Granular filtration backwash

A typical physical-chemical treatment system incorporates three "dual" medial (sand anthracite) filters connected in parallel in its treatment train. The major maintenance consideration with granular medial filtration is the handling of the backwash. The backwash will generally contain a high concentration of contaminants and require subsequent treatment. 

Sand filters vary in sophistication. A simple filter will remove most particles down to 5 pm. Multi-media filters which use sand and anthracite, and possibly a third medium, in discrete layers, can yield very efficient filtration down to 2 pm. Granular activated carbon can be used instead of sand to add some measure of organic removal to the filtration process. The quality produced by any filter depends largely on the efficiency of the backwash. Sand filters in some form provide a satisfactory solution for the majority of water-filtration problems. 

Granular bed filters are used in porcelain enameling wastewater treatment to remove residual solids from clarifier effluent (sedimentation effluent or flotation effluent). Filtration polishes the effluent and reduces suspended solids and insoluble precipitated metals to very low levels. Fine sand and coal are media commonly utilized in granular bed filtration. The filter is backwashed after becoming loaded with solids and the backwash is returned to the treatment plant influent for removal of solids in the clarification step.10-12... 

Cleasby JL, Baumann AR. Backwash of granular filters used in waste water filtration. Final Report on EPA Project R 802140, Engineering Research Institute, Iowa State University, 1976. 

The granular media filters are backwashed using filtered water or, if acceptable, concentrate from the SWRO membrane system. The backwash rate is typically designed to provide 30-50% media bed expansion. The number of individual filter units is usually selected such that the filtration system can operate at fiiU production capacity with one filter out of service in backwash and one out of service for maintenance. 

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