Trial investigating the effects of different water treatments on RAS water quality. Photo: Kim Gregersen.

PhD defence about micro particles in recirculating aquaculture systems

Monday 09 Nov 20


Kim Joao de Jesus Gregersen
DTU Aqua
+45 35 88 32 04


Anne Johanne Tang Dalsgaard
Senior Researcher
DTU Aqua
+45 35 88 32 16

Time & Place

The defence takes place on Tuesday 17 November 2020 at 10:00 (CEST) via Zoom.

Please register for the Zoom meeting no later than 16 November at 10:00 by e-mail to Karin Stubgaard,

On 17 November 2020, Kim João de Jesus Gregersen will defend his PhD thesis. The defence can be watched online.

The use of recirculating aquaculture systems (RAS) for fish farming is developing rapidly. These systems have a lower water usages and longer retention times, which has led to a need for increased focus on understanding the micro particles that build-up in the water and cause an impaired water quality. 

PhD Student Kim João de Jesus Gregersen from DTU Aqua and his colleagues have investigated the origins of micro particles and how micro particles can be controlled in recirculating aquaculture systems. 

In his PhD thesis, “Beyond water quality: Micro particles in Recirculation Aquaculture Systems”, Kim Gregersen demonstrates that the micro particles are intrinsically connected to bacteria. In low intensity recirculating aquaculture systems, surface area provided by micro particles seems to partly control the amount of bacterial activity in the system, while in higher intensity systems most micro particle dynamics seems to be the result of changes in bacterial populations. The main driver behind the large fluctuations in micro particles seems to be organic matter build-up, the studies show.

Furthermore, Kim Gregersen has tested the ability of different well-known technologies to reduce the amount of micro particles. In these tests, he has used ultra violet radiation and ozone to remove bacteria and has used micro filtration and freshwater foam fractionation to remove organic matter. He shows that combinations of the methods can produce very large reductions in micro particles, with approximately 90 percent reduction of micro particle numbers and bacterial activity, as well as large reductions in organic matter.

About the defence

Kim Gregersen will defend his PhD thesis "Beyond water quality: Micro particles in Recirculation Aquaculture Systems” on Tuesday 17 November 2020 at 10:00 (CEST) via Zoom.


  • Principal supervisor: Senior Researcher Anne Johanne Tang Nielsen, DTU Aqua
  • Co-supervisor: Senior Researcher Lars-Flemming Pedersen, DTU Aqua
  • Co-supervisor: Head of Section Per Bovbjerg Pedersen, DTU Aqua


  • Senior Researcher Ivar Lund, DTU Aqua
  • Senior Advisor Alexander Brinker, Fisheries Research Station, Baden-Württemberg, Germany
  • Professor Uwe Waller, Hochschule für Tecknik und Wirtschaft des Saarlandes, Germany

Chairperson at defence

  • Associate Professor Peter Vilhelm Skov, DTU Aqua

Please register for the Zoom meeting no later than 16 November at 10:00 by e-mail to Karin Stubgaard, You will receive an e-mail with a link to Zoom the day before the defence.

Learn more

A popular science summery of the thesis can be downloaded here

A copy of the thesis is available by e-mail. Please contact Karin Stubgaard,