Green pond scum isn’t enticing, especially when mixed with effluent. Indeed, it’s difficult to imagine a less appealing mixture.
Yet in a series of long, narrow ponds at Chiclana de la Frontera in Southern Spain, a region better known for its golf courses than its sewage treatment, this is exactly what engineers are mixing up in order to harvest a crop of biomethane fuel.
The €12 million EU-funded All-gas project passed a major milestone last summer when its ponds produced its first algae biomass crop. By next year, engineers on the project plan to have scaled up the plant to 10 hectares. They hope to be able to produce 3,000kg of biomass a day, yielding around 2000m3 of biomethane, enough to fuel about 150 vehicles a day. This will make it the largest algae biomass production plant in the world that uses wastewater as a feedstock.
This is a niche title to hold - at this stage in time. Frank Rogalia, the project’s coordinator and director of innovation and technology at Spanish water company FCC Aqualia, believes algae-derived biofuel has great potential as a renewable source of energy in the future. “This new approach means that Spain’s 40 million population could power 200,000 vehicles every year from a single toilet flush,” he says.
“There are many efforts to develop algae biofuels, in total worth billions of euros. I have no doubt, given the magnitude of investments into research and development, successful results will ensue - but the timeframe to reach economical returns is the big unknown.”
Rogalia is sensible to limit his prescience. When start-up technology companies first began to talk about the growing and refining of algae to make biofuels last decade, many over-promised the potential and played down the development time required. Over-ambitious start-ups were quick to fail and some were subsequently accused of fraud, mainly by disgruntled investors (http://algaelink-bioking-scam.blogspot.co.uk/). Technical critics were quick to point out the challenges involved in commercialising the technology and that vast amounts of water, energy and chemicals that would be required.
However, enough companies and academics survived the initial period of over-optimism to form a strong research base, mainly in Europe, the US and Australia. Several companies have partnerships with major multinationals such as Boeing, Daimler and Exxon Mobil. As Mary Rosenthal, executive director of the World Algae Biomass Organisation, writes in the trade association’s newsletter, the algae industry is “warmed up and ready to go”. “After spending years laying the groundwork in research, financing and testing we are now entering a period of commercialisation,” she enthuses.
There are two main techniques used to cultivate algae, open-air ponds, the option used by the All-Gas project, or photobioreactors, long tubes made of plastic or glass that offer an environment conducive to algae-growth. Both methods require sunshine and space.
Typical photbioreactors being used to grow algae
Algae expert Steve Skill has more than 30 years experience of growing algae and developed his own algae photobioreactor 20 years ago. He says his system is today “dramatically less energy intensive than other systems being developed”.
“Normally people use pumps to maintain suspension and give turbulence. The system I designed uses a low energy tube and a device inside that rotates. It is dramatically less energy intensive.”
“The other main technical challenge to using algae as a source of energy is integrating it into other processes. Algae requires nitrates and phosphates and the best source for that is wastewater treatment. So, what is being investigated more is using algae and bacterial communities together. The algae provides the oxygen and the bacteria provides the CO2 from the waste.”
Conventional sewage processes rely on bacteria to oxidise the organic component of sewage. In a typical sewage works the oxidation process occurs in large circular tanks where the wastewater is agitated using large stirrers. This is a hugely energy intensive process. In addition the bacteria in the process emits a lot of CO2. It is a “lose-lose” situation in energy terms.
The integration of algae growth into conventional sewage treatment turns the process into a “win-win”. A closed cycle process uses sunlight to provide photons for the algae to produce oxygen, explains Skill. The oxygen is used by the bacteria to feed on the waste. The CO2 produced is used by the algae and it also stores the nitrates and phosphates from the waste that can be used in other processes.
The algae can then undergo a process called hydrothermal liquefaction, Skill says, a type of low pressure, low temperature pyrolysis that produces bio-crude oil for biofuels, as well as nitrogen and phosphates for fertiliser. “Potentially we could produce as much bio-crude oil as sewage sludge produced,” he says. “This is the future of the way in which algae will produce biofuels.”
“You are reducing the energy that is required to process sewage, recovering the nitrogen and phosphorous which is normally flushed out, where it makes algae anyway, and you are recovering a biofuel.”
A commercial-scale demonstrator of a sewage treatment process that uses algae to produce biodiesel and fertiliser could be less than five years away, Skill adds.
Water utilities have been quick to realise that the waste they are paid to deal with could be transformed into a profitable revenue stream and are beginning to dabble in the technology. Many sewage works now have anaerobic digestion (AD) processes that enable them to produce biomethane. Spanish water company FCC Aquila, the lead of the consortium of companies and academic institutions working on the All-gas project has 20. The crucial difference compared to conventional AD is that All-gas introduces a process stage where algae uses the wastewater to provide nitrogen and phosphate for the biogas. This and the resultant stream of clean water, is what makes the All-as process sustainable, says Frank Rogalia.
He says: “Artificial nitrogen fertilizer, used for many algae and biofuel cultures, is energy intensive and phosphorus is a limited resource. We separate them by flotation, then use anaerobic digestion of the harvested biomass to for biogas production. This is all very common wastewater treatment technology.”
The biogas is upgraded to methane by a purification process supplied by Dutch company Hygear called pressure swing adsorption and the CO2 is recovered. Any scrubbing technology could be used for this, says Rogalia.
Engineers are preparing the use of larger 2 x 500m2 ponds for the All-gas project this year before building ponds that will cover 10,000 hectares.
A prototype pond at the All-gas project in Spain
However, the largest engineering challenge to using algae for biofuels is the perennial problem of getting more energy out than in. Laid end-to-end, the algae growth and then the refining process requires a lot of energy. Rogalia says that All-gas’ preliminary energy balance is positive. “Normally the removal of nutrients from wastewater consumes about 0.3 to 0.5 kwh per m3 of electricity. We avoid that expense, and the maximum production of 2000m3 of biomethane would signify a thermal output of 20,000kwh per day for a water input of 5000m3 per day, or the production of thermal energy of 4kwh per m3.”
The fuel will be used for vehicles, but it could also be transformed into electricity, he adds, yielding about 1.35 kwh per m3 with typical efficiencies of 35%. Integrated equipment for algae harvesting and processing, pond mixers and CO2 blowers, should not consume more than 25% of the energy produced.
All-gas is to produce biogas for vehicles, but the project is still investigating the production of biodiesel with several partners. Rogalia admits that it is “not yet clear under what circumstances liquid algae fuels would be cost competitive” and if a positive energy balance can be reached because of the need to dewater the biomass to produce oil. But, he says that the potential growth rate of algae is impressive enough compared to existing biofuel feedstocks such as sugar cane and palm oil, the “synergies” with wastewater beneficial enough and the use of anaerobic digestion to produce biogas makes it a sustainable option for small towns to consider converting their effluents to “run school buses or public vehicles”.
Algae expert Steve Skill agrees the future for liquid fuels from algae is far from clear. He says: “Algae in biodiesel is never going to happen. People growing algae to make fuel and using transesterification to make biodiesel - its far too expensive and you will never be able to compete with petroleum. You can do it in the lab and it works great. But put it in the real world and it gets contaminated and all crashes.
“You have to develop a process that is usable in the real world. It has to be robust. And improving the process efficiency of waste water treatment makes it a win-win.”