- Filtration means the accumulation of molecules on and within the membrane
- Flow technology influences the structure of the fouling layer
- Reversible fouling = concentration polarisation
- Irreversible fouling = protein gel
Why are other measures valid for the cleaning strategy of hygienically clean and optimally functioning membrane systems than is the case for conventional CIP cleaning?
During the filtration, molecules accumulate on and within the membrane. On the surface of the membrane a boundary layer forms that is only a few micrometres thick. This boundary layer is of vital importance for the performance of the membrane, because primarily it has an effect on the mass flow through the membrane.
Besides this known surface contamination, molecules also accumulate in the pores of the membrane, which also have a decisive effect on the filtration process. The deposits in the pores are primarily responsible for an altered retention capacity of the membrane, because they change the loading and surface properties of the membrane. This in turn changes the filtration properties of the membrane and thus the possible result of the whole filtration plant. Fouling is a natural and unavoidable phenomenon of filtration.
Practically all membranes, in all applications, are subject to fouling. We differentiate between reversible and irreversible fouling, based on the principle of how firmly the molecules adhere.
- After a certain time, convection and back diffusion come into equilibrium.
- The wall concentration (Cwall) is much higher than the concentration in the core stream (Cbulk) !
As the filtration continues, more and more molecules are transported to the membrane. There the larger molecules are retained. They build up a second layer on the membrane. The dynamic build-up of this layer is called concentration polarisation. Concentration polarisation has an effect on the performance and retention of the filtration.
This effect can be recognised by a drop in the permeate throughput in the first minutes of the filtration process.
In spite of its very low thickness of only a few micrometres, the influence of the fouling layer on the filtration is enormous. The performance of the plant, i.e. the throughput of permeate, is very severely reduced, because the fouling layer acts as an additional filtration layer. This additional layer increases the retention of other molecules, which can have an undesirable effect.
Fouling cannot be avoided, but through appropriate cleaning strategies, suitable membrane selection and well-aligned operating conditions, it can be minimized.
- Water as carrier
- Enormous surfaces
- Plant-related concentration determination = liquor ratio
- Quantity per surface unit
- Cleaning of the surface and the membrane pores
- Sensitivity of the membrane material
- No visual assessment possible
- Moistening requirement promotes bacterial growth
- Only the filtration performance gives an indication of the cleaning Performance
Sustainable maintenance of membranes involves the removal of the irreversible layer through the targeted application of cleaning chemicals. However, there are restrictive limitations when applying chemical cleaning agents in membrane systems.