Algae nutrient recycling undertaken by Sandia biologists
LIVERMORE, Calif. — Nitrogen and phosphate nutrients are among the biggest costs in cultivating algae for biofuels. Sandia National Laboratories molecular biologists Todd Lane and Ryan Davis have shown they can recycle about two-thirds of those critical nutrients, and aim to raise the recycling rate to close to 100%.
Recycling nitrogen and phosphate has benefits that go far beyond cost. While nitrogen can be produced through a costly artificial nitrogen fixation process using natural gas and atmospheric nitrogen, phosphate is a limited natural resource that can be toxic at high concentration.
“We have a finite amount of phosphate in the world, but it’s in high demand as a fertilizer. Half of the phosphates that go into our crops in the form of fertilizer end up in the Gulf of Mexico, contributing to hypoxic zones,” said Lane. Better known as “dead zones,” hypoxic zones are areas of low oxygen concentration that kill or drive out marine life.
Economic models show that replacing just 10% of liquid transportation fuels with algal-derived fuels, though beneficial to the environment in many ways, could double fertilizer consumption, which, in turn, would drive up the cost of food.
But recycling phosphates means everyone wins: algal-derived biofuels producers, farmers and the environment. “By recycling phosphates from one batch of algae to the next, we save money, no longer compete with agriculture for a non-renewable resource and keep those phosphates out of the environment,” said Lane.
Lane and Davis are considering other applications for their closed-loop algae nutrient recycling methods. “Our method could be used to strip phosphates from the agricultural runoff before it reaches the Salton Sea,” said Davis. Fertilizer runoff into the saltwater sea, California’s largest lake, has led to dead zones that threaten fish and other wildlife. “Those nutrients that would otherwise further contribute to the dead zone could be used to grow algae intentionally for biofuels and other biobased commodities.”
Lane and Davis found their nutrient recycling method works on many different algae feedstocks, even mixed feedstocks. Because algae have more genetic diversity than any other organism, many methods developed in the past haven’t worked universally.
The researchers use a fairly simple process, osmotic shock, to liberate phosphate from the cultivated algae. “We shock the algae with fresh water while controlling certain conditions like pH and temperature. This disrupts the internal structure of the cell and releases naturally occurring enzymes,” explained Lane. “These enzymes chew up the cell and rapidly release the phosphates.”
The next step is fermentation to convert the nitrogen, which is mostly in the form of amino acids, into ammonia. The phosphates and ammonia are then recombined — with help from magnesium, present in great quantities in the algal biomass — to form struvite, a solid salt.