Jul 19, 2021

Fine Bubble Lagoon Aeration

Funding support & lower energy costs support cleaner water outcome

air diffusion systems
Air Diffiusion Systems technician Chris Milton stands over an aeration unit.

Air Diffusion Systems fine bubble diffusion has had a transformative effect on the treatment at Annawan, Illinois, Wastewater Lagoon System. With 90% of system funding coming from the Illinois EPA, Annawan has been able to provide improved water quality to the rural community at a fraction of the electrical cost to operate the aeration system.

Incorporated in 1853, the Village of Annawan, Illinois, in Henry County likely gets its name from a Native American chief. A channel developed at the turn of the 20th century in a nearby river turned the area into productive farmland and made way for the development of Annawan’s commercial areas.

The original wastewater treatment plant (WWTP) was permitted and constructed in 1980, which was during an era of population growth and industrial expansion for Annawan. This system utilized static tubes and coarse bubble aeration for wastewater treatment. As time progressed, village managers realized the system they designed was oversized, and the technologies used had become outdated.

The original lagoon plans were designed to treat a flow of 165,000 gallons per day, more than double the approximately 80,000 gallons that the village presently sends to the system each day. Because it was designed in anticipation of a much higher population growth rate and subsequent flow rates than the town experienced, the old lagoon system was no longer treating the wastewater efficiently.

The system managers and surrounding community had concerns regarding the high energy costs, total suspended solids discharge limits and sludge accumulation within the lagoon system, prompting Annawan to look for opportunities to improve the lagoon treatment.

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Smart Energy Award

In 2018, the Village of Annawan operator, Mark Crosby, contacted the Smart Energy Design Assistance Center (SEDAC), an organization that provides cost-free energy evaluations to wastewater treatment plants in Illinois for an assessment to identify improvement opportunities for the village’s lagoon system.

SEDAC recommended “fine bubble aeration with blower controls, and downsizing blowers.” This recommendation led the village to Air Diffusion Systems (ADS) for its wastewater treatment. Using these recommendations, village management applied for funding from the Illinois EPA (ILEPA) Wastewater Treatment Plant Energy Assessment Program, designed to fund energy-efficient projects at public-owned WWTPs. The ILEPA Office of Energy has partnered with the Illinois Sustainable Technology Center (ISTC) and SEDAC to provide funding solutions for improved wastewater treatment at a reduced cost to local municipalities. Due to the power savings associated with ADS technologies, there are many comparable federal and state funding opportunities for most ADS installations.

The village of Annawan was awarded funding for 90% of the total project costs for its wastewater treatment facility upgrade, equal to $130,821. Additionally, Annawan will continue to obtain approximately 101,000 kWh annual power savings along with associated cost savings (estimated $9,600 per year) by replacing the two 30-hp blowers used by the coarse bubble aeration system. These were replaced with two 15-hp blower packages with variable frequency drives (VFD). This new aeration system will operate at an estimated brake horsepower (BHP) of 10, creating ongoing annual power savings for the community.

Design

ADS engineers designed the aeration solution to address specific treatment concerns at Annawan WWTP. Annawan’s treatment solution uses 26 stainless steel “LTC” diffuser disk modules installed into two of the system’s three lagoons. The third lagoon is quiescent to settle solids prior to discharge. It also has a floating cover of Rhombo poly-tiles. This cover blocks sunlight, which prevents the photosynthetic growth of green water, and provides thermal retention to the water.

Annawan’s fine bubble diffusion system is designed to treat a maximum of 0.12 million gallons a day (mgd) of domestic wastewater, which equates to the estimated daily flow for a population of 1,200. The design influent loading values are approximately 200 mg/L biological oxygen demand (BOD), 200 mg/L total suspended solids (TSS), and 30 mg/L ammonia (NH3).

The ILEPA requires that the effluent discharge contains less than 25 mg/L BOD and 37 mg/L TSS. Additionally, the ILEPA requires that operators report effluent nitrogen and ammonia limits. The ADS solution is designed to keep Annawan’s effluent in compliance with these standards while also improving the local air and water quality.

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Air Supply

With bubbles, bigger does not mean better. ADS replaced the existing coarse bubble aeration system with fine bubble aeration and blowers connected to variable frequency drives (VFDs). VFDs improve the efficiency of the air supply by allowing the operator to turn down the amount of power supplied to the system. Each installed blower supplies 211 cubic feet per minute of oil-free air to reinforced self-sink feeder tubing. This is connected to ADS “LTC” Disk modules, which each produce billions of microbubbles daily of diffused air into the lagoon system.

According to SEDAC’s evaluation from a published case study, “Fine bubble aeration has roughly double the oxygen transfer efficiency of coarse bubble aeration, which can cut energy consumption by half.” Fine bubble diffusion provides superior oxygenation and mixing rates compared to coarse bubble diffusion, requiring less SCFM and BHP with better treatment results (Metcalf and Eddy). Using fine bubble diffusion, the system’s total energy use was reduced from 30 hp to 10 BHP, eliminating 66% of the daily energy use.

Aeration Equipment Installation

In December 2019, ADS engineers designed and installed the treatment solution of 22 “LTC” disk diffusers into the first lagoon and 4 “LTC” disk diffusers into the second lagoon at an approximate water depth of 10 feet.

This equipment can be installed in either filled or empty lagoons, providing maximum ease of installation. ADS installed the Annawan equipment over the course of three days. The installation was supervised by ADS technicians, who guided the installation team through the proper assembly of air supply, header and feeder pipe, and aeration equipment.

Depending on the site, these systems include either onshore or floating header pipes. The Annawan aeration lagoons use floating header pipes, which are easily adjustable with turnbuckles, equipped to handle more than 100 mile-per-hour winds, and they are secured to shore using stainless steel wire rope. Annawan provided the labor to assist the ADS supervisor with the installation components. The aeration system was supplemented with sludge reducing bacteria every week for four months to help with the digestion of accumulated solids.

Sludge Reducing Bacteria

Sludge Reducing Bacteria (SRB) is a non-pathogenic formulation of microorganisms selected for their ability to digest organic matter. Proper application doses of SRB vary for each lagoon system.

SRB is applied into the bottom areas of thick sludge from a work boat using a liquid air delivery system. SRB in an aerobic process provides a cost-effective and efficient way to reduce unwanted organic matter buildup in lagoon systems. This treatment provides substantial cost savings compared to removing, hauling and disposing of sludge. Additionally, with proper safety data sheets, SRB applications do not require permitting.

Cleaner Water, Lower Costs

The upgrades to Annawan WWTP have proved to be a successful treatment solution for this small rural community. The average effluent concentrations have been reduced substantially from previous treatment records, are well within the ILEPA permitted levels and are meeting monthly NPDES limits.

“TSS has been brought down drastically within the first 9 months of ADS aeration treatment,” said Mark Crosby, Annawan operator. Additionally, the sludge in the primary lagoon decreased by approximately 8 inches over the initial four-month treatment period.

About the author

Sarah Porter is water and waste specialist for Air Diffusion Systems. Porter can  be reached at [email protected] or 847.782.0044.     

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