixed liquor suspended solids (MLSS) will not re-suspend if the aeration system is shut down. All scenarios are case-specific, but many facilities can re-suspend the solids even if the air has been off for an hour or so. In general, the required DO for solids processing is 0.5 mg/L in the floc and 2.0 mg/L in the mixed liquor.

What to do 

If you are like the many operators who enjoy finding creative and better ways to do things, fine-tuning aeration may be another opportunity to make process improvements and save money. Aeration design and control can get very complex, but it can also be done in low-tech, energy-efficient ways. Let’s start with the basics.

Monitor DO

Probably the easiest way to monitor DO is with a hand-held meter with a data logging function. Set the probe in the mixed liquor for as long as the data logger will collect data (one reading per half hour, 48 readings per day as a starting point). Plot the DO for a week or so on a chart. If the DO is over or under your ideal setpoint, reprogram the controls to increase or decrease aeration, or even turn off the aeration at a high DO setpoint for a certain period. Where you put the probe in the aeration tank matters. Think about the ideal setpoint for the location you are measuring to get the desired treatment. Also, ask your consulting engineer what your DO setpoints should be.

If you nitrify, why not denitrify?

What kind of question is this? Why would you want to do more work? The short answer is that adding a denitrification step may save energy and chemicals and benefit the environment. The nitrification process consumes a lot of energy through aeration and also consumes alkalinity. What’s needed to nitrify?

  • DO range: >2 mg/L [1]
  • ORP range: +100 mv to +350 mv [2]
  • About 4.57 pounds of oxygen consumed per pound of ammonia nitrified
  • About 7.14 pounds of alkalinity consumed per pound of ammonia nitrified
  • Time

On the other hand, denitrification occurs under anoxic conditions. By decreasing the DO, nitrate is further reduced to nitrogen gas. Important points about denitrification:

  • DO range: <0.2 mg/L [3]
  • ORP range: +50 mv to -50 mv [3]
  • About 2.86 pounds of oxygen released per pound of nitrate denitrified
  • About 3.57 pounds of alkalinity released per pound of nitrate denitrified

Denitrification is often thought of as a method to decrease effluent nitrogen, but it’s also a great way to gain back some of the oxygen and alkalinity consumed in the nitrification process.

Success stories

Here are a few real-life examples of treatment plants improving aeration efficiency:

1. Hamilton Township

The operator of the Hamilton Township Wastewater Treatment Plant in Ludlow, Pennsylvania, applied knowledge gained from an energy efficiency training conducted by the U.S. EPA Region 3 and the Pennsylvania Department of Environmental Protection. The plant’s 0.07 mgd activated sludge process includes ammonia removal. The operator chose to cut DO from 6-9 mg/L to 2 mg/L.

He first cut back the runtime of two 15 hp blowers from 24 hours per day to 12 hours by alternating the blowers on and off. Over time, he saw that he could cut runtime to nine hours per day while maintaining permit compliance. The electric bill dropped almost 40 percent, from $13,000 to $8,000 per year.

2. Exeter Township Authority

Optimization was more complex for this 7.1 mgd plant in Pennsylvania, which also has an activated sludge process with ammonia removal. During the warm summer months, the aeration tank detention time was more than enough to achieve ammonia compliance. The operations manager knew that if he could denitrify at the front end of the aeration tank by closing some air-supply valves, he would bring back alkalinity, reducing chemical costs and saving energy.

The aeration system consisted of a multistage centrifugal blower with an automated inlet valve controlled by the header pressure. By closing the aeration drop leg ball valves to about 90 percent (to allow some air for mixing) in the first third of the tank, a successful anoxic zone was created. Closing off one-third of the diffusers raised the air header pressure, thus automatically closing the blower inlet valve somewhat to keep the header pressure constant. A partially closed inlet valve admits less air, reducing energy required by the blower.

A power logger installed on the blower during the cold season (100 percent aerated tank) and warm season (66 percent aerated, 34 percent anoxic tank) recorded the savings. Energy use was cut almost in half, saving about $5,000 per month. To achieve further savings, the plant is investing $100,000 to upgrade its aeration system and controls to automated DO control and “most open valve” control technology.

