Halfway - Fighting for resources

Mari Winkler: “I succeeded in allowing good PAOs to maintain the upper hand.” (Photo: Tomas van Dijk)

Name: Mari Winkler (30)
Nationality: German
Supervisor: Professor Mark van Loosdrecht (faculty of Applied
Sciences)
Subject: Microbial population engineering with aerobic granular sludge
Thesis defense: In 18 months

“Look at these granules; inside is a very complicated ecosystem with different bacteria fighting for resources. If you get the system right, then all aspects of the water purification process occur in a single granule, which is very special. Normally, to purify water you must let the water flow through a series of tanks containing different prevailing conditions. A varying condition in these tanks would be, for instance, the amount of oxygen.
Generally, in order to clean sewage water, you must first let bacteria perform oxidation reactions. The resulting chemicals can then be reduced by other bacteria in tanks that are low in oxygen. But by using granules, both types of reactions can occur in one reactor, because oxygen hardly enters the inner part of the granules.
Using a technique called ‘Fluorescence in Situ Hybridization’, I attached fluorescing chemicals of differing colours to different bacteria. I then observed, under the microscope, a very interesting segregation of organisms, with bacteria that use oxygen (aerobic bacteria) in the outer layer and anaerobic bacteria in the inner.
The technique is very promising. Next year full scale experiments will be conducted on aerobic granular sludge in Epe.
For warm weather countries however this method is problematic. At 30 C°, the bacteria that accumulate phosphate - the so-called Phosphate Accumulating Organisms (PAO) - are outcompeted by Glycogen Accumulating Organisms (GAOs). Yet what I found out is that these GAOs are mainly present in granules found in the sludge’s upper layer. PAOs, on the other hand, gather phosphates, making them heavier, and hence PAOs sink to the bottom.
By frequently removing water from the reactor’s top layer, I succeeded in allowing the good PAOs to maintain the upper hand. Normally, in order to control the competition between the various bacteria, you would play with temperature, acidity and oxygen concentration.
What you see in this second reactor is quite new. The reactor contains granules with Anammox bacteria. These microorganisms can be very useful for cleaning water that is rich in ammonium and poor in organic carbon compounds. These microorganisms oxidize ammonium. In the reactor, there must be some oxygen but not too much, because otherwise nitrite oxidizing bacteria gain the upper hand. The bacteria are playing a game: who can pull hardest on the nitrite.”