Biofouling in membrane filtration systems and control strategies

Biofouling in membrane filtration systems and control strategies

The use of RO membrane for water treatment is usually accompanied with the accumulation of foulants on the membrane that negatively affects the quality of the product and the operation cost. The accumulated foulants on the RO membrane can be categorized into four groups: 1- crystalline fouling such as mineral deposition 2- organic fouling such as the deposition of the organic matter 3- particle and colloidal fouling such as the deposition of clay and slit 4-Microbiological fouling which involves the adhesion and subsequent growth of microorganisms on the membrane. The first three types of fouling can be controlled by reducing the concentration of the foulants in the RO feed water. In comparison. biofouling control is a much more complicated process. In addition to reducing the concentration of the existing microorganisms in the RO feed water, biofouling control requires additional efforts represented by maintaining proper monitoring strategies and controlling the factors affecting biofilm developments such as nutrient concentration and the physio-chemical interactions between microorganisms and membrane surface Biofouling is defined as the accumulation of microorganisms accompanied with agglomeration of extracellular materials on the membrane surface. When the microorganisms adhere to the membrane surface. they start building up their aggregates biofilm . Biofilm is regarded as one of the oldest and most robust forms of life on the earth. Biofilm has a strong structure that results from the capacity of microorganisms on developing layers of polymer-like materials called Extra-cellar Polymeric Substances (EPS), which can protect the microorganisms from biocides and toxins. The microorganisms presented in the matrix of biofilm can obtain nutrients for maintaining their life functions by secluding organic or inorganic substances from the surrounding environment. Biofouling problem can render the advanced filtration techniques such as RO very costly process. The cost associated with the biofouling problem in water treatment plants is a combination of a number of losses including the following: the deterioration of plant performance, decrease in the quality and quantity of the product, the damage in the construction materials of the plant, need for extensive cleaning processes, addition of biocides, the need of additional process to treat the contaminated water by antifouling chemicals, labor cost involved in cleaning or replacing the membranes and cost associated with the down-time of the plant. This has been reported that the estimated cost required to combat biofouling problem in desalination industry is about 10 billion pound sterling. Recently, it was shown that the biofouling problem has reduced the working lifetime of nanofiltration membrane used for sea water treatment from 3 to 1 year. This action led to increase the cost associated with replacing the membrane from 0.58 million Euro to 1.75 million Euro. In addition to the adverse effect of biofouling on the cost and the performance of water treatment process, biofouling forms a great threat to the public health. For instance, the formed biofilm on a membrane surface acts as a host for the pathogenic microorganisms that can settle and survive in the integrated environment of the biofilm. So biofilm plays a role of inaccessible source for contamination in the water treatment systems that have the capacity to release microorganisms to the flowing water in the system. Furthermore, the health risk associated with biofilm is extended to include the release of the harmful compounds from the metabolic activities of the microorganisms in the biofilm matrix such as endotoxins. Owing to the enormous threat of biofouling on the environmental economical and hygienic aspects of water treatment system and the strong structure of biofilm, it is of utmost importance to plan the combat against such recalcitrant problem carefully. Many techniques have been reported to decrease the deposition of microorganisms and biotic material on the RO membrane including: 1-techniques which are capable of reducing the formation of biofouling on the membrane surface through adjusting the characteristics of the membrane surface in a way that decreases membrane propensity towards biofouling 2- techniques that have the potential to degrade the structure of an established biofilm 3-decreasing the deposition of microorganisms on the membrane through adjusting the pH of the membrane fed solution 4- techniques that can reduce the formation of biofouling on the RO membrane by removing or deactivating the planktonic microorganisms existing in the upstream of the membrane. Some of these techniques can also decrease biofouling activities through secluding the available nutrients for microorganisms in the membrane feed. Consequently, the combination of physical disinfection, nutrient limitation and membrane surface modification appears to be an efficient integrated system that has the potential to control biofouling in RO systems in an environmentally friendly way. Owning to the stringent regulations of health organization over the quality of the treated water, the use of environmentally friendly techniques for controlling biofouling is preferable. For this reason, the application of enzymes as new emerging method in the aim of reducing biofouling in the membrane system was used.