Using wastewater to grow algae

Researchers from Rice University have found they can easily grow high-value strains of oil-rich algae while simultaneously removing more than 90% of nitrates and more than 50% of phosphorous from wastewater. Their study, which was one of the first to examine the potential for using municipal wastewater as a feedstock for algae-based biofuels, has been published in the journal Algae.

Lead author Meenakshi Bhattacharjee said algae producers currently remain heavily dependent on chemical fertilisers, which both “reduces profit margins and puts them in competition with food producers for fertilisers”. She said algae producers “must address sustainability if they are to progress toward producing higher-volume products, ‘green’ petrochemical substitutes and fuels”.

A 2012 National Research Council report found that with current technologies, scaling up production of algal biofuels to meet 5% of US transportation fuel needs could create unsustainable demands for energy, water and nutrient resources. However, the report suggests wastewater-based cultivation as a potential way to make algae production sustainable and, at the same time, address nutrient pollution from excess nitrogen and phosphorous in US waterways.

“The idea has been on the books for quite a while, but there are questions, including whether it can be done in open tanks and whether it will be adaptable for monoculture - a preferred process where producers grow one algal strain that’s optimised to yield particular products,” said study co-author Evan Siemann. “We were surprised at how little had been done to test these questions. There are a number of laboratory studies, but we found only one previous large-scale study, which was conducted at a wastewater facility in Kansas.”

Rice’s research team set up a test involving 12 open-topped 2270 L tanks at one of the city’s satellite wastewater treatment plants. The tanks were fed with filtered wastewater from the plant’s clarifiers, which remove suspended solids from sewage. Various formulations of algae were tested in each tank; some were monocultures of oil-rich algal strains and others contained mixed cultures, including some with local algal strains from Houston bayous. Some tanks contained fish that preyed on algae-eating zooplankton.

“We recorded prolific algal growth in all 12 tanks,” Siemann said. “Our results are likely to be very encouraging to algae producers, because the case they would prefer - monocultures with no fish and no cross-contamination - was the case where we saw optimal performance.”

Bhattacharjee said more research is needed to determine whether wastewater-based algaculture will be cost-effective and under what circumstances. For instance, the algae in the Rice study was four times more effective at removing phosphorous than were the algae in the Kansas study. Bhattacharjee said that could be because the Houston test was performed in summer and autumn, so the tanks were warmer than those in Kansas.

“Using wastewater would be one of the best solutions to make algaculture sustainable,” she said. “If temperature is key, then cultivation may be more economical in the southeast and southwest.”

Bhattacharjee added that other factors, like starting levels of nitrogen and phosphorous, might have caused a rate-limiting effect. “These are the kinds of questions future studies would need to address to optimise this process and make it more attractive for investors,” she said.

Image credit: E Siemann/Rice University.