Big Island Welcomes Small Footprint Aeration Solution

An aging basin liner potentially causing untreated water to leak into the ground? Concerns of environmental contamination, violations or a cease-and-desist letter?

All sounds rather dramatic, but there are a worryingly large number of small treatment plants (<500,000 gallons per day) that have just one aeration pond, one clarifier, and an aging liner approaching the end of its design life. Many of these systems have no redundancy or back-up whatsoever.

Easily remedied you might say by digging a new pond, but that’s almost certainly not the most economical solution, and as with many smaller treatment plants, there often isn’t enough space on site to do so anyway. So, when you only have one aeration basin, how can you take it out of service for cleaning, maintenance and relining?

In a recently completed project, WSI International, a Denver based equipment manufacturer and engineering services company, demonstrated how to economically provide the treatment redundancy necessary to safely renovate a 300,000 GPD extended aeration activated sludge facility’s aging infrastructure.

The Utility Manager at the facility took a proactive approach and engaged with WSI to design and manufacture a system to provide the necessary redundancy to take the activated sludge lagoon offline.

From conceptual design, all the way through project delivery, WSI was challenged with ensuring that the WWTP (built in 1995) on the Big Island of Hawaii would, for the first time, have the option to take its aerated lagoon offline for cleaning and relining without interrupting the overall treatment process.

Furthermore, the treatment plant needed to add redundancy for both aeration and clarification, but without expanding the site footprint. The liner for the existing aeration lagoon (443,000 gallons) at the 30-year-old treatment plant was showing signs of deterioration from the weather along the outer edges. Also, operations had identified an accumulation of grit which needed to be removed to maintain treatment capacity. Removing the lagoon from service to inspect the liner and carry out repairs/replacement and cleaning would all be fine if the wastewater could be diverted to a temporary basin, but the challenge was the absence of redundancy for the aeration basin and a secondary clarifier.

Redundancy for the aeration basin was not as critical, but a clarifier failure due to mechanical problems, such as a broken rake or main shaft would create a significant operational challenge and may cause an effluent exceedance. WSI saw that it could create redundancy within the small footprint of the facility by repurposing the existing secondary clarifier as an aeration tank for an activated sludge system, as its Vice President of Engineering, Ben Garcia, explains:

“As we set out to design a compact, integrated system, we needed to develop a DAF unit capable of managing Mixed Liquor Suspended Solids (MLSS) from the existing activated sludge treatment process within a much smaller footprint—while still providing the same level of clarification as the significantly larger existing clarifier. Additionally, the DAF system, being located down grade, required an effluent pump system to send clarified water uphill to the existing chlorine contact tank, as well as a sludge transfer system for the RAS and WAS operations of the activated sludge treatment process.

The WSI integrated DAF system had to function as a clarifier for the existing system while seamlessly transitioning between temporary conversion, operation, and back to the original WWTP configuration.

While the aeration lagoon rework got underway, WSI’s DAF unit replaced the secondary clarifier, returning RAS (Return Activated Sludge) to the aeration basin and discharging WAS (Waste Activated Sludge) to the solids processing system to balance biological mass in the treatment system.

Further challenging WSI was the site's topography, which featured steep terrain that the original designers had utilized for a gravity-flow-based system. WSI was able to utilize a small (38’x14’) flat area next to the control building for the integrated DAF system.

‘Landia allowed us the flexibility to install the air induction nozzles outside of the clarifier.’
WSI’s Ben Garcia, added: “Since this was a temporary project, we set up a system of interconnecting hoses from the sites existing equipment to connection points on the integrated DAF system for influent, effluent, and sludge disposal. Once the DAF was operational, the site took down the clarifier to prepare its conversion as the WWTP temporary home for the biological treatment process.”

With existing clarifier scum rake, sludge scrapper, and sloped base, the thought of installing a traditional blower and aeration grid, even if temporary, was out of the question.

Ben Garcia said: “We looked long and hard at various air eductor/mixing pump options, even though some meet the criteria of supplying 40 lbs O2/hr, they had larger air induction nozzles that required installation inside the clarifier, which as with an aeration grid, this would have been incredibly difficult to install. The AirJet diffuser from Landia allowed us the flexibility to install the air induction nozzles outside of the clarifier.

Designed as a venturi system, the Landia AirJet entrains atmospheric air into a recirculating flow stream that is drawn from the clarifier, aerated, and then discharged back into the clarifier, thereby both mixing and aerated it to function as an activated sludge basin. The aeration nozzles also provide mixing flow and velocity to ensure that adequate mixing energy is provided to prevent the drop-out of solids.

WSI’s Ben Garcia continued: “The Landia AirJet could be introduced far more conveniently just by adding penetrations through the clarifier wall. Landia worked with WSI to configure the angles of aerator injection nozzles that best suited the project. The aspirator and venturi section are positioned outside of the walls so as to not interfere with the scum-rake assembly at the water surface or sludge rake assembly at the base of the clarifier.

‘Creates exactly the right amount of mixing needed’.
The nozzles simply poke through the clarifier wall a few inches, while the heart of the unit, the Landia Chopper Pump, remains on the outside, which also simplifies maintenance. The Landia AirJet was optimal fit for this clarifier to aeration basin conversion project. Not only is this jet aerator providing air for the biological process, but it also creates exactly the right amount of mixing needed. Everything is comprehensively agitated, with solids unable to settle at the base of the tank. Landia also allowed for direct OEM and Engineering working relationship, listening closely to our needs so that we could reach the best possible outcome. The Landia AirJet worked exactly as designed, providing the oxygen needed for biological nutrient removal that was required. It runs very smoothly. We and our customers are very happy.”