TEP and biofilm fouling on membranes


Biological fouling on membranes is a major problem in desalination and water purification plants. Oceanographers and limnologists have found that most marine and fresh waters are full of microscopic Transparent Exopolymer Particles, otherwise known as TEP. In this article, aquatic microbiologist Tom Berman and Filtration Specialist Marina Holenberg (Amiad Filtration Systems, Amiad, Israel) propose that TEP in source waters is a prime factor leading to biofilm growth on membrane surfaces. Because TEP levels can be lowered by appropriate pre-filtration, they suggest measuring TEP concentrations to determine the efficiency of pre-filtration arrays upstream from membranes.

Biological fouling caused by the growth of biofilm on RO and NF membranes can be a major problem in desalination and water purification plants. Biofilm development lowers filtration efficiency and eventually the membranes must be replaced. Biofilm is usually made up of layers assorted microbial populations, mostly bacteria, held together in a sticky matrix of extracellular polymeric substances, EPS. These substances are largely mucopolysaccharides, long-chain polymers of amino-sugars.

It is generally accepted that the growth of biofilm on membranes is fueled by dissolved organic material, DOM, in the input water. Current thinking is that much of the DOM used by biofilm microorganisms derives from the cells of algae and other plankton that are mechanically disrupted or damaged as a result of pre-filtration upstream of the membranes. As a result, these organisms leak their cellular contents of DOM into the water that flows over the membranes and thus provide readily available nutrition for the biofilm bacteria and their associates. Bits and pieces of cells and other detritus that also stick to the membrane surface may spur further biofilm development.

We doubt that the above scenario accurately or adequately explains how biofilm fouling gets its start and grows on membrane surfaces. Most input waters to desalination or water purification plants have naturally high DOM levels. For example, in many marine coastal waters, about 80% of all the nitrogen content (apart from gaseous nitrogen) is in the form of dissolved organic nitrogen, while only about 10% consists of dissolved inorganic nitrogen (nitrate and ammonia). The remaining 10% of nitrogen is in particle form, mostly within the cells of algae, bacteria and zooplankton and in detritus. In a freshwater example, (Lake Kinneret, the Biblical Sea of Galilee, Israel) between 40 and 65% of total nitrogen is dissolved organic nitrogen, about 8 to 10% is dissolved inorganic nitrogen and the remainder is particulate N.

Not all of the large DOM pool is readily used by microorganisms. Nevertheless we stress that most source waters used for desalination or water treatment have initially high concentrations of DOM, even prior to any addition resulting from pre-filtration. But, even more significantly, most input waters carry high loads of TEP.

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