BIOFOULING IN MEMBRANE SYSTEMS - A REVIEW

The use of microbiocides, particularly chlorine may be advantageous to operation but can also exacerbate biofouling problems. Micro-organism s subjected to low levels of biocides often exude large amounts of ext racellular polysaccharides (EPS) as a protection, and it is this EPS m aterial that forms the biofilm. This paper examines the causes and eff ects of obstinate biofilms in membrane elements. In these cases proble ms of increased differential pressure have proven difficult to correct during routine cleaning cycles. Consequently, regrowth rates, as indi cated by this differential pressure for such biofilms have been rapid. Experimental data has been taken from more than a hundred membrane au topsies from around the world. These autopsies have been undertaken wh en such problematic biofilms are encountered. These confirm that biofi lms formed in process and membrane systems are comparable except that membrane biofilms contain greater numbers of fungi. Some operating sys tems have reported the cessation of chlorination with a significant re duction of biofouling. The paper considers possible causes for this. I t also considers the increasing use of proprietary non-oxidising micro biocides. Conclusions are that biofouling is endemic within membrane s ystems, yet many systems operate satisfactorily even with a biofilm. F oulant layers can be 'conditioned' or 'hardened' by the repeated use o f cleaning programmes and there is a strong case for alternating clean ers and biocides as used by the cooling water treatment industry.