The sulphurous springs of Yellowstone national park are scalding, tainted with heavy metals and acidic enough to eat through clothing. But their murky depths are also home to an algae that scientists claim could one day help provide cleaner, healthier water around the world.

“Galdieria sulphuraria is one of the most interesting microorganisms on the planet,” says Peter Lammers, a professor in algal bioenergy at Arizona State University. “It grows in a witches brew, can degrade over 50 organic molecules and even photosynthesise like a plant.” That makes it ideal, Lammers says, to use somewhere even more unpleasant than Yellowstone’s volcanic springs: urban sewage farms.

At a pilot site in Las Cruces, New Mexico, Lammers and researchers at New Mexico State University are diverting effluent from the city’s wastewater treatment plant into row upon row of long plastic bags primed with Galdieria sulphuraria. Air enriched with carbon dioxide is pumped gently through the tubes, while plastic wing-like foils move slowly up and down to mix the concoction.

The aim? To build an energy-positive wastewater treatment that helps preserve rivers, lakes and estuaries, reclaims the chemical energy in sewage, utilises sunlight to expand that energy footprint and ultimately pays for itself.

Environmental benefits

Wastewater is a growing environmental issue. Excess nitrogen and phosphorus from fertilisers cause excessive growth of aquatic plants, a process called eutrophication. These blooms deplete oxygen in the water, creating “dead zones” where fish and shellfish cannot survive. Globally, there are hundreds of dead zones, including one at the mouth of the Mississippi River that can reach 20,000 sq kilometres in size.

Conventional treatment removes only about 10-15% of the nitrogen and 20-30% of the phosphorus in raw wastewater. And existing technologies to reduce the remainder are extremely expensive, doubling or tripling a plant’s electricity consumption. In the US, for example, up to 4% of the country’s electricity consumption is already used for the movement and treatment of water and wastewater.

Utilities have come to realise that wastewater is not just a problem to deal with but, potentially, a resource to exploit. The organic material in sewage actually contains more energy than is needed to decontaminate it.

“The line where we traditionally differentiated wastewater utilities from energy utilities is becoming more permeable,” says Sharlene Leurig of Ceres, a nonprofit organisation helping institutional investors integrate sustainability into US capital markets. “Alexandria Sanitation District in Virginia rebranded itself to Alexandria Renew Enterprises, creating a revenue stream from nutrients collected at their facilities, selling reclaimed water and generating electricity from their digesters.”

Anaerobic digesters, adopted by many cities around the world, already use bacteria to break down sewage into methane (the biggest component of natural gas) and carbon dioxide. But existing digesters still leave behind a nasty residue packed with nitrogen and phosphorus.

At the Las Cruces pilot project, the algae use sunlight and carbon dioxide to grow, breaking down over 95% of the nitrogen and phosphates in a couple of days. The high temperatures and acid conditions are also tough on bacteria, viruses and parasites. Sewage treated like this should require 10 times less chemical disinfectant than usual.

Thanks to the algae’s photosynthetic growth, the system also creates around four times as much rich organic sludge as traditional sewage treatment. That sludge can then be turned into biofuel oil using processes called hydrothermal liquefaction and catalytic hydrothermal gasification, both well established in the biofuels industry.

Scaling up

Lammers believes that algal systems could ultimately eliminate sewage farms’ electricity bills, which can account for anything up to 60% of operating costs today, or even generate a surplus. The Las Cruces pilot is going well and a similar project in Phoenix, Arizona, is due to start within a month.

However, scaling up to a city-size project presents tough engineering challenges, including handling the acidic wastewater, mixing the solution efficiently and recycling millions of plastic bags.

“If we’re talking about commercialisation, then harvesting [the algal sludge] becomes a very big problem,” says Meenakshi Bhattacharjee, executive director of Rice University’s Center for Applied Algal Research in Texas, where she is using wastewater and algae to produce biodiesel. Even after algae has been growing for days, it still makes up only a few grams in every litre of water. “We have chemical and electric ways to harvest it but we need to design something that is much less expensive,” says Bhattacharjee.

Space is likely to be another issue. Lammers calculates that a farm big enough to handle the sewage of a million people would need around 15,000 acres of plastic bags, as well as plenty of sunlight to keep the photosynthesis ticking along. “That wouldn’t work in the most of the US,” he admits, “but it will work beautifully here in the desert southwest.”

A sustainable, self-financing wastewater system could improve health and prevent environmental damage in dozens of sunny countries across Africa, South America and Asia. According to the UN, 90% of the sewage in developing countries today is discharged without any treatment.

Given the problems in scaling algal technologies in the past, with many startups struggling to produce biofuels at competitive prices, a focus on tackling farm waste might make sense, says Leurig: “So much of the discharge is coming from agricultural producers, so the cost of implementing technologies in the agricultural context might be more economically efficient than doing it at the city scale.”

“I’m hoping some enterprising company will see this as a package, almost a franchise opportunity, worldwide,” says Lammers. “In many parts of the world, they’re denuding forests because they need fuel for cooking. That’s stupid and self-defeating when you can produce cooking gas from wastewater. And if we can do this with primary sewage, let’s do it with waste from dairies, swine and poultry production, aquaculture, even food processing. Let’s capture it all and turn it into clean water and energy.”