Algae can make sewage treatment plants more effective

Economists would certainly call it a win-win situation: in the future algae may well be used to clean contaminated water while simultaneously serving as an energy source.  For now, this remains only a remote prospect. A pilot study conducted by researchers at Leuphana University of Lüneburg and Ostfalia University of Applied Sciences in Suderburg shows, however, that this scenario does not have to remain in the distant future.  Algae can after all remove phosphate from water with relative ease.  Biotechnology experts have for some time now considered using algae as a potential low-impact water treatment method.

In the summer of 2010 photographs taken from orbit created a media sensation: these satellite pictures showed a vast algae blanket in the Baltic Sea, whose surface area was as large as Germany. Environmentalists were concerned about its harmful impact on the sea’s delicate ecosystem: the algal bloom, they feared, could lead to a massive oxygen deficit in the ocean water, thereby threatening many plants and animals.

The explosive increase in algae had two causes: firstly, the warm summer weather and, secondly, contamination of the Baltic with algae nutrients, such as phosphate and nitrate.  Both of these are used in agriculture as fertilizer, for instance, that eventually flows through rivers into the ocean. This over-fertilization provides algae with ideal living conditions.  And this is exactly why researchers want to utilize algae in water treatment processing:             

As they grow, the plants bind nitrate and phosphate. The latter is normally very difficult to remove from water.  “In large sewage treatment plants this is currently done with chemicals and the phosphate eventually ends up in the sewage sludge,” explained Professor Dr. Artur Mennerich of the Ostfalia University of Applied Sciences in Suderburg.  This procedure is too complicated and expensive for small treatment plants—they simply do not remove the phosphate. 

Controlled Algal Blooms in Sedimentation Tanks

Algae on the other hand could remove phosphate from the water without too much trouble.  Biotechnology experts have for some time now considered using algae as a potential low-impact water treatment method.  The idea is to use controlled algal blooms in sedimentation tanks to bind with the undesirable nutrient.  These single cell organisms would also conveniently absorb a considerable amount of carbon dioxide and thereby help to reduce the greenhouse effect.      

 “Furthermore, we could continually harvest the algae for use as an energy source in, for example, biogas plants,” Mennerich said.  Yet, it is not quite as simple as it sounds.  The main problem:  The algae used in water purification cannot be allowed to leave the treatment plant (certainly not in large quantities) or they eventually would wind up in rivers and a situation similar to the one in the Baltic might threaten the ecosystem once more.  The microscopically small single cell creatures are typically widely dispersed in the water and it takes considerable effort to filter them out.  Researchers have however succeeded in making their helpers more sedentary.  Professor Dr. Brigitte Urban of Leuphana University explained the principle, “We mixed them with activated sludge from the treatment plant.”

“The sludge consists primarily of bacteria that removes organic matter from water.”  These bacteria form large clumps that on their own sink to the bottom and drag the algae down with them.  “In our laboratory experiments the algae precipitated much more quickly if they had been mixed with activated sludge beforehand,” explained Yanyan Su, Professor Urban’s doctoral student.  Another advantage:  in the right mixture, algae and bacteria achieve a significantly higher purification rate than if they are applied individually.  One hitch remains however: in our latitude the climate is usually too cold and dark.  Certainly during the winter it is unrealistic to use algae in treatment plants, Mennerich explained.  “Yet for many countries with a more advantageous climate than ours, this could be a promising and cost-effective approach to getting water pollution under control,” he added.