At night it looks like a huge tower, humming quietly and glowing with a green light. it is the first Biocoil, and it is being test operated by a water utility as a means of cleaning up contaminated water. it relies on microscopic algae called ^UChlorella^u, who have a voracious appetite for nitrogen, iron, phosphorus and other pollutants. Once they have fed on polluted water in open-air ponds, the algae are introduced into the Biocoil, a spiral of tubing in which they multiply and transform the pollutants into new algal cells, ready to start the cycle again. And excess algae makes a nutrient-rich food for fish or animals.