A demonstration project by Silicon Valley Clean Water with Stanford University researchers shows that anaerobic secondary treatment of wastewater can be effective and more efficient than traditional aerobic processes.

Anaerobic treatment has several sustainability advantages, including lower power consumption, reduced biosolids volume and a smaller footprint. The process also produces biogas similar to anaerobic digesters at traditional plants.

The project was funded by a grant from the California Energy Commission and contributions from utility, industry and academic partners. The project is led by Silicon Valley Clean Water, which operates a large wastewater treatment plant in Redwood City. Next to the main plant, a team including Stanford’s Codiga Resource Recovery Center built a 24,000 gpd anaerobic plant, which receives primary-treated wastewater. 

Two stages

The demonstration plant is called a staged anaerobic fluidized-bed membrane bioreactor (SAF-MBR). The first stage is a 6-foot-square, 20-foot-tall fluidized bed reactor filled with granular activated carbon. The wastewater is pumped up through the carbon bed, which becomes a habitat for biofilm containing anaerobic bacteria.

The second stage is a membrane tank that uses Suez membranes with a porosity of 0.04 microns. In a traditional system, the membranes would be kept clean with charges of air. In this system, biogas collected from the first stage is used to prevent the membranes from fouling.

After several months of operation, the team was satisfied with the reductions in COD, says Sebastien Tilmans, Ph.D., PE, executive director of the Codiga Center. “The water going in is averaging close to 600 mg/L COD, but the effluent averages 30 mg/L,” Tilmans says. “That’s a 95% reduction in COD that is compliant with U.S. secondary effluent standards.”

The effluent BOD was averaging 15 mg/L, TSS was about 1 mg/L. Biogas is collected from the fluidized bed and membrane tank. In the demonstration project, the gas was measured and then flared, but it could be used to fuel a combined heat and power process.

“Our numbers are showing that this system is actually net energy positive,” Tilmans says. “The electricity you could generate from the biogas is greater than the power it takes to run the plant.”

Reduced solids

The anaerobic treatment produces far lower biosolids volume than aerobic treatment because the anaerobic bacteria grow slowly and have to consume more organic material in order to reproduce. Tilmans estimates that the secondary biosolids volume is reduced by 90%.

“You still have your primary solids,” he says, “but if you are close to eliminating the secondary solids, you are cutting down the total solids by 30-50%. It’s a pretty large reduction in the number of trucks that would be leaving your facility. That’s a big cost savings.”

The bacteria that live on the biofilm in the fluidized bed are naturally occurring in wastewater but are slow-growing. To jump-start the process, the Stanford team seeded the fluidized bed with bacteria from a wastewater treatment plant at a winery and from the digesters at the Silicon Valley Clean Water plant.

Tilmans says the bacteria from the winery had especially good adhesion to the activated carbon.

“The advantage of the bacteria from the winery is they are already in a biofilm form to attach to the carbon,” he says. “The advantage of the ones from the wastewater treatment plant is they are acclimated to the wastewater already flowing in that community. They are locals.” 

Additional issues

Although the demonstration has been successful, some issues need to be addressed. For example, some biogas remains dissolved in the effluent leaving the membrane tanks. “We are testing efficient ways to extract the gas so we can use it,” he says.

Another issue is nutrient removal. The SAF-MBR process doesn’t remove phosphorus or nitrogen, so additional steps would be necessary, just as with a traditional plant. “We’re starting to think about what the full treatment process would look like for plants that have nutrient limits in their permit,” Tilmans says.

A third issue is water reuse, especially important in California. “We’re thinking about what downstream treatment processes, including reverse osmosis and advanced oxidation, that could take the water from our system to irrigation or drinking water quality.”

One bonus with anaerobic treatment is that the bacteria appear more capable than aerobic bacteria to consume some chemicals of emerging concern, such as certain pharmaceuticals. Odor at the demonstration plant has not been a problem because the entire process is sealed.

Making it bigger

There are several ways to scale up the technology. “One way to envision scale-up is simply to multiply the reactor,” says Tilmans. “The current system processes 24,000 gpd; a 240,000 gpd system would use 10 of these reactors in parallel, or one reactor 60 by 6 feet and 20 feet tall.

“Ideally, we would build the reactors taller, which would reduce the footprint.”

Tilmans says that for many years, effective anaerobic treatment was considered impossible.

“Because anaerobic organisms grow more slowly, and because they don’t settle well, it was traditionally considered impossible to achieve the high solids retention times necessary to grow the organisms and achieve low effluent organic concentrations.

The advent of membranes, along with the innovation of using the granular activated carbon as biofilm media, enabled us to overcome that challenge, achieving long solids retention times without provoking excessive fouling of the membranes.”

The total hydraulic retention time for the SAF-MBR systems, including the membrane tank, is less than six hours. “This would mean the treatment process would happen in a footprint competitive with typical aerobic activated sludge systems,” Tilmans says.

“However, the lower biosolids production means the biosolids handling footprint could be reduced by 30 to 50%, and dual-media filtration facilities could be eliminated because of the membrane filtration achieved within the system.”

The bottom line is that anaerobic secondary treatment is a potential option for wastewater treatment plants. “We thought we knew that it was impossible,” says Tilmans. “But it turns out that it’s very possible.”