Science: Combining micro-organisms and ozone to effectively break down micro-pollutants

Carefully, scientist Koen van Gijn attaches a clamping ring around a glass cylinder filled with a brownish liquid. The bottom part is filled with sand. Two neighboring cylinders contain a similar fluid, but with different solids. In the middle cylinder, little plastic pieces dance randomly in the brownish liquid as air bubbles through. ‘Bacteria have different preferences for different substrates. Therefore, we are testing activated carbon, plastic pieces with a large surface area, and just sand as growth surface’, says van Gijn, while pointing at the three glass cylinders. Van Gijn’s research to find more effective ways of cleaning micro-pollutants from wastewater has just started. In this first experiment, he is evaluating the most effective way to reduce the amount of background organic compounds in wastewater using microorganisms. This cleaning step precedes the ozone treatment and reduces the amount of ozone needed.

Complex mixtures
Many chemical compounds used in cosmetics, medication, and also agriculture may end up in wastewater in low concentrations. These micro-pollutants consist of complex mixtures of micro plastics, breakdown products from medicine, cosmetics, as well as pesticides. Due to improved analytical methods, their presence in wastewater has become clear. In The Netherlands alone, about 1250 tons of medication was used in 2007; that is roughly 160 pills per person per year. In the future, medicine use will only grow, with an expected increase of more than a third by 2050. This will inevitably result in increased exposure to the environment and also
our drinking water. ‘To determine effects of these mixtures on the environment is complex and expensive, while removal of these compounds could be cheaper than monitoring’, states van Gijn. ‘It is therefore important to develop technology to remove these compounds from wastewater before they are discharged into surface waters.’

New cleaning technology
Current methods to remove micro-pollutants from wastewater include treatment with activated carbon or ozone. Activated carbon and ozone treatment both have the disadvantage that they require quite some energy to be produced. Reducing the amount of ozone needed could make this method more cost-effective and sustainable.  Therefore, van Gijn is developing a new, more efficient cleaning technology.

Microorganisms play a key role in this new method. ‘I think ozone and microorganisms fit well together’, van Gijn explains. ‘Ozone can be used to break down the most recalcitrant micro-pollutants into smaller pieces, where after microorganisms take care of these smaller pieces.’ But that’s only half of the method. ‘When the ozone treatment starts, a lot of this reactive compound is used to break down background organic material, making the treatment less efficient’, van Gijn explains. ‘By including an extra bacteriological cleaning step before the ozone treatment, we try to effectively get rid of this organic material and save on ozone.’ To make this first bacteriological step as efficient as possible, the scientist is optimizing reactor conditions for the microorganisms involved, for example by comparing different growth surfaces for the bacteria. In separate experiments, van Gijn is also trying to find out which groups of these organic compounds are responsible for the unwanted ozone consumption. Van Gijn: ‘With this knowledge we can start investigating how to breakdown these specific compounds and really increase the efficiency of ozonation.’  

Safe effluent
After reducing the amount of background organic material, the next cleaning step exposes wastewater to ozone by simply bubbling ozone gas through it. This results in breakdown of the more biologically persistent micro-pollutants such as painkiller diclofenac. ‘Finally, during the third step, different microorganisms play a main role in degrading the resulting breakdown products. Together with the department of Aquatic Ecology and Water Quality, van Gijn will also assess the toxicity of the cleaned effluent on different layers of the ecosystem to check the effectivity of our cleaning method. Van Gijn: ‘When we have developed the most effective cleaning method, resulting in a safe effluent, we will test our method in a scaled-up pilot plant.’