In 2015, California’s Fallbrook Public Utility District completed a two-year overhaul of its 2.7 mgd wastewater treatment plant.

The district wasn’t in the market for any significant upgrades but was recently approached by Moleaer, a manufacturer of nanobubble technology. The company offered its system as a solution to a problem endemic to nearly every wastewater treatment stream: surfactants (soap and detergents) in the influent that inhibit aeration in activated sludge systems.

Owni Toma, chief plant operator, embodies the district’s policy of staying on top of new technology that can create more efficient and cost-effective operations. The progressive culture encourages working with local educational institutions. Toma invited Moleaer’s application engineer, Ph.D. student Federico Pasini, to enlighten him on the company’s equipment.

The result was a co-funded pilot program to test the effect of injecting nanobubbles in a functioning plant environment. It showed strong potential for the technology to reduce operating costs and improve treatment effectiveness and capacity.

Testing the waters

“We wanted to test this system of injecting nanobubbles into the headworks, upstream of the aeration system, to reduce oxygen demand downstream,” says Toma. “Every treatment plant is interested in curbing energy demand, and improving oxygen transfer helps that.

“If you reduce aeration demand downstream, where it’s expensive, like the blowers at aeration basins, by injecting nanobubbles upstream, then you’re looking at improved oxygen efficiency and reduced costs. Federico did a wonderful job designing this application for us.”

Pasini adds, “Based on the results of internal laboratory testing data we collected, and seeking to validate results we had observed over the years for wastewater applications, we wanted to figure out what nanobubbles could do in a wastewater treatment plant.

“The collaboration with Fallbrook was born from their reviewing the way they provide air to the preliminary headworks tank, and to the activated sludge process. We all decided that our best attempt injecting nanobubbles was to do it before primary treatment, as soon as the raw wastewater enters the plant.”

Treatment on wheels

Moleaer brought in one of its proprietary mobile units, installed inside a trailer for easy transport. On site, the company constructed an external piping system that allowed the unit to reach the water in the 35-by-15-foot, 21-foot-deep headworks basin.

They installed a liquid pump, a gas compressor and a piping system to circulate water from the basin through the nanobubble generator. This injected nanobubbles — part gas and part dissolved oxygen — into the wastewater, which then discharged to primary clarification and the activated sludge system.

“Our main focus was to evaluate how much the removal of surfactants would increase what we define as the oxygen transfer efficiency in the aeration basin,” says Pasini. “We know that soap in general reduces the ability to transfer oxygen from gas to liquid form.

“Bacteria in the activators require a minimum dissolved oxygen level to be able to assimilate the oxygen and perform uptake of the contaminants, along with carbon or ammonia produced by the surfactants.

“This dissolving of oxygen in the activated sludge is one of the main costs of wastewater treatment. We wanted to assess our ability to remove at least some surfactants and determine how that removal would affect oxygen transfer efficiency, and thereby the energy intensity and cost of that process.”

A big difference

It would be easy to confuse nanobubbles with industry-standard fine bubble aeration, but the difference between them and how they work is significant. In a fine bubble system, depth is important because the bubbles will ultimately rise and escape at the surface. The faster they rise, the lower the oxygen transfer to the water.

Nanobubbles are so small that they can’t be seen by the naked eye, and they are not subject to buoyancy. Nanobubbles never rise because their tiny size allows them to move around through Brownian forces — weak electric interactions at an infinitesimal scale. 

Nanobubbles also don’t coalesce. They will stay indefinitely dispersed in the water, transferring oxygen even when no further aeration is provided. The bubbles will still be there days, weeks, and in some cases even months after injection. For that whole time they slowly release oxygen into the water.

Favorable results

To their surprise, the team observed that the injection of nanobubbles in preliminary treatment also affected other processes. One was bacteria biomass activity. “It turns out we had a beneficial effect on the activated sludge process on one end because we allowed the existing aeration system to transfer more oxygen for the same energy intensity,” Pasini says.

“And on the other side, we saw that we created healthier bacteria because surfactants also inhibit biological processes that happen in an activated sludge system.” The removal of surfactants allowed faster oxygen uptake and production of needed effluent quality.

The pilot project, in which the district paid for the energy cost to run the system and Moleaer covered everything else, brought three measurable results:

The nanobubble treatment of influent allowed removal of surfactants. The activated sludge kinetic increased due to elimination of the inhibitory effect of quaternary ammonia compounds routinely released by surfactants.

The aeration efficiency of the fine-bubble system in secondary treatment increased by an average of 45% with surfactant removal, and 60% more oxygen was transferred to the aeration basin, for the same cost, when influent was pretreated with nanobubbles.

There was increased wastewater emulsification — the physical separation of particulate and colloidal contaminants — that made them more digestible in secondary treatment.

About the money

Final results showed that nanobubbles transferred twice the oxygen for the same energy consumption if the generator power requirement isn’t considered. If it is, savings are reduced to 15-20%. However, that was for the pilot test installation.

Pasini and his colleagues are now designing systems that require low to zero net energy consumption. Increased biomass activity was recorded at 20-25%, meaning that contaminants are removed from the wastewater at a faster rate.

This showed potential to expand capacity in the secondary treatment basin, allowing the plant to operate with a smaller footprint, because the retention time of the wastewater in the activated sludge basin could be reduced, or the process could operate at lower biomass concentrations.

Faster biomass oxygen uptake rate means less treatment time required and less treatment space needed. This makes nanobubble technology a great option for plants that need retrofitting or experience chronic overloading with no room to expand their footprint.

The study indicates that operation and maintenance costs could be reduced by 30-50% with nanobubble injection at the headworks, using existing pumping energy. There is opportunity for further efficiency increases with the injection of nanobubbles at several steps in the process. With projected payback on capital investment in less than 18 months, Pasini is bullish on nanobubble technology’s future.

“We will come up with other pilot applications for biosolids dewatering, disinfection and increasing biogas production, all processes we should be able to affect with nanobubble injection,” he says. “We’ll validate its use in a lot of applications in wastewater treatment. We’re reproducing this experience in other plants, so we’re confident that what we’ve seen at Fallbrook wasn’t a fluke.”