Algae can be a problem for waterways, feeding on excess nutrients and clogging water bodies with algal bloom.
But researchers at the National University of Singapore (NUS) believe they can harness that very growth to treat wastewater, and cut the energy used in treatment by as much as 80 per cent.
That is because wastewater treatment takes a lot of energy, most of it to bubble air through, or aerate, sludge ponds so bacteria can eat the waste.
Instead, associate professors Loh Kai Chee, 47, and Tong Yen Wah, 38, keep the bacteria, but replace the aeration process with algae which produce oxygen. In turn, the algae gobble up the carbon dioxide that the bacteria produce, as well as any waste products the latter do not eat.
Prof Loh and Prof Tong, of NUS’ chemical and bioengineering department, started devising their system about two years ago.
They were inspired by a previous government grant call asking for research to improve the energy efficiency of wastewater treatment.
They did not get the grant, but kept plugging away at the system.
Now, they have got as far as a lab-bench experimental setup - a 250ml flask full of algae and an identical flask full of bacteria. The liquids in the twin flasks never mix, meeting only in a reactor where a membrane made of plastic fibres lets the necessary gases through.
So far, they have tested on a high-concentration sugar solution that mimics the concentration of wastewater, but have not yet tried it on real wastewater. That is one of the next steps, explained Prof Loh.
Another would be genetically engineering strains of algae that grow faster and thus consume more carbon dioxide, a project Prof Tong is working on.
After that, scaling up to a pilot or demonstration plant should be straightforward, as the membranes used are commercially available.
They reckon the process could even be a net producer of energy, if the algae are digested at the end of their life to harvest methane gas which is then burned to generate power.
Currently, the sludge left over from wastewater treatment can also be digested for such biogas, which can generate about half of the energy needed to run the treatment process. But Prof Loh and Prof Tong say their system is better because using algae saves energy on aeration.
Their idea did not come out of the blue. The NUS researchers worked with algae for four years, at first trying to get biofuel from it.
In fact, researchers at the Rochester Institute of Technology in New York are growing algae in wastewater, both helping to clean the wastewater and harvest the algae for biofuel.
But other research groups trying to use algae for water treatment mixed the algae and wastewater, said Prof Loh.
For that, the strains of algae and treatment bacteria had to be carefully picked so they would not compete with or kill each other.
So the realisation that algae and bacteria could exchange gases and nutrients through a membrane was a ‘eureka moment’ of sorts, Prof Loh explained.
He said: ‘Maybe people were too wedded to the idea that we need the two micro-organisms to be in the same broth, and we need to select them to live with each other.’