March 2014, Vol. 26, No.3

Problem Solvers

Fine-bubble aeration upgrade saves energy and costs

Problem: An aging jet aeration system led to ammonia concentrations exceeding permit requirements. 
Solution: Installing a fine-bubble diffuser aeration system improved energy- and cost-efficiency.
 

Faced with a thriving agriculture industry and a growing population, the Water Reclamation Division of Montgomery County Environmental Services (MCES) continually searches for ways to optimize operating efficiency within its facilities.

MCES manages water and wastewater services for the businesses and 537,602 residents spread throughout 18 cities surrounding Dayton, Ohio. The division operates the Western Regional Water Reclamation Facility in West Carrollton and the Eastern Regional Water Reclamation Facility in Kettering. It also coordinates the disposal of 3018 Mg/yr (3300 ton/yr) of biosolids and regulates certain industrial waste discharges into the collection system.

When the Western Regional Water Reclamation Facility began having issues with its aging jet aeration system, MCES recognized a new opportunity for optimization. The facility, which produces 57 ML/d (15 mgd) of purified water for Montgomery County, has eight 2.27-million L (600,000-gal) side-by-side wastewater aeration tanks that use jet aeration systems. Each of the 33-m-long (109-ft-long) × 15-m-wide (50-ft-wide) × 5-m-deep (15-ft-deep) open-top tanks contains jet aeration manifolds made of reinforced fiberglass.

After 35 years of use, the fiberglass had deteriorated, rendering half of the eight tanks inoperative. Efforts to repair cracks were unsuccessful, and the failing jet aeration system led to inadequate oxygen supply, and, thus, effluent with ammonia concentrations exceeding levels required by permits.

The facility had to replace the aging system, and in September 2012, the Montgomery County Board of Commissioners received approval for replacement.

MCES began evaluating different aeration systems. Because aeration systems in water resource recovery facilities can account for 50% to 70% of the energy required to operate an entire facility, the county recognized an opportunity to save both energy and cost.

“Montgomery County recognized an opportunity to increase their plant’s efficiency by utilizing a fine-bubble diffused aeration system,” said Mark Livengood, water reclamation manager at MCES.

Fine-bubble diffuser aeration systems produce smaller bubbles that have a larger surface area and a longer residence time in the water being treated than larger bubbles. They also eliminate the need for pumps required as part of the jet aeration system, improving efficiency. The result is higher oxygen transfer efficiency, which lowers the energy needed and the cost to operate the system when compared to alternative aeration technologies.

To deliver the same mass of oxygen in one aeration tank, for example, fine-bubble diffuser aeration systems require 48 m3/min (1700 ft3/min) of air, while jet aeration systems require 96 m3/min (3400 ft3/min), Livengood said.

The facility purchased the Xylem/Sanitaire® (Brown Deer, Wis.) Silver Series Membrane Disc System that employs fine-bubble diffused aeration because of the product’s reputation for being reliable and energy efficient, Livengood said. Local contractor, Dugan and Meyers Construction Co. (Cincinnati), installed the system in 3 weeks.

Prior to start-up, Xylem sent a trained service technician to ensure the system had been installed properly and to train operators on routine operation and maintenance procedures. “This ensured a smooth transition … and gave the Montgomery County plant the expertise and tools necessary to operate and maintain the plant’s new aeration system,” Livengood said.

Operation of the new equipment began on June 4, 2013, and within 72 hours, plant managers saw benefits. The settling velocity of the solids greatly improved — visible in a side-by side comparison between the jet-aeration and fine-bubble systems. In addition, the system achieved superior ammonia removal with less air being injected into the tank, Livengood said.

After 3 months of successful run time, MCES has experienced significant energy and cost savings from operating only the fine-bubble systems. The experience resulted in the purchase of six more membrane disc systems to replace the remaining jet aeration equipment. And with this plan, the county expects to have additional savings when the facility is fully converted to fine-bubble systems.

“We have identified cost savings will be achieved by using [48 m3/min] 1700 ft3/min less air per tank in comparison to the jet aeration system, equating to an estimated $111,500 in annual energy savings. In addition, two 14-hp mixing pumps required for the jet aeration system will be taken off-line, equating to an additional $68,600 in annual energy savings,” Livengood said.

The jet aeration system, in use since 1978, provided a long service life but the increasing failure rate of the fiberglass headers led to further risk of noncompliance.

“Our goal is 100% compliance. The new Sanitaire/Xylem system works with our existing blowers and motors, and we will realize a cost savings by a significant reduction in air demand. Plus, the Xylem/Sanitaire system has been designed to allow for additional diffusers to be added (or removed) in case our facility must meet new nutrient removal limitations,” Livengood said.

Because of the smooth installation last year, the county does not expect installation of six new systems to interfere with normal facility operation or affect effluent compliance.  

“Sanitaire’s highly efficient aeration systems have been so successful that the Western Regional Water Reclamation Facilityis considering reducing the number of its fully operational tanks from eight to six. We feel confident that Montgomery County will continue to experience reduced energy cost and increased plant efficiency with the addition of the final six fine-bubble diffused aeration systems,” Livengood said.