Membrane fouling systems are a well-known phenomenon. The systems can suffer from fouling of organics, suspended matter or inorganics. In most cases the first two sorts of membrane fouling are located in the feed element of the membrane plants first stage. On the other hand the inorganic solids tend to form and precipitate at that spot in the system where the concentration is the highest. Typically this occurs in the last elements of the last stage of the plant.
Organic fouling
The deposition of organics as well as suspended matter, which are not adequately removed in the pre-treatment. But adherence and growth of bacteria and subsequently slime formation, will take place in the first moments when the feed water enters the membrane system. This is due to the filtering effect of the membranes. When the suspended matter and organic substances such as humic acids and polysaccharides enter the membrane plants first elements, they will be directly drawn to the membrane surface and remain there as a layer while the pure water is produced.
Accumulation of this substances over time can lead to severe membrane fouling, leading to loss of production and lower quality of the purified water. Bacteria in the incoming water, when it is in contact with any surface of feed spacer and the membrane tend to adhere and start growing. At a certain point the bacteria colonies will excrete Extracellular Poly Saccharides (EPS) which is a immobile slimy layer that is voluminous and thus leading to the same results as organics and suspended matter. The only difference being that the latter are fed to the fouling membrane system and the EPS being produced there.
Inorganic Fouling
Because of the production of pure water throughout the membrane fouling system the salt ions, that are rejected by the membrane, will concentrate in the remaining water. The maximum concentration of these salts therefore will occur in the last element close to the membrane surface where the mixing effects with the bulk water are lowest.
Sparingly soluble salts can reach a concentration above there saturation level and start to initiate the first phase of crystallization known as pre-clustering. These tiny crystals of around 100 molecules which act as trigger to other super saturated molecules and will grow into bigger crystals. These will deposit on the membrane surface and affect the performance of the fouling system. The dynamics of these processes are still poorly understood. The influence of temperature, pH and salts composition to the time of forming actual crystals of different salts, generally called induction time, is under investigation.
Salts of Calcium, Barium and Strontium with Carbonate, Sulphate and Phosphate as well as Silica are commonly found in membrane fouling systems. Since the saturation levels of these salts are known and the actual concentration of the ions at any point in the system can be calculated form the feed water composition and the plant data. The appearance of this fouling is predictable. Therefore it can be prevented easily by adjusting the plants parameters and with the use of additives such as anti scalents. scaling is the expression mostly used for inorganic fouling.