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Surface waters are prone to seasonal and annual changes in the concentration of the suspended matter. This is a concern for conventional media filters since breakthrough of solids will typically occur if the solids holding capacity is exceeded. There will not be any solids breakthrough if the solids holding capacity of the media in the Micro Media Filter is exceeded due an increase in the feed suspended solids concentration. The pressure drop across the filter will merely increase to the pre-determined value that automatically initiates the backwash sequence. This operational feature ensures there will not be any contamination of downstream storage volumes or treatment equipment.
Removal of organics.
Coagulation and fine filtration with the Micro Media Filter is a proven method for removing naturally occurring organics. Studies have shown a reduction in tannic acid concentration from around 2.0 ppm to 0.35 ppm or less. This represents greater than 80% reduction. The negatively charged organic molecules attach themselves to the positively charged metal ions from the coagulant during the agglomeration of the small particles (i.e., during floc formation). These organics are then removed with the agglomerated solids in the filter. An organic polymer can also be added to improve the coagulation and filtration processes by promoting the formation of a stronger floc that will remain intact within the filter bed. This prevents floc fragmentation and breakthrough into the filtrate stream.
Micro vs. Conventional Dual Layer Media Filter
Naturally occurring organics, such as tannic, humic or fulvic acids, enter the feed water supply from decaying vegetation or agricultural runoff. These organics tend to accumulate on the surfaces of reverse osmosis membranes or on anion resin in demineralization equipment. This gradual fouling will lead to poor performance and increased operating costs. The superficial flow rate (i.e., flow per surface area) of the Micro Media Filter is two to three times that of a conventional multi-media filter. This increased unit productivity results in a much smaller filter vessel diameter and an associated reduction in capital cost and required floor space.
In other media filters, the force of a high flow rate drives solids through the filter and into the filtrate stream. The fine media layer at the bottom of the Micro Media Filter ensures solids will not breakthrough, even at the higher operational flow rate. In fact, the filtrate quality of the Micro Media Filter at its high superficial flow is superior to the filtrate quality of a conventional dual media filter at the lower flow rate.
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Compare with ultra filtration or micro filtration.
The Micro Media Filter provides filtered water quality that meets or exceeds the requirements of R.O. membrane manufacturers. Along with good operating practice, this ensures long R.O. membrane life. While the use of membrane-based prefiltration (ultrafiltration or micro-filtration) provides marginally higher filtered water quality, the incremental benefit to R.O. membrane life does not justify the significant additional cost of membrane prefilters. The Micro Media Filter is able to achieve its high level of performance by using two backwashable particulate media layers.
An upper layer of coarse media allows for an extended service cycle by providing depth filtration of large particles while a lower layer of very dense micromedia provides surface filtration of small coagulated particles
Micro-Media Filter Design.
An advanced media filter configuration utilizes a very fine 'polishing' or 'micro' media has been developed. The micro-media filter attempts to address some of the limitations of conventional media filters while improving filtration efficiency to levels approaching that previously achievable only with membranes. The filter is basically a two layer depth media filter. There are a number of features which depart significantly from the conventional design, however.
Its main design features are as follows:
Coarse upper layer-the top layer consists of approximately 30 inches (76 cm) of coarse anthracite. This material is similar to, but somewhat finer than that used in a conventional dual media filter. As with conventional dual layer filters, the top layer provides the bulk of the solids retention and therefore defines the run length.
Fine Micro-media lower layer. The lower layer of the filter is a significant departure from the conventional design. Whereas dual media filters typically employ about 8 inches (20 cm) of silica sand with an effective size of about 0.35 mm, the filter uses a layer of high density media with an effective size of less than 0.1 mm. The lower layer removes the residual quantity of fine suspended solids not retained in the upper layer and therefore effectively defines the filtration efficiency.