Remember the digesters

Aeration efficiency improvements also apply to solids digestion. The main function of aerobic digesters is to reduce the amount of solids to be managed. To reduce the solids in the digester, operators must treat it in a manner similar to the activated sludge process: maintain 1-2 mg/L DO and the proper pH and alkalinity.

Aerobic digesters often accumulate a large amount of nitrate. As the bugs consume each other (endogenous respiration), there is a release of nitrogen compounds. These compounds are oxidized, nitrate is formed and acid is released (alkalinity consumed). This represents another opportunity for on/off aeration to reduce the nitrate and maintain alkalinity levels.

Here are two success stories:

1. Hallstead Great Bend Joint Sewer Authority

This agency in Pennsylvania upgraded from conventional secondary treatment to a four-stage Bardenpho process with chemical phosphorus removal and denitrification filters. The digester decant levels for nitrate-nitrogen (NO3-N) during summer 2014 were over 80 mg/L. The operator experimented with on/off instead of continuous aeration, setting the blowers to run two hours on and two hours off. This reduced the NO3-N discharged back to the main process to 0.18 mg/L, saving 54,000 kW ($4,300) per year. Added benefits are a reduction in concentrated nitrate in the supernatant and the restoration of alkalinity to the system.

2. Borough of Pottstown

This 12.85 mgd plant in Pennsylvania has been investing in energy efficiency for several years.

In 2008, the borough replaced the aerobic digester coarse-bubble diffusers with fine-bubble diffusers. By cutting the need for two 250 hp blowers in half, the plant saved $72,000 per year and achieved payback in four months. In 2010, new positive displacement blowers with variable-frequency drive controls were installed in the aerobic digester. The more efficient blowers supply air at lower horsepower. The added aeration capacity enabled the plant to haul in more waste and increase revenue. Net savings are $36,000 per year.

The latest investment was a $200 timer for the activated sludge blower. The plant electrician wired the timer and set the “on” cycle for four hours and the “off” cycle for four hours, reducing energy usage by half. This enabled the creation of an anoxic environment for denitrification. The change saved $10,000 per year on energy and $50,000 per year by eliminating lime and soda ash addition for pH and alkalinity adjustment. To top it off, mixed liquor settling characteristics improved.

Monitor the changes

With any process change, it is necessary to involve the plant staff, the regulatory agency and the consulting engineer. It is best to make small, incremental changes and wait for the results. It is important to collect data, including DO levels, ORP readings and alkalinity concentrations to be sure you’re on track. When aiming for low to no DO in anoxic zones, low-DO filaments could begin to thrive, slowing down settling. Changes require frequent monitoring.

When considering a project to save on energy and chemicals, discuss it with management. One way to amplify the savings is to invest them in other efficiency projects. It is important to have a champion to lead each project, to communicate and win buy-in up and down the chain of command, and to adhere to a “plan, do, check, act” approach.

Although decreasing aeration to improve performance may seem counterintuitive, the success stories show what can be achieved. There are numerous benefits to implementing energy- and chemical-saving projects sooner rather than later.


Find out more

For more information about optimizing aeration processes to save energy and improve process performance, readers may contact the U.S. EPA Region 3 Energy/Optimization Team.

  • • Walter Higgins, 215/814-5476, higgins.walter@epa.gov
  • • Jim Kern, P.E., 215/814-5788, kern.jim@epa.gov

Other resources include:

  • • Website: water.epa.gov/infrastructure/sustain/water efficiency.cfm.
  • • Ensuring a Sustainable Future: An Energy Management Guidebook for Wastewater and Water Utilities: http://www.deq.virginia.gov/Portals/0/DEQ/PollutionPrevention/EPA_WWTP_guidebook_si_energymanagement.pdf

For funding opportunities, readers may contact their State Revolving Fund office: water.epa.gov/grants_funding/cwsrf/contacts.cfm.



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