Self-Cleaning Filtration for Pretreatment of Membrane, UV and Ozone Water Treatment Systems
Recent field data has shown the effectiveness of automatic self-cleaning filters in removing suspended solids down to less than 3 microns in size without flocculents, pre-coats or granulated media. Pre-treatment of ozone systems is often needed to remove organic solids that would utilize high amounts of ozone during oxidation leaving little for complete disinfection. Selfcleaning screen filters can physically remove much of that organic matter efficiently and economically resulting in the need for lower ozone concentrations. The use of ultraviolet light water treatment has been growing rapidly. An effective job of disinfection requires that microorganisms be directly exposed to light of the appropriate wavelength. If suspended particles of sufficient size are present, shadowing can protect microorganisms from inactivating doses of ultraviolet light. Self-cleaning filters remove the likelihood of shadowing. Tests conducted over many days have demonstrated the removal of particles 10-micron and larger from raw lake-water prior to UV treatment. The ratio of water needed in cleaning the screen to process water varied between 0.31% and 0.40%. Membrane systems of differing types (micro, ultra, nano, RO) require different degrees of pre-filtration. Self-cleaning screen filters with weavewire elements down to 10 microns are available for most membrane pre-treatment applications while recently developed 'thread filtration' technology can remove suspended solids down to below 3 microns for reverse osmosis systems. One application of pre-filtration for RO at a coal fired power plant in Nevada showed raw water from a well to have TSS = 10.5 ppm. Laboratory analyses of the particle size distribution after a 10-micron screen filter showed 99% removal of particles 5 micron and larger. Screen filtration of canal water using a 10-micron weavewire screen showed a TSS reduction of 97% in California. Use of a thread filter in Newfoundland has given substantial cryptosporidium removal at a municipal water treatment plant. These examples and others demonstrate the successful and dependable use of automatic self-cleaning mechanical screen and thread filters for suspended solids removal well below the visible limit of the naked eye.