Research

Structure of the fouling layer

Characterizations by confocal laser microscopy indicate that a fouling layer is formed on the membrane. This fouling layer is penetrated by channels and cavities which form a complex structure in the layer. The heterogeneity of this layer increases over filtration time. The development of this channel structure in the fouling layer prevents the membrane from complete clogging. Not only the heterogeneity, but also the thickness of the fouling layer increases over time. This occurs due to accumulation of particles, organic matter and microorganisms. A thick fouling layer causes high resistance and low flux. High heterogeneity has an opposite effect. Stable flux is reached because an increase of heterogeneity counteracts the decline of flux.

Channels and cavities are visible on the cross-sections of fouled membranes imaged by confocal laser microscopy. The arrows show the separation plane between the membrane and the fouling layer. In the fouling layer, purple indicates particles, green – bacteria and red – biopolymers.

The role of biological processes

When biological processes in the fouling layer are suppressed, for example by addition of a disinfectant or at low temperatures, flux does not stabilize. In this case, complex channel structures are not formed and the layer is rather homogeneous and compact. The thickness and resistance of this layer increases over time, which leads to a decline of flux.
The exact nature of these biological processes which cause structural changes in the fouling layer has not been conclusively established so far. Predators, degradation of biopolymers in the fouling layer and biomass die-off are some of factors which may influence the process.

Fouling layer after 1 and 3 month of filtration with diluted wastewater.